diff options
| author | Araq <rumpf_a@web.de> | 2014-10-02 02:33:59 +0200 |
|---|---|---|
| committer | Araq <rumpf_a@web.de> | 2014-10-02 02:33:59 +0200 |
| commit | 2c1f3f75f5d37db810113cf37e1b38c3b7b09ee7 (patch) | |
| tree | 70b805d8f0e4d62c2e84384d605c084661dd5334 | |
| parent | e9dec2feedc25afd8af8fc3db3131ffbb4b284a7 (diff) | |
| download | Nim-2c1f3f75f5d37db810113cf37e1b38c3b7b09ee7.tar.gz | |
manual split up into multiple files; documented the new concurrency system
31 files changed, 6198 insertions, 5901 deletions
diff --git a/doc/backends.txt b/doc/backends.txt index 307f141b2..eb16217cd 100644 --- a/doc/backends.txt +++ b/doc/backends.txt @@ -103,7 +103,7 @@ component?). Nim code calling the backend --------------------------------- +---------------------------- Nim code can interface with the backend through the `Foreign function interface <manual.html#foreign-function-interface>`_ mainly through the @@ -211,7 +211,7 @@ javascript, you should see the value ``10``. In JavaScript the `echo proc Backend code calling Nim ---------------------------- +------------------------ Backend code can interface with Nim code exposed through the `exportc pragma <manual.html#exportc-pragma>`_. The ``exportc`` pragma is the *generic* @@ -235,7 +235,7 @@ Nim code. Nim invocation example from C -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Create a ``fib.nim`` file with the following content: @@ -291,7 +291,7 @@ use ``-ldl`` too to link in required dlopen functionality. Nim invocation example from JavaScript -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Create a ``mhost.html`` file with the following content: diff --git a/doc/manual.txt b/doc/manual.txt index 8b0f5a5e6..5c6f64344 100644 --- a/doc/manual.txt +++ b/doc/manual.txt @@ -12,5838 +12,27 @@ Nim Manual user to one/some of the other players, but the total amount seems to remain pretty much constant for a given task. -- Ran - - -About this document -=================== - -**Note**: This document is a draft! Several of Nim's features need more -precise wording. This manual will evolve into a proper specification some -day. - -This document describes the lexis, the syntax, and the semantics of Nim. - -The language constructs are explained using an extended BNF, in -which ``(a)*`` means 0 or more ``a``'s, ``a+`` means 1 or more ``a``'s, and -``(a)?`` means an optional *a*. Parentheses may be used to group elements. - -``&`` is the lookahead operator; ``&a`` means that an ``a`` is expected but -not consumed. It will be consumed in the following rule. - -The ``|``, ``/`` symbols are used to mark alternatives and have the lowest -precedence. ``/`` is the ordered choice that requires the parser to try the -alternatives in the given order. ``/`` is often used to ensure the grammar -is not ambiguous. - -Non-terminals start with a lowercase letter, abstract terminal symbols are in -UPPERCASE. Verbatim terminal symbols (including keywords) are quoted -with ``'``. An example:: - - ifStmt = 'if' expr ':' stmts ('elif' expr ':' stmts)* ('else' stmts)? - -The binary ``^*`` operator is used as a shorthand for 0 or more occurances -separated by its second argument; likewise ``^+`` means 1 or more -occurances: ``a ^+ b`` is short for ``a (b a)*`` -and ``a ^* b`` is short for ``(a (b a)*)?``. Example:: - - arrayConstructor = '[' expr ^* ',' ']' - -Other parts of Nim - like scoping rules or runtime semantics are only -described in an informal manner for now. - - -Definitions -=========== - -A Nim program specifies a computation that acts on a memory consisting of -components called `locations`:idx:. A variable is basically a name for a -location. Each variable and location is of a certain `type`:idx:. The -variable's type is called `static type`:idx:, the location's type is called -`dynamic type`:idx:. If the static type is not the same as the dynamic type, -it is a super-type or subtype of the dynamic type. - -An `identifier`:idx: is a symbol declared as a name for a variable, type, -procedure, etc. The region of the program over which a declaration applies is -called the `scope`:idx: of the declaration. Scopes can be nested. The meaning -of an identifier is determined by the smallest enclosing scope in which the -identifier is declared unless overloading resolution rules suggest otherwise. - -An expression specifies a computation that produces a value or location. -Expressions that produce locations are called `l-values`:idx:. An l-value -can denote either a location or the value the location contains, depending on -the context. Expressions whose values can be determined statically are called -`constant expressions`:idx:; they are never l-values. - -A `static error`:idx: is an error that the implementation detects before -program execution. Unless explicitly classified, an error is a static error. - -A `checked runtime error`:idx: is an error that the implementation detects -and reports at runtime. The method for reporting such errors is via *raising -exceptions* or *dying with a fatal error*. However, the implementation -provides a means to disable these runtime checks. See the section pragmas_ -for details. - -Wether a checked runtime error results in an exception or in a fatal error at -runtime is implementation specific. Thus the following program is always -invalid: - -.. code-block:: nim - var a: array[0..1, char] - let i = 5 - try: - a[i] = 'N' - except EInvalidIndex: - echo "invalid index" - -An `unchecked runtime error`:idx: is an error that is not guaranteed to be -detected, and can cause the subsequent behavior of the computation to -be arbitrary. Unchecked runtime errors cannot occur if only `safe`:idx: -language features are used. - - -Lexical Analysis -================ - -Encoding --------- - -All Nim source files are in the UTF-8 encoding (or its ASCII subset). Other -encodings are not supported. Any of the standard platform line termination -sequences can be used - the Unix form using ASCII LF (linefeed), the Windows -form using the ASCII sequence CR LF (return followed by linefeed), or the old -Macintosh form using the ASCII CR (return) character. All of these forms can be -used equally, regardless of platform. - - -Indentation ------------ - -Nim's standard grammar describes an `indentation sensitive`:idx: language. -This means that all the control structures are recognized by indentation. -Indentation consists only of spaces; tabulators are not allowed. - -The indentation handling is implemented as follows: The lexer annotates the -following token with the preceding number of spaces; indentation is not -a separate token. This trick allows parsing of Nim with only 1 token of -lookahead. - -The parser uses a stack of indentation levels: the stack consists of integers -counting the spaces. The indentation information is queried at strategic -places in the parser but ignored otherwise: The pseudo terminal ``IND{>}`` -denotes an indentation that consists of more spaces than the entry at the top -of the stack; IND{=} an indentation that has the same number of spaces. ``DED`` -is another pseudo terminal that describes the *action* of popping a value -from the stack, ``IND{>}`` then implies to push onto the stack. - -With this notation we can now easily define the core of the grammar: A block of -statements (simplified example):: - - ifStmt = 'if' expr ':' stmt - (IND{=} 'elif' expr ':' stmt)* - (IND{=} 'else' ':' stmt)? - - simpleStmt = ifStmt / ... - - stmt = IND{>} stmt ^+ IND{=} DED # list of statements - / simpleStmt # or a simple statement - - - -Comments --------- - -Comments start anywhere outside a string or character literal with the -hash character ``#``. -Comments consist of a concatenation of `comment pieces`:idx:. A comment piece -starts with ``#`` and runs until the end of the line. The end of line characters -belong to the piece. If the next line only consists of a comment piece with -no other tokens between it and the preceding one, it does not start a new -comment: - - -.. code-block:: nim - i = 0 # This is a single comment over multiple lines. - # The scanner merges these two pieces. - # The comment continues here. - - -`Documentation comments`:idx: are comments that start with two ``##``. -Documentation comments are tokens; they are only allowed at certain places in -the input file as they belong to the syntax tree! - - -Identifiers & Keywords ----------------------- - -Identifiers in Nim can be any string of letters, digits -and underscores, beginning with a letter. Two immediate following -underscores ``__`` are not allowed:: - - letter ::= 'A'..'Z' | 'a'..'z' | '\x80'..'\xff' - digit ::= '0'..'9' - IDENTIFIER ::= letter ( ['_'] (letter | digit) )* - -Currently any unicode character with an ordinal value > 127 (non ASCII) is -classified as a ``letter`` and may thus be part of an identifier but later -versions of the language may assign some Unicode characters to belong to the -operator characters instead. - -The following keywords are reserved and cannot be used as identifiers: - -.. code-block:: nim - :file: keywords.txt - -Some keywords are unused; they are reserved for future developments of the -language. - -Nim is a `style-insensitive`:idx: language. This means that it is not -case-sensitive and even underscores are ignored: -**type** is a reserved word, and so is **TYPE** or **T_Y_P_E**. The idea behind -this is that this allows programmers to use their own preferred spelling style -and libraries written by different programmers cannot use incompatible -conventions. A Nim-aware editor or IDE can show the identifiers as -preferred. Another advantage is that it frees the programmer from remembering -the exact spelling of an identifier. - - -String literals ---------------- - -Terminal symbol in the grammar: ``STR_LIT``. - -String literals can be delimited by matching double quotes, and can -contain the following `escape sequences`:idx:\ : - -================== =================================================== - Escape sequence Meaning -================== =================================================== - ``\n`` `newline`:idx: - ``\r``, ``\c`` `carriage return`:idx: - ``\l`` `line feed`:idx: - ``\f`` `form feed`:idx: - ``\t`` `tabulator`:idx: - ``\v`` `vertical tabulator`:idx: - ``\\`` `backslash`:idx: - ``\"`` `quotation mark`:idx: - ``\'`` `apostrophe`:idx: - ``\`` '0'..'9'+ `character with decimal value d`:idx:; - all decimal digits directly - following are used for the character - ``\a`` `alert`:idx: - ``\b`` `backspace`:idx: - ``\e`` `escape`:idx: `[ESC]`:idx: - ``\x`` HH `character with hex value HH`:idx:; - exactly two hex digits are allowed -================== =================================================== - - -Strings in Nim may contain any 8-bit value, even embedded zeros. However -some operations may interpret the first binary zero as a terminator. - - -Triple quoted string literals ------------------------------ - -Terminal symbol in the grammar: ``TRIPLESTR_LIT``. - -String literals can also be delimited by three double quotes -``"""`` ... ``"""``. -Literals in this form may run for several lines, may contain ``"`` and do not -interpret any escape sequences. -For convenience, when the opening ``"""`` is followed by a newline (there may -be whitespace between the opening ``"""`` and the newline), -the newline (and the preceding whitespace) is not included in the string. The -ending of the string literal is defined by the pattern ``"""[^"]``, so this: - -.. code-block:: nim - """"long string within quotes"""" - -Produces:: - - "long string within quotes" - - -Raw string literals -------------------- - -Terminal symbol in the grammar: ``RSTR_LIT``. - -There are also raw string literals that are preceded with the -letter ``r`` (or ``R``) and are delimited by matching double quotes (just -like ordinary string literals) and do not interpret the escape sequences. -This is especially convenient for regular expressions or Windows paths: - -.. code-block:: nim - - var f = openFile(r"C:\texts\text.txt") # a raw string, so ``\t`` is no tab - -To produce a single ``"`` within a raw string literal, it has to be doubled: - -.. code-block:: nim - - r"a""b" - -Produces:: - - a"b - -``r""""`` is not possible with this notation, because the three leading -quotes introduce a triple quoted string literal. ``r"""`` is the same -as ``"""`` since triple quoted string literals do not interpret escape -sequences either. - - -Generalized raw string literals -------------------------------- - -Terminal symbols in the grammar: ``GENERALIZED_STR_LIT``, -``GENERALIZED_TRIPLESTR_LIT``. - -The construct ``identifier"string literal"`` (without whitespace between the -identifier and the opening quotation mark) is a -generalized raw string literal. It is a shortcut for the construct -``identifier(r"string literal")``, so it denotes a procedure call with a -raw string literal as its only argument. Generalized raw string literals -are especially convenient for embedding mini languages directly into Nim -(for example regular expressions). - -The construct ``identifier"""string literal"""`` exists too. It is a shortcut -for ``identifier("""string literal""")``. - - -Character literals ------------------- - -Character literals are enclosed in single quotes ``''`` and can contain the -same escape sequences as strings - with one exception: `newline`:idx: (``\n``) -is not allowed as it may be wider than one character (often it is the pair -CR/LF for example). Here are the valid `escape sequences`:idx: for character -literals: - -================== =================================================== - Escape sequence Meaning -================== =================================================== - ``\r``, ``\c`` `carriage return`:idx: - ``\l`` `line feed`:idx: - ``\f`` `form feed`:idx: - ``\t`` `tabulator`:idx: - ``\v`` `vertical tabulator`:idx: - ``\\`` `backslash`:idx: - ``\"`` `quotation mark`:idx: - ``\'`` `apostrophe`:idx: - ``\`` '0'..'9'+ `character with decimal value d`:idx:; - all decimal digits directly - following are used for the character - ``\a`` `alert`:idx: - ``\b`` `backspace`:idx: - ``\e`` `escape`:idx: `[ESC]`:idx: - ``\x`` HH `character with hex value HH`:idx:; - exactly two hex digits are allowed -================== =================================================== - -A character is not an Unicode character but a single byte. The reason for this -is efficiency: for the overwhelming majority of use-cases, the resulting -programs will still handle UTF-8 properly as UTF-8 was specially designed for -this. Another reason is that Nim can thus support ``array[char, int]`` or -``set[char]`` efficiently as many algorithms rely on this feature. The `TRune` -type is used for Unicode characters, it can represent any Unicode character. -``TRune`` is declared in the `unicode module <unicode.html>`_. - - -Numerical constants -------------------- - -Numerical constants are of a single type and have the form:: - - hexdigit = digit | 'A'..'F' | 'a'..'f' - octdigit = '0'..'7' - bindigit = '0'..'1' - HEX_LIT = '0' ('x' | 'X' ) hexdigit ( ['_'] hexdigit )* - DEC_LIT = digit ( ['_'] digit )* - OCT_LIT = '0o' octdigit ( ['_'] octdigit )* - BIN_LIT = '0' ('b' | 'B' ) bindigit ( ['_'] bindigit )* - - INT_LIT = HEX_LIT - | DEC_LIT - | OCT_LIT - | BIN_LIT - - INT8_LIT = INT_LIT ['\''] ('i' | 'I') '8' - INT16_LIT = INT_LIT ['\''] ('i' | 'I') '16' - INT32_LIT = INT_LIT ['\''] ('i' | 'I') '32' - INT64_LIT = INT_LIT ['\''] ('i' | 'I') '64' - - UINT8_LIT = INT_LIT ['\''] ('u' | 'U') - UINT8_LIT = INT_LIT ['\''] ('u' | 'U') '8' - UINT16_LIT = INT_LIT ['\''] ('u' | 'U') '16' - UINT32_LIT = INT_LIT ['\''] ('u' | 'U') '32' - UINT64_LIT = INT_LIT ['\''] ('u' | 'U') '64' - - exponent = ('e' | 'E' ) ['+' | '-'] digit ( ['_'] digit )* - FLOAT_LIT = digit (['_'] digit)* (('.' (['_'] digit)* [exponent]) |exponent) - FLOAT32_LIT = HEX_LIT '\'' ('f'|'F') '32' - | (FLOAT_LIT | DEC_LIT | OCT_LIT | BIN_LIT) ['\''] ('f'|'F') '32' - FLOAT64_LIT = HEX_LIT '\'' ('f'|'F') '64' - | (FLOAT_LIT | DEC_LIT | OCT_LIT | BIN_LIT) ['\''] ('f'|'F') '64' - - -As can be seen in the productions, numerical constants can contain underscores -for readability. Integer and floating point literals may be given in decimal (no -prefix), binary (prefix ``0b``), octal (prefix ``0o``) and hexadecimal -(prefix ``0x``) notation. - -There exists a literal for each numerical type that is -defined. The suffix starting with an apostrophe ('\'') is called a -`type suffix`:idx:. Literals without a type suffix are of the type ``int``, -unless the literal contains a dot or ``E|e`` in which case it is of -type ``float``. For notational convenience the apostrophe of a type suffix -is optional if it is not ambiguous (only hexadecimal floating point literals -with a type suffix can be ambiguous). - - -The type suffixes are: - -================= ========================= - Type Suffix Resulting type of literal -================= ========================= - ``'i8`` int8 - ``'i16`` int16 - ``'i32`` int32 - ``'i64`` int64 - ``'u`` uint - ``'u8`` uint8 - ``'u16`` uint16 - ``'u32`` uint32 - ``'u64`` uint64 - ``'f32`` float32 - ``'f64`` float64 -================= ========================= - -Floating point literals may also be in binary, octal or hexadecimal -notation: -``0B0_10001110100_0000101001000111101011101111111011000101001101001001'f64`` -is approximately 1.72826e35 according to the IEEE floating point standard. - - -Operators ---------- - -In Nim one can define his own operators. An operator is any -combination of the following characters:: - - = + - * / < > - @ $ ~ & % | - ! ? ^ . : \ - -These keywords are also operators: -``and or not xor shl shr div mod in notin is isnot of``. - -`=`:tok:, `:`:tok:, `::`:tok: are not available as general operators; they -are used for other notational purposes. - -``*:`` is as a special case the two tokens `*`:tok: and `:`:tok: -(to support ``var v*: T``). - - -Other tokens ------------- - -The following strings denote other tokens:: - - ` ( ) { } [ ] , ; [. .] {. .} (. .) - - -The `slice`:idx: operator `..`:tok: takes precedence over other tokens that -contain a dot: `{..}`:tok: are the three tokens `{`:tok:, `..`:tok:, `}`:tok: -and not the two tokens `{.`:tok:, `.}`:tok:. - - -Syntax -====== - -This section lists Nim's standard syntax. How the parser handles -the indentation is already described in the `Lexical Analysis`_ section. - -Nim allows user-definable operators. -Binary operators have 10 different levels of precedence. - -Relevant character ------------------- - -An operator symbol's *relevant character* is its first -character unless the first character is ``\`` and its length is greater than 1 -then it is the second character. - -This rule allows to escape operator symbols with ``\`` and keeps the operator's -precedence and associativity; this is useful for meta programming. - - -Associativity -------------- - -Binary operators whose relevant character is ``^`` are right-associative, all -other binary operators are left-associative. - -Precedence ----------- - -Unary operators always bind stronger than any binary -operator: ``$a + b`` is ``($a) + b`` and not ``$(a + b)``. - -If an unary operator's relevant character is ``@`` it is a `sigil-like`:idx: -operator which binds stronger than a ``primarySuffix``: ``@x.abc`` is parsed -as ``(@x).abc`` whereas ``$x.abc`` is parsed as ``$(x.abc)``. - - -For binary operators that are not keywords the precedence is determined by the -following rules: - -If the operator ends with ``=`` and its relevant character is none of -``<``, ``>``, ``!``, ``=``, ``~``, ``?``, it is an *assignment operator* which -has the lowest precedence. - -Otherwise precedence is determined by the relevant character. - -================ =============================================== ================== =============== -Precedence level Operators Relevant character Terminal symbol -================ =============================================== ================== =============== - 9 (highest) ``$ ^`` OP9 - 8 ``* / div mod shl shr %`` ``* % \ /`` OP8 - 7 ``+ -`` ``+ ~ |`` OP7 - 6 ``&`` ``&`` OP6 - 5 ``..`` ``.`` OP5 - 4 ``== <= < >= > != in notin is isnot not of`` ``= < > !`` OP4 - 3 ``and`` OP3 - 2 ``or xor`` OP2 - 1 ``@ : ?`` OP1 - 0 (lowest) *assignment operator* (like ``+=``, ``*=``) OP0 -================ =============================================== ================== =============== - - -Strong spaces -------------- - -The number of spaces preceeding a non-keyword operator affects precedence -if the experimental parser directive ``#!strongSpaces`` is used. Indentation -is not used to determine the number of spaces. If 2 or more operators have the -same number of preceding spaces the precedence table applies, so ``1 + 3 * 4`` -is still parsed as ``1 + (3 * 4)``, but ``1+3 * 4`` is parsed as ``(1+3) * 4``: - -.. code-block:: nim - #! strongSpaces - if foo+4 * 4 == 8 and b&c | 9 ++ - bar: - echo "" - # is parsed as - if ((foo+4)*4 == 8) and (((b&c) | 9) ++ bar): echo "" - - -Furthermore whether an operator is used a prefix operator is affected by the -number of spaces: - -.. code-block:: nim - #! strongSpaces - echo $foo - # is parsed as - echo($foo) - -This also affects whether ``[]``, ``{}``, ``()`` are parsed as constructors -or as accessors: - -.. code-block:: nim - #! strongSpaces - echo (1,2) - # is parsed as - echo((1,2)) - -Only 0, 1, 2, 4 or 8 spaces are allowed to specify precedence and it is -enforced that infix operators have the same amount of spaces before and after -them. This rules does not apply when a newline follows after the operator, -then only the preceding spaces are considered. - - -Grammar -------- - -The grammar's start symbol is ``module``. - -.. include:: grammar.txt - :literal: - - - -Types -===== - -All expressions have a type which is known at compile time. Nim -is statically typed. One can declare new types, which is in essence defining -an identifier that can be used to denote this custom type. - -These are the major type classes: - -* ordinal types (consist of integer, bool, character, enumeration - (and subranges thereof) types) -* floating point types -* string type -* structured types -* reference (pointer) type -* procedural type -* generic type - - -Ordinal types -------------- -Ordinal types have the following characteristics: - -- Ordinal types are countable and ordered. This property allows - the operation of functions as ``inc``, ``ord``, ``dec`` on ordinal types to - be defined. -- Ordinal values have a smallest possible value. Trying to count further - down than the smallest value gives a checked runtime or static error. -- Ordinal values have a largest possible value. Trying to count further - than the largest value gives a checked runtime or static error. - -Integers, bool, characters and enumeration types (and subranges of these -types) belong to ordinal types. For reasons of simplicity of implementation -the types ``uint`` and ``uint64`` are not ordinal types. - - -Pre-defined integer types -------------------------- -These integer types are pre-defined: - -``int`` - the generic signed integer type; its size is platform dependent and has the - same size as a pointer. This type should be used in general. An integer - literal that has no type suffix is of this type. - -intXX - additional signed integer types of XX bits use this naming scheme - (example: int16 is a 16 bit wide integer). - The current implementation supports ``int8``, ``int16``, ``int32``, ``int64``. - Literals of these types have the suffix 'iXX. - -``uint`` - the generic `unsigned integer`:idx: type; its size is platform dependent and - has the same size as a pointer. An integer literal with the type - suffix ``'u`` is of this type. - -uintXX - additional signed integer types of XX bits use this naming scheme - (example: uint16 is a 16 bit wide unsigned integer). - The current implementation supports ``uint8``, ``uint16``, ``uint32``, - ``uint64``. Literals of these types have the suffix 'uXX. - Unsigned operations all wrap around; they cannot lead to over- or - underflow errors. - - -In addition to the usual arithmetic operators for signed and unsigned integers -(``+ - *`` etc.) there are also operators that formally work on *signed* -integers but treat their arguments as *unsigned*: They are mostly provided -for backwards compatibility with older versions of the language that lacked -unsigned integer types. These unsigned operations for signed integers use -the ``%`` suffix as convention: - - -====================== ====================================================== -operation meaning -====================== ====================================================== -``a +% b`` unsigned integer addition -``a -% b`` unsigned integer subtraction -``a *% b`` unsigned integer multiplication -``a /% b`` unsigned integer division -``a %% b`` unsigned integer modulo operation -``a <% b`` treat ``a`` and ``b`` as unsigned and compare -``a <=% b`` treat ``a`` and ``b`` as unsigned and compare -``ze(a)`` extends the bits of ``a`` with zeros until it has the - width of the ``int`` type -``toU8(a)`` treats ``a`` as unsigned and converts it to an - unsigned integer of 8 bits (but still the - ``int8`` type) -``toU16(a)`` treats ``a`` as unsigned and converts it to an - unsigned integer of 16 bits (but still the - ``int16`` type) -``toU32(a)`` treats ``a`` as unsigned and converts it to an - unsigned integer of 32 bits (but still the - ``int32`` type) -====================== ====================================================== - -`Automatic type conversion`:idx: is performed in expressions where different -kinds of integer types are used: the smaller type is converted to the larger. - -A `narrowing type conversion`:idx: converts a larger to a smaller type (for -example ``int32 -> int16``. A `widening type conversion`:idx: converts a -smaller type to a larger type (for example ``int16 -> int32``). In Nim only -widening type conversions are *implicit*: - -.. code-block:: nim - var myInt16 = 5i16 - var myInt: int - myInt16 + 34 # of type ``int16`` - myInt16 + myInt # of type ``int`` - myInt16 + 2i32 # of type ``int32`` - -However, ``int`` literals are implicitly convertible to a smaller integer type -if the literal's value fits this smaller type and such a conversion is less -expensive than other implicit conversions, so ``myInt16 + 34`` produces -an ``int16`` result. - -For further details, see `Convertible relation`_. - - -Subrange types --------------- -A subrange type is a range of values from an ordinal type (the base -type). To define a subrange type, one must specify it's limiting values: the -lowest and highest value of the type: - -.. code-block:: nim - type - Subrange = range[0..5] - - -``Subrange`` is a subrange of an integer which can only hold the values 0 -to 5. Assigning any other value to a variable of type ``Subrange`` is a -checked runtime error (or static error if it can be statically -determined). Assignments from the base type to one of its subrange types -(and vice versa) are allowed. - -A subrange type has the same size as its base type (``int`` in the example). - -Nim requires `interval arithmetic`:idx: for subrange types over a set -of built-in operators that involve constants: ``x %% 3`` is of -type ``range[0..2]``. The following built-in operators for integers are -affected by this rule: ``-``, ``+``, ``*``, ``min``, ``max``, ``succ``, -``pred``, ``mod``, ``div``, ``%%``, ``and`` (bitwise ``and``). - -Bitwise ``and`` only produces a ``range`` if one of its operands is a -constant *x* so that (x+1) is a number of two. -(Bitwise ``and`` is then a ``%%`` operation.) - -This means that the following code is accepted: - -.. code-block:: nim - case (x and 3) + 7 - of 7: echo "A" - of 8: echo "B" - of 9: echo "C" - of 10: echo "D" - # note: no ``else`` required as (x and 3) + 7 has the type: range[7..10] - - -Pre-defined floating point types --------------------------------- - -The following floating point types are pre-defined: - -``float`` - the generic floating point type; its size is platform dependent - (the compiler chooses the processor's fastest floating point type). - This type should be used in general. - -floatXX - an implementation may define additional floating point types of XX bits using - this naming scheme (example: float64 is a 64 bit wide float). The current - implementation supports ``float32`` and ``float64``. Literals of these types - have the suffix 'fXX. - - -Automatic type conversion in expressions with different kinds -of floating point types is performed: See `Convertible relation`_ for further -details. Arithmetic performed on floating point types follows the IEEE -standard. Integer types are not converted to floating point types automatically -and vice versa. - -The IEEE standard defines five types of floating-point exceptions: - -* Invalid: operations with mathematically invalid operands, - for example 0.0/0.0, sqrt(-1.0), and log(-37.8). -* Division by zero: divisor is zero and dividend is a finite nonzero number, - for example 1.0/0.0. -* Overflow: operation produces a result that exceeds the range of the exponent, - for example MAXDOUBLE+0.0000000000001e308. -* Underflow: operation produces a result that is too small to be represented - as a normal number, for example, MINDOUBLE * MINDOUBLE. -* Inexact: operation produces a result that cannot be represented with infinite - precision, for example, 2.0 / 3.0, log(1.1) and 0.1 in input. - -The IEEE exceptions are either ignored at runtime or mapped to the -Nim exceptions: `EFloatInvalidOp`:idx:, `EFloatDivByZero`:idx:, -`EFloatOverflow`:idx:, `EFloatUnderflow`:idx:, and `EFloatInexact`:idx:. -These exceptions inherit from the `EFloatingPoint`:idx: base class. - -Nim provides the pragmas `NaNChecks`:idx: and `InfChecks`:idx: to control -whether the IEEE exceptions are ignored or trap a Nim exception: - -.. code-block:: nim - {.NanChecks: on, InfChecks: on.} - var a = 1.0 - var b = 0.0 - echo b / b # raises EFloatInvalidOp - echo a / b # raises EFloatOverflow - -In the current implementation ``EFloatDivByZero`` and ``EFloatInexact`` are -never raised. ``EFloatOverflow`` is raised instead of ``EFloatDivByZero``. -There is also a `floatChecks`:idx: pragma that is a short-cut for the -combination of ``NaNChecks`` and ``InfChecks`` pragmas. ``floatChecks`` are -turned off as default. - -The only operations that are affected by the ``floatChecks`` pragma are -the ``+``, ``-``, ``*``, ``/`` operators for floating point types. - -An implementation should always use the maximum precision available to evaluate -floating pointer values at compile time; this means expressions like -``0.09'f32 + 0.01'f32 == 0.09'f64 + 0.01'f64`` are true. - - -Boolean type ------------- -The boolean type is named `bool`:idx: in Nim and can be one of the two -pre-defined values ``true`` and ``false``. Conditions in while, -if, elif, when statements need to be of type bool. - -This condition holds:: - - ord(false) == 0 and ord(true) == 1 - -The operators ``not, and, or, xor, <, <=, >, >=, !=, ==`` are defined -for the bool type. The ``and`` and ``or`` operators perform short-cut -evaluation. Example: - -.. code-block:: nim - - while p != nil and p.name != "xyz": - # p.name is not evaluated if p == nil - p = p.next - - -The size of the bool type is one byte. - - -Character type --------------- -The character type is named ``char`` in Nim. Its size is one byte. -Thus it cannot represent an UTF-8 character, but a part of it. -The reason for this is efficiency: for the overwhelming majority of use-cases, -the resulting programs will still handle UTF-8 properly as UTF-8 was specially -designed for this. -Another reason is that Nim can support ``array[char, int]`` or -``set[char]`` efficiently as many algorithms rely on this feature. The -`TRune` type is used for Unicode characters, it can represent any Unicode -character. ``TRune`` is declared in the `unicode module <unicode.html>`_. - - - - -Enumeration types ------------------ -Enumeration types define a new type whose values consist of the ones -specified. The values are ordered. Example: - -.. code-block:: nim - - type - TDirection = enum - north, east, south, west - - -Now the following holds:: - - ord(north) == 0 - ord(east) == 1 - ord(south) == 2 - ord(west) == 3 - -Thus, north < east < south < west. The comparison operators can be used -with enumeration types. - -For better interfacing to other programming languages, the fields of enum -types can be assigned an explicit ordinal value. However, the ordinal values -have to be in ascending order. A field whose ordinal value is not -explicitly given is assigned the value of the previous field + 1. - -An explicit ordered enum can have *holes*: - -.. code-block:: nim - type - TTokenType = enum - a = 2, b = 4, c = 89 # holes are valid - -However, it is then not an ordinal anymore, so it is not possible to use these -enums as an index type for arrays. The procedures ``inc``, ``dec``, ``succ`` -and ``pred`` are not available for them either. - - -The compiler supports the built-in stringify operator ``$`` for enumerations. -The stringify's result can be controlled by explicitly giving the string -values to use: - -.. code-block:: nim - - type - TMyEnum = enum - valueA = (0, "my value A"), - valueB = "value B", - valueC = 2, - valueD = (3, "abc") - -As can be seen from the example, it is possible to both specify a field's -ordinal value and its string value by using a tuple. It is also -possible to only specify one of them. - -An enum can be marked with the ``pure`` pragma so that it's fields are not -added to the current scope, so they always need to be accessed -via ``TMyEnum.value``: - -.. code-block:: nim - - type - TMyEnum {.pure.} = enum - valueA, valueB, valueC, valueD - - echo valueA # error: Unknown identifier - echo TMyEnum.valueA # works - - -String type ------------ -All string literals are of the type ``string``. A string in Nim is very -similar to a sequence of characters. However, strings in Nim are both -zero-terminated and have a length field. One can retrieve the length with the -builtin ``len`` procedure; the length never counts the terminating zero. -The assignment operator for strings always copies the string. -The ``&`` operator concatenates strings. - -Strings are compared by their lexicographical order. All comparison operators -are available. Strings can be indexed like arrays (lower bound is 0). Unlike -arrays, they can be used in case statements: - -.. code-block:: nim - - case paramStr(i) - of "-v": incl(options, optVerbose) - of "-h", "-?": incl(options, optHelp) - else: write(stdout, "invalid command line option!\n") - -Per convention, all strings are UTF-8 strings, but this is not enforced. For -example, when reading strings from binary files, they are merely a sequence of -bytes. The index operation ``s[i]`` means the i-th *char* of ``s``, not the -i-th *unichar*. The iterator ``runes`` from the `unicode module -<unicode.html>`_ can be used for iteration over all Unicode characters. - - -CString type ------------- -The ``cstring`` type represents a pointer to a zero-terminated char array -compatible to the type ``char*`` in Ansi C. Its primary purpose lies in easy -interfacing with C. The index operation ``s[i]`` means the i-th *char* of -``s``; however no bounds checking for ``cstring`` is performed making the -index operation unsafe. - -A Nim ``string`` is implicitly convertible -to ``cstring`` for convenience. If a Nim string is passed to a C-style -variadic proc, it is implicitly converted to ``cstring`` too: - -.. code-block:: nim - proc printf(formatstr: cstring) {.importc: "printf", varargs, - header: "<stdio.h>".} - - printf("This works %s", "as expected") - -Even though the conversion is implicit, it is not *safe*: The garbage collector -does not consider a ``cstring`` to be a root and may collect the underlying -memory. However in practice this almost never happens as the GC considers -stack roots conservatively. One can use the builtin procs ``GC_ref`` and -``GC_unref`` to keep the string data alive for the rare cases where it does -not work. - -A `$` proc is defined for cstrings that returns a string. Thus to get a nim -string from a cstring: - -.. code-block:: nim - var str: string = "Hello!" - var cstr: cstring = s - var newstr: string = $cstr - - -Structured types ----------------- -A variable of a structured type can hold multiple values at the same -time. Structured types can be nested to unlimited levels. Arrays, sequences, -tuples, objects and sets belong to the structured types. - -Array and sequence types ------------------------- -Arrays are a homogeneous type, meaning that each element in the array -has the same type. Arrays always have a fixed length which is specified at -compile time (except for open arrays). They can be indexed by any ordinal type. -A parameter ``A`` may be an *open array*, in which case it is indexed by -integers from 0 to ``len(A)-1``. An array expression may be constructed by the -array constructor ``[]``. - -Sequences are similar to arrays but of dynamic length which may change -during runtime (like strings). Sequences are implemented as growable arrays, -allocating pieces of memory as items are added. A sequence ``S`` is always -indexed by integers from 0 to ``len(S)-1`` and its bounds are checked. -Sequences can be constructed by the array constructor ``[]`` in conjunction -with the array to sequence operator ``@``. Another way to allocate space for a -sequence is to call the built-in ``newSeq`` procedure. - -A sequence may be passed to a parameter that is of type *open array*. - -Example: - -.. code-block:: nim - - type - TIntArray = array[0..5, int] # an array that is indexed with 0..5 - TIntSeq = seq[int] # a sequence of integers - var - x: TIntArray - y: TIntSeq - x = [1, 2, 3, 4, 5, 6] # [] is the array constructor - y = @[1, 2, 3, 4, 5, 6] # the @ turns the array into a sequence - -The lower bound of an array or sequence may be received by the built-in proc -``low()``, the higher bound by ``high()``. The length may be -received by ``len()``. ``low()`` for a sequence or an open array always returns -0, as this is the first valid index. -One can append elements to a sequence with the ``add()`` proc or the ``&`` -operator, and remove (and get) the last element of a sequence with the -``pop()`` proc. - -The notation ``x[i]`` can be used to access the i-th element of ``x``. - -Arrays are always bounds checked (at compile-time or at runtime). These -checks can be disabled via pragmas or invoking the compiler with the -``--boundChecks:off`` command line switch. - - -Open arrays ------------ - -Often fixed size arrays turn out to be too inflexible; procedures should -be able to deal with arrays of different sizes. The `openarray`:idx: type -allows this; it can only be used for parameters. Openarrays are always -indexed with an ``int`` starting at position 0. The ``len``, ``low`` -and ``high`` operations are available for open arrays too. Any array with -a compatible base type can be passed to an openarray parameter, the index -type does not matter. In addition to arrays sequences can also be passed -to an open array parameter. - -The openarray type cannot be nested: multidimensional openarrays are not -supported because this is seldom needed and cannot be done efficiently. - - -Varargs -------- - -A ``varargs`` parameter is an openarray parameter that additionally -allows to pass a variable number of arguments to a procedure. The compiler -converts the list of arguments to an array implicitly: - -.. code-block:: nim - proc myWriteln(f: TFile, a: varargs[string]) = - for s in items(a): - write(f, s) - write(f, "\n") - - myWriteln(stdout, "abc", "def", "xyz") - # is transformed to: - myWriteln(stdout, ["abc", "def", "xyz"]) - -This transformation is only done if the varargs parameter is the -last parameter in the procedure header. It is also possible to perform -type conversions in this context: - -.. code-block:: nim - proc myWriteln(f: TFile, a: varargs[string, `$`]) = - for s in items(a): - write(f, s) - write(f, "\n") - - myWriteln(stdout, 123, "abc", 4.0) - # is transformed to: - myWriteln(stdout, [$123, $"def", $4.0]) - -In this example ``$`` is applied to any argument that is passed to the -parameter ``a``. (Note that ``$`` applied to strings is a nop.) - - - -Tuples and object types ------------------------ -A variable of a tuple or object type is a heterogeneous storage -container. -A tuple or object defines various named *fields* of a type. A tuple also -defines an *order* of the fields. Tuples are meant for heterogeneous storage -types with no overhead and few abstraction possibilities. The constructor ``()`` -can be used to construct tuples. The order of the fields in the constructor -must match the order of the tuple's definition. Different tuple-types are -*equivalent* if they specify the same fields of the same type in the same -order. The *names* of the fields also have to be identical but this might -change in a future version of the language. - -The assignment operator for tuples copies each component. -The default assignment operator for objects copies each component. Overloading -of the assignment operator for objects is not possible, but this will change -in future versions of the compiler. - -.. code-block:: nim - - type - TPerson = tuple[name: string, age: int] # type representing a person: - # a person consists of a name - # and an age - var - person: TPerson - person = (name: "Peter", age: 30) - # the same, but less readable: - person = ("Peter", 30) - -The implementation aligns the fields for best access performance. The alignment -is compatible with the way the C compiler does it. - -For consistency with ``object`` declarations, tuples in a ``type`` section -can also be defined with indentation instead of ``[]``: - -.. code-block:: nim - type - TPerson = tuple # type representing a person - name: string # a person consists of a name - age: natural # and an age - -Objects provide many features that tuples do not. Object provide inheritance -and information hiding. Objects have access to their type at runtime, so that -the ``of`` operator can be used to determine the object's type. - -.. code-block:: nim - type - TPerson {.inheritable.} = object - name*: string # the * means that `name` is accessible from other modules - age: int # no * means that the field is hidden - - TStudent = object of TPerson # a student is a person - id: int # with an id field - - var - student: TStudent - person: TPerson - assert(student of TStudent) # is true - -Object fields that should be visible from outside the defining module, have to -be marked by ``*``. In contrast to tuples, different object types are -never *equivalent*. Objects that have no ancestor are implicitly ``final`` -and thus have no hidden type field. One can use the ``inheritable`` pragma to -introduce new object roots apart from ``system.TObject``. - - -Object construction -------------------- - -Objects can also be created with an `object construction expression`:idx: that -has the syntax ``T(fieldA: valueA, fieldB: valueB, ...)`` where ``T`` is -an ``object`` type or a ``ref object`` type: - -.. code-block:: nim - var student = TStudent(name: "Anton", age: 5, id: 3) - -For a ``ref object`` type ``system.new`` is invoked implicitly. - - -Object variants ---------------- -Often an object hierarchy is overkill in certain situations where simple -variant types are needed. - -An example: - -.. code-block:: nim - - # This is an example how an abstract syntax tree could be modelled in Nim - type - TNodeKind = enum # the different node types - nkInt, # a leaf with an integer value - nkFloat, # a leaf with a float value - nkString, # a leaf with a string value - nkAdd, # an addition - nkSub, # a subtraction - nkIf # an if statement - PNode = ref TNode - TNode = object - case kind: TNodeKind # the ``kind`` field is the discriminator - of nkInt: intVal: int - of nkFloat: floatVal: float - of nkString: strVal: string - of nkAdd, nkSub: - leftOp, rightOp: PNode - of nkIf: - condition, thenPart, elsePart: PNode - - # create a new case object: - var n = PNode(kind: nkIf, condition: nil) - # accessing n.thenPart is valid because the ``nkIf`` branch is active: - n.thenPart = PNode(kind: nkFloat, floatVal: 2.0) - - # the following statement raises an `EInvalidField` exception, because - # n.kind's value does not fit and the ``nkString`` branch is not active: - n.strVal = "" - - # invalid: would change the active object branch: - n.kind = nkInt - - var x = PNode(kind: nkAdd, leftOp: PNode(kind: nkInt, intVal: 4), - rightOp: PNode(kind: nkInt, intVal: 2)) - # valid: does not change the active object branch: - x.kind = nkSub - -As can been seen from the example, an advantage to an object hierarchy is that -no casting between different object types is needed. Yet, access to invalid -object fields raises an exception. - -The syntax of ``case`` in an object declaration follows closely the syntax of -the ``case`` statement: The branches in a ``case`` section may be indented too. - -In the example the ``kind`` field is called the `discriminator`:idx:\: For -safety its address cannot be taken and assignments to it are restricted: The -new value must not lead to a change of the active object branch. For an object -branch switch ``system.reset`` has to be used. - - -Set type --------- - -.. include:: sets_fragment.txt - -Reference and pointer types ---------------------------- -References (similar to pointers in other programming languages) are a -way to introduce many-to-one relationships. This means different references can -point to and modify the same location in memory (also called `aliasing`:idx:). - -Nim distinguishes between `traced`:idx: and `untraced`:idx: references. -Untraced references are also called *pointers*. Traced references point to -objects of a garbage collected heap, untraced references point to -manually allocated objects or to objects somewhere else in memory. Thus -untraced references are *unsafe*. However for certain low-level operations -(accessing the hardware) untraced references are unavoidable. - -Traced references are declared with the **ref** keyword, untraced references -are declared with the **ptr** keyword. - -An empty subscript ``[]`` notation can be used to derefer a reference, -the ``addr`` procedure returns the address of an item. An address is always -an untraced reference. -Thus the usage of ``addr`` is an *unsafe* feature. - -The ``.`` (access a tuple/object field operator) -and ``[]`` (array/string/sequence index operator) operators perform implicit -dereferencing operations for reference types: - -.. code-block:: nim - - type - PNode = ref TNode - TNode = object - le, ri: PNode - data: int - - var - n: PNode - new(n) - n.data = 9 - # no need to write n[].data; in fact n[].data is highly discouraged! - -As a syntactical extension ``object`` types can be anonymous if -declared in a type section via the ``ref object`` or ``ptr object`` notations. -This feature is useful if an object should only gain reference semantics: - -.. code-block:: nim - - type - Node = ref object - le, ri: Node - data: int - - -To allocate a new traced object, the built-in procedure ``new`` has to be used. -To deal with untraced memory, the procedures ``alloc``, ``dealloc`` and -``realloc`` can be used. The documentation of the system module contains -further information. - -If a reference points to *nothing*, it has the value ``nil``. - -Special care has to be taken if an untraced object contains traced objects like -traced references, strings or sequences: in order to free everything properly, -the built-in procedure ``GCunref`` has to be called before freeing the untraced -memory manually: - -.. code-block:: nim - type - TData = tuple[x, y: int, s: string] - - # allocate memory for TData on the heap: - var d = cast[ptr TData](alloc0(sizeof(TData))) - - # create a new string on the garbage collected heap: - d.s = "abc" - - # tell the GC that the string is not needed anymore: - GCunref(d.s) - - # free the memory: - dealloc(d) - -Without the ``GCunref`` call the memory allocated for the ``d.s`` string would -never be freed. The example also demonstrates two important features for low -level programming: the ``sizeof`` proc returns the size of a type or value -in bytes. The ``cast`` operator can circumvent the type system: the compiler -is forced to treat the result of the ``alloc0`` call (which returns an untyped -pointer) as if it would have the type ``ptr TData``. Casting should only be -done if it is unavoidable: it breaks type safety and bugs can lead to -mysterious crashes. - -**Note**: The example only works because the memory is initialized to zero -(``alloc0`` instead of ``alloc`` does this): ``d.s`` is thus initialized to -``nil`` which the string assignment can handle. One needs to know low level -details like this when mixing garbage collected data with unmanaged memory. - -.. XXX finalizers for traced objects - - -Not nil annotation ------------------- - -All types for that ``nil`` is a valid value can be annotated to -exclude ``nil`` as a valid value with the ``not nil`` annotation: - -.. code-block:: nim - type - PObject = ref TObj not nil - TProc = (proc (x, y: int)) not nil - - proc p(x: PObject) = - echo "not nil" - - # compiler catches this: - p(nil) - - # and also this: - var x: PObject - p(x) - -The compiler ensures that every code path initializes variables which contain -not nilable pointers. The details of this analysis are still to be specified -here. - - -Memory regions --------------- - -The types ``ref`` and ``ptr`` can get an optional ``region`` annotation. -A region has to be an object type. - -Regions are very useful to separate user space and kernel memory in the -development of OS kernels: - -.. code-block:: nim - type - Kernel = object - Userspace = object - - var a: Kernel ptr Stat - var b: Userspace ptr Stat - - # the following does not compile as the pointer types are incompatible: - a = b - -As the example shows ``ptr`` can also be used as a binary -operator, ``region ptr T`` is a shortcut for ``ptr[region, T]``. - -In order to make generic code easier to write ``ptr T`` is a subtype -of ``ptr[R, T]`` for any ``R``. - -Furthermore the subtype relation of the region object types is lifted to -the pointer types: If ``A <: B`` then ``ptr[A, T] <: ptr[B, T]``. This can be -used to model subregions of memory. As a special typing rule ``ptr[R, T]`` is -not compatible to ``pointer`` to prevent the following from compiling: - -.. code-block:: nim - # from system - proc dealloc(p: pointer) - - # wrap some scripting language - type - PythonsHeap = object - PyObjectHeader = object - rc: int - typ: pointer - PyObject = ptr[PythonsHeap, PyObjectHeader] - - proc createPyObject(): PyObject {.importc: "...".} - proc destroyPyObject(x: PyObject) {.importc: "...".} - - var foo = createPyObject() - # type error here, how convenient: - dealloc(foo) - - -Future directions: - -* Memory regions might become available for ``string`` and ``seq`` too. -* Builtin regions like ``private``, ``global`` and ``local`` will - prove very useful for the upcoming OpenCL target. -* Builtin "regions" can model ``lent`` and ``unique`` pointers. - - - -Procedural type ---------------- -A procedural type is internally a pointer to a procedure. ``nil`` is -an allowed value for variables of a procedural type. Nim uses procedural -types to achieve `functional`:idx: programming techniques. - -Examples: - -.. code-block:: nim - - proc printItem(x: int) = ... - - proc forEach(c: proc (x: int) {.cdecl.}) = - ... - - forEach(printItem) # this will NOT compile because calling conventions differ - - -.. code-block:: nim - - type - TOnMouseMove = proc (x, y: int) {.closure.} - - proc onMouseMove(mouseX, mouseY: int) = - # has default calling convention - echo "x: ", mouseX, " y: ", mouseY - - proc setOnMouseMove(mouseMoveEvent: TOnMouseMove) = discard - - # ok, 'onMouseMove' has the default calling convention, which is compatible - # to 'closure': - setOnMouseMove(onMouseMove) - - -A subtle issue with procedural types is that the calling convention of the -procedure influences the type compatibility: procedural types are only -compatible if they have the same calling convention. As a special extension, -a procedure of the calling convention ``nimcall`` can be passed to a parameter -that expects a proc of the calling convention ``closure``. - -Nim supports these `calling conventions`:idx:\: - -`nimcall`:idx: - is the default convention used for a Nim **proc**. It is the - same as ``fastcall``, but only for C compilers that support ``fastcall``. - -`closure`:idx: - is the default calling convention for a **procedural type** that lacks - any pragma annotations. It indicates that the procedure has a hidden - implicit parameter (an *environment*). Proc vars that have the calling - convention ``closure`` take up two machine words: One for the proc pointer - and another one for the pointer to implicitly passed environment. - -`stdcall`:idx: - This the stdcall convention as specified by Microsoft. The generated C - procedure is declared with the ``__stdcall`` keyword. - -`cdecl`:idx: - The cdecl convention means that a procedure shall use the same convention - as the C compiler. Under windows the generated C procedure is declared with - the ``__cdecl`` keyword. - -`safecall`:idx: - This is the safecall convention as specified by Microsoft. The generated C - procedure is declared with the ``__safecall`` keyword. The word *safe* - refers to the fact that all hardware registers shall be pushed to the - hardware stack. - -`inline`:idx: - The inline convention means the the caller should not call the procedure, - but inline its code directly. Note that Nim does not inline, but leaves - this to the C compiler; it generates ``__inline`` procedures. This is - only a hint for the compiler: it may completely ignore it and - it may inline procedures that are not marked as ``inline``. - -`fastcall`:idx: - Fastcall means different things to different C compilers. One gets whatever - the C ``__fastcall`` means. - -`syscall`:idx: - The syscall convention is the same as ``__syscall`` in C. It is used for - interrupts. - -`noconv`:idx: - The generated C code will not have any explicit calling convention and thus - use the C compiler's default calling convention. This is needed because - Nim's default calling convention for procedures is ``fastcall`` to - improve speed. - -Most calling conventions exist only for the Windows 32-bit platform. - -Assigning/passing a procedure to a procedural variable is only allowed if one -of the following conditions hold: -1) The procedure that is accessed resides in the current module. -2) The procedure is marked with the ``procvar`` pragma (see `procvar pragma`_). -3) The procedure has a calling convention that differs from ``nimcall``. -4) The procedure is anonymous. - -The rules' purpose is to prevent the case that extending a non-``procvar`` -procedure with default parameters breaks client code. - -The default calling convention is ``nimcall``, unless it is an inner proc (a -proc inside of a proc). For an inner proc an analysis is performed whether it -accesses its environment. If it does so, it has the calling convention -``closure``, otherwise it has the calling convention ``nimcall``. - - -Distinct type -------------- - -A ``distinct`` type is new type derived from a `base type`:idx: that is -incompatible with its base type. In particular, it is an essential property -of a distinct type that it **does not** imply a subtype relation between it -and its base type. Explicit type conversions from a distinct type to its -base type and vice versa are allowed. - - -Modelling currencies -~~~~~~~~~~~~~~~~~~~~ - -A distinct type can be used to model different physical `units`:idx: with a -numerical base type, for example. The following example models currencies. - -Different currencies should not be mixed in monetary calculations. Distinct -types are a perfect tool to model different currencies: - -.. code-block:: nim - type - TDollar = distinct int - TEuro = distinct int - - var - d: TDollar - e: TEuro - - echo d + 12 - # Error: cannot add a number with no unit and a ``TDollar`` - -Unfortunately, ``d + 12.TDollar`` is not allowed either, -because ``+`` is defined for ``int`` (among others), not for ``TDollar``. So -a ``+`` for dollars needs to be defined: - -.. code-block:: - proc `+` (x, y: TDollar): TDollar = - result = TDollar(int(x) + int(y)) - -It does not make sense to multiply a dollar with a dollar, but with a -number without unit; and the same holds for division: - -.. code-block:: - proc `*` (x: TDollar, y: int): TDollar = - result = TDollar(int(x) * y) - - proc `*` (x: int, y: TDollar): TDollar = - result = TDollar(x * int(y)) - - proc `div` ... - -This quickly gets tedious. The implementations are trivial and the compiler -should not generate all this code only to optimize it away later - after all -``+`` for dollars should produce the same binary code as ``+`` for ints. -The pragma `borrow`:idx: has been designed to solve this problem; in principle -it generates the above trivial implementations: - -.. code-block:: nim - proc `*` (x: TDollar, y: int): TDollar {.borrow.} - proc `*` (x: int, y: TDollar): TDollar {.borrow.} - proc `div` (x: TDollar, y: int): TDollar {.borrow.} - -The ``borrow`` pragma makes the compiler use the same implementation as -the proc that deals with the distinct type's base type, so no code is -generated. - -But it seems all this boilerplate code needs to be repeated for the ``TEuro`` -currency. This can be solved with templates_. - -.. code-block:: nim - template additive(typ: typedesc): stmt = - proc `+` *(x, y: typ): typ {.borrow.} - proc `-` *(x, y: typ): typ {.borrow.} - - # unary operators: - proc `+` *(x: typ): typ {.borrow.} - proc `-` *(x: typ): typ {.borrow.} - - template multiplicative(typ, base: typedesc): stmt = - proc `*` *(x: typ, y: base): typ {.borrow.} - proc `*` *(x: base, y: typ): typ {.borrow.} - proc `div` *(x: typ, y: base): typ {.borrow.} - proc `mod` *(x: typ, y: base): typ {.borrow.} - - template comparable(typ: typedesc): stmt = - proc `<` * (x, y: typ): bool {.borrow.} - proc `<=` * (x, y: typ): bool {.borrow.} - proc `==` * (x, y: typ): bool {.borrow.} - - template defineCurrency(typ, base: expr): stmt = - type - typ* = distinct base - additive(typ) - multiplicative(typ, base) - comparable(typ) - - defineCurrency(TDollar, int) - defineCurrency(TEuro, int) - - -The borrow pragma can also be used to annotate the distinct type to allow -certain builtin operations to be lifted: - -.. code-block:: nim - type - Foo = object - a, b: int - s: string - - Bar {.borrow: `.`.} = distinct Foo - - var bb: ref Bar - new bb - # field access now valid - bb.a = 90 - bb.s = "abc" - -Currently only the dot accessor can be borrowed in this way. - - -Avoiding SQL injection attacks -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - -An SQL statement that is passed from Nim to an SQL database might be -modelled as a string. However, using string templates and filling in the -values is vulnerable to the famous `SQL injection attack`:idx:\: - -.. code-block:: nim - import strutils - - proc query(db: TDbHandle, statement: string) = ... - - var - username: string - - db.query("SELECT FROM users WHERE name = '$1'" % username) - # Horrible security hole, but the compiler does not mind! - -This can be avoided by distinguishing strings that contain SQL from strings -that don't. Distinct types provide a means to introduce a new string type -``TSQL`` that is incompatible with ``string``: - -.. code-block:: nim - type - TSQL = distinct string - - proc query(db: TDbHandle, statement: TSQL) = ... - - var - username: string - - db.query("SELECT FROM users WHERE name = '$1'" % username) - # Error at compile time: `query` expects an SQL string! - - -It is an essential property of abstract types that they **do not** imply a -subtype relation between the abtract type and its base type. Explict type -conversions from ``string`` to ``TSQL`` are allowed: - -.. code-block:: nim - import strutils, sequtils - - proc properQuote(s: string): TSQL = - # quotes a string properly for an SQL statement - return TSQL(s) - - proc `%` (frmt: TSQL, values: openarray[string]): TSQL = - # quote each argument: - let v = values.mapIt(TSQL, properQuote(it)) - # we need a temporary type for the type conversion :-( - type TStrSeq = seq[string] - # call strutils.`%`: - result = TSQL(string(frmt) % TStrSeq(v)) - - db.query("SELECT FROM users WHERE name = '$1'".TSQL % [username]) - -Now we have compile-time checking against SQL injection attacks. Since -``"".TSQL`` is transformed to ``TSQL("")`` no new syntax is needed for nice -looking ``TSQL`` string literals. The hypothetical ``TSQL`` type actually -exists in the library as the `TSqlQuery type <db_sqlite.html#TSqlQuery>`_ of -modules like `db_sqlite <db_sqlite.html>`_. - - -Void type ---------- - -The ``void`` type denotes the absense of any type. Parameters of -type ``void`` are treated as non-existent, ``void`` as a return type means that -the procedure does not return a value: - -.. code-block:: nim - proc nothing(x, y: void): void = - echo "ha" - - nothing() # writes "ha" to stdout - -The ``void`` type is particularly useful for generic code: - -.. code-block:: nim - proc callProc[T](p: proc (x: T), x: T) = - when T is void: - p() - else: - p(x) - - proc intProc(x: int) = discard - proc emptyProc() = discard - - callProc[int](intProc, 12) - callProc[void](emptyProc) - -However, a ``void`` type cannot be inferred in generic code: - -.. code-block:: nim - callProc(emptyProc) - # Error: type mismatch: got (proc ()) - # but expected one of: - # callProc(p: proc (T), x: T) - -The ``void`` type is only valid for parameters and return types; other symbols -cannot have the type ``void``. - - -Type relations -============== - -The following section defines several relations on types that are needed to -describe the type checking done by the compiler. - - -Type equality -------------- -Nim uses structural type equivalence for most types. Only for objects, -enumerations and distinct types name equivalence is used. The following -algorithm (in pseudo-code) determines type equality: - -.. code-block:: nim - proc typeEqualsAux(a, b: PType, - s: var set[PType * PType]): bool = - if (a,b) in s: return true - incl(s, (a,b)) - if a.kind == b.kind: - case a.kind - of int, intXX, float, floatXX, char, string, cstring, pointer, - bool, nil, void: - # leaf type: kinds identical; nothing more to check - result = true - of ref, ptr, var, set, seq, openarray: - result = typeEqualsAux(a.baseType, b.baseType, s) - of range: - result = typeEqualsAux(a.baseType, b.baseType, s) and - (a.rangeA == b.rangeA) and (a.rangeB == b.rangeB) - of array: - result = typeEqualsAux(a.baseType, b.baseType, s) and - typeEqualsAux(a.indexType, b.indexType, s) - of tuple: - if a.tupleLen == b.tupleLen: - for i in 0..a.tupleLen-1: - if not typeEqualsAux(a[i], b[i], s): return false - result = true - of object, enum, distinct: - result = a == b - of proc: - result = typeEqualsAux(a.parameterTuple, b.parameterTuple, s) and - typeEqualsAux(a.resultType, b.resultType, s) and - a.callingConvention == b.callingConvention - - proc typeEquals(a, b: PType): bool = - var s: set[PType * PType] = {} - result = typeEqualsAux(a, b, s) - -Since types are graphs which can have cycles, the above algorithm needs an -auxiliary set ``s`` to detect this case. - - -Type equality modulo type distinction -------------------------------------- - -The following algorithm (in pseudo-code) determines whether two types -are equal with no respect to ``distinct`` types. For brevity the cycle check -with an auxiliary set ``s`` is omitted: - -.. code-block:: nim - proc typeEqualsOrDistinct(a, b: PType): bool = - if a.kind == b.kind: - case a.kind - of int, intXX, float, floatXX, char, string, cstring, pointer, - bool, nil, void: - # leaf type: kinds identical; nothing more to check - result = true - of ref, ptr, var, set, seq, openarray: - result = typeEqualsOrDistinct(a.baseType, b.baseType) - of range: - result = typeEqualsOrDistinct(a.baseType, b.baseType) and - (a.rangeA == b.rangeA) and (a.rangeB == b.rangeB) - of array: - result = typeEqualsOrDistinct(a.baseType, b.baseType) and - typeEqualsOrDistinct(a.indexType, b.indexType) - of tuple: - if a.tupleLen == b.tupleLen: - for i in 0..a.tupleLen-1: - if not typeEqualsOrDistinct(a[i], b[i]): return false - result = true - of distinct: - result = typeEqualsOrDistinct(a.baseType, b.baseType) - of object, enum: - result = a == b - of proc: - result = typeEqualsOrDistinct(a.parameterTuple, b.parameterTuple) and - typeEqualsOrDistinct(a.resultType, b.resultType) and - a.callingConvention == b.callingConvention - elif a.kind == distinct: - result = typeEqualsOrDistinct(a.baseType, b) - elif b.kind == distinct: - result = typeEqualsOrDistinct(a, b.baseType) - - -Subtype relation ----------------- -If object ``a`` inherits from ``b``, ``a`` is a subtype of ``b``. This subtype -relation is extended to the types ``var``, ``ref``, ``ptr``: - -.. code-block:: nim - proc isSubtype(a, b: PType): bool = - if a.kind == b.kind: - case a.kind - of object: - var aa = a.baseType - while aa != nil and aa != b: aa = aa.baseType - result = aa == b - of var, ref, ptr: - result = isSubtype(a.baseType, b.baseType) - -.. XXX nil is a special value! - - -Convertible relation --------------------- -A type ``a`` is **implicitly** convertible to type ``b`` iff the following -algorithm returns true: - -.. code-block:: nim - # XXX range types? - proc isImplicitlyConvertible(a, b: PType): bool = - case a.kind - of int: result = b in {int8, int16, int32, int64, uint, uint8, uint16, - uint32, uint64, float, float32, float64} - of int8: result = b in {int16, int32, int64, int} - of int16: result = b in {int32, int64, int} - of int32: result = b in {int64, int} - of uint: result = b in {uint32, uint64} - of uint8: result = b in {uint16, uint32, uint64} - of uint16: result = b in {uint32, uint64} - of uint32: result = b in {uint64} - of float: result = b in {float32, float64} - of float32: result = b in {float64, float} - of float64: result = b in {float32, float} - of seq: - result = b == openArray and typeEquals(a.baseType, b.baseType) - of array: - result = b == openArray and typeEquals(a.baseType, b.baseType) - if a.baseType == char and a.indexType.rangeA == 0: - result = b = cstring - of cstring, ptr: - result = b == pointer - of string: - result = b == cstring - -A type ``a`` is **explicitly** convertible to type ``b`` iff the following -algorithm returns true: - -.. code-block:: nim - proc isIntegralType(t: PType): bool = - result = isOrdinal(t) or t.kind in {float, float32, float64} - - proc isExplicitlyConvertible(a, b: PType): bool = - result = false - if isImplicitlyConvertible(a, b): return true - if typeEqualsOrDistinct(a, b): return true - if isIntegralType(a) and isIntegralType(b): return true - if isSubtype(a, b) or isSubtype(b, a): return true - -The convertible relation can be relaxed by a user-defined type -`converter`:idx:. - -.. code-block:: nim - converter toInt(x: char): int = result = ord(x) - - var - x: int - chr: char = 'a' - - # implicit conversion magic happens here - x = chr - echo x # => 97 - # you can use the explicit form too - x = chr.toInt - echo x # => 97 - -The type conversion ``T(a)`` is an L-value if ``a`` is an L-value and -``typeEqualsOrDistinct(T, type(a))`` holds. - - -Assignment compatibility ------------------------- - -An expression ``b`` can be assigned to an expression ``a`` iff ``a`` is an -`l-value` and ``isImplicitlyConvertible(b.typ, a.typ)`` holds. - - -Overloading resolution ----------------------- - -To be written. - - -Statements and expressions -========================== - -Nim uses the common statement/expression paradigm: Statements do not -produce a value in contrast to expressions. However, some expressions are -statements. - -Statements are separated into `simple statements`:idx: and -`complex statements`:idx:. -Simple statements are statements that cannot contain other statements like -assignments, calls or the ``return`` statement; complex statements can -contain other statements. To avoid the `dangling else problem`:idx:, complex -statements always have to be intended. The details can be found in the grammar. - - -Statement list expression -------------------------- - -Statements can also occur in an expression context that looks -like ``(stmt1; stmt2; ...; ex)``. This is called -an statement list expression or ``(;)``. The type -of ``(stmt1; stmt2; ...; ex)`` is the type of ``ex``. All the other statements -must be of type ``void``. (One can use ``discard`` to produce a ``void`` type.) -``(;)`` does not introduce a new scope. - - -Discard statement ------------------ - -Example: - -.. code-block:: nim - proc p(x, y: int): int = - result = x + y - - discard p(3, 4) # discard the return value of `p` - -The ``discard`` statement evaluates its expression for side-effects and -throws the expression's resulting value away. - -Ignoring the return value of a procedure without using a discard statement is -a static error. - -The return value can be ignored implicitly if the called proc/iterator has -been declared with the `discardable`:idx: pragma: - -.. code-block:: nim - proc p(x, y: int): int {.discardable.} = - result = x + y - - p(3, 4) # now valid - -An empty ``discard`` statement is often used as a null statement: - -.. code-block:: nim - proc classify(s: string) = - case s[0] - of SymChars, '_': echo "an identifier" - of '0'..'9': echo "a number" - else: discard - - -Var statement -------------- - -Var statements declare new local and global variables and -initialize them. A comma separated list of variables can be used to specify -variables of the same type: - -.. code-block:: nim - - var - a: int = 0 - x, y, z: int - -If an initializer is given the type can be omitted: the variable is then of the -same type as the initializing expression. Variables are always initialized -with a default value if there is no initializing expression. The default -value depends on the type and is always a zero in binary. - -============================ ============================================== -Type default value -============================ ============================================== -any integer type 0 -any float 0.0 -char '\\0' -bool false -ref or pointer type nil -procedural type nil -sequence nil (*not* ``@[]``) -string nil (*not* "") -tuple[x: A, y: B, ...] (default(A), default(B), ...) - (analogous for objects) -array[0..., T] [default(T), ...] -range[T] default(T); this may be out of the valid range -T = enum cast[T](0); this may be an invalid value -============================ ============================================== - - -The implicit initialization can be avoided for optimization reasons with the -`noinit`:idx: pragma: - -.. code-block:: nim - var - a {.noInit.}: array [0..1023, char] - -If a proc is annotated with the ``noinit`` pragma this refers to its implicit -``result`` variable: - -.. code-block:: nim - proc returnUndefinedValue: int {.noinit.} = discard - - -The implicit initialization can be also prevented by the `requiresInit`:idx: -type pragma. The compiler requires an explicit initialization then. However -it does a `control flow analysis`:idx: to prove the variable has been -initialized and does not rely on syntactic properties: - -.. code-block:: nim - type - TMyObject = object {.requiresInit.} - - proc p() = - # the following is valid: - var x: TMyObject - if someCondition(): - x = a() - else: - x = a() - use x - -let statement -------------- - -A ``let`` statement declares new local and global `single assignment`:idx: -variables and binds a value to them. The syntax is the of the ``var`` -statement, except that the keyword ``var`` is replaced by the keyword ``let``. -Let variables are not l-values and can thus not be passed to ``var`` parameters -nor can their address be taken. They cannot be assigned new values. - -For let variables the same pragmas are available as for ordinary variables. - - -Const section -------------- - -`Constants`:idx: are symbols which are bound to a value. The constant's value -cannot change. The compiler must be able to evaluate the expression in a -constant declaration at compile time. - -Nim contains a sophisticated compile-time evaluator, so procedures which -have no side-effect can be used in constant expressions too: - -.. code-block:: nim - import strutils - const - constEval = contains("abc", 'b') # computed at compile time! - - -The rules for compile-time computability are: - -1. Literals are compile-time computable. -2. Type conversions are compile-time computable. -3. Procedure calls of the form ``p(X)`` are compile-time computable if - ``p`` is a proc without side-effects (see the `noSideEffect pragma`_ - for details) and if ``X`` is a (possibly empty) list of compile-time - computable arguments. - - -Constants cannot be of type ``ptr``, ``ref``, ``var`` or ``object``, nor can -they contain such a type. - - -Static statement/expression ---------------------------- - -A static statement/expression can be used to enforce compile -time evaluation explicitly. Enforced compile time evaluation can even evaluate -code that has side effects: - -.. code-block:: - - static: - echo "echo at compile time" - -It's a static error if the compiler cannot perform the evaluation at compile -time. - -The current implementation poses some restrictions for compile time -evaluation: Code which contains ``cast`` or makes use of the foreign function -interface cannot be evaluated at compile time. Later versions of Nim will -support the FFI at compile time. - - -If statement ------------- - -Example: - -.. code-block:: nim - - var name = readLine(stdin) - - if name == "Andreas": - echo("What a nice name!") - elif name == "": - echo("Don't you have a name?") - else: - echo("Boring name...") - -The ``if`` statement is a simple way to make a branch in the control flow: -The expression after the keyword ``if`` is evaluated, if it is true -the corresponding statements after the ``:`` are executed. Otherwise -the expression after the ``elif`` is evaluated (if there is an -``elif`` branch), if it is true the corresponding statements after -the ``:`` are executed. This goes on until the last ``elif``. If all -conditions fail, the ``else`` part is executed. If there is no ``else`` -part, execution continues with the statement after the ``if`` statement. - -The scoping for an ``if`` statement is slightly subtle to support an important -use case. A new scope starts for the ``if``/``elif`` condition and ends after -the corresponding *then* block: - -.. code-block:: nim - if {| (let m = input =~ re"(\w+)=\w+"; m.isMatch): - echo "key ", m[0], " value ", m[1] |} - elif {| (let m = input =~ re""; m.isMatch): - echo "new m in this scope" |} - else: - # 'm' not declared here - -In the example the scopes have been enclosed in ``{| |}``. - - -Case statement --------------- - -Example: - -.. code-block:: nim - - case readline(stdin) - of "delete-everything", "restart-computer": - echo("permission denied") - of "go-for-a-walk": echo("please yourself") - else: echo("unknown command") - - # indentation of the branches is also allowed; and so is an optional colon - # after the selecting expression: - case readline(stdin): - of "delete-everything", "restart-computer": - echo("permission denied") - of "go-for-a-walk": echo("please yourself") - else: echo("unknown command") - - -The ``case`` statement is similar to the if statement, but it represents -a multi-branch selection. The expression after the keyword ``case`` is -evaluated and if its value is in a *slicelist* the corresponding statements -(after the ``of`` keyword) are executed. If the value is not in any -given *slicelist* the ``else`` part is executed. If there is no ``else`` -part and not all possible values that ``expr`` can hold occur in a -``slicelist``, a static error occurs. This holds only for expressions of -ordinal types. "All possible values" of ``expr`` are determined by ``expr``'s -type. - -If the expression is not of an ordinal type, and no ``else`` part is -given, control passes after the ``case`` statement. - -To suppress the static error in the ordinal case an ``else`` part with an -empty ``discard`` statement can be used. - -As a special semantic extension, an expression in an ``of`` branch of a case -statement may evaluate to a set or array constructor; the set or array is then -expanded into a list of its elements: - -.. code-block:: nim - const - SymChars: set[char] = {'a'..'z', 'A'..'Z', '\x80'..'\xFF'} - - proc classify(s: string) = - case s[0] - of SymChars, '_': echo "an identifier" - of '0'..'9': echo "a number" - else: echo "other" - - # is equivalent to: - proc classify(s: string) = - case s[0] - of 'a'..'z', 'A'..'Z', '\x80'..'\xFF', '_': echo "an identifier" - of '0'..'9': echo "a number" - else: echo "other" - - -When statement --------------- - -Example: - -.. code-block:: nim - - when sizeof(int) == 2: - echo("running on a 16 bit system!") - elif sizeof(int) == 4: - echo("running on a 32 bit system!") - elif sizeof(int) == 8: - echo("running on a 64 bit system!") - else: - echo("cannot happen!") - -The ``when`` statement is almost identical to the ``if`` statement with some -exceptions: - -* Each condition (``expr``) has to be a constant expression (of type ``bool``). -* The statements do not open a new scope. -* The statements that belong to the expression that evaluated to true are - translated by the compiler, the other statements are not checked for - semantics! However, each condition is checked for semantics. - -The ``when`` statement enables conditional compilation techniques. As -a special syntactic extension, the ``when`` construct is also available -within ``object`` definitions. - - -Return statement ----------------- - -Example: - -.. code-block:: nim - return 40+2 - -The ``return`` statement ends the execution of the current procedure. -It is only allowed in procedures. If there is an ``expr``, this is syntactic -sugar for: - -.. code-block:: nim - result = expr - return result - - -``return`` without an expression is a short notation for ``return result`` if -the proc has a return type. The `result`:idx: variable is always the return -value of the procedure. It is automatically declared by the compiler. As all -variables, ``result`` is initialized to (binary) zero: - -.. code-block:: nim - proc returnZero(): int = - # implicitly returns 0 - - -Yield statement ---------------- - -Example: - -.. code-block:: nim - yield (1, 2, 3) - -The ``yield`` statement is used instead of the ``return`` statement in -iterators. It is only valid in iterators. Execution is returned to the body -of the for loop that called the iterator. Yield does not end the iteration -process, but execution is passed back to the iterator if the next iteration -starts. See the section about iterators (`Iterators and the for statement`_) -for further information. - - -Block statement ---------------- - -Example: - -.. code-block:: nim - var found = false - block myblock: - for i in 0..3: - for j in 0..3: - if a[j][i] == 7: - found = true - break myblock # leave the block, in this case both for-loops - echo(found) - -The block statement is a means to group statements to a (named) ``block``. -Inside the block, the ``break`` statement is allowed to leave the block -immediately. A ``break`` statement can contain a name of a surrounding -block to specify which block is to leave. - - -Break statement ---------------- - -Example: - -.. code-block:: nim - break - -The ``break`` statement is used to leave a block immediately. If ``symbol`` -is given, it is the name of the enclosing block that is to leave. If it is -absent, the innermost block is left. - - -While statement ---------------- - -Example: - -.. code-block:: nim - echo("Please tell me your password: \n") - var pw = readLine(stdin) - while pw != "12345": - echo("Wrong password! Next try: \n") - pw = readLine(stdin) - - -The ``while`` statement is executed until the ``expr`` evaluates to false. -Endless loops are no error. ``while`` statements open an `implicit block`, -so that they can be left with a ``break`` statement. - - -Continue statement ------------------- - -A ``continue`` statement leads to the immediate next iteration of the -surrounding loop construct. It is only allowed within a loop. A continue -statement is syntactic sugar for a nested block: - -.. code-block:: nim - while expr1: - stmt1 - continue - stmt2 - -Is equivalent to: - -.. code-block:: nim - while expr1: - block myBlockName: - stmt1 - break myBlockName - stmt2 - - -Assembler statement -------------------- - -The direct embedding of assembler code into Nim code is supported -by the unsafe ``asm`` statement. Identifiers in the assembler code that refer to -Nim identifiers shall be enclosed in a special character which can be -specified in the statement's pragmas. The default special character is ``'`'``: - -.. code-block:: nim - {.push stackTrace:off.} - proc addInt(a, b: int): int = - # a in eax, and b in edx - asm """ - mov eax, `a` - add eax, `b` - jno theEnd - call `raiseOverflow` - theEnd: - """ - {.pop.} - -If the GNU assembler is used, quotes and newlines are inserted automatically: - -.. code-block:: nim - proc addInt(a, b: int): int = - asm """ - addl %%ecx, %%eax - jno 1 - call `raiseOverflow` - 1: - :"=a"(`result`) - :"a"(`a`), "c"(`b`) - """ - -Instead of: - -.. code-block:: nim - proc addInt(a, b: int): int = - asm """ - "addl %%ecx, %%eax\n" - "jno 1\n" - "call `raiseOverflow`\n" - "1: \n" - :"=a"(`result`) - :"a"(`a`), "c"(`b`) - """ - -Using statement ---------------- - -**Warning**: The ``using`` statement is highly experimental! - -The using statement provides syntactic convenience for procs that -heavily use a single contextual parameter. When applied to a variable or a -constant, it will instruct Nim to automatically consider the used symbol as -a hidden leading parameter for any procedure calls, following the using -statement in the current scope. Thus, it behaves much like the hidden `this` -parameter available in some object-oriented programming languages. - -.. code-block:: nim - - var s = socket() - using s - - connect(host, port) - send(data) - - while true: - let line = readLine(timeout) - ... - - -When applied to a callable symbol, it brings the designated symbol in the -current scope. Thus, it can be used to disambiguate between imported symbols -from different modules having the same name. - -.. code-block:: nim - import windows, sdl - using sdl.SetTimer - -Note that ``using`` only *adds* to the current context, it doesn't remove or -replace, **neither** does it create a new scope. What this means is that if one -applies this to multiple variables the compiler will find conflicts in what -variable to use: - -.. code-block:: nim - var a, b = "kill it" - using a - add(" with fire") - using b - add(" with water") - echo a - echo b - -When the compiler reaches the second ``add`` call, both ``a`` and ``b`` could -be used with the proc, so one gets ``Error: expression '(a|b)' has no type (or -is ambiguous)``. To solve this one would need to nest ``using`` with a -``block`` statement so as to control the reach of the ``using`` statement. - -If expression -------------- - -An `if expression` is almost like an if statement, but it is an expression. -Example: - -.. code-block:: nim - var y = if x > 8: 9 else: 10 - -An if expression always results in a value, so the ``else`` part is -required. ``Elif`` parts are also allowed. - -When expression ---------------- - -Just like an `if expression`, but corresponding to the when statement. - -Case expression ---------------- - -The `case expression` is again very similar to the case statement: - -.. code-block:: nim - var favoriteFood = case animal - of "dog": "bones" - of "cat": "mice" - elif animal.endsWith"whale": "plankton" - else: - echo "I'm not sure what to serve, but everybody loves ice cream" - "ice cream" - -As seen in the above example, the case expression can also introduce side -effects. When multiple statements are given for a branch, Nim will use -the last expression as the result value, much like in an `expr` template. - -Table constructor ------------------ - -A table constructor is syntactic sugar for an array constructor: - -.. code-block:: nim - {"key1": "value1", "key2", "key3": "value2"} - - # is the same as: - [("key1", "value1"), ("key2", "value2"), ("key3", "value2")] - - -The empty table can be written ``{:}`` (in contrast to the empty set -which is ``{}``) which is thus another way to write as the empty array -constructor ``[]``. This slightly unusal way of supporting tables -has lots of advantages: - -* The order of the (key,value)-pairs is preserved, thus it is easy to - support ordered dicts with for example ``{key: val}.newOrderedTable``. -* A table literal can be put into a ``const`` section and the compiler - can easily put it into the executable's data section just like it can - for arrays and the generated data section requires a minimal amount - of memory. -* Every table implementation is treated equal syntactically. -* Apart from the minimal syntactic sugar the language core does not need to - know about tables. - - -Type conversions ----------------- -Syntactically a `type conversion` is like a procedure call, but a -type name replaces the procedure name. A type conversion is always -safe in the sense that a failure to convert a type to another -results in an exception (if it cannot be determined statically). - - -Type casts ----------- -Example: - -.. code-block:: nim - cast[int](x) - -Type casts are a crude mechanism to interpret the bit pattern of -an expression as if it would be of another type. Type casts are -only needed for low-level programming and are inherently unsafe. - - -The addr operator ------------------ -The ``addr`` operator returns the address of an l-value. If the type of the -location is ``T``, the `addr` operator result is of the type ``ptr T``. An -address is always an untraced reference. Taking the address of an object that -resides on the stack is **unsafe**, as the pointer may live longer than the -object on the stack and can thus reference a non-existing object. One can get -the address of variables, but one can't use it on variables declared through -``let`` statements: - -.. code-block:: nim - - let t1 = "Hello" - var - t2 = t1 - t3 : pointer = addr(t2) - echo repr(addr(t2)) - # --> ref 0x7fff6b71b670 --> 0x10bb81050"Hello" - echo cast[ptr string](t3)[] - # --> Hello - # The following line doesn't compile: - echo repr(addr(t1)) - # Error: expression has no address - - -Procedures -========== - -What most programming languages call `methods`:idx: or `functions`:idx: are -called `procedures`:idx: in Nim (which is the correct terminology). A -procedure declaration defines an identifier and associates it with a block -of code. -A procedure may call itself recursively. A parameter may be given a default -value that is used if the caller does not provide a value for this parameter. - -If the proc declaration has no body, it is a `forward`:idx: declaration. If -the proc returns a value, the procedure body can access an implicitly declared -variable named `result`:idx: that represents the return value. Procs can be -overloaded. The overloading resolution algorithm tries to find the proc that is -the best match for the arguments. Example: - -.. code-block:: nim - - proc toLower(c: Char): Char = # toLower for characters - if c in {'A'..'Z'}: - result = chr(ord(c) + (ord('a') - ord('A'))) - else: - result = c - - proc toLower(s: string): string = # toLower for strings - result = newString(len(s)) - for i in 0..len(s) - 1: - result[i] = toLower(s[i]) # calls toLower for characters; no recursion! - -Calling a procedure can be done in many different ways: - -.. code-block:: nim - proc callme(x, y: int, s: string = "", c: char, b: bool = false) = ... - - # call with positional arguments # parameter bindings: - callme(0, 1, "abc", '\t', true) # (x=0, y=1, s="abc", c='\t', b=true) - # call with named and positional arguments: - callme(y=1, x=0, "abd", '\t') # (x=0, y=1, s="abd", c='\t', b=false) - # call with named arguments (order is not relevant): - callme(c='\t', y=1, x=0) # (x=0, y=1, s="", c='\t', b=false) - # call as a command statement: no () needed: - callme 0, 1, "abc", '\t' - - -A procedure cannot modify its parameters (unless the parameters have the type -`var`). - -`Operators`:idx: are procedures with a special operator symbol as identifier: - -.. code-block:: nim - proc `$` (x: int): string = - # converts an integer to a string; this is a prefix operator. - result = intToStr(x) - -Operators with one parameter are prefix operators, operators with two -parameters are infix operators. (However, the parser distinguishes these from -the operator's position within an expression.) There is no way to declare -postfix operators: all postfix operators are built-in and handled by the -grammar explicitly. - -Any operator can be called like an ordinary proc with the '`opr`' -notation. (Thus an operator can have more than two parameters): - -.. code-block:: nim - proc `*+` (a, b, c: int): int = - # Multiply and add - result = a * b + c - - assert `*+`(3, 4, 6) == `*`(a, `+`(b, c)) - - -Method call syntax ------------------- - -For object oriented programming, the syntax ``obj.method(args)`` can be used -instead of ``method(obj, args)``. The parentheses can be omitted if there are no -remaining arguments: ``obj.len`` (instead of ``len(obj)``). - -This method call syntax is not restricted to objects, it can be used -to supply any type of first argument for procedures: - -.. code-block:: nim - - echo("abc".len) # is the same as echo(len("abc")) - echo("abc".toUpper()) - echo({'a', 'b', 'c'}.card) - stdout.writeln("Hallo") # the same as writeln(stdout, "Hallo") - -Another way to look at the method call syntax is that it provides the missing -postfix notation. - - -Properties ----------- -Nim has no need for *get-properties*: Ordinary get-procedures that are called -with the *method call syntax* achieve the same. But setting a value is -different; for this a special setter syntax is needed: - -.. code-block:: nim - - type - TSocket* = object of TObject - FHost: int # cannot be accessed from the outside of the module - # the `F` prefix is a convention to avoid clashes since - # the accessors are named `host` - - proc `host=`*(s: var TSocket, value: int) {.inline.} = - ## setter of hostAddr - s.FHost = value - - proc host*(s: TSocket): int {.inline.} = - ## getter of hostAddr - s.FHost - - var - s: TSocket - s.host = 34 # same as `host=`(s, 34) - - -Command invocation syntax -------------------------- - -Routines can be invoked without the ``()`` if the call is syntatically -a statement. This command invocation syntax also works for -expressions, but then only a single argument may follow. This restriction -means ``echo f 1, f 2`` is parsed as ``echo(f(1), f(2))`` and not as -``echo(f(1, f(2)))``. The method call syntax may be used to provide one -more argument in this case: - -.. code-block:: nim - proc optarg(x:int, y:int = 0):int = x + y - proc singlearg(x:int):int = 20*x - - echo optarg 1, " ", singlearg 2 # prints "1 40" - - let fail = optarg 1, optarg 8 # Wrong. Too many arguments for a command call - let x = optarg(1, optarg 8) # traditional procedure call with 2 arguments - let y = 1.optarg optarg 8 # same thing as above, w/o the parenthesis - assert x == y - -The command invocation syntax also can't have complex expressions as arguments. -For example: (`anonymous procs`_), ``if``, ``case`` or ``try``. The (`do -notation`_) is limited, but usable for a single proc (see the example in the -corresponding section). Function calls with no arguments still needs () to -distinguish between a call and the function itself as a first class value. - - -Closures --------- - -Procedures can appear at the top level in a module as well as inside other -scopes, in which case they are called nested procs. A nested proc can access -local variables from its enclosing scope and if it does so it becomes a -closure. Any captured variables are stored in a hidden additional argument -to the closure (its environment) and they are accessed by reference by both -the closure and its enclosing scope (i.e. any modifications made to them are -visible in both places). The closure environment may be allocated on the heap -or on the stack if the compiler determines that this would be safe. - - -Anonymous Procs ---------------- - -Procs can also be treated as expressions, in which case it's allowed to omit -the proc's name. - -.. code-block:: nim - var cities = @["Frankfurt", "Tokyo", "New York"] - - cities.sort(proc (x,y: string): int = - cmp(x.len, y.len)) - - -Procs as expressions can appear both as nested procs and inside top level -executable code. - - -Do notation ------------ - -As a special more convenient notation, proc expressions involved in procedure -calls can use the ``do`` keyword: - -.. code-block:: nim - sort(cities) do (x,y: string) -> int: - cmp(x.len, y.len) - # Less parenthesis using the method plus command syntax: - cities = cities.map do (x:string) -> string: - "City of " & x - -``do`` is written after the parentheses enclosing the regular proc params. -The proc expression represented by the do block is appended to them. - -More than one ``do`` block can appear in a single call: - -.. code-block:: nim - proc performWithUndo(task: proc(), undo: proc()) = ... - - performWithUndo do: - # multiple-line block of code - # to perform the task - do: - # code to undo it - -For compatibility with ``stmt`` templates and macros, the ``do`` keyword can be -omitted if the supplied proc doesn't have any parameters and return value. -The compatibility works in the other direction too as the ``do`` syntax can be -used with macros and templates expecting ``stmt`` blocks. - - -Nonoverloadable builtins ------------------------- - -The following builtin procs cannot be overloaded for reasons of implementation -simplicity (they require specialized semantic checking):: - - defined, definedInScope, compiles, low, high, sizeOf, - is, of, echo, shallowCopy, getAst, spawn - -Thus they act more like keywords than like ordinary identifiers; unlike a -keyword however, a redefinition may `shadow`:idx: the definition in -the ``system`` module. - - -Var parameters --------------- -The type of a parameter may be prefixed with the ``var`` keyword: - -.. code-block:: nim - proc divmod(a, b: int; res, remainder: var int) = - res = a div b - remainder = a mod b - - var - x, y: int - - divmod(8, 5, x, y) # modifies x and y - assert x == 1 - assert y == 3 - -In the example, ``res`` and ``remainder`` are `var parameters`. -Var parameters can be modified by the procedure and the changes are -visible to the caller. The argument passed to a var parameter has to be -an l-value. Var parameters are implemented as hidden pointers. The -above example is equivalent to: - -.. code-block:: nim - proc divmod(a, b: int; res, remainder: ptr int) = - res[] = a div b - remainder[] = a mod b - - var - x, y: int - divmod(8, 5, addr(x), addr(y)) - assert x == 1 - assert y == 3 - -In the examples, var parameters or pointers are used to provide two -return values. This can be done in a cleaner way by returning a tuple: - -.. code-block:: nim - proc divmod(a, b: int): tuple[res, remainder: int] = - (a div b, a mod b) - - var t = divmod(8, 5) - - assert t.res == 1 - assert t.remainder == 3 - -One can use `tuple unpacking`:idx: to access the tuple's fields: - -.. code-block:: nim - var (x, y) = divmod(8, 5) # tuple unpacking - assert x == 1 - assert y == 3 - - -Var return type ---------------- - -A proc, converter or iterator may return a ``var`` type which means that the -returned value is an l-value and can be modified by the caller: - -.. code-block:: nim - var g = 0 - - proc WriteAccessToG(): var int = - result = g - - WriteAccessToG() = 6 - assert g == 6 - -It is a compile time error if the implicitly introduced pointer could be -used to access a location beyond its lifetime: - -.. code-block:: nim - proc WriteAccessToG(): var int = - var g = 0 - result = g # Error! - -For iterators, a component of a tuple return type can have a ``var`` type too: - -.. code-block:: nim - iterator mpairs(a: var seq[string]): tuple[key: int, val: var string] = - for i in 0..a.high: - yield (i, a[i]) - -In the standard library every name of a routine that returns a ``var`` type -starts with the prefix ``m`` per convention. - - -Overloading of the subscript operator -------------------------------------- - -The ``[]`` subscript operator for arrays/openarrays/sequences can be overloaded. - - -Multi-methods -============= - -Procedures always use static dispatch. Multi-methods use dynamic -dispatch. - -.. code-block:: nim - type - TExpr = object ## abstract base class for an expression - TLiteral = object of TExpr - x: int - TPlusExpr = object of TExpr - a, b: ref TExpr - - method eval(e: ref TExpr): int = - # override this base method - quit "to override!" - - method eval(e: ref TLiteral): int = return e.x - - method eval(e: ref TPlusExpr): int = - # watch out: relies on dynamic binding - result = eval(e.a) + eval(e.b) - - proc newLit(x: int): ref TLiteral = - new(result) - result.x = x - - proc newPlus(a, b: ref TExpr): ref TPlusExpr = - new(result) - result.a = a - result.b = b - - echo eval(newPlus(newPlus(newLit(1), newLit(2)), newLit(4))) - -In the example the constructors ``newLit`` and ``newPlus`` are procs -because they should use static binding, but ``eval`` is a method because it -requires dynamic binding. - -In a multi-method all parameters that have an object type are used for the -dispatching: - -.. code-block:: nim - type - TThing = object - TUnit = object of TThing - x: int - - method collide(a, b: TThing) {.inline.} = - quit "to override!" - - method collide(a: TThing, b: TUnit) {.inline.} = - echo "1" - - method collide(a: TUnit, b: TThing) {.inline.} = - echo "2" - - var - a, b: TUnit - collide(a, b) # output: 2 - - -Invocation of a multi-method cannot be ambiguous: collide 2 is preferred over -collide 1 because the resolution works from left to right. -In the example ``TUnit, TThing`` is preferred over ``TThing, TUnit``. - -**Performance note**: Nim does not produce a virtual method table, but -generates dispatch trees. This avoids the expensive indirect branch for method -calls and enables inlining. However, other optimizations like compile time -evaluation or dead code elimination do not work with methods. - - -Iterators and the for statement -=============================== - -The `for`:idx: statement is an abstract mechanism to iterate over the elements -of a container. It relies on an `iterator`:idx: to do so. Like ``while`` -statements, ``for`` statements open an `implicit block`:idx:, so that they -can be left with a ``break`` statement. - -The ``for`` loop declares iteration variables - their scope reaches until the -end of the loop body. The iteration variables' types are inferred by the -return type of the iterator. - -An iterator is similar to a procedure, except that it can be called in the -context of a ``for`` loop. Iterators provide a way to specify the iteration over -an abstract type. A key role in the execution of a ``for`` loop plays the -``yield`` statement in the called iterator. Whenever a ``yield`` statement is -reached the data is bound to the ``for`` loop variables and control continues -in the body of the ``for`` loop. The iterator's local variables and execution -state are automatically saved between calls. Example: - -.. code-block:: nim - # this definition exists in the system module - iterator items*(a: string): char {.inline.} = - var i = 0 - while i < len(a): - yield a[i] - inc(i) - - for ch in items("hello world"): # `ch` is an iteration variable - echo(ch) - -The compiler generates code as if the programmer would have written this: - -.. code-block:: nim - var i = 0 - while i < len(a): - var ch = a[i] - echo(ch) - inc(i) - -If the iterator yields a tuple, there can be as many iteration variables -as there are components in the tuple. The i'th iteration variable's type is -the type of the i'th component. In other words, implicit tuple unpacking in a -for loop context is supported. - -Implict items/pairs invocations -------------------------------- - -If the for loop expression ``e`` does not denote an iterator and the for loop -has exactly 1 variable, the for loop expression is rewritten to ``items(e)``; -ie. an ``items`` iterator is implicitly invoked: - -.. code-block:: nim - for x in [1,2,3]: echo x - -If the for loop has exactly 2 variables, a ``pairs`` iterator is implicitly -invoked. - -Symbol lookup of the identifiers ``items``/``pairs`` is performed after -the rewriting step, so that all overloadings of ``items``/``pairs`` are taken -into account. - - -First class iterators ---------------------- - -There are 2 kinds of iterators in Nim: *inline* and *closure* iterators. -An `inline iterator`:idx: is an iterator that's always inlined by the compiler -leading to zero overhead for the abstraction, but may result in a heavy -increase in code size. Inline iterators are second class citizens; -They can be passed as parameters only to other inlining code facilities like -templates, macros and other inline iterators. - -In contrast to that, a `closure iterator`:idx: can be passed around more freely: - -.. code-block:: nim - iterator count0(): int {.closure.} = - yield 0 - - iterator count2(): int {.closure.} = - var x = 1 - yield x - inc x - yield x - - proc invoke(iter: iterator(): int {.closure.}) = - for x in iter(): echo x - - invoke(count0) - invoke(count2) - -Closure iterators have other restrictions than inline iterators: - -1. ``yield`` in a closure iterator can not occur in a ``try`` statement. -2. For now, a closure iterator cannot be evaluated at compile time. -3. ``return`` is allowed in a closure iterator (but rarely useful). -4. Both inline and closure iterators cannot be recursive. - -Iterators that are neither marked ``{.closure.}`` nor ``{.inline.}`` explicitly -default to being inline, but that this may change in future versions of the -implementation. - -The ``iterator`` type is always of the calling convention ``closure`` -implicitly; the following example shows how to use iterators to implement -a `collaborative tasking`:idx: system: - -.. code-block:: nim - # simple tasking: - type - TTask = iterator (ticker: int) - - iterator a1(ticker: int) {.closure.} = - echo "a1: A" - yield - echo "a1: B" - yield - echo "a1: C" - yield - echo "a1: D" - - iterator a2(ticker: int) {.closure.} = - echo "a2: A" - yield - echo "a2: B" - yield - echo "a2: C" - - proc runTasks(t: varargs[TTask]) = - var ticker = 0 - while true: - let x = t[ticker mod t.len] - if finished(x): break - x(ticker) - inc ticker - - runTasks(a1, a2) - -The builtin ``system.finished`` can be used to determine if an iterator has -finished its operation; no exception is raised on an attempt to invoke an -iterator that has already finished its work. - -Closure iterators are *resumable functions* and so one has to provide the -arguments to every call. To get around this limitation one can capture -parameters of an outer factory proc: - -.. code-block:: nim - proc mycount(a, b: int): iterator (): int = - result = iterator (): int = - var x = a - while x <= b: - yield x - inc x - - let foo = mycount(1, 4) - - for f in foo(): - echo f - -Implicit return type --------------------- - -Since inline interators must always produce values that will be consumed in -a for loop, the compiler will implicity use the ``auto`` return type if no -type is given by the user. In contrast, since closure iterators can be used -as a collaborative tasking system, ``void`` is a valid return type for them. - - -Type sections -============= - -Example: - -.. code-block:: nim - type # example demonstrating mutually recursive types - PNode = ref TNode # a traced pointer to a TNode - TNode = object - le, ri: PNode # left and right subtrees - sym: ref TSym # leaves contain a reference to a TSym - - TSym = object # a symbol - name: string # the symbol's name - line: int # the line the symbol was declared in - code: PNode # the symbol's abstract syntax tree - -A type section begins with the ``type`` keyword. It contains multiple -type definitions. A type definition binds a type to a name. Type definitions -can be recursive or even mutually recursive. Mutually recursive types are only -possible within a single ``type`` section. Nominal types like ``objects`` -or ``enums`` can only be defined in a ``type`` section. - - -Exception handling -================== - -Try statement -------------- - -Example: - -.. code-block:: nim - # read the first two lines of a text file that should contain numbers - # and tries to add them - var - f: TFile - if open(f, "numbers.txt"): - try: - var a = readLine(f) - var b = readLine(f) - echo("sum: " & $(parseInt(a) + parseInt(b))) - except EOverflow: - echo("overflow!") - except EInvalidValue: - echo("could not convert string to integer") - except EIO: - echo("IO error!") - except: - echo("Unknown exception!") - finally: - close(f) - - -The statements after the ``try`` are executed in sequential order unless -an exception ``e`` is raised. If the exception type of ``e`` matches any -listed in an ``except`` clause the corresponding statements are executed. -The statements following the ``except`` clauses are called -`exception handlers`:idx:. - -The empty `except`:idx: clause is executed if there is an exception that is -not listed otherwise. It is similar to an ``else`` clause in ``if`` statements. - -If there is a `finally`:idx: clause, it is always executed after the -exception handlers. - -The exception is *consumed* in an exception handler. However, an -exception handler may raise another exception. If the exception is not -handled, it is propagated through the call stack. This means that often -the rest of the procedure - that is not within a ``finally`` clause - -is not executed (if an exception occurs). - - -Except and finally statements ------------------------------ - -``except`` and ``finally`` can also be used as a stand-alone statements. -Any statements following them in the current block will be considered to be -in an implicit try block: - -.. code-block:: nim - var f = open("numbers.txt") - finally: close(f) - ... - -The ``except`` statement has a limitation in this form: one can't specify the -type of the exception, one has to catch everything. Also, if one wants to use -both ``finally`` and ``except`` one needs to reverse the usual sequence of the -statements. Example: - -.. code-block:: nim - proc test() = - raise newException(E_base, "Hey ho") - - proc tester() = - finally: echo "3. Finally block" - except: echo "2. Except block" - echo "1. Pre exception" - test() - echo "4. Post exception" - # --> 1, 2, 3 is printed, 4 is never reached - - -Raise statement ---------------- - -Example: - -.. code-block:: nim - raise newEOS("operating system failed") - -Apart from built-in operations like array indexing, memory allocation, etc. -the ``raise`` statement is the only way to raise an exception. - -.. XXX document this better! - -If no exception name is given, the current exception is `re-raised`:idx:. The -`ENoExceptionToReraise`:idx: exception is raised if there is no exception to -re-raise. It follows that the ``raise`` statement *always* raises an -exception (unless a raise hook has been provided). - - -OnRaise builtin ---------------- - -`system.onRaise() <system.html#onRaise>`_ can be used to override the -behaviour of ``raise`` for a single ``try`` statement. ``onRaise`` has to be -called within the ``try`` statement that should be affected. - -This allows for a Lisp-like `condition system`:idx:\: - -.. code-block:: nim - var myFile = open("broken.txt", fmWrite) - try: - onRaise do (e: ref E_Base)-> bool: - if e of EIO: - stdout.writeln "ok, writing to stdout instead" - else: - # do raise other exceptions: - result = true - myFile.writeln "writing to broken file" - finally: - myFile.close() - -``OnRaise`` can only *filter* raised exceptions, it cannot transform one -exception into another. (Nor should ``onRaise`` raise an exception though -this is currently not enforced.) This restriction keeps the exception tracking -analysis sound. - - -Effect system -============= - -Exception tracking ------------------- - -Nim supports exception tracking. The `raises`:idx: pragma can be used -to explicitly define which exceptions a proc/iterator/method/converter is -allowed to raise. The compiler verifies this: - -.. code-block:: nim - proc p(what: bool) {.raises: [EIO, EOS].} = - if what: raise newException(EIO, "IO") - else: raise newException(EOS, "OS") - -An empty ``raises`` list (``raises: []``) means that no exception may be raised: - -.. code-block:: nim - proc p(): bool {.raises: [].} = - try: - unsafeCall() - result = true - except: - result = false - - -A ``raises`` list can also be attached to a proc type. This affects type -compatibility: - -.. code-block:: nim - type - TCallback = proc (s: string) {.raises: [EIO].} - var - c: TCallback - - proc p(x: string) = - raise newException(EOS, "OS") - - c = p # type error - - -For a routine ``p`` the compiler uses inference rules to determine the set of -possibly raised exceptions; the algorithm operates on ``p``'s call graph: - -1. Every indirect call via some proc type ``T`` is assumed to - raise ``system.E_Base`` (the base type of the exception hierarchy) and - thus any exception unless ``T`` has an explicit ``raises`` list. - However if the call is of the form ``f(...)`` where ``f`` is a parameter - of the currently analysed routine it is ignored. The call is optimistically - assumed to have no effect. Rule 2 compensates for this case. -2. Every expression of some proc type wihtin a call that is not a call - itself (and not nil) is assumed to be called indirectly somehow and thus - its raises list is added to ``p``'s raises list. -3. Every call to a proc ``q`` which has an unknown body (due to a forward - declaration or an ``importc`` pragma) is assumed to - raise ``system.E_Base`` unless ``q`` has an explicit ``raises`` list. -4. Every call to a method ``m`` is assumed to - raise ``system.E_Base`` unless ``m`` has an explicit ``raises`` list. -5. For every other call the analysis can determine an exact ``raises`` list. -6. For determining a ``raises`` list, the ``raise`` and ``try`` statements - of ``p`` are taken into consideration. - -Rules 1-2 ensure the following works: - -.. code-block:: nim - proc noRaise(x: proc()) {.raises: [].} = - # unknown call that might raise anything, but valid: - x() - - proc doRaise() {.raises: [EIO].} = - raise newException(EIO, "IO") - - proc use() {.raises: [].} = - # doesn't compile! Can raise EIO! - noRaise(doRaise) - -So in many cases a callback does not cause the compiler to be overly -conservative in its effect analysis. - - -Tag tracking ------------- - -The exception tracking is part of Nim's `effect system`:idx:. Raising an -exception is an *effect*. Other effects can also be defined. A user defined -effect is a means to *tag* a routine and to perform checks against this tag: - -.. code-block:: nim - type IO = object ## input/output effect - proc readLine(): string {.tags: [IO].} - - proc no_IO_please() {.tags: [].} = - # the compiler prevents this: - let x = readLine() - -A tag has to be a type name. A ``tags`` list - like a ``raises`` list - can -also be attached to a proc type. This affects type compatibility. - -The inference for tag tracking is analogous to the inference for -exception tracking. - - -Read/Write tracking -------------------- - -**Note**: Read/write tracking is not yet implemented! - -The inference for read/write tracking is analogous to the inference for -exception tracking. - - -Effects pragma --------------- - -The ``effects`` pragma has been designed to assist the programmer with the -effects analysis. It is a statement that makes the compiler output all inferred -effects up to the ``effects``'s position: - -.. code-block:: nim - proc p(what: bool) = - if what: - raise newException(EIO, "IO") - {.effects.} - else: - raise newException(EOS, "OS") - -The compiler produces a hint message that ``EIO`` can be raised. ``EOS`` is not -listed as it cannot be raised in the branch the ``effects`` pragma appears in. - - -Generics -======== - -Example: - -.. code-block:: nim - type - TBinaryTree[T] = object # TBinaryTree is a generic type with - # with generic param ``T`` - le, ri: ref TBinaryTree[T] # left and right subtrees; may be nil - data: T # the data stored in a node - PBinaryTree[T] = ref TBinaryTree[T] # a shorthand for notational convenience - - proc newNode[T](data: T): PBinaryTree[T] = # constructor for a node - new(result) - result.data = data - - proc add[T](root: var PBinaryTree[T], n: PBinaryTree[T]) = - if root == nil: - root = n - else: - var it = root - while it != nil: - var c = cmp(it.data, n.data) # compare the data items; uses - # the generic ``cmp`` proc that works for - # any type that has a ``==`` and ``<`` - # operator - if c < 0: - if it.le == nil: - it.le = n - return - it = it.le - else: - if it.ri == nil: - it.ri = n - return - it = it.ri - - iterator inorder[T](root: PBinaryTree[T]): T = - # inorder traversal of a binary tree - # recursive iterators are not yet implemented, so this does not work in - # the current compiler! - if root.le != nil: yield inorder(root.le) - yield root.data - if root.ri != nil: yield inorder(root.ri) - - var - root: PBinaryTree[string] # instantiate a PBinaryTree with the type string - add(root, newNode("hallo")) # instantiates generic procs ``newNode`` and - add(root, newNode("world")) # ``add`` - for str in inorder(root): - writeln(stdout, str) - -Generics are Nim's means to parametrize procs, iterators or types with -`type parameters`:idx:. Depending on context, the brackets are used either to -introduce type parameters or to instantiate a generic proc, iterator or type. - - -Is operator ------------ - -The ``is`` operator checks for type equivalence at compile time. It is -therefore very useful for type specialization within generic code: - -.. code-block:: nim - type - TTable[TKey, TValue] = object - keys: seq[TKey] - values: seq[TValue] - when not (TKey is string): # nil value for strings used for optimization - deletedKeys: seq[bool] - - -Type operator -------------- - -The ``type`` (in many other languages called `typeof`:idx:) operator can -be used to get the type of an expression: - -.. code-block:: nim - var x = 0 - var y: type(x) # y has type int - -If ``type`` is used to determine the result type of a proc/iterator/converter -call ``c(X)`` (where ``X`` stands for a possibly empty list of arguments), the -interpretation where ``c`` is an iterator is preferred over the -other interpretations: - -.. code-block:: nim - import strutils - - # strutils contains both a ``split`` proc and iterator, but since an - # an iterator is the preferred interpretation, `y` has the type ``string``: - var y: type("a b c".split) - - -Type Classes ------------- - -A type class is a special pseudo-type that can be used to match against -types in the context of overload resolution or the ``is`` operator. -Nim supports the following built-in type classes: - -================== =================================================== -type class matches -================== =================================================== -``object`` any object type -``tuple`` any tuple type - -``enum`` any enumeration -``proc`` any proc type -``ref`` any ``ref`` type -``ptr`` any ``ptr`` type -``var`` any ``var`` type -``distinct`` any distinct type -``array`` any array type -``set`` any set type -``seq`` any seq type -``auto`` any type -================== =================================================== - -Furthermore, every generic type automatically creates a type class of the same -name that will match any instantiation of the generic type. - -Type classes can be combined using the standard boolean operators to form -more complex type classes: - -.. code-block:: nim - # create a type class that will match all tuple and object types - type TRecordType = tuple or object - - proc printFields(rec: TRecordType) = - for key, value in fieldPairs(rec): - echo key, " = ", value - -Procedures utilizing type classes in such manner are considered to be -`implicitly generic`:idx:. They will be instantiated once for each unique -combination of param types used within the program. - -Nim also allows for type classes and regular types to be specified -as `type constraints`:idx: of the generic type parameter: - -.. code-block:: nim - proc onlyIntOrString[T: int|string](x, y: T) = discard - - onlyIntOrString(450, 616) # valid - onlyIntOrString(5.0, 0.0) # type mismatch - onlyIntOrString("xy", 50) # invalid as 'T' cannot be both at the same time - -By default, during overload resolution each named type class will bind to -exactly one concrete type. Here is an example taken directly from the system -module to illustrate this: - -.. code-block:: nim - proc `==`*(x, y: tuple): bool = - ## requires `x` and `y` to be of the same tuple type - ## generic ``==`` operator for tuples that is lifted from the components - ## of `x` and `y`. - result = true - for a, b in fields(x, y): - if a != b: result = false - -Alternatively, the ``distinct`` type modifier can be applied to the type class -to allow each param matching the type class to bind to a different type. - -If a proc param doesn't have a type specified, Nim will use the -``distinct auto`` type class (also known as ``any``): - -.. code-block:: nim - # allow any combination of param types - proc concat(a, b): string = $a & $b - -Procs written with the implicitly generic style will often need to refer to the -type parameters of the matched generic type. They can be easily accessed using -the dot syntax: - -.. code-block:: nim - type TMatrix[T, Rows, Columns] = object - ... - - proc `[]`(m: TMatrix, row, col: int): TMatrix.T = - m.data[col * high(TMatrix.Columns) + row] - -Alternatively, the `type` operator can be used over the proc params for similar -effect when anonymous or distinct type classes are used. - -When a generic type is instantiated with a type class instead of a concrete -type, this results in another more specific type class: - -.. code-block:: nim - seq[ref object] # Any sequence storing references to any object type - - type T1 = auto - proc foo(s: seq[T1], e: T1) - # seq[T1] is the same as just `seq`, but T1 will be allowed to bind - # to a single type, while the signature is being matched - - TMatrix[Ordinal] # Any TMatrix instantiation using integer values - -As seen in the previous example, in such instantiations, it's not necessary to -supply all type parameters of the generic type, because any missing ones will -be inferred to have the equivalent of the `any` type class and thus they will -match anything without discrimination. - - -User defined type classes -------------------------- - -**Note**: User defined type classes are still in development. - -The user-defined type classes are available in two flavours - declarative and -imperative. Both are used to specify an arbitrary set of requirements that the -matched type must satisfy. - -Declarative type classes are written in the following form: - -.. code-block:: nim - type - Comparable = generic x, y - (x < y) is bool - - Container[T] = generic c - c.len is ordinal - items(c) is iterator - for value in c: - type(value) is T - -The type class will be matched if: - -a) all of the expressions within the body can be compiled for the tested type -b) all statically evaluatable boolean expressions in the body must be true - -The identifiers following the `generic` keyword represent instances of the -currently matched type. These instances can act both as variables of the type, -when used in contexts where a value is expected, and as the type itself when -used in contexts where a type is expected. - -Please note that the ``is`` operator allows one to easily verify the precise -type signatures of the required operations, but since type inference and -default parameters are still applied in the provided block, it's also possible -to encode usage protocols that do not reveal implementation details. - -As a special rule providing further convenience when writing type classes, any -type value appearing in a callable expression will be treated as a variable of -the designated type for overload resolution purposes, unless the type value was -passed in its explicit ``typedesc[T]`` form: - -.. code-block:: nim - type - OutputStream = generic S - write(var S, string) - -Much like generics, the user defined type classes will be instantiated exactly -once for each tested type and any static code included within them will also be -executed once. - - -Type inference with type classes --------------------------------- - -If a type class is used as the return type of a proc and it won't be bound to -a concrete type by some of the proc params, Nim will infer the return type -from the proc body. This is usually used with the ``auto`` type class: - -.. code-block:: nim - proc makePair(a, b): auto = (first: a, second: b) - -The return type will be treated as an additional generic param and can be -explicitly specified at call sites as any other generic param. - -Future versions of Nim may also support overloading based on the return type -of the overloads. In such settings, the expected result type at call sites may -also influence the inferred return type. - -.. - Likewise, if a type class is used in another position where Nim expects a - concrete type (e.g. a variable declaration or a type coercion), Nim will try - to infer the concrete type by applying the matching algorithm that also used - in overload resolution. - - -Symbol lookup in generics -------------------------- - -The symbol binding rules in generics are slightly subtle: There are "open" and -"closed" symbols. A "closed" symbol cannot be re-bound in the instantiation -context, an "open" symbol can. Per default overloaded symbols are open -and every other symbol is closed. - -Open symbols are looked up in two different contexts: Both the context -at definition and the context at instantiation are considered: - -.. code-block:: nim - type - TIndex = distinct int - - proc `==` (a, b: TIndex): bool {.borrow.} - - var a = (0, 0.TIndex) - var b = (0, 0.TIndex) - - echo a == b # works! - -In the example the generic ``==`` for tuples (as defined in the system module) -uses the ``==`` operators of the tuple's components. However, the ``==`` for -the ``TIndex`` type is defined *after* the ``==`` for tuples; yet the example -compiles as the instantiation takes the currently defined symbols into account -too. - -A symbol can be forced to be open by a `mixin`:idx: declaration: - -.. code-block:: nim - proc create*[T](): ref T = - # there is no overloaded 'init' here, so we need to state that it's an - # open symbol explicitly: - mixin init - new result - init result - - -Bind statement --------------- - -The ``bind`` statement is the counterpart to the ``mixin`` statement. It -can be used to explicitly declare identifiers that should be bound early (i.e. -the identifiers should be looked up in the scope of the template/generic -definition): - -.. code-block:: nim - # Module A - var - lastId = 0 - - template genId*: expr = - bind lastId - inc(lastId) - lastId - -.. code-block:: nim - # Module B - import A - - echo genId() - -But a ``bind`` is rarely useful because symbol binding from the definition -scope is the default. - - -Templates -========= - -A template is a simple form of a macro: It is a simple substitution -mechanism that operates on Nim's abstract syntax trees. It is processed in -the semantic pass of the compiler. - -The syntax to *invoke* a template is the same as calling a procedure. - -Example: - -.. code-block:: nim - template `!=` (a, b: expr): expr = - # this definition exists in the System module - not (a == b) - - assert(5 != 6) # the compiler rewrites that to: assert(not (5 == 6)) - -The ``!=``, ``>``, ``>=``, ``in``, ``notin``, ``isnot`` operators are in fact -templates: - -| ``a > b`` is transformed into ``b < a``. -| ``a in b`` is transformed into ``contains(b, a)``. -| ``notin`` and ``isnot`` have the obvious meanings. - -The "types" of templates can be the symbols ``expr`` (stands for *expression*), -``stmt`` (stands for *statement*) or ``typedesc`` (stands for *type -description*). These are "meta types", they can only be used in certain -contexts. Real types can be used too; this implies that expressions are -expected. - - -Ordinary vs immediate templates -------------------------------- - -There are two different kinds of templates: immediate templates and -ordinary templates. Ordinary templates take part in overloading resolution. As -such their arguments need to be type checked before the template is invoked. -So ordinary templates cannot receive undeclared identifiers: - -.. code-block:: nim - - template declareInt(x: expr) = - var x: int - - declareInt(x) # error: unknown identifier: 'x' - -An ``immediate`` template does not participate in overload resolution and so -its arguments are not checked for semantics before invocation. So they can -receive undeclared identifiers: - -.. code-block:: nim - - template declareInt(x: expr) {.immediate.} = - var x: int - - declareInt(x) # valid - - -Passing a code block to a template ----------------------------------- - -If there is a ``stmt`` parameter it should be the last in the template -declaration, because statements are passed to a template via a -special ``:`` syntax: - -.. code-block:: nim - - template withFile(f, fn, mode: expr, actions: stmt): stmt {.immediate.} = - var f: TFile - if open(f, fn, mode): - try: - actions - finally: - close(f) - else: - quit("cannot open: " & fn) - - withFile(txt, "ttempl3.txt", fmWrite): - txt.writeln("line 1") - txt.writeln("line 2") - -In the example the two ``writeln`` statements are bound to the ``actions`` -parameter. - - -Symbol binding in templates ---------------------------- - -A template is a `hygienic`:idx: macro and so opens a new scope. Most symbols are -bound from the definition scope of the template: - -.. code-block:: nim - # Module A - var - lastId = 0 - - template genId*: expr = - inc(lastId) - lastId - -.. code-block:: nim - # Module B - import A - - echo genId() # Works as 'lastId' has been bound in 'genId's defining scope - -As in generics symbol binding can be influenced via ``mixin`` or ``bind`` -statements. - - - -Identifier construction ------------------------ - -In templates identifiers can be constructed with the backticks notation: - -.. code-block:: nim - - template typedef(name: expr, typ: typedesc) {.immediate.} = - type - `T name`* {.inject.} = typ - `P name`* {.inject.} = ref `T name` - - typedef(myint, int) - var x: PMyInt - -In the example ``name`` is instantiated with ``myint``, so \`T name\` becomes -``Tmyint``. - - -Lookup rules for template parameters ------------------------------------- - -A parameter ``p`` in a template is even substituted in the expression ``x.p``. -Thus template arguments can be used as field names and a global symbol can be -shadowed by the same argument name even when fully qualified: - -.. code-block:: nim - # module 'm' - - type - TLev = enum - levA, levB - - var abclev = levB - - template tstLev(abclev: TLev) = - echo abclev, " ", m.abclev - - tstLev(levA) - # produces: 'levA levA' - -But the global symbol can properly be captured by a ``bind`` statement: - -.. code-block:: nim - # module 'm' - - type - TLev = enum - levA, levB - - var abclev = levB - - template tstLev(abclev: TLev) = - bind m.abclev - echo abclev, " ", m.abclev - - tstLev(levA) - # produces: 'levA levB' - - -Hygiene in templates --------------------- - -Per default templates are `hygienic`:idx:\: Local identifiers declared in a -template cannot be accessed in the instantiation context: - -.. code-block:: nim - - template newException*(exceptn: typedesc, message: string): expr = - var - e: ref exceptn # e is implicitly gensym'ed here - new(e) - e.msg = message - e - - # so this works: - let e = "message" - raise newException(EIO, e) - - -Whether a symbol that is declared in a template is exposed to the instantiation -scope is controlled by the `inject`:idx: and `gensym`:idx: pragmas: gensym'ed -symbols are not exposed but inject'ed are. - -The default for symbols of entity ``type``, ``var``, ``let`` and ``const`` -is ``gensym`` and for ``proc``, ``iterator``, ``converter``, ``template``, -``macro`` is ``inject``. However, if the name of the entity is passed as a -template parameter, it is an inject'ed symbol: - -.. code-block:: nim - template withFile(f, fn, mode: expr, actions: stmt): stmt {.immediate.} = - block: - var f: TFile # since 'f' is a template param, it's injected implicitly - ... - - withFile(txt, "ttempl3.txt", fmWrite): - txt.writeln("line 1") - txt.writeln("line 2") - - -The ``inject`` and ``gensym`` pragmas are second class annotations; they have -no semantics outside of a template definition and cannot be abstracted over: - -.. code-block:: nim - {.pragma myInject: inject.} - - template t() = - var x {.myInject.}: int # does NOT work - - -To get rid of hygiene in templates, one can use the `dirty`:idx: pragma for -a template. ``inject`` and ``gensym`` have no effect in ``dirty`` templates. - - - -Macros -====== - -A macro is a special kind of low level template. Macros can be used -to implement `domain specific languages`:idx:. Like templates, macros come in -the 2 flavors *immediate* and *ordinary*. - -While macros enable advanced compile-time code transformations, they -cannot change Nim's syntax. However, this is no real restriction because -Nim's syntax is flexible enough anyway. - -To write macros, one needs to know how the Nim concrete syntax is converted -to an abstract syntax tree. - -There are two ways to invoke a macro: -(1) invoking a macro like a procedure call (`expression macros`) -(2) invoking a macro with the special ``macrostmt`` syntax (`statement macros`) - - -Expression Macros ------------------ - -The following example implements a powerful ``debug`` command that accepts a -variable number of arguments: - -.. code-block:: nim - # to work with Nim syntax trees, we need an API that is defined in the - # ``macros`` module: - import macros - - macro debug(n: varargs[expr]): stmt = - # `n` is a Nim AST that contains the whole macro invocation - # this macro returns a list of statements: - result = newNimNode(nnkStmtList, n) - # iterate over any argument that is passed to this macro: - for i in 0..n.len-1: - # add a call to the statement list that writes the expression; - # `toStrLit` converts an AST to its string representation: - add(result, newCall("write", newIdentNode("stdout"), toStrLit(n[i]))) - # add a call to the statement list that writes ": " - add(result, newCall("write", newIdentNode("stdout"), newStrLitNode(": "))) - # add a call to the statement list that writes the expressions value: - add(result, newCall("writeln", newIdentNode("stdout"), n[i])) - - var - a: array [0..10, int] - x = "some string" - a[0] = 42 - a[1] = 45 - - debug(a[0], a[1], x) - -The macro call expands to: - -.. code-block:: nim - write(stdout, "a[0]") - write(stdout, ": ") - writeln(stdout, a[0]) - - write(stdout, "a[1]") - write(stdout, ": ") - writeln(stdout, a[1]) - - write(stdout, "x") - write(stdout, ": ") - writeln(stdout, x) - - -Arguments that are passed to a ``varargs`` parameter are wrapped in an array -constructor expression. This is why ``debug`` iterates over all of ``n``'s -children. - - -BindSym -------- - -The above ``debug`` macro relies on the fact that ``write``, ``writeln`` and -``stdout`` are declared in the system module and thus visible in the -instantiating context. There is a way to use bound identifiers -(aka `symbols`:idx:) instead of using unbound identifiers. The ``bindSym`` -builtin can be used for that: - -.. code-block:: nim - import macros - - macro debug(n: varargs[expr]): stmt = - result = newNimNode(nnkStmtList, n) - for i in 0..n.len-1: - # we can bind symbols in scope via 'bindSym': - add(result, newCall(bindSym"write", bindSym"stdout", toStrLit(n[i]))) - add(result, newCall(bindSym"write", bindSym"stdout", newStrLitNode(": "))) - add(result, newCall(bindSym"writeln", bindSym"stdout", n[i])) - - var - a: array [0..10, int] - x = "some string" - a[0] = 42 - a[1] = 45 - - debug(a[0], a[1], x) - -The macro call expands to: - -.. code-block:: nim - write(stdout, "a[0]") - write(stdout, ": ") - writeln(stdout, a[0]) - - write(stdout, "a[1]") - write(stdout, ": ") - writeln(stdout, a[1]) - - write(stdout, "x") - write(stdout, ": ") - writeln(stdout, x) - -However, the symbols ``write``, ``writeln`` and ``stdout`` are already bound -and are not looked up again. As the example shows, ``bindSym`` does work with -overloaded symbols implicitly. - - -Statement Macros ----------------- - -Statement macros are defined just as expression macros. However, they are -invoked by an expression following a colon. - -The following example outlines a macro that generates a lexical analyzer from -regular expressions: - -.. code-block:: nim - import macros - - macro case_token(n: stmt): stmt = - # creates a lexical analyzer from regular expressions - # ... (implementation is an exercise for the reader :-) - discard - - case_token: # this colon tells the parser it is a macro statement - of r"[A-Za-z_]+[A-Za-z_0-9]*": - return tkIdentifier - of r"0-9+": - return tkInteger - of r"[\+\-\*\?]+": - return tkOperator - else: - return tkUnknown - - -**Style note**: For code readability, it is the best idea to use the least -powerful programming construct that still suffices. So the "check list" is: - -(1) Use an ordinary proc/iterator, if possible. -(2) Else: Use a generic proc/iterator, if possible. -(3) Else: Use a template, if possible. -(4) Else: Use a macro. - - -Macros as pragmas ------------------ - -Whole routines (procs, iterators etc.) can also be passed to a template or -a macro via the pragma notation: - -.. code-block:: nim - template m(s: stmt) = discard - - proc p() {.m.} = discard - -This is a simple syntactic transformation into: - -.. code-block:: nim - template m(s: stmt) = discard - - m: - proc p() = discard - - -Special Types -============= - -static[T] ---------- - -**Note**: static[T] is still in development. - -As their name suggests, static params must be known at compile-time: - -.. code-block:: nim - - proc precompiledRegex(pattern: static[string]): TRegEx = - var res {.global.} = re(pattern) - return res - - precompiledRegex("/d+") # Replaces the call with a precompiled - # regex, stored in a global variable - - precompiledRegex(paramStr(1)) # Error, command-line options - # are not known at compile-time - - -For the purposes of code generation, all static params are treated as -generic params - the proc will be compiled separately for each unique -supplied value (or combination of values). - -Furthermore, the system module defines a `semistatic[T]` type than can be -used to declare procs accepting both static and run-time values, which can -optimize their body according to the supplied param using the `isStatic(p)` -predicate: - -.. code-block:: nim - - # The following proc will be compiled once for each unique static - # value and also once for the case handling all run-time values: - - proc re(pattern: semistatic[string]): TRegEx = - when isStatic(pattern): - result = precompiledRegex(pattern) - else: - result = compile(pattern) - -Static params can also appear in the signatures of generic types: - -.. code-block:: nim - - type - Matrix[M,N: static[int]; T: Number] = array[0..(M*N - 1), T] - # Note how `Number` is just a type constraint here, while - # `static[int]` requires us to supply a compile-time int value - - AffineTransform2D[T] = Matrix[3, 3, T] - AffineTransform3D[T] = Matrix[4, 4, T] - - var m1: AffineTransform3D[float] # OK - var m2: AffineTransform2D[string] # Error, `string` is not a `Number` - - -typedesc --------- - -`typedesc` is a special type allowing one to treat types as compile-time values -(i.e. if types are compile-time values and all values have a type, then -typedesc must be their type). - -When used as a regular proc param, typedesc acts as a type class. The proc -will be instantiated for each unique type parameter and one can refer to the -instantiation type using the param name: - -.. code-block:: nim - - proc new(T: typedesc): ref T = - echo "allocating ", T.name - new(result) - - var n = TNode.new - var tree = new(TBinaryTree[int]) - -When multiple typedesc params are present, they act like a distinct type class -(i.e. they will bind freely to different types). To force a bind-once behavior -one can use a named alias or an explicit `typedesc` generic param: - -.. code-block:: nim - - # `type1` and `type2` are aliases for typedesc available from system.nim - proc acceptOnlyTypePairs(A, B: type1; C, D: type2) - proc acceptOnlyTypePairs[T: typedesc, U: typedesc](A, B: T; C, D: U) - -Once bound, typedesc params can appear in the rest of the proc signature: - -.. code-block:: nim - - template declareVariableWithType(T: typedesc, value: T) = - var x: T = value - - declareVariableWithType int, 42 - -When used with macros and .compileTime. procs on the other hand, the compiler -does not need to instantiate the code multiple times, because types then can be -manipulated using the unified internal symbol representation. In such context -typedesc acts as any other type. One can create variables, store typedesc -values inside containers and so on. For example, here is how one can create -a type-safe wrapper for the unsafe `printf` function from C: - -.. code-block:: nim - macro safePrintF(formatString: string{lit}, args: varargs[expr]): expr = - var i = 0 - for c in formatChars(formatString): - var expectedType = case c - of 'c': char - of 'd', 'i', 'x', 'X': int - of 'f', 'e', 'E', 'g', 'G': float - of 's': string - of 'p': pointer - else: EOutOfRange - - var actualType = args[i].getType - inc i - - if expectedType == EOutOfRange: - error c & " is not a valid format character" - elif expectedType != actualType: - error "type mismatch for argument ", i, ". expected type: ", - expectedType.name, ", actual type: ", actualType.name - - # keep the original callsite, but use cprintf instead - result = callsite() - result[0] = newIdentNode(!"cprintf") - - -Overload resolution can be further influenced by constraining the set of -types that will match the typedesc param: - -.. code-block:: nim - - template maxval(T: typedesc[int]): int = high(int) - template maxval(T: typedesc[float]): float = Inf - - var i = int.maxval - var f = float.maxval - var s = string.maxval # error, maxval is not implemented for string - -The constraint can be a concrete type or a type class. - - -Special Operators -================= - -dot operators -------------- - -Nim offers a special family of dot operators that can be used to -intercept and rewrite proc call and field access attempts, referring -to previously undeclared symbol names. They can be used to provide a -fluent interface to objects lying outside the static confines of the -type system such as values from dynamic scripting languages -or dynamic file formats such as JSON or XML. - -When Nim encounters an expression that cannot be resolved by the -standard overload resolution rules, the current scope will be searched -for a dot operator that can be matched against a re-written form of -the expression, where the unknown field or proc name is converted to -an additional static string parameter: - -.. code-block:: nim - a.b # becomes `.`(a, "b") - a.b(c, d) # becomes `.`(a, "b", c, d) - -The matched dot operators can be symbols of any callable kind (procs, -templates and macros), depending on the desired effect: - -.. code-block:: nim - proc `.` (js: PJsonNode, field: string): JSON = js[field] - - var js = parseJson("{ x: 1, y: 2}") - echo js.x # outputs 1 - echo js.y # outputs 2 - -The following dot operators are available: - -operator `.` ------------- -This operator will be matched against both field accesses and method calls. - -operator `.()` ---------------- -This operator will be matched exclusively against method calls. It has higher -precedence than the `.` operator and this allows one to handle expressions like -`x.y` and `x.y()` differently if one is interfacing with a scripting language -for example. - -operator `.=` -------------- -This operator will be matched against assignments to missing fields. - -.. code-block:: nim - a.b = c # becomes `.=`(a, "b", c) - - -Type bound operations -===================== - -There are 3 operations that are bound to a type: - -1. Assignment -2. Destruction -3. Deep copying for communication between threads - -These operations can be *overriden* instead of *overloaded*. This means the -implementation is automatically lifted to structured types. For instance if type -``T`` has an overriden assignment operator ``=`` this operator is also used -for assignments of the type ``seq[T]``. Since these operations are bound to a -type they have to be bound to a nominal type for reasons of simplicity of -implementation: This means an overriden ``deepCopy`` for ``ref T`` is really -bound to ``T`` and not to ``ref T``. This also means that one cannot override -``deepCopy`` for both ``ptr T`` and ``ref T`` at the same time; instead a -helper distinct or object type has to be used for one pointer type. - - -operator `=` ------------- - -This operator is the assignment operator. Note that in the contexts -like ``let v = expr``, ``var v = expr``, ``parameter = defaultValue`` or for -parameter passing no assignment is performed. The ``override`` pragma is -optional for overriding ``=``. - -**Note**: Overriding of operator ``=`` is not yet implemented. - - -destructors ------------ - -A destructor must have a single parameter with a concrete type (the name of a -generic type is allowed too). The name of the destructor has to be ``destroy`` -and it need to be annotated with the ``override`` pragma. - -``destroy(v)`` will be automatically invoked for every local stack -variable ``v`` that goes out of scope. - -If a structured type features a field with destructable type and -the user has not provided an explicit implementation, a destructor for the -structured type will be automatically generated. Calls to any base class -destructors in both user-defined and generated destructors will be inserted. - -A destructor is attached to the type it destructs; expressions of this type -can then only be used in *destructible contexts* and as parameters: - -.. code-block:: nim - type - TMyObj = object - x, y: int - p: pointer - - proc destroy(o: var TMyObj) {.override.} = - if o.p != nil: dealloc o.p - - proc open: TMyObj = - result = TMyObj(x: 1, y: 2, p: alloc(3)) - - proc work(o: TMyObj) = - echo o.x - # No destructor invoked here for 'o' as 'o' is a parameter. - - proc main() = - # destructor automatically invoked at the end of the scope: - var x = open() - # valid: pass 'x' to some other proc: - work(x) - - # Error: usage of a type with a destructor in a non destructible context - echo open() - -A destructible context is currently only the following: - -1. The ``expr`` in ``var x = expr``. -2. The ``expr`` in ``let x = expr``. -3. The ``expr`` in ``return expr``. -4. The ``expr`` in ``result = expr`` where ``result`` is the special symbol - introduced by the compiler. - -These rules ensure that the construction is tied to a variable and can easily -be destructed at its scope exit. Later versions of the language will improve -the support of destructors. - -Be aware that destructors are not called for objects allocated with ``new``. -This may change in future versions of language, but for now the ``finalizer`` -parameter to ``new`` has to be used. - -**Note**: Destructors are still experimental and the spec might change -significantly in order to incorporate an escape analysis. - - -deepCopy --------- - -``deepCopy`` is a builtin that is invoked whenever data is passed to -a ``spawn``'ed proc to ensure memory safety. The programmer can override its -behaviour for a specific ``ref`` or ``ptr`` type ``T``. (Later versions of the -language may weaken this restriction.) - -The signature has to be: - -.. code-block:: nim - proc deepCopy(x: T): T {.override.} - -This mechanism is used by most data structures that support shared memory like -channels to implement thread safe automatic memory management. - -The builtin ``deepCopy`` can even clone closures and their environments. See -the documentation of `spawn`_ for details. - - -Term rewriting macros -===================== - -Term rewriting macros are macros or templates that have not only -a *name* but also a *pattern* that is searched for after the semantic checking -phase of the compiler: This means they provide an easy way to enhance the -compilation pipeline with user defined optimizations: - -.. code-block:: nim - template optMul{`*`(a, 2)}(a: int): int = a+a - - let x = 3 - echo x * 2 - -The compiler now rewrites ``x * 2`` as ``x + x``. The code inside the -curlies is the pattern to match against. The operators ``*``, ``**``, -``|``, ``~`` have a special meaning in patterns if they are written in infix -notation, so to match verbatim against ``*`` the ordinary function call syntax -needs to be used. - - -Unfortunately optimizations are hard to get right and even the tiny example -is **wrong**: - -.. code-block:: nim - template optMul{`*`(a, 2)}(a: int): int = a+a - - proc f(): int = - echo "side effect!" - result = 55 - - echo f() * 2 - -We cannot duplicate 'a' if it denotes an expression that has a side effect! -Fortunately Nim supports side effect analysis: - -.. code-block:: nim - template optMul{`*`(a, 2)}(a: int{noSideEffect}): int = a+a - - proc f(): int = - echo "side effect!" - result = 55 - - echo f() * 2 # not optimized ;-) - -So what about ``2 * a``? We should tell the compiler ``*`` is commutative. We -cannot really do that however as the following code only swaps arguments -blindly: - -.. code-block:: nim - template mulIsCommutative{`*`(a, b)}(a, b: int): int = b*a - -What optimizers really need to do is a *canonicalization*: - -.. code-block:: nim - template canonMul{`*`(a, b)}(a: int{lit}, b: int): int = b*a - -The ``int{lit}`` parameter pattern matches against an expression of -type ``int``, but only if it's a literal. - - - -Parameter constraints ---------------------- - -The `parameter constraint`:idx: expression can use the operators ``|`` (or), -``&`` (and) and ``~`` (not) and the following predicates: - -=================== ===================================================== -Predicate Meaning -=================== ===================================================== -``atom`` The matching node has no children. -``lit`` The matching node is a literal like "abc", 12. -``sym`` The matching node must be a symbol (a bound - identifier). -``ident`` The matching node must be an identifier (an unbound - identifier). -``call`` The matching AST must be a call/apply expression. -``lvalue`` The matching AST must be an lvalue. -``sideeffect`` The matching AST must have a side effect. -``nosideeffect`` The matching AST must have no side effect. -``param`` A symbol which is a parameter. -``genericparam`` A symbol which is a generic parameter. -``module`` A symbol which is a module. -``type`` A symbol which is a type. -``var`` A symbol which is a variable. -``let`` A symbol which is a ``let`` variable. -``const`` A symbol which is a constant. -``result`` The special ``result`` variable. -``proc`` A symbol which is a proc. -``method`` A symbol which is a method. -``iterator`` A symbol which is an iterator. -``converter`` A symbol which is a converter. -``macro`` A symbol which is a macro. -``template`` A symbol which is a template. -``field`` A symbol which is a field in a tuple or an object. -``enumfield`` A symbol which is a field in an enumeration. -``forvar`` A for loop variable. -``label`` A label (used in ``block`` statements). -``nk*`` The matching AST must have the specified kind. - (Example: ``nkIfStmt`` denotes an ``if`` statement.) -``alias`` States that the marked parameter needs to alias - with *some* other parameter. -``noalias`` States that *every* other parameter must not alias - with the marked parameter. -=================== ===================================================== - -The ``alias`` and ``noalias`` predicates refer not only to the matching AST, -but also to every other bound parameter; syntactially they need to occur after -the ordinary AST predicates: - -.. code-block:: nim - template ex{a = b + c}(a: int{noalias}, b, c: int) = - # this transformation is only valid if 'b' and 'c' do not alias 'a': - a = b - inc a, c - - -Pattern operators ------------------ - -The operators ``*``, ``**``, ``|``, ``~`` have a special meaning in patterns -if they are written in infix notation. - - -The ``|`` operator -~~~~~~~~~~~~~~~~~~ - -The ``|`` operator if used as infix operator creates an ordered choice: - -.. code-block:: nim - template t{0|1}(): expr = 3 - let a = 1 - # outputs 3: - echo a - -The matching is performed after the compiler performed some optimizations like -constant folding, so the following does not work: - -.. code-block:: nim - template t{0|1}(): expr = 3 - # outputs 1: - echo 1 - -The reason is that the compiler already transformed the 1 into "1" for -the ``echo`` statement. However, a term rewriting macro should not change the -semantics anyway. In fact they can be deactived with the ``--patterns:off`` -command line option or temporarily with the ``patterns`` pragma. - - -The ``{}`` operator -~~~~~~~~~~~~~~~~~~~ - -A pattern expression can be bound to a pattern parameter via the ``expr{param}`` -notation: - -.. code-block:: nim - template t{(0|1|2){x}}(x: expr): expr = x+1 - let a = 1 - # outputs 2: - echo a - - -The ``~`` operator -~~~~~~~~~~~~~~~~~~ - -The ``~`` operator is the **not** operator in patterns: - -.. code-block:: nim - template t{x = (~x){y} and (~x){z}}(x, y, z: bool): stmt = - x = y - if x: x = z - - var - a = false - b = true - c = false - a = b and c - echo a - - -The ``*`` operator -~~~~~~~~~~~~~~~~~~ - -The ``*`` operator can *flatten* a nested binary expression like ``a & b & c`` -to ``&(a, b, c)``: - -.. code-block:: nim - var - calls = 0 - - proc `&&`(s: varargs[string]): string = - result = s[0] - for i in 1..len(s)-1: result.add s[i] - inc calls - - template optConc{ `&&` * a }(a: string): expr = &&a - - let space = " " - echo "my" && (space & "awe" && "some " ) && "concat" - - # check that it's been optimized properly: - doAssert calls == 1 - - -The second operator of `*` must be a parameter; it is used to gather all the -arguments. The expression ``"my" && (space & "awe" && "some " ) && "concat"`` -is passed to ``optConc`` in ``a`` as a special list (of kind ``nkArgList``) -which is flattened into a call expression; thus the invocation of ``optConc`` -produces: - -.. code-block:: nim - `&&`("my", space & "awe", "some ", "concat") - - -The ``**`` operator -~~~~~~~~~~~~~~~~~~~ - -The ``**`` is much like the ``*`` operator, except that it gathers not only -all the arguments, but also the matched operators in reverse polish notation: - -.. code-block:: nim - import macros - - type - TMatrix = object - dummy: int - - proc `*`(a, b: TMatrix): TMatrix = discard - proc `+`(a, b: TMatrix): TMatrix = discard - proc `-`(a, b: TMatrix): TMatrix = discard - proc `$`(a: TMatrix): string = result = $a.dummy - proc mat21(): TMatrix = - result.dummy = 21 - - macro optM{ (`+`|`-`|`*`) ** a }(a: TMatrix): expr = - echo treeRepr(a) - result = newCall(bindSym"mat21") - - var x, y, z: TMatrix - - echo x + y * z - x - -This passes the expression ``x + y * z - x`` to the ``optM`` macro as -an ``nnkArgList`` node containing:: - - Arglist - Sym "x" - Sym "y" - Sym "z" - Sym "*" - Sym "+" - Sym "x" - Sym "-" - -(Which is the reverse polish notation of ``x + y * z - x``.) - - -Parameters ----------- - -Parameters in a pattern are type checked in the matching process. If a -parameter is of the type ``varargs`` it is treated specially and it can match -0 or more arguments in the AST to be matched against: - -.. code-block:: nim - template optWrite{ - write(f, x) - ((write|writeln){w})(f, y) - }(x, y: varargs[expr], f: TFile, w: expr) = - w(f, x, y) - - - -Example: Partial evaluation ---------------------------- - -The following example shows how some simple partial evaluation can be -implemented with term rewriting: - -.. code-block:: nim - proc p(x, y: int; cond: bool): int = - result = if cond: x + y else: x - y - - template optP1{p(x, y, true)}(x, y: expr): expr = x + y - template optP2{p(x, y, false)}(x, y: expr): expr = x - y - - -Example: Hoisting ------------------ - -The following example shows how some form of hoisting can be implemented: - -.. code-block:: nim - import pegs - - template optPeg{peg(pattern)}(pattern: string{lit}): TPeg = - var gl {.global, gensym.} = peg(pattern) - gl - - for i in 0 .. 3: - echo match("(a b c)", peg"'(' @ ')'") - echo match("W_HI_Le", peg"\y 'while'") - -The ``optPeg`` template optimizes the case of a peg constructor with a string -literal, so that the pattern will only be parsed once at program startup and -stored in a global ``gl`` which is then re-used. This optimization is called -hoisting because it is comparable to classical loop hoisting. - - -AST based overloading -===================== - -Parameter constraints can also be used for ordinary routine parameters; these -constraints affect ordinary overloading resolution then: - -.. code-block:: nim - proc optLit(a: string{lit|`const`}) = - echo "string literal" - proc optLit(a: string) = - echo "no string literal" - - const - constant = "abc" - - var - variable = "xyz" - - optLit("literal") - optLit(constant) - optLit(variable) - -However, the constraints ``alias`` and ``noalias`` are not available in -ordinary routines. - - -Move optimization ------------------ - -The ``call`` constraint is particularly useful to implement a move -optimization for types that have copying semantics: - -.. code-block:: nim - proc `[]=`*(t: var TTable, key: string, val: string) = - ## puts a (key, value)-pair into `t`. The semantics of string require - ## a copy here: - let idx = findInsertionPosition(key) - t[idx] = key - t[idx] = val - - proc `[]=`*(t: var TTable, key: string{call}, val: string{call}) = - ## puts a (key, value)-pair into `t`. Optimized version that knows that - ## the strings are unique and thus don't need to be copied: - let idx = findInsertionPosition(key) - shallowCopy t[idx], key - shallowCopy t[idx], val - - var t: TTable - # overloading resolution ensures that the optimized []= is called here: - t[f()] = g() - - - -Modules -======= -Nim supports splitting a program into pieces by a module concept. -Each module needs to be in its own file and has its own `namespace`:idx:. -Modules enable `information hiding`:idx: and `separate compilation`:idx:. -A module may gain access to symbols of another module by the `import`:idx: -statement. `Recursive module dependencies`:idx: are allowed, but slightly -subtle. Only top-level symbols that are marked with an asterisk (``*``) are -exported. - -The algorithm for compiling modules is: - -- compile the whole module as usual, following import statements recursively - -- if there is a cycle only import the already parsed symbols (that are - exported); if an unknown identifier occurs then abort - -This is best illustrated by an example: - -.. code-block:: nim - # Module A - type - T1* = int # Module A exports the type ``T1`` - import B # the compiler starts parsing B - - proc main() = - var i = p(3) # works because B has been parsed completely here - - main() - - -.. code-block:: nim - # Module B - import A # A is not parsed here! Only the already known symbols - # of A are imported. - - proc p*(x: A.T1): A.T1 = - # this works because the compiler has already - # added T1 to A's interface symbol table - result = x + 1 - - -Import statement -~~~~~~~~~~~~~~~~ - -After the ``import`` statement a list of module names can follow or a single -module name followed by an ``except`` to prevent some symbols to be imported: - -.. code-block:: nim - import strutils except `%` - - # doesn't work then: - echo "$1" % "abc" - - -Module names in imports -~~~~~~~~~~~~~~~~~~~~~~~ - -A module alias can be introduced via the ``as`` keyword: - -.. code-block:: nim - import strutils as su, sequtils as qu - - echo su.format("$1", "lalelu") - -The original module name is then not accessible. The -notations ``path/to/module`` or ``path.to.module`` or ``"path/to/module"`` -can be used to refer to a module in subdirectories: - -.. code-block:: nim - import lib.pure.strutils, lib/pure/os, "lib/pure/times" - -Note that the module name is still ``strutils`` and not ``lib.pure.strutils`` -and so one **cannot** do: - -.. code-block:: nim - import lib.pure.strutils - echo lib.pure.strutils - -Likewise the following does not make sense as the name is ``strutils`` already: - -.. code-block:: nim - import lib.pure.strutils as strutils - - -From import statement -~~~~~~~~~~~~~~~~~~~~~ - -After the ``from`` statement a module name follows followed by -an ``import`` to list the symbols one likes to use without explict -full qualification: - -.. code-block:: nim - from strutils import `%` - - echo "$1" % "abc" - # always possible: full qualification: - echo strutils.replace("abc", "a", "z") - -It's also possible to use ``from module import nil`` if one wants to import -the module but wants to enforce fully qualified access to every symbol -in ``module``. - - -Export statement -~~~~~~~~~~~~~~~~ - -An ``export`` statement can be used for symbol fowarding so that client -modules don't need to import a module's dependencies: - -.. code-block:: nim - # module B - type TMyObject* = object - -.. code-block:: nim - # module A - import B - export B.TMyObject - - proc `$`*(x: TMyObject): string = "my object" - - -.. code-block:: nim - # module C - import A - - # B.TMyObject has been imported implicitly here: - var x: TMyObject - echo($x) - - -Scope rules ------------ -Identifiers are valid from the point of their declaration until the end of -the block in which the declaration occurred. The range where the identifier -is known is the scope of the identifier. The exact scope of an -identifier depends on the way it was declared. - -Block scope -~~~~~~~~~~~ -The *scope* of a variable declared in the declaration part of a block -is valid from the point of declaration until the end of the block. If a -block contains a second block, in which the identifier is redeclared, -then inside this block, the second declaration will be valid. Upon -leaving the inner block, the first declaration is valid again. An -identifier cannot be redefined in the same block, except if valid for -procedure or iterator overloading purposes. - - -Tuple or object scope -~~~~~~~~~~~~~~~~~~~~~ -The field identifiers inside a tuple or object definition are valid in the -following places: - -* To the end of the tuple/object definition. -* Field designators of a variable of the given tuple/object type. -* In all descendant types of the object type. - -Module scope -~~~~~~~~~~~~ -All identifiers of a module are valid from the point of declaration until -the end of the module. Identifiers from indirectly dependent modules are *not* -available. The `system`:idx: module is automatically imported in every other -module. - -If a module imports an identifier by two different modules, each occurrence of -the identifier has to be qualified, unless it is an overloaded procedure or -iterator in which case the overloading resolution takes place: - -.. code-block:: nim - # Module A - var x*: string - -.. code-block:: nim - # Module B - var x*: int - -.. code-block:: nim - # Module C - import A, B - write(stdout, x) # error: x is ambiguous - write(stdout, A.x) # no error: qualifier used - - var x = 4 - write(stdout, x) # not ambiguous: uses the module C's x - - -Compiler Messages -================= - -The Nim compiler emits different kinds of messages: `hint`:idx:, -`warning`:idx:, and `error`:idx: messages. An *error* message is emitted if -the compiler encounters any static error. - - -Pragmas -======= - -Pragmas are Nim's method to give the compiler additional information / -commands without introducing a massive number of new keywords. Pragmas are -processed on the fly during semantic checking. Pragmas are enclosed in the -special ``{.`` and ``.}`` curly brackets. Pragmas are also often used as a -first implementation to play with a language feature before a nicer syntax -to access the feature becomes available. - - -noSideEffect pragma -------------------- -The ``noSideEffect`` pragma is used to mark a proc/iterator to have no side -effects. This means that the proc/iterator only changes locations that are -reachable from its parameters and the return value only depends on the -arguments. If none of its parameters have the type ``var T`` -or ``ref T`` or ``ptr T`` this means no locations are modified. It is a static -error to mark a proc/iterator to have no side effect if the compiler cannot -verify this. - -As a special semantic rule, the built-in `debugEcho <system.html#debugEcho>`_ -pretends to be free of side effects, so that it can be used for debugging -routines marked as ``noSideEffect``. - -**Future directions**: ``func`` may become a keyword and syntactic sugar for a -proc with no side effects: - -.. code-block:: nim - func `+` (x, y: int): int - - -destructor pragma ------------------ - -The ``destructor`` pragma is used to mark a proc to act as a type destructor. -Its usage is deprecated, use the ``override`` pragma instead. -See `type bound operations`_. - - -override pragma ---------------- - -See `type bound operations`_ instead. - -procvar pragma --------------- -The ``procvar`` pragma is used to mark a proc that it can be passed to a -procedural variable. - - -compileTime pragma ------------------- -The ``compileTime`` pragma is used to mark a proc to be used at compile -time only. No code will be generated for it. Compile time procs are useful -as helpers for macros. - - -noReturn pragma ---------------- -The ``noreturn`` pragma is used to mark a proc that never returns. - - -Acyclic pragma --------------- -The ``acyclic`` pragma can be used for object types to mark them as acyclic -even though they seem to be cyclic. This is an **optimization** for the garbage -collector to not consider objects of this type as part of a cycle: - -.. code-block:: nim - type - PNode = ref TNode - TNode {.acyclic, final.} = object - left, right: PNode - data: string - -In the example a tree structure is declared with the ``TNode`` type. Note that -the type definition is recursive and the GC has to assume that objects of -this type may form a cyclic graph. The ``acyclic`` pragma passes the -information that this cannot happen to the GC. If the programmer uses the -``acyclic`` pragma for data types that are in reality cyclic, the GC may leak -memory, but nothing worse happens. - -**Future directions**: The ``acyclic`` pragma may become a property of a -``ref`` type: - -.. code-block:: nim - type - PNode = acyclic ref TNode - TNode = object - left, right: PNode - data: string - - -Final pragma ------------- -The ``final`` pragma can be used for an object type to specify that it -cannot be inherited from. - - -shallow pragma --------------- -The ``shallow`` pragma affects the semantics of a type: The compiler is -allowed to make a shallow copy. This can cause serious semantic issues and -break memory safety! However, it can speed up assignments considerably, -because the semantics of Nim require deep copying of sequences and strings. -This can be expensive, especially if sequences are used to build a tree -structure: - -.. code-block:: nim - type - TNodeKind = enum nkLeaf, nkInner - TNode {.final, shallow.} = object - case kind: TNodeKind - of nkLeaf: - strVal: string - of nkInner: - children: seq[TNode] - - -Pure pragma ------------ -An object type can be marked with the ``pure`` pragma so that its type -field which is used for runtime type identification is omitted. This is -necessary for binary compatibility with other compiled languages. - - -AsmNoStackFrame pragma ----------------------- -A proc can be marked with the ``AsmNoStackFrame`` pragma to tell the compiler -it should not generate a stack frame for the proc. There are also no exit -statements like ``return result;`` generated and the generated C function is -declared as ``__declspec(naked)`` or ``__attribute__((naked))`` (depending on -the used C compiler). - -**Note**: This pragma should only be used by procs which consist solely of -assembler statements. - -error pragma ------------- -The ``error`` pragma is used to make the compiler output an error message -with the given content. Compilation does not necessarily abort after an error -though. - -The ``error`` pragma can also be used to -annotate a symbol (like an iterator or proc). The *usage* of the symbol then -triggers a compile-time error. This is especially useful to rule out that some -operation is valid due to overloading and type conversions: - -.. code-block:: nim - ## check that underlying int values are compared and not the pointers: - proc `==`(x, y: ptr int): bool {.error.} - - -fatal pragma ------------- -The ``fatal`` pragma is used to make the compiler output an error message -with the given content. In contrast to the ``error`` pragma, compilation -is guaranteed to be aborted by this pragma. Example: - -.. code-block:: nim - when not defined(objc): - {.fatal: "Compile this program with the objc command!".} - -warning pragma --------------- -The ``warning`` pragma is used to make the compiler output a warning message -with the given content. Compilation continues after the warning. - -hint pragma ------------ -The ``hint`` pragma is used to make the compiler output a hint message with -the given content. Compilation continues after the hint. - -line pragma ------------ -The ``line`` pragma can be used to affect line information of the annotated -statement as seen in stack backtraces: - -.. code-block:: nim - - template myassert*(cond: expr, msg = "") = - if not cond: - # change run-time line information of the 'raise' statement: - {.line: InstantiationInfo().}: - raise newException(EAssertionFailed, msg) - -If the ``line`` pragma is used with a parameter, the parameter needs be a -``tuple[filename: string, line: int]``. If it is used without a parameter, -``system.InstantiationInfo()`` is used. - - -linearScanEnd pragma --------------------- -The ``linearScanEnd`` pragma can be used to tell the compiler how to -compile a Nim `case`:idx: statement. Syntactically it has to be used as a -statement: - -.. code-block:: nim - case myInt - of 0: - echo "most common case" - of 1: - {.linearScanEnd.} - echo "second most common case" - of 2: echo "unlikely: use branch table" - else: echo "unlikely too: use branch table for ", myInt - -In the example, the case branches ``0`` and ``1`` are much more common than -the other cases. Therefore the generated assembler code should test for these -values first, so that the CPU's branch predictor has a good chance to succeed -(avoiding an expensive CPU pipeline stall). The other cases might be put into a -jump table for O(1) overhead, but at the cost of a (very likely) pipeline -stall. - -The ``linearScanEnd`` pragma should be put into the last branch that should be -tested against via linear scanning. If put into the last branch of the -whole ``case`` statement, the whole ``case`` statement uses linear scanning. - - -computedGoto pragma -------------------- -The ``computedGoto`` pragma can be used to tell the compiler how to -compile a Nim `case`:idx: in a ``while true`` statement. -Syntactically it has to be used as a statement inside the loop: - -.. code-block:: nim - - type - MyEnum = enum - enumA, enumB, enumC, enumD, enumE - - proc vm() = - var instructions: array [0..100, MyEnum] - instructions[2] = enumC - instructions[3] = enumD - instructions[4] = enumA - instructions[5] = enumD - instructions[6] = enumC - instructions[7] = enumA - instructions[8] = enumB - - instructions[12] = enumE - var pc = 0 - while true: - {.computedGoto.} - let instr = instructions[pc] - case instr - of enumA: - echo "yeah A" - of enumC, enumD: - echo "yeah CD" - of enumB: - echo "yeah B" - of enumE: - break - inc(pc) - - vm() - -As the example shows ``computedGoto`` is mostly useful for interpreters. If -the underlying backend (C compiler) does not support the computed goto -extension the pragma is simply ignored. - - -unroll pragma -------------- -The ``unroll`` pragma can be used to tell the compiler that it should unroll -a `for`:idx: or `while`:idx: loop for runtime efficiency: - -.. code-block:: nim - proc searchChar(s: string, c: char): int = - for i in 0 .. s.high: - {.unroll: 4.} - if s[i] == c: return i - result = -1 - -In the above example, the search loop is unrolled by a factor 4. The unroll -factor can be left out too; the compiler then chooses an appropriate unroll -factor. - -**Note**: Currently the compiler recognizes but ignores this pragma. - - -immediate pragma ----------------- - -See `Ordinary vs immediate templates`_. - - -compilation option pragmas --------------------------- -The listed pragmas here can be used to override the code generation options -for a proc/method/converter. - -The implementation currently provides the following possible options (various -others may be added later). - -=============== =============== ============================================ -pragma allowed values description -=============== =============== ============================================ -checks on|off Turns the code generation for all runtime - checks on or off. -boundChecks on|off Turns the code generation for array bound - checks on or off. -overflowChecks on|off Turns the code generation for over- or - underflow checks on or off. -nilChecks on|off Turns the code generation for nil pointer - checks on or off. -assertions on|off Turns the code generation for assertions - on or off. -warnings on|off Turns the warning messages of the compiler - on or off. -hints on|off Turns the hint messages of the compiler - on or off. -optimization none|speed|size Optimize the code for speed or size, or - disable optimization. -patterns on|off Turns the term rewriting templates/macros - on or off. -callconv cdecl|... Specifies the default calling convention for - all procedures (and procedure types) that - follow. -=============== =============== ============================================ - -Example: - -.. code-block:: nim - {.checks: off, optimization: speed.} - # compile without runtime checks and optimize for speed - - -push and pop pragmas --------------------- -The `push/pop`:idx: pragmas are very similar to the option directive, -but are used to override the settings temporarily. Example: - -.. code-block:: nim - {.push checks: off.} - # compile this section without runtime checks as it is - # speed critical - # ... some code ... - {.pop.} # restore old settings - - -register pragma ---------------- -The ``register`` pragma is for variables only. It declares the variable as -``register``, giving the compiler a hint that the variable should be placed -in a hardware register for faster access. C compilers usually ignore this -though and for good reasons: Often they do a better job without it anyway. - -In highly specific cases (a dispatch loop of an bytecode interpreter for -example) it may provide benefits, though. - - -global pragma -------------- -The ``global`` pragma can be applied to a variable within a proc to instruct -the compiler to store it in a global location and initialize it once at program -startup. - -.. code-block:: nim - proc isHexNumber(s: string): bool = - var pattern {.global.} = re"[0-9a-fA-F]+" - result = s.match(pattern) - -When used within a generic proc, a separate unique global variable will be -created for each instantiation of the proc. The order of initialization of -the created global variables within a module is not defined, but all of them -will be initialized after any top-level variables in their originating module -and before any variable in a module that imports it. - -DeadCodeElim pragma -------------------- -The ``deadCodeElim`` pragma only applies to whole modules: It tells the -compiler to activate (or deactivate) dead code elimination for the module the -pragma appears in. - -The ``--deadCodeElim:on`` command line switch has the same effect as marking -every module with ``{.deadCodeElim:on}``. However, for some modules such as -the GTK wrapper it makes sense to *always* turn on dead code elimination - -no matter if it is globally active or not. - -Example: - -.. code-block:: nim - {.deadCodeElim: on.} - - -.. - NoForward pragma - ---------------- - The ``noforward`` pragma can be used to turn on and off a special compilation - mode that to large extent eliminates the need for forward declarations. In this - mode, the proc definitions may appear out of order and the compiler will postpone - their semantic analysis and compilation until it actually needs to generate code - using the definitions. In this regard, this mode is similar to the modus operandi - of dynamic scripting languages, where the function calls are not resolved until - the code is executed. Here is the detailed algorithm taken by the compiler: - - 1. When a callable symbol is first encountered, the compiler will only note the - symbol callable name and it will add it to the appropriate overload set in the - current scope. At this step, it won't try to resolve any of the type expressions - used in the signature of the symbol (so they can refer to other not yet defined - symbols). - - 2. When a top level call is encountered (usually at the very end of the module), - the compiler will try to determine the actual types of all of the symbols in the - matching overload set. This is a potentially recursive process as the signatures - of the symbols may include other call expressions, whoose types will be resolved - at this point too. - - 3. Finally, after the best overload is picked, the compiler will start compiling - the body of the respective symbol. This in turn will lead the compiler to discover - more call expresions that need to be resolved and steps 2 and 3 will be repeated - as necessary. - - Please note that if a callable symbol is never used in this scenario, its body - will never be compiled. This is the default behavior leading to best compilation - times, but if exhaustive compilation of all definitions is required, using - ``nim check`` provides this option as well. - - Example: - - .. code-block:: nim - - {.noforward: on.} - - proc foo(x: int) = - bar x - - proc bar(x: int) = - echo x - - foo(10) - - -Pragma pragma -------------- - -The ``pragma`` pragma can be used to declare user defined pragmas. This is -useful because Nim's templates and macros do not affect pragmas. User -defined pragmas are in a different module-wide scope than all other symbols. -They cannot be imported from a module. - -Example: - -.. code-block:: nim - when appType == "lib": - {.pragma: rtl, exportc, dynlib, cdecl.} - else: - {.pragma: rtl, importc, dynlib: "client.dll", cdecl.} - - proc p*(a, b: int): int {.rtl.} = - result = a+b - -In the example a new pragma named ``rtl`` is introduced that either imports -a symbol from a dynamic library or exports the symbol for dynamic library -generation. - - -Disabling certain messages --------------------------- -Nim generates some warnings and hints ("line too long") that may annoy the -user. A mechanism for disabling certain messages is provided: Each hint -and warning message contains a symbol in brackets. This is the message's -identifier that can be used to enable or disable it: - -.. code-block:: Nim - {.hint[LineTooLong]: off.} # turn off the hint about too long lines - -This is often better than disabling all warnings at once. - - -Foreign function interface -========================== - -Nim's `FFI`:idx: (foreign function interface) is extensive and only the -parts that scale to other future backends (like the LLVM/JavaScript backends) -are documented here. - - -Importc pragma --------------- -The ``importc`` pragma provides a means to import a proc or a variable -from C. The optional argument is a string containing the C identifier. If -the argument is missing, the C name is the Nim identifier *exactly as -spelled*: - -.. code-block:: - proc printf(formatstr: cstring) {.header: "<stdio.h>", importc: "printf", varargs.} - -Note that this pragma is somewhat of a misnomer: Other backends will provide -the same feature under the same name. Also, if one is interfacing with C++ -the `ImportCpp pragma <nimc.html#importcpp-pragma>`_ and -interfacing with Objective-C the `ImportObjC pragma -<nimc.html#importobjc-pragma>`_ can be used. - - -Exportc pragma --------------- -The ``exportc`` pragma provides a means to export a type, a variable, or a -procedure to C. Enums and constants can't be exported. The optional argument -is a string containing the C identifier. If the argument is missing, the C -name is the Nim identifier *exactly as spelled*: - -.. code-block:: Nim - proc callme(formatstr: cstring) {.exportc: "callMe", varargs.} - -Note that this pragma is somewhat of a misnomer: Other backends will provide -the same feature under the same name. - - -Extern pragma -------------- -Like ``exportc`` or ``importc``, the ``extern`` pragma affects name -mangling. The string literal passed to ``extern`` can be a format string: - -.. code-block:: Nim - proc p(s: string) {.extern: "prefix$1".} = - echo s - -In the example the external name of ``p`` is set to ``prefixp``. - - -Bycopy pragma -------------- - -The ``bycopy`` pragma can be applied to an object or tuple type and -instructs the compiler to pass the type by value to procs: - -.. code-block:: nim - type - TVector {.bycopy, pure.} = object - x, y, z: float - - -Byref pragma ------------- - -The ``byref`` pragma can be applied to an object or tuple type and instructs -the compiler to pass the type by reference (hidden pointer) to procs. - - -Varargs pragma --------------- -The ``varargs`` pragma can be applied to procedures only (and procedure -types). It tells Nim that the proc can take a variable number of parameters -after the last specified parameter. Nim string values will be converted to C -strings automatically: - -.. code-block:: Nim - proc printf(formatstr: cstring) {.nodecl, varargs.} - - printf("hallo %s", "world") # "world" will be passed as C string - - -Union pragma ------------- -The ``union`` pragma can be applied to any ``object`` type. It means all -of the object's fields are overlaid in memory. This produces a ``union`` -instead of a ``struct`` in the generated C/C++ code. The object declaration -then must not use inheritance or any GC'ed memory but this is currently not -checked. - -**Future directions**: GC'ed memory should be allowed in unions and the GC -should scan unions conservatively. - -Packed pragma -------------- -The ``packed`` pragma can be applied to any ``object`` type. It ensures -that the fields of an object are packed back-to-back in memory. It is useful -to store packets or messages from/to network or hardware drivers, and for -interoperability with C. Combining packed pragma with inheritance is not -defined, and it should not be used with GC'ed memory (ref's). - -**Future directions**: Using GC'ed memory in packed pragma will result in -compile-time error. Usage with inheritance should be defined and documented. - -Unchecked pragma ----------------- -The ``unchecked`` pragma can be used to mark a named array as ``unchecked`` -meaning its bounds are not checked. This is often useful when one wishes to -implement his own flexibly sized arrays. Additionally an unchecked array is -translated into a C array of undetermined size: - -.. code-block:: nim - type - ArrayPart{.unchecked.} = array[0..0, int] - MySeq = object - len, cap: int - data: ArrayPart - -Produces roughly this C code: - -.. code-block:: C - typedef struct { - NI len; - NI cap; - NI data[]; - } MySeq; - -The bounds checking done at compile time is not disabled for now, so to access -``s.data[C]`` (where ``C`` is a constant) the array's index needs needs to -include ``C``. - -The base type of the unchecked array may not contain any GC'ed memory but this -is currently not checked. - -**Future directions**: GC'ed memory should be allowed in unchecked arrays and -there should be an explicit annotation of how the GC is to determine the -runtime size of the array. - - -Dynlib pragma for import ------------------------- -With the ``dynlib`` pragma a procedure or a variable can be imported from -a dynamic library (``.dll`` files for Windows, ``lib*.so`` files for UNIX). -The non-optional argument has to be the name of the dynamic library: - -.. code-block:: Nim - proc gtk_image_new(): PGtkWidget - {.cdecl, dynlib: "libgtk-x11-2.0.so", importc.} - -In general, importing a dynamic library does not require any special linker -options or linking with import libraries. This also implies that no *devel* -packages need to be installed. - -The ``dynlib`` import mechanism supports a versioning scheme: - -.. code-block:: nim - proc Tcl_Eval(interp: pTcl_Interp, script: cstring): int {.cdecl, - importc, dynlib: "libtcl(|8.5|8.4|8.3).so.(1|0)".} - -At runtime the dynamic library is searched for (in this order):: - - libtcl.so.1 - libtcl.so.0 - libtcl8.5.so.1 - libtcl8.5.so.0 - libtcl8.4.so.1 - libtcl8.4.so.0 - libtcl8.3.so.1 - libtcl8.3.so.0 - -The ``dynlib`` pragma supports not only constant strings as argument but also -string expressions in general: - -.. code-block:: nim - import os - - proc getDllName: string = - result = "mylib.dll" - if existsFile(result): return - result = "mylib2.dll" - if existsFile(result): return - quit("could not load dynamic library") - - proc myImport(s: cstring) {.cdecl, importc, dynlib: getDllName().} - -**Note**: Patterns like ``libtcl(|8.5|8.4).so`` are only supported in constant -strings, because they are precompiled. - -**Note**: Passing variables to the ``dynlib`` pragma will fail at runtime -because of order of initialization problems. - -**Note**: A ``dynlib`` import can be overriden with -the ``--dynlibOverride:name`` command line option. The Compiler User Guide -contains further information. - - -Dynlib pragma for export ------------------------- - -With the ``dynlib`` pragma a procedure can also be exported to -a dynamic library. The pragma then has no argument and has to be used in -conjunction with the ``exportc`` pragma: - -.. code-block:: Nim - proc exportme(): int {.cdecl, exportc, dynlib.} - -This is only useful if the program is compiled as a dynamic library via the -``--app:lib`` command line option. - - -Threads -======= - -To enable thread support the ``--threads:on`` command line switch needs to -be used. The ``system`` module then contains several threading primitives. -See the `threads <threads.html>`_ and `channels <channels.html>`_ modules -for the thread API. - -Nim's memory model for threads is quite different than that of other common -programming languages (C, Pascal, Java): Each thread has its own (garbage -collected) heap and sharing of memory is restricted to global variables. This -helps to prevent race conditions. GC efficiency is improved quite a lot, -because the GC never has to stop other threads and see what they reference. -Memory allocation requires no lock at all! This design easily scales to massive -multicore processors that will become the norm in the future. - - -Thread pragma -------------- - -A proc that is executed as a new thread of execution should be marked by the -``thread`` pragma. The compiler checks procedures marked as ``thread`` for -violations of the `no heap sharing restriction`:idx:\: This restriction implies -that it is invalid to construct a data structure that consists of memory -allocated from different (thread local) heaps. - -A thread proc is passed to ``createThread`` or ``spawn`` and invoked -indirectly; so the ``thread`` pragma implies ``procvar``. - - -GC safety ---------- - -We call a proc ``p`` `GC safe`:idx: when it doesn't access any global variable -that contains GC'ed memory (``string``, ``seq``, ``ref`` or a closure) either -directly or indirectly through a call to a GC unsafe proc. - -The `gcsafe`:idx: annotation can be used to mark a proc to be gcsafe -otherwise this property is inferred by the compiler. Note that ``noSideEfect`` -implies ``gcsafe``. The only way to create a thread is via ``spawn`` or -``createThead``. ``spawn`` is usually the preferable method. Either way -the invoked proc must not use ``var`` parameters nor must any of its parameters -contain a ``ref`` or ``closure`` type. This enforces -the *no heap sharing restriction*. - -Routines that are imported from C are always assumed to be ``gcsafe``. -To enable the GC-safety checking the ``--threadAnalysis:on`` command line -switch must be used. This is a temporary workaround to ease the porting effort -from old code to the new threading model. In the future the thread analysis -will always be performed. - - -Future directions: - -- For structured fork&join parallelism more efficient parameter passing can - be performed and much more can be proven safe. -- A shared GC'ed heap is planned. - - -Threadvar pragma ----------------- - -A global variable can be marked with the ``threadvar`` pragma; it is -a `thread-local`:idx: variable then: - -.. code-block:: nim - var checkpoints* {.threadvar.}: seq[string] - -Due to implementation restrictions thread local variables cannot be -initialized within the ``var`` section. (Every thread local variable needs to -be replicated at thread creation.) - - -Threads and exceptions ----------------------- - -The interaction between threads and exceptions is simple: A *handled* exception -in one thread cannot affect any other thread. However, an *unhandled* -exception in one thread terminates the whole *process*! - - -Spawn ------ - -Nim has a builtin thread pool that can be used for CPU intensive tasks. For -IO intensive tasks the upcoming ``async`` and ``await`` features should be -used. `spawn`:idx: is used to pass a task to the thread pool: - -.. code-block:: nim - proc processLine(line: string) = - # do some heavy lifting here: - discard - - for x in lines("myinput.txt"): - spawn processLine(x) - sync() - -Currently the expression that ``spawn`` takes is however quite restricted: - -* It must be a call expression ``f(a, ...)``. -* ``f`` must be ``gcsafe``. -* ``f`` must not have the calling convention ``closure``. -* ``f``'s parameters may not be of type ``var``. - This means one has to use raw ``ptr``'s for data passing reminding the - programmer to be careful. -* ``ref`` parameters are deeply copied which is a subtle semantic change and - can cause performance problems but ensures memory safety. -* For *safe* data exchange between ``f`` and the caller a global ``TChannel`` - needs to be used. Other means will be provided soon. - - - -Taint mode -========== - -The Nim compiler and most parts of the standard library support -a taint mode. Input strings are declared with the `TaintedString`:idx: -string type declared in the ``system`` module. - -If the taint mode is turned on (via the ``--taintMode:on`` command line -option) it is a distinct string type which helps to detect input -validation errors: - -.. code-block:: nim - echo "your name: " - var name: TaintedString = stdin.readline - # it is safe here to output the name without any input validation, so - # we simply convert `name` to string to make the compiler happy: - echo "hi, ", name.string - -If the taint mode is turned off, ``TaintedString`` is simply an alias for -``string``. +.. include:: manual/about.txt +.. include:: manual/definitions.txt +.. include:: manual/lexing.txt +.. include:: manual/syntax.txt +.. include:: manual/types.txt +.. include:: manual/type_rel.txt +.. include:: manual/stmts.txt +.. include:: manual/procs.txt +.. include:: manual/type_sections.txt +.. include:: manual/exceptions.txt +.. include:: manual/effects.txt +.. include:: manual/generics.txt +.. include:: manual/templates.txt +.. include:: manual/typedesc.txt +.. include:: manual/special_ops.txt +.. include:: manual/type_bound_ops.txt +.. include:: manual/trmacros.txt +.. include:: manual/modules.txt +.. include:: manual/compiler_msgs.txt +.. include:: manual/pragmas.txt +.. include:: manual/ffi.txt +.. include:: manual/threads.txt +.. include:: manual/locking.txt +.. include:: manual/taint.txt diff --git a/doc/manual/about.txt b/doc/manual/about.txt new file mode 100644 index 000000000..13307279b --- /dev/null +++ b/doc/manual/about.txt @@ -0,0 +1,37 @@ +About this document +=================== + +**Note**: This document is a draft! Several of Nim's features need more +precise wording. This manual will evolve into a proper specification some +day. + +This document describes the lexis, the syntax, and the semantics of Nim. + +The language constructs are explained using an extended BNF, in +which ``(a)*`` means 0 or more ``a``'s, ``a+`` means 1 or more ``a``'s, and +``(a)?`` means an optional *a*. Parentheses may be used to group elements. + +``&`` is the lookahead operator; ``&a`` means that an ``a`` is expected but +not consumed. It will be consumed in the following rule. + +The ``|``, ``/`` symbols are used to mark alternatives and have the lowest +precedence. ``/`` is the ordered choice that requires the parser to try the +alternatives in the given order. ``/`` is often used to ensure the grammar +is not ambiguous. + +Non-terminals start with a lowercase letter, abstract terminal symbols are in +UPPERCASE. Verbatim terminal symbols (including keywords) are quoted +with ``'``. An example:: + + ifStmt = 'if' expr ':' stmts ('elif' expr ':' stmts)* ('else' stmts)? + +The binary ``^*`` operator is used as a shorthand for 0 or more occurances +separated by its second argument; likewise ``^+`` means 1 or more +occurances: ``a ^+ b`` is short for ``a (b a)*`` +and ``a ^* b`` is short for ``(a (b a)*)?``. Example:: + + arrayConstructor = '[' expr ^* ',' ']' + +Other parts of Nim - like scoping rules or runtime semantics are only +described in an informal manner for now. + diff --git a/doc/manual/compiler_msgs.txt b/doc/manual/compiler_msgs.txt new file mode 100644 index 000000000..3cf8417b0 --- /dev/null +++ b/doc/manual/compiler_msgs.txt @@ -0,0 +1,7 @@ +Compiler Messages +================= + +The Nim compiler emits different kinds of messages: `hint`:idx:, +`warning`:idx:, and `error`:idx: messages. An *error* message is emitted if +the compiler encounters any static error. + diff --git a/doc/manual/definitions.txt b/doc/manual/definitions.txt new file mode 100644 index 000000000..687980dbc --- /dev/null +++ b/doc/manual/definitions.txt @@ -0,0 +1,49 @@ + +Definitions +=========== + +A Nim program specifies a computation that acts on a memory consisting of +components called `locations`:idx:. A variable is basically a name for a +location. Each variable and location is of a certain `type`:idx:. The +variable's type is called `static type`:idx:, the location's type is called +`dynamic type`:idx:. If the static type is not the same as the dynamic type, +it is a super-type or subtype of the dynamic type. + +An `identifier`:idx: is a symbol declared as a name for a variable, type, +procedure, etc. The region of the program over which a declaration applies is +called the `scope`:idx: of the declaration. Scopes can be nested. The meaning +of an identifier is determined by the smallest enclosing scope in which the +identifier is declared unless overloading resolution rules suggest otherwise. + +An expression specifies a computation that produces a value or location. +Expressions that produce locations are called `l-values`:idx:. An l-value +can denote either a location or the value the location contains, depending on +the context. Expressions whose values can be determined statically are called +`constant expressions`:idx:; they are never l-values. + +A `static error`:idx: is an error that the implementation detects before +program execution. Unless explicitly classified, an error is a static error. + +A `checked runtime error`:idx: is an error that the implementation detects +and reports at runtime. The method for reporting such errors is via *raising +exceptions* or *dying with a fatal error*. However, the implementation +provides a means to disable these runtime checks. See the section pragmas_ +for details. + +Wether a checked runtime error results in an exception or in a fatal error at +runtime is implementation specific. Thus the following program is always +invalid: + +.. code-block:: nim + var a: array[0..1, char] + let i = 5 + try: + a[i] = 'N' + except EInvalidIndex: + echo "invalid index" + +An `unchecked runtime error`:idx: is an error that is not guaranteed to be +detected, and can cause the subsequent behavior of the computation to +be arbitrary. Unchecked runtime errors cannot occur if only `safe`:idx: +language features are used. + diff --git a/doc/manual/effects.txt b/doc/manual/effects.txt new file mode 100644 index 000000000..73934ab34 --- /dev/null +++ b/doc/manual/effects.txt @@ -0,0 +1,129 @@ +Effect system +============= + +Exception tracking +------------------ + +Nim supports exception tracking. The `raises`:idx: pragma can be used +to explicitly define which exceptions a proc/iterator/method/converter is +allowed to raise. The compiler verifies this: + +.. code-block:: nim + proc p(what: bool) {.raises: [IOError, OSError].} = + if what: raise newException(IOError, "IO") + else: raise newException(OSError, "OS") + +An empty ``raises`` list (``raises: []``) means that no exception may be raised: + +.. code-block:: nim + proc p(): bool {.raises: [].} = + try: + unsafeCall() + result = true + except: + result = false + + +A ``raises`` list can also be attached to a proc type. This affects type +compatibility: + +.. code-block:: nim + type + TCallback = proc (s: string) {.raises: [IOError].} + var + c: TCallback + + proc p(x: string) = + raise newException(OSError, "OS") + + c = p # type error + + +For a routine ``p`` the compiler uses inference rules to determine the set of +possibly raised exceptions; the algorithm operates on ``p``'s call graph: + +1. Every indirect call via some proc type ``T`` is assumed to + raise ``system.Exception`` (the base type of the exception hierarchy) and + thus any exception unless ``T`` has an explicit ``raises`` list. + However if the call is of the form ``f(...)`` where ``f`` is a parameter + of the currently analysed routine it is ignored. The call is optimistically + assumed to have no effect. Rule 2 compensates for this case. +2. Every expression of some proc type wihtin a call that is not a call + itself (and not nil) is assumed to be called indirectly somehow and thus + its raises list is added to ``p``'s raises list. +3. Every call to a proc ``q`` which has an unknown body (due to a forward + declaration or an ``importc`` pragma) is assumed to + raise ``system.Exception`` unless ``q`` has an explicit ``raises`` list. +4. Every call to a method ``m`` is assumed to + raise ``system.Exception`` unless ``m`` has an explicit ``raises`` list. +5. For every other call the analysis can determine an exact ``raises`` list. +6. For determining a ``raises`` list, the ``raise`` and ``try`` statements + of ``p`` are taken into consideration. + +Rules 1-2 ensure the following works: + +.. code-block:: nim + proc noRaise(x: proc()) {.raises: [].} = + # unknown call that might raise anything, but valid: + x() + + proc doRaise() {.raises: [IOError].} = + raise newException(IOError, "IO") + + proc use() {.raises: [].} = + # doesn't compile! Can raise IOError! + noRaise(doRaise) + +So in many cases a callback does not cause the compiler to be overly +conservative in its effect analysis. + + +Tag tracking +------------ + +The exception tracking is part of Nim's `effect system`:idx:. Raising an +exception is an *effect*. Other effects can also be defined. A user defined +effect is a means to *tag* a routine and to perform checks against this tag: + +.. code-block:: nim + type IO = object ## input/output effect + proc readLine(): string {.tags: [IO].} + + proc no_IO_please() {.tags: [].} = + # the compiler prevents this: + let x = readLine() + +A tag has to be a type name. A ``tags`` list - like a ``raises`` list - can +also be attached to a proc type. This affects type compatibility. + +The inference for tag tracking is analogous to the inference for +exception tracking. + + +Read/Write tracking +------------------- + +**Note**: Read/write tracking is not yet implemented! + +The inference for read/write tracking is analogous to the inference for +exception tracking. + + +Effects pragma +-------------- + +The ``effects`` pragma has been designed to assist the programmer with the +effects analysis. It is a statement that makes the compiler output all inferred +effects up to the ``effects``'s position: + +.. code-block:: nim + proc p(what: bool) = + if what: + raise newException(IOError, "IO") + {.effects.} + else: + raise newException(OSError, "OS") + +The compiler produces a hint message that ``IOError`` can be raised. ``OSError`` +is not listed as it cannot be raised in the branch the ``effects`` pragma +appears in. diff --git a/doc/manual/exceptions.txt b/doc/manual/exceptions.txt new file mode 100644 index 000000000..ba2ea30a0 --- /dev/null +++ b/doc/manual/exceptions.txt @@ -0,0 +1,132 @@ +Exception handling +================== + +Try statement +------------- + +Example: + +.. code-block:: nim + # read the first two lines of a text file that should contain numbers + # and tries to add them + var + f: File + if open(f, "numbers.txt"): + try: + var a = readLine(f) + var b = readLine(f) + echo("sum: " & $(parseInt(a) + parseInt(b))) + except OverflowError: + echo("overflow!") + except ValueError: + echo("could not convert string to integer") + except IOError: + echo("IO error!") + except: + echo("Unknown exception!") + finally: + close(f) + + +The statements after the ``try`` are executed in sequential order unless +an exception ``e`` is raised. If the exception type of ``e`` matches any +listed in an ``except`` clause the corresponding statements are executed. +The statements following the ``except`` clauses are called +`exception handlers`:idx:. + +The empty `except`:idx: clause is executed if there is an exception that is +not listed otherwise. It is similar to an ``else`` clause in ``if`` statements. + +If there is a `finally`:idx: clause, it is always executed after the +exception handlers. + +The exception is *consumed* in an exception handler. However, an +exception handler may raise another exception. If the exception is not +handled, it is propagated through the call stack. This means that often +the rest of the procedure - that is not within a ``finally`` clause - +is not executed (if an exception occurs). + + +Except and finally statements +----------------------------- + +``except`` and ``finally`` can also be used as a stand-alone statements. +Any statements following them in the current block will be considered to be +in an implicit try block: + +.. code-block:: nim + var f = open("numbers.txt") + finally: close(f) + ... + +The ``except`` statement has a limitation in this form: one can't specify the +type of the exception, one has to catch everything. Also, if one wants to use +both ``finally`` and ``except`` one needs to reverse the usual sequence of the +statements. Example: + +.. code-block:: nim + proc test() = + raise newException(Exception, "Hey ho") + + proc tester() = + finally: echo "3. Finally block" + except: echo "2. Except block" + echo "1. Pre exception" + test() + echo "4. Post exception" + # --> 1, 2, 3 is printed, 4 is never reached + + +Raise statement +--------------- + +Example: + +.. code-block:: nim + raise newEOS("operating system failed") + +Apart from built-in operations like array indexing, memory allocation, etc. +the ``raise`` statement is the only way to raise an exception. + +.. XXX document this better! + +If no exception name is given, the current exception is `re-raised`:idx:. The +`ReraiseError`:idx: exception is raised if there is no exception to +re-raise. It follows that the ``raise`` statement *always* raises an +exception (unless a raise hook has been provided). + + +onRaise builtin +--------------- + +`system.onRaise() <system.html#onRaise>`_ can be used to override the +behaviour of ``raise`` for a single ``try`` statement. ``onRaise`` has to be +called within the ``try`` statement that should be affected. + +This allows for a Lisp-like `condition system`:idx:\: + +.. code-block:: nim + var myFile = open("broken.txt", fmWrite) + try: + onRaise do (e: ref Exception)-> bool: + if e of IOError: + stdout.writeln "ok, writing to stdout instead" + else: + # do raise other exceptions: + result = true + myFile.writeln "writing to broken file" + finally: + myFile.close() + +``onRaise`` can only *filter* raised exceptions, it cannot transform one +exception into another. (Nor should ``onRaise`` raise an exception though +this is currently not enforced.) This restriction keeps the exception tracking +analysis sound. + + +Exception hierarchy +------------------- + +The exception tree is defined in the `system <system.html>`_ module: + +.. include:: exception_hierarchy_fragment.txt diff --git a/doc/manual/ffi.txt b/doc/manual/ffi.txt new file mode 100644 index 000000000..0ad4ebba9 --- /dev/null +++ b/doc/manual/ffi.txt @@ -0,0 +1,209 @@ +Foreign function interface +========================== + +Nim's `FFI`:idx: (foreign function interface) is extensive and only the +parts that scale to other future backends (like the LLVM/JavaScript backends) +are documented here. + + +Importc pragma +-------------- +The ``importc`` pragma provides a means to import a proc or a variable +from C. The optional argument is a string containing the C identifier. If +the argument is missing, the C name is the Nim identifier *exactly as +spelled*: + +.. code-block:: + proc printf(formatstr: cstring) {.header: "<stdio.h>", importc: "printf", varargs.} + +Note that this pragma is somewhat of a misnomer: Other backends will provide +the same feature under the same name. Also, if one is interfacing with C++ +the `ImportCpp pragma <nimc.html#importcpp-pragma>`_ and +interfacing with Objective-C the `ImportObjC pragma +<nimc.html#importobjc-pragma>`_ can be used. + + +Exportc pragma +-------------- +The ``exportc`` pragma provides a means to export a type, a variable, or a +procedure to C. Enums and constants can't be exported. The optional argument +is a string containing the C identifier. If the argument is missing, the C +name is the Nim identifier *exactly as spelled*: + +.. code-block:: Nim + proc callme(formatstr: cstring) {.exportc: "callMe", varargs.} + +Note that this pragma is somewhat of a misnomer: Other backends will provide +the same feature under the same name. + + +Extern pragma +------------- +Like ``exportc`` or ``importc``, the ``extern`` pragma affects name +mangling. The string literal passed to ``extern`` can be a format string: + +.. code-block:: Nim + proc p(s: string) {.extern: "prefix$1".} = + echo s + +In the example the external name of ``p`` is set to ``prefixp``. + + +Bycopy pragma +------------- + +The ``bycopy`` pragma can be applied to an object or tuple type and +instructs the compiler to pass the type by value to procs: + +.. code-block:: nim + type + TVector {.bycopy, pure.} = object + x, y, z: float + + +Byref pragma +------------ + +The ``byref`` pragma can be applied to an object or tuple type and instructs +the compiler to pass the type by reference (hidden pointer) to procs. + + +Varargs pragma +-------------- +The ``varargs`` pragma can be applied to procedures only (and procedure +types). It tells Nim that the proc can take a variable number of parameters +after the last specified parameter. Nim string values will be converted to C +strings automatically: + +.. code-block:: Nim + proc printf(formatstr: cstring) {.nodecl, varargs.} + + printf("hallo %s", "world") # "world" will be passed as C string + + +Union pragma +------------ +The ``union`` pragma can be applied to any ``object`` type. It means all +of the object's fields are overlaid in memory. This produces a ``union`` +instead of a ``struct`` in the generated C/C++ code. The object declaration +then must not use inheritance or any GC'ed memory but this is currently not +checked. + +**Future directions**: GC'ed memory should be allowed in unions and the GC +should scan unions conservatively. + +Packed pragma +------------- +The ``packed`` pragma can be applied to any ``object`` type. It ensures +that the fields of an object are packed back-to-back in memory. It is useful +to store packets or messages from/to network or hardware drivers, and for +interoperability with C. Combining packed pragma with inheritance is not +defined, and it should not be used with GC'ed memory (ref's). + +**Future directions**: Using GC'ed memory in packed pragma will result in +compile-time error. Usage with inheritance should be defined and documented. + +Unchecked pragma +---------------- +The ``unchecked`` pragma can be used to mark a named array as ``unchecked`` +meaning its bounds are not checked. This is often useful when one wishes to +implement his own flexibly sized arrays. Additionally an unchecked array is +translated into a C array of undetermined size: + +.. code-block:: nim + type + ArrayPart{.unchecked.} = array[0..0, int] + MySeq = object + len, cap: int + data: ArrayPart + +Produces roughly this C code: + +.. code-block:: C + typedef struct { + NI len; + NI cap; + NI data[]; + } MySeq; + +The bounds checking done at compile time is not disabled for now, so to access +``s.data[C]`` (where ``C`` is a constant) the array's index needs needs to +include ``C``. + +The base type of the unchecked array may not contain any GC'ed memory but this +is currently not checked. + +**Future directions**: GC'ed memory should be allowed in unchecked arrays and +there should be an explicit annotation of how the GC is to determine the +runtime size of the array. + + +Dynlib pragma for import +------------------------ +With the ``dynlib`` pragma a procedure or a variable can be imported from +a dynamic library (``.dll`` files for Windows, ``lib*.so`` files for UNIX). +The non-optional argument has to be the name of the dynamic library: + +.. code-block:: Nim + proc gtk_image_new(): PGtkWidget + {.cdecl, dynlib: "libgtk-x11-2.0.so", importc.} + +In general, importing a dynamic library does not require any special linker +options or linking with import libraries. This also implies that no *devel* +packages need to be installed. + +The ``dynlib`` import mechanism supports a versioning scheme: + +.. code-block:: nim + proc Tcl_Eval(interp: pTcl_Interp, script: cstring): int {.cdecl, + importc, dynlib: "libtcl(|8.5|8.4|8.3).so.(1|0)".} + +At runtime the dynamic library is searched for (in this order):: + + libtcl.so.1 + libtcl.so.0 + libtcl8.5.so.1 + libtcl8.5.so.0 + libtcl8.4.so.1 + libtcl8.4.so.0 + libtcl8.3.so.1 + libtcl8.3.so.0 + +The ``dynlib`` pragma supports not only constant strings as argument but also +string expressions in general: + +.. code-block:: nim + import os + + proc getDllName: string = + result = "mylib.dll" + if existsFile(result): return + result = "mylib2.dll" + if existsFile(result): return + quit("could not load dynamic library") + + proc myImport(s: cstring) {.cdecl, importc, dynlib: getDllName().} + +**Note**: Patterns like ``libtcl(|8.5|8.4).so`` are only supported in constant +strings, because they are precompiled. + +**Note**: Passing variables to the ``dynlib`` pragma will fail at runtime +because of order of initialization problems. + +**Note**: A ``dynlib`` import can be overriden with +the ``--dynlibOverride:name`` command line option. The Compiler User Guide +contains further information. + + +Dynlib pragma for export +------------------------ + +With the ``dynlib`` pragma a procedure can also be exported to +a dynamic library. The pragma then has no argument and has to be used in +conjunction with the ``exportc`` pragma: + +.. code-block:: Nim + proc exportme(): int {.cdecl, exportc, dynlib.} + +This is only useful if the program is compiled as a dynamic library via the +``--app:lib`` command line option. diff --git a/doc/manual/generics.txt b/doc/manual/generics.txt new file mode 100644 index 000000000..6f1b2ee0d --- /dev/null +++ b/doc/manual/generics.txt @@ -0,0 +1,346 @@ +Generics +======== + +Generics are Nim's means to parametrize procs, iterators or types with +`type parameters`:idx:. Depending on context, the brackets are used either to +introduce type parameters or to instantiate a generic proc, iterator or type. + +The following example shows a generic binary tree can be modelled: + +.. code-block:: nim + type + TBinaryTree[T] = object # TBinaryTree is a generic type with + # with generic param ``T`` + le, ri: ref TBinaryTree[T] # left and right subtrees; may be nil + data: T # the data stored in a node + PBinaryTree[T] = ref TBinaryTree[T] # a shorthand for notational convenience + + proc newNode[T](data: T): PBinaryTree[T] = # constructor for a node + new(result) + result.data = data + + proc add[T](root: var PBinaryTree[T], n: PBinaryTree[T]) = + if root == nil: + root = n + else: + var it = root + while it != nil: + var c = cmp(it.data, n.data) # compare the data items; uses + # the generic ``cmp`` proc that works for + # any type that has a ``==`` and ``<`` + # operator + if c < 0: + if it.le == nil: + it.le = n + return + it = it.le + else: + if it.ri == nil: + it.ri = n + return + it = it.ri + + iterator inorder[T](root: PBinaryTree[T]): T = + # inorder traversal of a binary tree + # recursive iterators are not yet implemented, so this does not work in + # the current compiler! + if root.le != nil: yield inorder(root.le) + yield root.data + if root.ri != nil: yield inorder(root.ri) + + var + root: PBinaryTree[string] # instantiate a PBinaryTree with the type string + add(root, newNode("hallo")) # instantiates generic procs ``newNode`` and + add(root, newNode("world")) # ``add`` + for str in inorder(root): + writeln(stdout, str) + + +Is operator +----------- + +The ``is`` operator checks for type equivalence at compile time. It is +therefore very useful for type specialization within generic code: + +.. code-block:: nim + type + TTable[TKey, TValue] = object + keys: seq[TKey] + values: seq[TValue] + when not (TKey is string): # nil value for strings used for optimization + deletedKeys: seq[bool] + + +Type operator +------------- + +The ``type`` (in many other languages called `typeof`:idx:) operator can +be used to get the type of an expression: + +.. code-block:: nim + var x = 0 + var y: type(x) # y has type int + +If ``type`` is used to determine the result type of a proc/iterator/converter +call ``c(X)`` (where ``X`` stands for a possibly empty list of arguments), the +interpretation where ``c`` is an iterator is preferred over the +other interpretations: + +.. code-block:: nim + import strutils + + # strutils contains both a ``split`` proc and iterator, but since an + # an iterator is the preferred interpretation, `y` has the type ``string``: + var y: type("a b c".split) + + +Type Classes +------------ + +A type class is a special pseudo-type that can be used to match against +types in the context of overload resolution or the ``is`` operator. +Nim supports the following built-in type classes: + +================== =================================================== +type class matches +================== =================================================== +``object`` any object type +``tuple`` any tuple type + +``enum`` any enumeration +``proc`` any proc type +``ref`` any ``ref`` type +``ptr`` any ``ptr`` type +``var`` any ``var`` type +``distinct`` any distinct type +``array`` any array type +``set`` any set type +``seq`` any seq type +``auto`` any type +================== =================================================== + +Furthermore, every generic type automatically creates a type class of the same +name that will match any instantiation of the generic type. + +Type classes can be combined using the standard boolean operators to form +more complex type classes: + +.. code-block:: nim + # create a type class that will match all tuple and object types + type TRecordType = tuple or object + + proc printFields(rec: TRecordType) = + for key, value in fieldPairs(rec): + echo key, " = ", value + +Procedures utilizing type classes in such manner are considered to be +`implicitly generic`:idx:. They will be instantiated once for each unique +combination of param types used within the program. + +Nim also allows for type classes and regular types to be specified +as `type constraints`:idx: of the generic type parameter: + +.. code-block:: nim + proc onlyIntOrString[T: int|string](x, y: T) = discard + + onlyIntOrString(450, 616) # valid + onlyIntOrString(5.0, 0.0) # type mismatch + onlyIntOrString("xy", 50) # invalid as 'T' cannot be both at the same time + +By default, during overload resolution each named type class will bind to +exactly one concrete type. Here is an example taken directly from the system +module to illustrate this: + +.. code-block:: nim + proc `==`*(x, y: tuple): bool = + ## requires `x` and `y` to be of the same tuple type + ## generic ``==`` operator for tuples that is lifted from the components + ## of `x` and `y`. + result = true + for a, b in fields(x, y): + if a != b: result = false + +Alternatively, the ``distinct`` type modifier can be applied to the type class +to allow each param matching the type class to bind to a different type. + +If a proc param doesn't have a type specified, Nim will use the +``distinct auto`` type class (also known as ``any``): + +.. code-block:: nim + # allow any combination of param types + proc concat(a, b): string = $a & $b + +Procs written with the implicitly generic style will often need to refer to the +type parameters of the matched generic type. They can be easily accessed using +the dot syntax: + +.. code-block:: nim + type TMatrix[T, Rows, Columns] = object + ... + + proc `[]`(m: TMatrix, row, col: int): TMatrix.T = + m.data[col * high(TMatrix.Columns) + row] + +Alternatively, the `type` operator can be used over the proc params for similar +effect when anonymous or distinct type classes are used. + +When a generic type is instantiated with a type class instead of a concrete +type, this results in another more specific type class: + +.. code-block:: nim + seq[ref object] # Any sequence storing references to any object type + + type T1 = auto + proc foo(s: seq[T1], e: T1) + # seq[T1] is the same as just `seq`, but T1 will be allowed to bind + # to a single type, while the signature is being matched + + TMatrix[Ordinal] # Any TMatrix instantiation using integer values + +As seen in the previous example, in such instantiations, it's not necessary to +supply all type parameters of the generic type, because any missing ones will +be inferred to have the equivalent of the `any` type class and thus they will +match anything without discrimination. + + +User defined type classes +------------------------- + +**Note**: User defined type classes are still in development. + +The user-defined type classes are available in two flavours - declarative and +imperative. Both are used to specify an arbitrary set of requirements that the +matched type must satisfy. + +Declarative type classes are written in the following form: + +.. code-block:: nim + type + Comparable = generic x, y + (x < y) is bool + + Container[T] = generic c + c.len is ordinal + items(c) is iterator + for value in c: + type(value) is T + +The type class will be matched if: + +a) all of the expressions within the body can be compiled for the tested type +b) all statically evaluatable boolean expressions in the body must be true + +The identifiers following the `generic` keyword represent instances of the +currently matched type. These instances can act both as variables of the type, +when used in contexts where a value is expected, and as the type itself when +used in contexts where a type is expected. + +Please note that the ``is`` operator allows one to easily verify the precise +type signatures of the required operations, but since type inference and +default parameters are still applied in the provided block, it's also possible +to encode usage protocols that do not reveal implementation details. + +As a special rule providing further convenience when writing type classes, any +type value appearing in a callable expression will be treated as a variable of +the designated type for overload resolution purposes, unless the type value was +passed in its explicit ``typedesc[T]`` form: + +.. code-block:: nim + type + OutputStream = generic S + write(var S, string) + +Much like generics, the user defined type classes will be instantiated exactly +once for each tested type and any static code included within them will also be +executed once. + + +Type inference with type classes +-------------------------------- + +If a type class is used as the return type of a proc and it won't be bound to +a concrete type by some of the proc params, Nim will infer the return type +from the proc body. This is usually used with the ``auto`` type class: + +.. code-block:: nim + proc makePair(a, b): auto = (first: a, second: b) + +The return type will be treated as an additional generic param and can be +explicitly specified at call sites as any other generic param. + +Future versions of Nim may also support overloading based on the return type +of the overloads. In such settings, the expected result type at call sites may +also influence the inferred return type. + +.. + Likewise, if a type class is used in another position where Nim expects a + concrete type (e.g. a variable declaration or a type coercion), Nim will try + to infer the concrete type by applying the matching algorithm that also used + in overload resolution. + + +Symbol lookup in generics +------------------------- + +The symbol binding rules in generics are slightly subtle: There are "open" and +"closed" symbols. A "closed" symbol cannot be re-bound in the instantiation +context, an "open" symbol can. Per default overloaded symbols are open +and every other symbol is closed. + +Open symbols are looked up in two different contexts: Both the context +at definition and the context at instantiation are considered: + +.. code-block:: nim + type + TIndex = distinct int + + proc `==` (a, b: TIndex): bool {.borrow.} + + var a = (0, 0.TIndex) + var b = (0, 0.TIndex) + + echo a == b # works! + +In the example the generic ``==`` for tuples (as defined in the system module) +uses the ``==`` operators of the tuple's components. However, the ``==`` for +the ``TIndex`` type is defined *after* the ``==`` for tuples; yet the example +compiles as the instantiation takes the currently defined symbols into account +too. + +A symbol can be forced to be open by a `mixin`:idx: declaration: + +.. code-block:: nim + proc create*[T](): ref T = + # there is no overloaded 'init' here, so we need to state that it's an + # open symbol explicitly: + mixin init + new result + init result + + +Bind statement +-------------- + +The ``bind`` statement is the counterpart to the ``mixin`` statement. It +can be used to explicitly declare identifiers that should be bound early (i.e. +the identifiers should be looked up in the scope of the template/generic +definition): + +.. code-block:: nim + # Module A + var + lastId = 0 + + template genId*: expr = + bind lastId + inc(lastId) + lastId + +.. code-block:: nim + # Module B + import A + + echo genId() + +But a ``bind`` is rarely useful because symbol binding from the definition +scope is the default. diff --git a/doc/manual/lexing.txt b/doc/manual/lexing.txt new file mode 100644 index 000000000..c3894b13d --- /dev/null +++ b/doc/manual/lexing.txt @@ -0,0 +1,356 @@ +Lexical Analysis +================ + +Encoding +-------- + +All Nim source files are in the UTF-8 encoding (or its ASCII subset). Other +encodings are not supported. Any of the standard platform line termination +sequences can be used - the Unix form using ASCII LF (linefeed), the Windows +form using the ASCII sequence CR LF (return followed by linefeed), or the old +Macintosh form using the ASCII CR (return) character. All of these forms can be +used equally, regardless of platform. + + +Indentation +----------- + +Nim's standard grammar describes an `indentation sensitive`:idx: language. +This means that all the control structures are recognized by indentation. +Indentation consists only of spaces; tabulators are not allowed. + +The indentation handling is implemented as follows: The lexer annotates the +following token with the preceding number of spaces; indentation is not +a separate token. This trick allows parsing of Nim with only 1 token of +lookahead. + +The parser uses a stack of indentation levels: the stack consists of integers +counting the spaces. The indentation information is queried at strategic +places in the parser but ignored otherwise: The pseudo terminal ``IND{>}`` +denotes an indentation that consists of more spaces than the entry at the top +of the stack; IND{=} an indentation that has the same number of spaces. ``DED`` +is another pseudo terminal that describes the *action* of popping a value +from the stack, ``IND{>}`` then implies to push onto the stack. + +With this notation we can now easily define the core of the grammar: A block of +statements (simplified example):: + + ifStmt = 'if' expr ':' stmt + (IND{=} 'elif' expr ':' stmt)* + (IND{=} 'else' ':' stmt)? + + simpleStmt = ifStmt / ... + + stmt = IND{>} stmt ^+ IND{=} DED # list of statements + / simpleStmt # or a simple statement + + + +Comments +-------- + +Comments start anywhere outside a string or character literal with the +hash character ``#``. +Comments consist of a concatenation of `comment pieces`:idx:. A comment piece +starts with ``#`` and runs until the end of the line. The end of line characters +belong to the piece. If the next line only consists of a comment piece with +no other tokens between it and the preceding one, it does not start a new +comment: + + +.. code-block:: nim + i = 0 # This is a single comment over multiple lines. + # The scanner merges these two pieces. + # The comment continues here. + + +`Documentation comments`:idx: are comments that start with two ``##``. +Documentation comments are tokens; they are only allowed at certain places in +the input file as they belong to the syntax tree! + + +Identifiers & Keywords +---------------------- + +Identifiers in Nim can be any string of letters, digits +and underscores, beginning with a letter. Two immediate following +underscores ``__`` are not allowed:: + + letter ::= 'A'..'Z' | 'a'..'z' | '\x80'..'\xff' + digit ::= '0'..'9' + IDENTIFIER ::= letter ( ['_'] (letter | digit) )* + +Currently any unicode character with an ordinal value > 127 (non ASCII) is +classified as a ``letter`` and may thus be part of an identifier but later +versions of the language may assign some Unicode characters to belong to the +operator characters instead. + +The following keywords are reserved and cannot be used as identifiers: + +.. code-block:: nim + :file: keywords.txt + +Some keywords are unused; they are reserved for future developments of the +language. + +Nim is a `style-insensitive`:idx: language. This means that it is not +case-sensitive and even underscores are ignored: +**type** is a reserved word, and so is **TYPE** or **T_Y_P_E**. The idea behind +this is that this allows programmers to use their own preferred spelling style +and libraries written by different programmers cannot use incompatible +conventions. A Nim-aware editor or IDE can show the identifiers as +preferred. Another advantage is that it frees the programmer from remembering +the exact spelling of an identifier. + + +String literals +--------------- + +Terminal symbol in the grammar: ``STR_LIT``. + +String literals can be delimited by matching double quotes, and can +contain the following `escape sequences`:idx:\ : + +================== =================================================== + Escape sequence Meaning +================== =================================================== + ``\n`` `newline`:idx: + ``\r``, ``\c`` `carriage return`:idx: + ``\l`` `line feed`:idx: + ``\f`` `form feed`:idx: + ``\t`` `tabulator`:idx: + ``\v`` `vertical tabulator`:idx: + ``\\`` `backslash`:idx: + ``\"`` `quotation mark`:idx: + ``\'`` `apostrophe`:idx: + ``\`` '0'..'9'+ `character with decimal value d`:idx:; + all decimal digits directly + following are used for the character + ``\a`` `alert`:idx: + ``\b`` `backspace`:idx: + ``\e`` `escape`:idx: `[ESC]`:idx: + ``\x`` HH `character with hex value HH`:idx:; + exactly two hex digits are allowed +================== =================================================== + + +Strings in Nim may contain any 8-bit value, even embedded zeros. However +some operations may interpret the first binary zero as a terminator. + + +Triple quoted string literals +----------------------------- + +Terminal symbol in the grammar: ``TRIPLESTR_LIT``. + +String literals can also be delimited by three double quotes +``"""`` ... ``"""``. +Literals in this form may run for several lines, may contain ``"`` and do not +interpret any escape sequences. +For convenience, when the opening ``"""`` is followed by a newline (there may +be whitespace between the opening ``"""`` and the newline), +the newline (and the preceding whitespace) is not included in the string. The +ending of the string literal is defined by the pattern ``"""[^"]``, so this: + +.. code-block:: nim + """"long string within quotes"""" + +Produces:: + + "long string within quotes" + + +Raw string literals +------------------- + +Terminal symbol in the grammar: ``RSTR_LIT``. + +There are also raw string literals that are preceded with the +letter ``r`` (or ``R``) and are delimited by matching double quotes (just +like ordinary string literals) and do not interpret the escape sequences. +This is especially convenient for regular expressions or Windows paths: + +.. code-block:: nim + + var f = openFile(r"C:\texts\text.txt") # a raw string, so ``\t`` is no tab + +To produce a single ``"`` within a raw string literal, it has to be doubled: + +.. code-block:: nim + + r"a""b" + +Produces:: + + a"b + +``r""""`` is not possible with this notation, because the three leading +quotes introduce a triple quoted string literal. ``r"""`` is the same +as ``"""`` since triple quoted string literals do not interpret escape +sequences either. + + +Generalized raw string literals +------------------------------- + +Terminal symbols in the grammar: ``GENERALIZED_STR_LIT``, +``GENERALIZED_TRIPLESTR_LIT``. + +The construct ``identifier"string literal"`` (without whitespace between the +identifier and the opening quotation mark) is a +generalized raw string literal. It is a shortcut for the construct +``identifier(r"string literal")``, so it denotes a procedure call with a +raw string literal as its only argument. Generalized raw string literals +are especially convenient for embedding mini languages directly into Nim +(for example regular expressions). + +The construct ``identifier"""string literal"""`` exists too. It is a shortcut +for ``identifier("""string literal""")``. + + +Character literals +------------------ + +Character literals are enclosed in single quotes ``''`` and can contain the +same escape sequences as strings - with one exception: `newline`:idx: (``\n``) +is not allowed as it may be wider than one character (often it is the pair +CR/LF for example). Here are the valid `escape sequences`:idx: for character +literals: + +================== =================================================== + Escape sequence Meaning +================== =================================================== + ``\r``, ``\c`` `carriage return`:idx: + ``\l`` `line feed`:idx: + ``\f`` `form feed`:idx: + ``\t`` `tabulator`:idx: + ``\v`` `vertical tabulator`:idx: + ``\\`` `backslash`:idx: + ``\"`` `quotation mark`:idx: + ``\'`` `apostrophe`:idx: + ``\`` '0'..'9'+ `character with decimal value d`:idx:; + all decimal digits directly + following are used for the character + ``\a`` `alert`:idx: + ``\b`` `backspace`:idx: + ``\e`` `escape`:idx: `[ESC]`:idx: + ``\x`` HH `character with hex value HH`:idx:; + exactly two hex digits are allowed +================== =================================================== + +A character is not an Unicode character but a single byte. The reason for this +is efficiency: for the overwhelming majority of use-cases, the resulting +programs will still handle UTF-8 properly as UTF-8 was specially designed for +this. Another reason is that Nim can thus support ``array[char, int]`` or +``set[char]`` efficiently as many algorithms rely on this feature. The `TRune` +type is used for Unicode characters, it can represent any Unicode character. +``TRune`` is declared in the `unicode module <unicode.html>`_. + + +Numerical constants +------------------- + +Numerical constants are of a single type and have the form:: + + hexdigit = digit | 'A'..'F' | 'a'..'f' + octdigit = '0'..'7' + bindigit = '0'..'1' + HEX_LIT = '0' ('x' | 'X' ) hexdigit ( ['_'] hexdigit )* + DEC_LIT = digit ( ['_'] digit )* + OCT_LIT = '0o' octdigit ( ['_'] octdigit )* + BIN_LIT = '0' ('b' | 'B' ) bindigit ( ['_'] bindigit )* + + INT_LIT = HEX_LIT + | DEC_LIT + | OCT_LIT + | BIN_LIT + + INT8_LIT = INT_LIT ['\''] ('i' | 'I') '8' + INT16_LIT = INT_LIT ['\''] ('i' | 'I') '16' + INT32_LIT = INT_LIT ['\''] ('i' | 'I') '32' + INT64_LIT = INT_LIT ['\''] ('i' | 'I') '64' + + UINT8_LIT = INT_LIT ['\''] ('u' | 'U') + UINT8_LIT = INT_LIT ['\''] ('u' | 'U') '8' + UINT16_LIT = INT_LIT ['\''] ('u' | 'U') '16' + UINT32_LIT = INT_LIT ['\''] ('u' | 'U') '32' + UINT64_LIT = INT_LIT ['\''] ('u' | 'U') '64' + + exponent = ('e' | 'E' ) ['+' | '-'] digit ( ['_'] digit )* + FLOAT_LIT = digit (['_'] digit)* (('.' (['_'] digit)* [exponent]) |exponent) + FLOAT32_LIT = HEX_LIT '\'' ('f'|'F') '32' + | (FLOAT_LIT | DEC_LIT | OCT_LIT | BIN_LIT) ['\''] ('f'|'F') '32' + FLOAT64_LIT = HEX_LIT '\'' ('f'|'F') '64' + | (FLOAT_LIT | DEC_LIT | OCT_LIT | BIN_LIT) ['\''] ('f'|'F') '64' + + +As can be seen in the productions, numerical constants can contain underscores +for readability. Integer and floating point literals may be given in decimal (no +prefix), binary (prefix ``0b``), octal (prefix ``0o``) and hexadecimal +(prefix ``0x``) notation. + +There exists a literal for each numerical type that is +defined. The suffix starting with an apostrophe ('\'') is called a +`type suffix`:idx:. Literals without a type suffix are of the type ``int``, +unless the literal contains a dot or ``E|e`` in which case it is of +type ``float``. For notational convenience the apostrophe of a type suffix +is optional if it is not ambiguous (only hexadecimal floating point literals +with a type suffix can be ambiguous). + + +The type suffixes are: + +================= ========================= + Type Suffix Resulting type of literal +================= ========================= + ``'i8`` int8 + ``'i16`` int16 + ``'i32`` int32 + ``'i64`` int64 + ``'u`` uint + ``'u8`` uint8 + ``'u16`` uint16 + ``'u32`` uint32 + ``'u64`` uint64 + ``'f32`` float32 + ``'f64`` float64 +================= ========================= + +Floating point literals may also be in binary, octal or hexadecimal +notation: +``0B0_10001110100_0000101001000111101011101111111011000101001101001001'f64`` +is approximately 1.72826e35 according to the IEEE floating point standard. + + +Operators +--------- + +In Nim one can define his own operators. An operator is any +combination of the following characters:: + + = + - * / < > + @ $ ~ & % | + ! ? ^ . : \ + +These keywords are also operators: +``and or not xor shl shr div mod in notin is isnot of``. + +`=`:tok:, `:`:tok:, `::`:tok: are not available as general operators; they +are used for other notational purposes. + +``*:`` is as a special case the two tokens `*`:tok: and `:`:tok: +(to support ``var v*: T``). + + +Other tokens +------------ + +The following strings denote other tokens:: + + ` ( ) { } [ ] , ; [. .] {. .} (. .) + + +The `slice`:idx: operator `..`:tok: takes precedence over other tokens that +contain a dot: `{..}`:tok: are the three tokens `{`:tok:, `..`:tok:, `}`:tok: +and not the two tokens `{.`:tok:, `.}`:tok:. + diff --git a/doc/manual/locking.txt b/doc/manual/locking.txt new file mode 100644 index 000000000..b646f0cee --- /dev/null +++ b/doc/manual/locking.txt @@ -0,0 +1,197 @@ +Guards and locks +================ + +Apart from ``spawn`` and ``parallel`` Nim also provides all the common low level +concurrency mechanisms like locks, atomic intristics or condition variables. + +Nim significantly improves on the safety of these features via additional +pragmas: + +1) A `guard`:idx: annotation is introduced to prevent data races. +2) Every access of a guarded memory location needs to happen in an + appropriate `locks`:idx: statement. +3) Locks and routines can be annotated with `lock levels`:idx: to prevent + deadlocks at compile time. + +Guards and the locks section +---------------------------- + +Protecting global variables +~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Global variables and object fields can be annotated via an ``guard`` pragma: + +.. code-block:: nim + var glock: TLock + var gdata {.guard: glock.}: int + +The compiler then ensures that every access of ``gdata`` is within a ``locks`` +section: + +.. code-block:: nim + proc invalid = + # invalid: unguarded access: + echo gdata + + proc valid = + # valid access: + {.locks: [glock].}: + echo gdata + +Top level accesses to ``gdata`` are always allowed so that it can be initialized +conveniently. It is *assumed* (but not enforced) that every top level statement +is executed before any concurrent action happens. + +The ``locks`` section deliberately looks ugly because it has no runtime +semantics and should not be used directly! It should only be used in templates +that also implement some form of locking at runtime: + +.. code-block:: nim + template lock(a: TLock; body: stmt) = + pthread_mutex_lock(a) + {.locks: [a].}: + try: + body + finally: + pthread_mutex_unlock(a) + + +The guard does not need to be of any particular type. It is flexible enough to +model low level lockfree mechanisms: + +.. code-block:: nim + var dummyLock {.compileTime.}: int + var atomicCounter {.guard: dummyLock.}: int + + template atomicRead(x): expr = + {.locks: [dummyLock].}: + memoryReadBarrier() + x + + echo atomicRead(atomicCounter) + + +The ``locks`` pragma takes a list of lock expressions ``locks: [a, b, ...]`` +in order to support *multi lock* statements. Why these are essential is +explained in the `lock levels`_ section. + + +Protecting general locations +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The ``guard`` annotation can also be used to protect fields within an object. +The guard then needs to be another field within the same object or a +global variable. + +Since objects can reside on the heap or on the stack this greatly enhances the +expressivity of the language: + +.. code-block:: nim + type + ProtectedCounter = object + v {.guard: L.}: int + L: TLock + + proc incCounters(counters: var openArray[ProtectedCounter]) = + for i in 0..counters.high: + lock counters[i].L: + inc counters[i].v + +The access to field ``x.v`` is allowed since its guard ``x.L`` is active. +After template expansion, this amounts to: + +.. code-block:: nim + proc incCounters(counters: var openArray[ProtectedCounter]) = + for i in 0..counters.high: + pthread_mutex_lock(counters[i].L) + {.locks: [counters[i].L].}: + try: + inc counters[i].v + finally: + pthread_mutex_unlock(counters[i].L) + +There is an analysis that checks that ``counters[i].L`` is the lock that +corresponds to the protected location ``counters[i].v``. This analysis is called +`path analysis`:idx: because it deals with paths to locations +like ``obj.field[i].fieldB[j]``. + +The path analysis is **currently unsound**, but that doesn't make it useless. +Two paths are considered equivalent if they are syntactically the same. + +This means the following compiles (for now) even though it really should not: + +.. code-block:: nim + {.locks: [a[i].L].}: + inc i + access a[i].v + + + +Lock levels +----------- + +Lock levels are used to enforce a global locking order in order to prevent +deadlocks at compile-time. A lock level is an constant integer in the range +0..1_000. Lock level 0 means that no lock is acquired at all. + +If a section of code holds a lock of level ``M`` than it can also acquire any +lock of level ``N < M``. Another lock of level ``M`` cannot be acquired. Locks +of the same level can only be acquired *at the same time* within a +single ``locks`` section: + +.. code-block:: nim + var a, b: TLock[2] + var x: TLock[1] + # invalid locking order: TLock[1] cannot be acquired before TLock[2]: + {.locks: [x].}: + {.locks: [a].}: + ... + # valid locking order: TLock[2] acquired before TLock[1]: + {.locks: [a].}: + {.locks: [x].}: + ... + + # invalid locking order: TLock[2] acquired before TLock[2]: + {.locks: [a].}: + {.locks: [b].}: + ... + + # valid locking order, locks of the same level acquired at the same time: + {.locks: [a, b].}: + ... + + +So here is how a typical multilock statement can be implemented in Nim: + +.. code-block:: nim + template multilock(a, b: ptr TLock; body: stmt) = + if cast[ByteAddress](a) < cast[ByteAddress](b): + pthread_mutex_lock(a) + pthread_mutex_lock(b) + else: + pthread_mutex_lock(b) + pthread_mutex_lock(a) + {.locks: [a, b].}: + try: + body + finally: + pthread_mutex_unlock(a) + pthread_mutex_unlock(b) + + +Whole routines can also be annotated with a ``locks`` pragma that takes a lock +level. This then means that the routine may acquire locks of up to this level. +This is essential so that procs can be called within a ``locks`` section: + +.. code-block:: nim + proc p() {.locks: 3.} = discard + + var a: TLock[4] + {.locks: [a].}: + # p's locklevel (3) is strictly less than a's (4) so the call is allowed: + p() + + +As usual ``locks`` is an inferred effect and there is a subtype +relation: ``proc () {.locks: N.}`` is a subtype of ``proc () {.locks: M.}`` +iff (M <= N). diff --git a/doc/manual/modules.txt b/doc/manual/modules.txt new file mode 100644 index 000000000..f412587db --- /dev/null +++ b/doc/manual/modules.txt @@ -0,0 +1,185 @@ +Modules +======= +Nim supports splitting a program into pieces by a module concept. +Each module needs to be in its own file and has its own `namespace`:idx:. +Modules enable `information hiding`:idx: and `separate compilation`:idx:. +A module may gain access to symbols of another module by the `import`:idx: +statement. `Recursive module dependencies`:idx: are allowed, but slightly +subtle. Only top-level symbols that are marked with an asterisk (``*``) are +exported. + +The algorithm for compiling modules is: + +- compile the whole module as usual, following import statements recursively + +- if there is a cycle only import the already parsed symbols (that are + exported); if an unknown identifier occurs then abort + +This is best illustrated by an example: + +.. code-block:: nim + # Module A + type + T1* = int # Module A exports the type ``T1`` + import B # the compiler starts parsing B + + proc main() = + var i = p(3) # works because B has been parsed completely here + + main() + + +.. code-block:: nim + # Module B + import A # A is not parsed here! Only the already known symbols + # of A are imported. + + proc p*(x: A.T1): A.T1 = + # this works because the compiler has already + # added T1 to A's interface symbol table + result = x + 1 + + +Import statement +~~~~~~~~~~~~~~~~ + +After the ``import`` statement a list of module names can follow or a single +module name followed by an ``except`` to prevent some symbols to be imported: + +.. code-block:: nim + import strutils except `%` + + # doesn't work then: + echo "$1" % "abc" + + +Module names in imports +~~~~~~~~~~~~~~~~~~~~~~~ + +A module alias can be introduced via the ``as`` keyword: + +.. code-block:: nim + import strutils as su, sequtils as qu + + echo su.format("$1", "lalelu") + +The original module name is then not accessible. The +notations ``path/to/module`` or ``path.to.module`` or ``"path/to/module"`` +can be used to refer to a module in subdirectories: + +.. code-block:: nim + import lib.pure.strutils, lib/pure/os, "lib/pure/times" + +Note that the module name is still ``strutils`` and not ``lib.pure.strutils`` +and so one **cannot** do: + +.. code-block:: nim + import lib.pure.strutils + echo lib.pure.strutils + +Likewise the following does not make sense as the name is ``strutils`` already: + +.. code-block:: nim + import lib.pure.strutils as strutils + + +From import statement +~~~~~~~~~~~~~~~~~~~~~ + +After the ``from`` statement a module name follows followed by +an ``import`` to list the symbols one likes to use without explict +full qualification: + +.. code-block:: nim + from strutils import `%` + + echo "$1" % "abc" + # always possible: full qualification: + echo strutils.replace("abc", "a", "z") + +It's also possible to use ``from module import nil`` if one wants to import +the module but wants to enforce fully qualified access to every symbol +in ``module``. + + +Export statement +~~~~~~~~~~~~~~~~ + +An ``export`` statement can be used for symbol fowarding so that client +modules don't need to import a module's dependencies: + +.. code-block:: nim + # module B + type TMyObject* = object + +.. code-block:: nim + # module A + import B + export B.TMyObject + + proc `$`*(x: TMyObject): string = "my object" + + +.. code-block:: nim + # module C + import A + + # B.TMyObject has been imported implicitly here: + var x: TMyObject + echo($x) + + +Scope rules +----------- +Identifiers are valid from the point of their declaration until the end of +the block in which the declaration occurred. The range where the identifier +is known is the scope of the identifier. The exact scope of an +identifier depends on the way it was declared. + +Block scope +~~~~~~~~~~~ +The *scope* of a variable declared in the declaration part of a block +is valid from the point of declaration until the end of the block. If a +block contains a second block, in which the identifier is redeclared, +then inside this block, the second declaration will be valid. Upon +leaving the inner block, the first declaration is valid again. An +identifier cannot be redefined in the same block, except if valid for +procedure or iterator overloading purposes. + + +Tuple or object scope +~~~~~~~~~~~~~~~~~~~~~ +The field identifiers inside a tuple or object definition are valid in the +following places: + +* To the end of the tuple/object definition. +* Field designators of a variable of the given tuple/object type. +* In all descendant types of the object type. + +Module scope +~~~~~~~~~~~~ +All identifiers of a module are valid from the point of declaration until +the end of the module. Identifiers from indirectly dependent modules are *not* +available. The `system`:idx: module is automatically imported in every other +module. + +If a module imports an identifier by two different modules, each occurrence of +the identifier has to be qualified, unless it is an overloaded procedure or +iterator in which case the overloading resolution takes place: + +.. code-block:: nim + # Module A + var x*: string + +.. code-block:: nim + # Module B + var x*: int + +.. code-block:: nim + # Module C + import A, B + write(stdout, x) # error: x is ambiguous + write(stdout, A.x) # no error: qualifier used + + var x = 4 + write(stdout, x) # not ambiguous: uses the module C's x diff --git a/doc/manual/pragmas.txt b/doc/manual/pragmas.txt new file mode 100644 index 000000000..1a19cb129 --- /dev/null +++ b/doc/manual/pragmas.txt @@ -0,0 +1,477 @@ +Pragmas +======= + +Pragmas are Nim's method to give the compiler additional information / +commands without introducing a massive number of new keywords. Pragmas are +processed on the fly during semantic checking. Pragmas are enclosed in the +special ``{.`` and ``.}`` curly brackets. Pragmas are also often used as a +first implementation to play with a language feature before a nicer syntax +to access the feature becomes available. + + +noSideEffect pragma +------------------- +The ``noSideEffect`` pragma is used to mark a proc/iterator to have no side +effects. This means that the proc/iterator only changes locations that are +reachable from its parameters and the return value only depends on the +arguments. If none of its parameters have the type ``var T`` +or ``ref T`` or ``ptr T`` this means no locations are modified. It is a static +error to mark a proc/iterator to have no side effect if the compiler cannot +verify this. + +As a special semantic rule, the built-in `debugEcho <system.html#debugEcho>`_ +pretends to be free of side effects, so that it can be used for debugging +routines marked as ``noSideEffect``. + +**Future directions**: ``func`` may become a keyword and syntactic sugar for a +proc with no side effects: + +.. code-block:: nim + func `+` (x, y: int): int + + +destructor pragma +----------------- + +The ``destructor`` pragma is used to mark a proc to act as a type destructor. +Its usage is deprecated, use the ``override`` pragma instead. +See `type bound operations`_. + + +override pragma +--------------- + +See `type bound operations`_ instead. + +procvar pragma +-------------- +The ``procvar`` pragma is used to mark a proc that it can be passed to a +procedural variable. + + +compileTime pragma +------------------ +The ``compileTime`` pragma is used to mark a proc to be used at compile +time only. No code will be generated for it. Compile time procs are useful +as helpers for macros. + + +noReturn pragma +--------------- +The ``noreturn`` pragma is used to mark a proc that never returns. + + +acyclic pragma +-------------- +The ``acyclic`` pragma can be used for object types to mark them as acyclic +even though they seem to be cyclic. This is an **optimization** for the garbage +collector to not consider objects of this type as part of a cycle: + +.. code-block:: nim + type + PNode = ref TNode + TNode {.acyclic, final.} = object + left, right: PNode + data: string + +In the example a tree structure is declared with the ``TNode`` type. Note that +the type definition is recursive and the GC has to assume that objects of +this type may form a cyclic graph. The ``acyclic`` pragma passes the +information that this cannot happen to the GC. If the programmer uses the +``acyclic`` pragma for data types that are in reality cyclic, the GC may leak +memory, but nothing worse happens. + +**Future directions**: The ``acyclic`` pragma may become a property of a +``ref`` type: + +.. code-block:: nim + type + PNode = acyclic ref TNode + TNode = object + left, right: PNode + data: string + + +final pragma +------------ +The ``final`` pragma can be used for an object type to specify that it +cannot be inherited from. + + +shallow pragma +-------------- +The ``shallow`` pragma affects the semantics of a type: The compiler is +allowed to make a shallow copy. This can cause serious semantic issues and +break memory safety! However, it can speed up assignments considerably, +because the semantics of Nim require deep copying of sequences and strings. +This can be expensive, especially if sequences are used to build a tree +structure: + +.. code-block:: nim + type + TNodeKind = enum nkLeaf, nkInner + TNode {.final, shallow.} = object + case kind: TNodeKind + of nkLeaf: + strVal: string + of nkInner: + children: seq[TNode] + + +pure pragma +----------- +An object type can be marked with the ``pure`` pragma so that its type +field which is used for runtime type identification is omitted. This used to be +necessary for binary compatibility with other compiled languages. + +An enum type can be marked as ``pure``. Then access of its fields always +requires full qualification. + + +asmNoStackFrame pragma +---------------------- +A proc can be marked with the ``AsmNoStackFrame`` pragma to tell the compiler +it should not generate a stack frame for the proc. There are also no exit +statements like ``return result;`` generated and the generated C function is +declared as ``__declspec(naked)`` or ``__attribute__((naked))`` (depending on +the used C compiler). + +**Note**: This pragma should only be used by procs which consist solely of +assembler statements. + +error pragma +------------ +The ``error`` pragma is used to make the compiler output an error message +with the given content. Compilation does not necessarily abort after an error +though. + +The ``error`` pragma can also be used to +annotate a symbol (like an iterator or proc). The *usage* of the symbol then +triggers a compile-time error. This is especially useful to rule out that some +operation is valid due to overloading and type conversions: + +.. code-block:: nim + ## check that underlying int values are compared and not the pointers: + proc `==`(x, y: ptr int): bool {.error.} + + +fatal pragma +------------ +The ``fatal`` pragma is used to make the compiler output an error message +with the given content. In contrast to the ``error`` pragma, compilation +is guaranteed to be aborted by this pragma. Example: + +.. code-block:: nim + when not defined(objc): + {.fatal: "Compile this program with the objc command!".} + +warning pragma +-------------- +The ``warning`` pragma is used to make the compiler output a warning message +with the given content. Compilation continues after the warning. + +hint pragma +----------- +The ``hint`` pragma is used to make the compiler output a hint message with +the given content. Compilation continues after the hint. + +line pragma +----------- +The ``line`` pragma can be used to affect line information of the annotated +statement as seen in stack backtraces: + +.. code-block:: nim + + template myassert*(cond: expr, msg = "") = + if not cond: + # change run-time line information of the 'raise' statement: + {.line: InstantiationInfo().}: + raise newException(EAssertionFailed, msg) + +If the ``line`` pragma is used with a parameter, the parameter needs be a +``tuple[filename: string, line: int]``. If it is used without a parameter, +``system.InstantiationInfo()`` is used. + + +linearScanEnd pragma +-------------------- +The ``linearScanEnd`` pragma can be used to tell the compiler how to +compile a Nim `case`:idx: statement. Syntactically it has to be used as a +statement: + +.. code-block:: nim + case myInt + of 0: + echo "most common case" + of 1: + {.linearScanEnd.} + echo "second most common case" + of 2: echo "unlikely: use branch table" + else: echo "unlikely too: use branch table for ", myInt + +In the example, the case branches ``0`` and ``1`` are much more common than +the other cases. Therefore the generated assembler code should test for these +values first, so that the CPU's branch predictor has a good chance to succeed +(avoiding an expensive CPU pipeline stall). The other cases might be put into a +jump table for O(1) overhead, but at the cost of a (very likely) pipeline +stall. + +The ``linearScanEnd`` pragma should be put into the last branch that should be +tested against via linear scanning. If put into the last branch of the +whole ``case`` statement, the whole ``case`` statement uses linear scanning. + + +computedGoto pragma +------------------- +The ``computedGoto`` pragma can be used to tell the compiler how to +compile a Nim `case`:idx: in a ``while true`` statement. +Syntactically it has to be used as a statement inside the loop: + +.. code-block:: nim + + type + MyEnum = enum + enumA, enumB, enumC, enumD, enumE + + proc vm() = + var instructions: array [0..100, MyEnum] + instructions[2] = enumC + instructions[3] = enumD + instructions[4] = enumA + instructions[5] = enumD + instructions[6] = enumC + instructions[7] = enumA + instructions[8] = enumB + + instructions[12] = enumE + var pc = 0 + while true: + {.computedGoto.} + let instr = instructions[pc] + case instr + of enumA: + echo "yeah A" + of enumC, enumD: + echo "yeah CD" + of enumB: + echo "yeah B" + of enumE: + break + inc(pc) + + vm() + +As the example shows ``computedGoto`` is mostly useful for interpreters. If +the underlying backend (C compiler) does not support the computed goto +extension the pragma is simply ignored. + + +unroll pragma +------------- +The ``unroll`` pragma can be used to tell the compiler that it should unroll +a `for`:idx: or `while`:idx: loop for runtime efficiency: + +.. code-block:: nim + proc searchChar(s: string, c: char): int = + for i in 0 .. s.high: + {.unroll: 4.} + if s[i] == c: return i + result = -1 + +In the above example, the search loop is unrolled by a factor 4. The unroll +factor can be left out too; the compiler then chooses an appropriate unroll +factor. + +**Note**: Currently the compiler recognizes but ignores this pragma. + + +immediate pragma +---------------- + +See `Ordinary vs immediate templates`_. + + +compilation option pragmas +-------------------------- +The listed pragmas here can be used to override the code generation options +for a proc/method/converter. + +The implementation currently provides the following possible options (various +others may be added later). + +=============== =============== ============================================ +pragma allowed values description +=============== =============== ============================================ +checks on|off Turns the code generation for all runtime + checks on or off. +boundChecks on|off Turns the code generation for array bound + checks on or off. +overflowChecks on|off Turns the code generation for over- or + underflow checks on or off. +nilChecks on|off Turns the code generation for nil pointer + checks on or off. +assertions on|off Turns the code generation for assertions + on or off. +warnings on|off Turns the warning messages of the compiler + on or off. +hints on|off Turns the hint messages of the compiler + on or off. +optimization none|speed|size Optimize the code for speed or size, or + disable optimization. +patterns on|off Turns the term rewriting templates/macros + on or off. +callconv cdecl|... Specifies the default calling convention for + all procedures (and procedure types) that + follow. +=============== =============== ============================================ + +Example: + +.. code-block:: nim + {.checks: off, optimization: speed.} + # compile without runtime checks and optimize for speed + + +push and pop pragmas +-------------------- +The `push/pop`:idx: pragmas are very similar to the option directive, +but are used to override the settings temporarily. Example: + +.. code-block:: nim + {.push checks: off.} + # compile this section without runtime checks as it is + # speed critical + # ... some code ... + {.pop.} # restore old settings + + +register pragma +--------------- +The ``register`` pragma is for variables only. It declares the variable as +``register``, giving the compiler a hint that the variable should be placed +in a hardware register for faster access. C compilers usually ignore this +though and for good reasons: Often they do a better job without it anyway. + +In highly specific cases (a dispatch loop of an bytecode interpreter for +example) it may provide benefits, though. + + +global pragma +------------- +The ``global`` pragma can be applied to a variable within a proc to instruct +the compiler to store it in a global location and initialize it once at program +startup. + +.. code-block:: nim + proc isHexNumber(s: string): bool = + var pattern {.global.} = re"[0-9a-fA-F]+" + result = s.match(pattern) + +When used within a generic proc, a separate unique global variable will be +created for each instantiation of the proc. The order of initialization of +the created global variables within a module is not defined, but all of them +will be initialized after any top-level variables in their originating module +and before any variable in a module that imports it. + +deadCodeElim pragma +------------------- +The ``deadCodeElim`` pragma only applies to whole modules: It tells the +compiler to activate (or deactivate) dead code elimination for the module the +pragma appears in. + +The ``--deadCodeElim:on`` command line switch has the same effect as marking +every module with ``{.deadCodeElim:on}``. However, for some modules such as +the GTK wrapper it makes sense to *always* turn on dead code elimination - +no matter if it is globally active or not. + +Example: + +.. code-block:: nim + {.deadCodeElim: on.} + + +.. + NoForward pragma + ---------------- + The ``noforward`` pragma can be used to turn on and off a special compilation + mode that to large extent eliminates the need for forward declarations. In this + mode, the proc definitions may appear out of order and the compiler will postpone + their semantic analysis and compilation until it actually needs to generate code + using the definitions. In this regard, this mode is similar to the modus operandi + of dynamic scripting languages, where the function calls are not resolved until + the code is executed. Here is the detailed algorithm taken by the compiler: + + 1. When a callable symbol is first encountered, the compiler will only note the + symbol callable name and it will add it to the appropriate overload set in the + current scope. At this step, it won't try to resolve any of the type expressions + used in the signature of the symbol (so they can refer to other not yet defined + symbols). + + 2. When a top level call is encountered (usually at the very end of the module), + the compiler will try to determine the actual types of all of the symbols in the + matching overload set. This is a potentially recursive process as the signatures + of the symbols may include other call expressions, whoose types will be resolved + at this point too. + + 3. Finally, after the best overload is picked, the compiler will start compiling + the body of the respective symbol. This in turn will lead the compiler to discover + more call expresions that need to be resolved and steps 2 and 3 will be repeated + as necessary. + + Please note that if a callable symbol is never used in this scenario, its body + will never be compiled. This is the default behavior leading to best compilation + times, but if exhaustive compilation of all definitions is required, using + ``nim check`` provides this option as well. + + Example: + + .. code-block:: nim + + {.noforward: on.} + + proc foo(x: int) = + bar x + + proc bar(x: int) = + echo x + + foo(10) + + +pragma pragma +------------- + +The ``pragma`` pragma can be used to declare user defined pragmas. This is +useful because Nim's templates and macros do not affect pragmas. User +defined pragmas are in a different module-wide scope than all other symbols. +They cannot be imported from a module. + +Example: + +.. code-block:: nim + when appType == "lib": + {.pragma: rtl, exportc, dynlib, cdecl.} + else: + {.pragma: rtl, importc, dynlib: "client.dll", cdecl.} + + proc p*(a, b: int): int {.rtl.} = + result = a+b + +In the example a new pragma named ``rtl`` is introduced that either imports +a symbol from a dynamic library or exports the symbol for dynamic library +generation. + + +Disabling certain messages +-------------------------- +Nim generates some warnings and hints ("line too long") that may annoy the +user. A mechanism for disabling certain messages is provided: Each hint +and warning message contains a symbol in brackets. This is the message's +identifier that can be used to enable or disable it: + +.. code-block:: Nim + {.hint[LineTooLong]: off.} # turn off the hint about too long lines + +This is often better than disabling all warnings at once. + + diff --git a/doc/manual/procs.txt b/doc/manual/procs.txt new file mode 100644 index 000000000..66f9ad58f --- /dev/null +++ b/doc/manual/procs.txt @@ -0,0 +1,554 @@ +Procedures +========== + +What most programming languages call `methods`:idx: or `functions`:idx: are +called `procedures`:idx: in Nim (which is the correct terminology). A +procedure declaration defines an identifier and associates it with a block +of code. +A procedure may call itself recursively. A parameter may be given a default +value that is used if the caller does not provide a value for this parameter. + +If the proc declaration has no body, it is a `forward`:idx: declaration. If +the proc returns a value, the procedure body can access an implicitly declared +variable named `result`:idx: that represents the return value. Procs can be +overloaded. The overloading resolution algorithm tries to find the proc that is +the best match for the arguments. Example: + +.. code-block:: nim + + proc toLower(c: Char): Char = # toLower for characters + if c in {'A'..'Z'}: + result = chr(ord(c) + (ord('a') - ord('A'))) + else: + result = c + + proc toLower(s: string): string = # toLower for strings + result = newString(len(s)) + for i in 0..len(s) - 1: + result[i] = toLower(s[i]) # calls toLower for characters; no recursion! + +Calling a procedure can be done in many different ways: + +.. code-block:: nim + proc callme(x, y: int, s: string = "", c: char, b: bool = false) = ... + + # call with positional arguments # parameter bindings: + callme(0, 1, "abc", '\t', true) # (x=0, y=1, s="abc", c='\t', b=true) + # call with named and positional arguments: + callme(y=1, x=0, "abd", '\t') # (x=0, y=1, s="abd", c='\t', b=false) + # call with named arguments (order is not relevant): + callme(c='\t', y=1, x=0) # (x=0, y=1, s="", c='\t', b=false) + # call as a command statement: no () needed: + callme 0, 1, "abc", '\t' + + +A procedure cannot modify its parameters (unless the parameters have the type +`var`). + +`Operators`:idx: are procedures with a special operator symbol as identifier: + +.. code-block:: nim + proc `$` (x: int): string = + # converts an integer to a string; this is a prefix operator. + result = intToStr(x) + +Operators with one parameter are prefix operators, operators with two +parameters are infix operators. (However, the parser distinguishes these from +the operator's position within an expression.) There is no way to declare +postfix operators: all postfix operators are built-in and handled by the +grammar explicitly. + +Any operator can be called like an ordinary proc with the '`opr`' +notation. (Thus an operator can have more than two parameters): + +.. code-block:: nim + proc `*+` (a, b, c: int): int = + # Multiply and add + result = a * b + c + + assert `*+`(3, 4, 6) == `*`(a, `+`(b, c)) + + +Method call syntax +------------------ + +For object oriented programming, the syntax ``obj.method(args)`` can be used +instead of ``method(obj, args)``. The parentheses can be omitted if there are no +remaining arguments: ``obj.len`` (instead of ``len(obj)``). + +This method call syntax is not restricted to objects, it can be used +to supply any type of first argument for procedures: + +.. code-block:: nim + + echo("abc".len) # is the same as echo(len("abc")) + echo("abc".toUpper()) + echo({'a', 'b', 'c'}.card) + stdout.writeln("Hallo") # the same as writeln(stdout, "Hallo") + +Another way to look at the method call syntax is that it provides the missing +postfix notation. + + +Properties +---------- +Nim has no need for *get-properties*: Ordinary get-procedures that are called +with the *method call syntax* achieve the same. But setting a value is +different; for this a special setter syntax is needed: + +.. code-block:: nim + + type + TSocket* = object of TObject + FHost: int # cannot be accessed from the outside of the module + # the `F` prefix is a convention to avoid clashes since + # the accessors are named `host` + + proc `host=`*(s: var TSocket, value: int) {.inline.} = + ## setter of hostAddr + s.FHost = value + + proc host*(s: TSocket): int {.inline.} = + ## getter of hostAddr + s.FHost + + var + s: TSocket + s.host = 34 # same as `host=`(s, 34) + + +Command invocation syntax +------------------------- + +Routines can be invoked without the ``()`` if the call is syntatically +a statement. This command invocation syntax also works for +expressions, but then only a single argument may follow. This restriction +means ``echo f 1, f 2`` is parsed as ``echo(f(1), f(2))`` and not as +``echo(f(1, f(2)))``. The method call syntax may be used to provide one +more argument in this case: + +.. code-block:: nim + proc optarg(x:int, y:int = 0):int = x + y + proc singlearg(x:int):int = 20*x + + echo optarg 1, " ", singlearg 2 # prints "1 40" + + let fail = optarg 1, optarg 8 # Wrong. Too many arguments for a command call + let x = optarg(1, optarg 8) # traditional procedure call with 2 arguments + let y = 1.optarg optarg 8 # same thing as above, w/o the parenthesis + assert x == y + +The command invocation syntax also can't have complex expressions as arguments. +For example: (`anonymous procs`_), ``if``, ``case`` or ``try``. The (`do +notation`_) is limited, but usable for a single proc (see the example in the +corresponding section). Function calls with no arguments still needs () to +distinguish between a call and the function itself as a first class value. + + +Closures +-------- + +Procedures can appear at the top level in a module as well as inside other +scopes, in which case they are called nested procs. A nested proc can access +local variables from its enclosing scope and if it does so it becomes a +closure. Any captured variables are stored in a hidden additional argument +to the closure (its environment) and they are accessed by reference by both +the closure and its enclosing scope (i.e. any modifications made to them are +visible in both places). The closure environment may be allocated on the heap +or on the stack if the compiler determines that this would be safe. + + +Anonymous Procs +--------------- + +Procs can also be treated as expressions, in which case it's allowed to omit +the proc's name. + +.. code-block:: nim + var cities = @["Frankfurt", "Tokyo", "New York"] + + cities.sort(proc (x,y: string): int = + cmp(x.len, y.len)) + + +Procs as expressions can appear both as nested procs and inside top level +executable code. + + +Do notation +----------- + +As a special more convenient notation, proc expressions involved in procedure +calls can use the ``do`` keyword: + +.. code-block:: nim + sort(cities) do (x,y: string) -> int: + cmp(x.len, y.len) + # Less parenthesis using the method plus command syntax: + cities = cities.map do (x:string) -> string: + "City of " & x + +``do`` is written after the parentheses enclosing the regular proc params. +The proc expression represented by the do block is appended to them. + +More than one ``do`` block can appear in a single call: + +.. code-block:: nim + proc performWithUndo(task: proc(), undo: proc()) = ... + + performWithUndo do: + # multiple-line block of code + # to perform the task + do: + # code to undo it + +For compatibility with ``stmt`` templates and macros, the ``do`` keyword can be +omitted if the supplied proc doesn't have any parameters and return value. +The compatibility works in the other direction too as the ``do`` syntax can be +used with macros and templates expecting ``stmt`` blocks. + + +Nonoverloadable builtins +------------------------ + +The following builtin procs cannot be overloaded for reasons of implementation +simplicity (they require specialized semantic checking):: + + defined, definedInScope, compiles, low, high, sizeOf, + is, of, echo, shallowCopy, getAst, spawn + +Thus they act more like keywords than like ordinary identifiers; unlike a +keyword however, a redefinition may `shadow`:idx: the definition in +the ``system`` module. + + +Var parameters +-------------- +The type of a parameter may be prefixed with the ``var`` keyword: + +.. code-block:: nim + proc divmod(a, b: int; res, remainder: var int) = + res = a div b + remainder = a mod b + + var + x, y: int + + divmod(8, 5, x, y) # modifies x and y + assert x == 1 + assert y == 3 + +In the example, ``res`` and ``remainder`` are `var parameters`. +Var parameters can be modified by the procedure and the changes are +visible to the caller. The argument passed to a var parameter has to be +an l-value. Var parameters are implemented as hidden pointers. The +above example is equivalent to: + +.. code-block:: nim + proc divmod(a, b: int; res, remainder: ptr int) = + res[] = a div b + remainder[] = a mod b + + var + x, y: int + divmod(8, 5, addr(x), addr(y)) + assert x == 1 + assert y == 3 + +In the examples, var parameters or pointers are used to provide two +return values. This can be done in a cleaner way by returning a tuple: + +.. code-block:: nim + proc divmod(a, b: int): tuple[res, remainder: int] = + (a div b, a mod b) + + var t = divmod(8, 5) + + assert t.res == 1 + assert t.remainder == 3 + +One can use `tuple unpacking`:idx: to access the tuple's fields: + +.. code-block:: nim + var (x, y) = divmod(8, 5) # tuple unpacking + assert x == 1 + assert y == 3 + + +Var return type +--------------- + +A proc, converter or iterator may return a ``var`` type which means that the +returned value is an l-value and can be modified by the caller: + +.. code-block:: nim + var g = 0 + + proc WriteAccessToG(): var int = + result = g + + WriteAccessToG() = 6 + assert g == 6 + +It is a compile time error if the implicitly introduced pointer could be +used to access a location beyond its lifetime: + +.. code-block:: nim + proc WriteAccessToG(): var int = + var g = 0 + result = g # Error! + +For iterators, a component of a tuple return type can have a ``var`` type too: + +.. code-block:: nim + iterator mpairs(a: var seq[string]): tuple[key: int, val: var string] = + for i in 0..a.high: + yield (i, a[i]) + +In the standard library every name of a routine that returns a ``var`` type +starts with the prefix ``m`` per convention. + + +Overloading of the subscript operator +------------------------------------- + +The ``[]`` subscript operator for arrays/openarrays/sequences can be overloaded. + + +Multi-methods +============= + +Procedures always use static dispatch. Multi-methods use dynamic +dispatch. + +.. code-block:: nim + type + TExpr = object ## abstract base class for an expression + TLiteral = object of TExpr + x: int + TPlusExpr = object of TExpr + a, b: ref TExpr + + method eval(e: ref TExpr): int = + # override this base method + quit "to override!" + + method eval(e: ref TLiteral): int = return e.x + + method eval(e: ref TPlusExpr): int = + # watch out: relies on dynamic binding + result = eval(e.a) + eval(e.b) + + proc newLit(x: int): ref TLiteral = + new(result) + result.x = x + + proc newPlus(a, b: ref TExpr): ref TPlusExpr = + new(result) + result.a = a + result.b = b + + echo eval(newPlus(newPlus(newLit(1), newLit(2)), newLit(4))) + +In the example the constructors ``newLit`` and ``newPlus`` are procs +because they should use static binding, but ``eval`` is a method because it +requires dynamic binding. + +In a multi-method all parameters that have an object type are used for the +dispatching: + +.. code-block:: nim + type + TThing = object + TUnit = object of TThing + x: int + + method collide(a, b: TThing) {.inline.} = + quit "to override!" + + method collide(a: TThing, b: TUnit) {.inline.} = + echo "1" + + method collide(a: TUnit, b: TThing) {.inline.} = + echo "2" + + var + a, b: TUnit + collide(a, b) # output: 2 + + +Invocation of a multi-method cannot be ambiguous: collide 2 is preferred over +collide 1 because the resolution works from left to right. +In the example ``TUnit, TThing`` is preferred over ``TThing, TUnit``. + +**Performance note**: Nim does not produce a virtual method table, but +generates dispatch trees. This avoids the expensive indirect branch for method +calls and enables inlining. However, other optimizations like compile time +evaluation or dead code elimination do not work with methods. + + +Iterators and the for statement +=============================== + +The `for`:idx: statement is an abstract mechanism to iterate over the elements +of a container. It relies on an `iterator`:idx: to do so. Like ``while`` +statements, ``for`` statements open an `implicit block`:idx:, so that they +can be left with a ``break`` statement. + +The ``for`` loop declares iteration variables - their scope reaches until the +end of the loop body. The iteration variables' types are inferred by the +return type of the iterator. + +An iterator is similar to a procedure, except that it can be called in the +context of a ``for`` loop. Iterators provide a way to specify the iteration over +an abstract type. A key role in the execution of a ``for`` loop plays the +``yield`` statement in the called iterator. Whenever a ``yield`` statement is +reached the data is bound to the ``for`` loop variables and control continues +in the body of the ``for`` loop. The iterator's local variables and execution +state are automatically saved between calls. Example: + +.. code-block:: nim + # this definition exists in the system module + iterator items*(a: string): char {.inline.} = + var i = 0 + while i < len(a): + yield a[i] + inc(i) + + for ch in items("hello world"): # `ch` is an iteration variable + echo(ch) + +The compiler generates code as if the programmer would have written this: + +.. code-block:: nim + var i = 0 + while i < len(a): + var ch = a[i] + echo(ch) + inc(i) + +If the iterator yields a tuple, there can be as many iteration variables +as there are components in the tuple. The i'th iteration variable's type is +the type of the i'th component. In other words, implicit tuple unpacking in a +for loop context is supported. + +Implict items/pairs invocations +------------------------------- + +If the for loop expression ``e`` does not denote an iterator and the for loop +has exactly 1 variable, the for loop expression is rewritten to ``items(e)``; +ie. an ``items`` iterator is implicitly invoked: + +.. code-block:: nim + for x in [1,2,3]: echo x + +If the for loop has exactly 2 variables, a ``pairs`` iterator is implicitly +invoked. + +Symbol lookup of the identifiers ``items``/``pairs`` is performed after +the rewriting step, so that all overloadings of ``items``/``pairs`` are taken +into account. + + +First class iterators +--------------------- + +There are 2 kinds of iterators in Nim: *inline* and *closure* iterators. +An `inline iterator`:idx: is an iterator that's always inlined by the compiler +leading to zero overhead for the abstraction, but may result in a heavy +increase in code size. Inline iterators are second class citizens; +They can be passed as parameters only to other inlining code facilities like +templates, macros and other inline iterators. + +In contrast to that, a `closure iterator`:idx: can be passed around more freely: + +.. code-block:: nim + iterator count0(): int {.closure.} = + yield 0 + + iterator count2(): int {.closure.} = + var x = 1 + yield x + inc x + yield x + + proc invoke(iter: iterator(): int {.closure.}) = + for x in iter(): echo x + + invoke(count0) + invoke(count2) + +Closure iterators have other restrictions than inline iterators: + +1. ``yield`` in a closure iterator can not occur in a ``try`` statement. +2. For now, a closure iterator cannot be evaluated at compile time. +3. ``return`` is allowed in a closure iterator (but rarely useful). +4. Both inline and closure iterators cannot be recursive. + +Iterators that are neither marked ``{.closure.}`` nor ``{.inline.}`` explicitly +default to being inline, but that this may change in future versions of the +implementation. + +The ``iterator`` type is always of the calling convention ``closure`` +implicitly; the following example shows how to use iterators to implement +a `collaborative tasking`:idx: system: + +.. code-block:: nim + # simple tasking: + type + TTask = iterator (ticker: int) + + iterator a1(ticker: int) {.closure.} = + echo "a1: A" + yield + echo "a1: B" + yield + echo "a1: C" + yield + echo "a1: D" + + iterator a2(ticker: int) {.closure.} = + echo "a2: A" + yield + echo "a2: B" + yield + echo "a2: C" + + proc runTasks(t: varargs[TTask]) = + var ticker = 0 + while true: + let x = t[ticker mod t.len] + if finished(x): break + x(ticker) + inc ticker + + runTasks(a1, a2) + +The builtin ``system.finished`` can be used to determine if an iterator has +finished its operation; no exception is raised on an attempt to invoke an +iterator that has already finished its work. + +Closure iterators are *resumable functions* and so one has to provide the +arguments to every call. To get around this limitation one can capture +parameters of an outer factory proc: + +.. code-block:: nim + proc mycount(a, b: int): iterator (): int = + result = iterator (): int = + var x = a + while x <= b: + yield x + inc x + + let foo = mycount(1, 4) + + for f in foo(): + echo f + +Implicit return type +-------------------- + +Since inline interators must always produce values that will be consumed in +a for loop, the compiler will implicity use the ``auto`` return type if no +type is given by the user. In contrast, since closure iterators can be used +as a collaborative tasking system, ``void`` is a valid return type for them. diff --git a/doc/manual/special_ops.txt b/doc/manual/special_ops.txt new file mode 100644 index 000000000..46135f129 --- /dev/null +++ b/doc/manual/special_ops.txt @@ -0,0 +1,54 @@ +Special Operators +================= + +dot operators +------------- + +Nim offers a special family of dot operators that can be used to +intercept and rewrite proc call and field access attempts, referring +to previously undeclared symbol names. They can be used to provide a +fluent interface to objects lying outside the static confines of the +type system such as values from dynamic scripting languages +or dynamic file formats such as JSON or XML. + +When Nim encounters an expression that cannot be resolved by the +standard overload resolution rules, the current scope will be searched +for a dot operator that can be matched against a re-written form of +the expression, where the unknown field or proc name is converted to +an additional static string parameter: + +.. code-block:: nim + a.b # becomes `.`(a, "b") + a.b(c, d) # becomes `.`(a, "b", c, d) + +The matched dot operators can be symbols of any callable kind (procs, +templates and macros), depending on the desired effect: + +.. code-block:: nim + proc `.` (js: PJsonNode, field: string): JSON = js[field] + + var js = parseJson("{ x: 1, y: 2}") + echo js.x # outputs 1 + echo js.y # outputs 2 + +The following dot operators are available: + +operator `.` +------------ +This operator will be matched against both field accesses and method calls. + +operator `.()` +--------------- +This operator will be matched exclusively against method calls. It has higher +precedence than the `.` operator and this allows one to handle expressions like +`x.y` and `x.y()` differently if one is interfacing with a scripting language +for example. + +operator `.=` +------------- +This operator will be matched against assignments to missing fields. + +.. code-block:: nim + a.b = c # becomes `.=`(a, "b", c) + + diff --git a/doc/manual/stmts.txt b/doc/manual/stmts.txt new file mode 100644 index 000000000..b706101e1 --- /dev/null +++ b/doc/manual/stmts.txt @@ -0,0 +1,647 @@ +Statements and expressions +========================== + +Nim uses the common statement/expression paradigm: Statements do not +produce a value in contrast to expressions. However, some expressions are +statements. + +Statements are separated into `simple statements`:idx: and +`complex statements`:idx:. +Simple statements are statements that cannot contain other statements like +assignments, calls or the ``return`` statement; complex statements can +contain other statements. To avoid the `dangling else problem`:idx:, complex +statements always have to be intended. The details can be found in the grammar. + + +Statement list expression +------------------------- + +Statements can also occur in an expression context that looks +like ``(stmt1; stmt2; ...; ex)``. This is called +an statement list expression or ``(;)``. The type +of ``(stmt1; stmt2; ...; ex)`` is the type of ``ex``. All the other statements +must be of type ``void``. (One can use ``discard`` to produce a ``void`` type.) +``(;)`` does not introduce a new scope. + + +Discard statement +----------------- + +Example: + +.. code-block:: nim + proc p(x, y: int): int = + result = x + y + + discard p(3, 4) # discard the return value of `p` + +The ``discard`` statement evaluates its expression for side-effects and +throws the expression's resulting value away. + +Ignoring the return value of a procedure without using a discard statement is +a static error. + +The return value can be ignored implicitly if the called proc/iterator has +been declared with the `discardable`:idx: pragma: + +.. code-block:: nim + proc p(x, y: int): int {.discardable.} = + result = x + y + + p(3, 4) # now valid + +An empty ``discard`` statement is often used as a null statement: + +.. code-block:: nim + proc classify(s: string) = + case s[0] + of SymChars, '_': echo "an identifier" + of '0'..'9': echo "a number" + else: discard + + +Var statement +------------- + +Var statements declare new local and global variables and +initialize them. A comma separated list of variables can be used to specify +variables of the same type: + +.. code-block:: nim + + var + a: int = 0 + x, y, z: int + +If an initializer is given the type can be omitted: the variable is then of the +same type as the initializing expression. Variables are always initialized +with a default value if there is no initializing expression. The default +value depends on the type and is always a zero in binary. + +============================ ============================================== +Type default value +============================ ============================================== +any integer type 0 +any float 0.0 +char '\\0' +bool false +ref or pointer type nil +procedural type nil +sequence nil (*not* ``@[]``) +string nil (*not* "") +tuple[x: A, y: B, ...] (default(A), default(B), ...) + (analogous for objects) +array[0..., T] [default(T), ...] +range[T] default(T); this may be out of the valid range +T = enum cast[T](0); this may be an invalid value +============================ ============================================== + + +The implicit initialization can be avoided for optimization reasons with the +`noinit`:idx: pragma: + +.. code-block:: nim + var + a {.noInit.}: array [0..1023, char] + +If a proc is annotated with the ``noinit`` pragma this refers to its implicit +``result`` variable: + +.. code-block:: nim + proc returnUndefinedValue: int {.noinit.} = discard + + +The implicit initialization can be also prevented by the `requiresInit`:idx: +type pragma. The compiler requires an explicit initialization then. However +it does a `control flow analysis`:idx: to prove the variable has been +initialized and does not rely on syntactic properties: + +.. code-block:: nim + type + TMyObject = object {.requiresInit.} + + proc p() = + # the following is valid: + var x: TMyObject + if someCondition(): + x = a() + else: + x = a() + use x + +let statement +------------- + +A ``let`` statement declares new local and global `single assignment`:idx: +variables and binds a value to them. The syntax is the of the ``var`` +statement, except that the keyword ``var`` is replaced by the keyword ``let``. +Let variables are not l-values and can thus not be passed to ``var`` parameters +nor can their address be taken. They cannot be assigned new values. + +For let variables the same pragmas are available as for ordinary variables. + + +Const section +------------- + +`Constants`:idx: are symbols which are bound to a value. The constant's value +cannot change. The compiler must be able to evaluate the expression in a +constant declaration at compile time. + +Nim contains a sophisticated compile-time evaluator, so procedures which +have no side-effect can be used in constant expressions too: + +.. code-block:: nim + import strutils + const + constEval = contains("abc", 'b') # computed at compile time! + + +The rules for compile-time computability are: + +1. Literals are compile-time computable. +2. Type conversions are compile-time computable. +3. Procedure calls of the form ``p(X)`` are compile-time computable if + ``p`` is a proc without side-effects (see the `noSideEffect pragma`_ + for details) and if ``X`` is a (possibly empty) list of compile-time + computable arguments. + + +Constants cannot be of type ``ptr``, ``ref``, ``var`` or ``object``, nor can +they contain such a type. + + +Static statement/expression +--------------------------- + +A static statement/expression can be used to enforce compile +time evaluation explicitly. Enforced compile time evaluation can even evaluate +code that has side effects: + +.. code-block:: + + static: + echo "echo at compile time" + +It's a static error if the compiler cannot perform the evaluation at compile +time. + +The current implementation poses some restrictions for compile time +evaluation: Code which contains ``cast`` or makes use of the foreign function +interface cannot be evaluated at compile time. Later versions of Nim will +support the FFI at compile time. + + +If statement +------------ + +Example: + +.. code-block:: nim + + var name = readLine(stdin) + + if name == "Andreas": + echo("What a nice name!") + elif name == "": + echo("Don't you have a name?") + else: + echo("Boring name...") + +The ``if`` statement is a simple way to make a branch in the control flow: +The expression after the keyword ``if`` is evaluated, if it is true +the corresponding statements after the ``:`` are executed. Otherwise +the expression after the ``elif`` is evaluated (if there is an +``elif`` branch), if it is true the corresponding statements after +the ``:`` are executed. This goes on until the last ``elif``. If all +conditions fail, the ``else`` part is executed. If there is no ``else`` +part, execution continues with the statement after the ``if`` statement. + +The scoping for an ``if`` statement is slightly subtle to support an important +use case. A new scope starts for the ``if``/``elif`` condition and ends after +the corresponding *then* block: + +.. code-block:: nim + if {| (let m = input =~ re"(\w+)=\w+"; m.isMatch): + echo "key ", m[0], " value ", m[1] |} + elif {| (let m = input =~ re""; m.isMatch): + echo "new m in this scope" |} + else: + # 'm' not declared here + +In the example the scopes have been enclosed in ``{| |}``. + + +Case statement +-------------- + +Example: + +.. code-block:: nim + + case readline(stdin) + of "delete-everything", "restart-computer": + echo("permission denied") + of "go-for-a-walk": echo("please yourself") + else: echo("unknown command") + + # indentation of the branches is also allowed; and so is an optional colon + # after the selecting expression: + case readline(stdin): + of "delete-everything", "restart-computer": + echo("permission denied") + of "go-for-a-walk": echo("please yourself") + else: echo("unknown command") + + +The ``case`` statement is similar to the if statement, but it represents +a multi-branch selection. The expression after the keyword ``case`` is +evaluated and if its value is in a *slicelist* the corresponding statements +(after the ``of`` keyword) are executed. If the value is not in any +given *slicelist* the ``else`` part is executed. If there is no ``else`` +part and not all possible values that ``expr`` can hold occur in a +``slicelist``, a static error occurs. This holds only for expressions of +ordinal types. "All possible values" of ``expr`` are determined by ``expr``'s +type. + +If the expression is not of an ordinal type, and no ``else`` part is +given, control passes after the ``case`` statement. + +To suppress the static error in the ordinal case an ``else`` part with an +empty ``discard`` statement can be used. + +As a special semantic extension, an expression in an ``of`` branch of a case +statement may evaluate to a set or array constructor; the set or array is then +expanded into a list of its elements: + +.. code-block:: nim + const + SymChars: set[char] = {'a'..'z', 'A'..'Z', '\x80'..'\xFF'} + + proc classify(s: string) = + case s[0] + of SymChars, '_': echo "an identifier" + of '0'..'9': echo "a number" + else: echo "other" + + # is equivalent to: + proc classify(s: string) = + case s[0] + of 'a'..'z', 'A'..'Z', '\x80'..'\xFF', '_': echo "an identifier" + of '0'..'9': echo "a number" + else: echo "other" + + +When statement +-------------- + +Example: + +.. code-block:: nim + + when sizeof(int) == 2: + echo("running on a 16 bit system!") + elif sizeof(int) == 4: + echo("running on a 32 bit system!") + elif sizeof(int) == 8: + echo("running on a 64 bit system!") + else: + echo("cannot happen!") + +The ``when`` statement is almost identical to the ``if`` statement with some +exceptions: + +* Each condition (``expr``) has to be a constant expression (of type ``bool``). +* The statements do not open a new scope. +* The statements that belong to the expression that evaluated to true are + translated by the compiler, the other statements are not checked for + semantics! However, each condition is checked for semantics. + +The ``when`` statement enables conditional compilation techniques. As +a special syntactic extension, the ``when`` construct is also available +within ``object`` definitions. + + +Return statement +---------------- + +Example: + +.. code-block:: nim + return 40+2 + +The ``return`` statement ends the execution of the current procedure. +It is only allowed in procedures. If there is an ``expr``, this is syntactic +sugar for: + +.. code-block:: nim + result = expr + return result + + +``return`` without an expression is a short notation for ``return result`` if +the proc has a return type. The `result`:idx: variable is always the return +value of the procedure. It is automatically declared by the compiler. As all +variables, ``result`` is initialized to (binary) zero: + +.. code-block:: nim + proc returnZero(): int = + # implicitly returns 0 + + +Yield statement +--------------- + +Example: + +.. code-block:: nim + yield (1, 2, 3) + +The ``yield`` statement is used instead of the ``return`` statement in +iterators. It is only valid in iterators. Execution is returned to the body +of the for loop that called the iterator. Yield does not end the iteration +process, but execution is passed back to the iterator if the next iteration +starts. See the section about iterators (`Iterators and the for statement`_) +for further information. + + +Block statement +--------------- + +Example: + +.. code-block:: nim + var found = false + block myblock: + for i in 0..3: + for j in 0..3: + if a[j][i] == 7: + found = true + break myblock # leave the block, in this case both for-loops + echo(found) + +The block statement is a means to group statements to a (named) ``block``. +Inside the block, the ``break`` statement is allowed to leave the block +immediately. A ``break`` statement can contain a name of a surrounding +block to specify which block is to leave. + + +Break statement +--------------- + +Example: + +.. code-block:: nim + break + +The ``break`` statement is used to leave a block immediately. If ``symbol`` +is given, it is the name of the enclosing block that is to leave. If it is +absent, the innermost block is left. + + +While statement +--------------- + +Example: + +.. code-block:: nim + echo("Please tell me your password: \n") + var pw = readLine(stdin) + while pw != "12345": + echo("Wrong password! Next try: \n") + pw = readLine(stdin) + + +The ``while`` statement is executed until the ``expr`` evaluates to false. +Endless loops are no error. ``while`` statements open an `implicit block`, +so that they can be left with a ``break`` statement. + + +Continue statement +------------------ + +A ``continue`` statement leads to the immediate next iteration of the +surrounding loop construct. It is only allowed within a loop. A continue +statement is syntactic sugar for a nested block: + +.. code-block:: nim + while expr1: + stmt1 + continue + stmt2 + +Is equivalent to: + +.. code-block:: nim + while expr1: + block myBlockName: + stmt1 + break myBlockName + stmt2 + + +Assembler statement +------------------- + +The direct embedding of assembler code into Nim code is supported +by the unsafe ``asm`` statement. Identifiers in the assembler code that refer to +Nim identifiers shall be enclosed in a special character which can be +specified in the statement's pragmas. The default special character is ``'`'``: + +.. code-block:: nim + {.push stackTrace:off.} + proc addInt(a, b: int): int = + # a in eax, and b in edx + asm """ + mov eax, `a` + add eax, `b` + jno theEnd + call `raiseOverflow` + theEnd: + """ + {.pop.} + +If the GNU assembler is used, quotes and newlines are inserted automatically: + +.. code-block:: nim + proc addInt(a, b: int): int = + asm """ + addl %%ecx, %%eax + jno 1 + call `raiseOverflow` + 1: + :"=a"(`result`) + :"a"(`a`), "c"(`b`) + """ + +Instead of: + +.. code-block:: nim + proc addInt(a, b: int): int = + asm """ + "addl %%ecx, %%eax\n" + "jno 1\n" + "call `raiseOverflow`\n" + "1: \n" + :"=a"(`result`) + :"a"(`a`), "c"(`b`) + """ + +Using statement +--------------- + +**Warning**: The ``using`` statement is highly experimental! + +The using statement provides syntactic convenience for procs that +heavily use a single contextual parameter. When applied to a variable or a +constant, it will instruct Nim to automatically consider the used symbol as +a hidden leading parameter for any procedure calls, following the using +statement in the current scope. Thus, it behaves much like the hidden `this` +parameter available in some object-oriented programming languages. + +.. code-block:: nim + + var s = socket() + using s + + connect(host, port) + send(data) + + while true: + let line = readLine(timeout) + ... + + +When applied to a callable symbol, it brings the designated symbol in the +current scope. Thus, it can be used to disambiguate between imported symbols +from different modules having the same name. + +.. code-block:: nim + import windows, sdl + using sdl.SetTimer + +Note that ``using`` only *adds* to the current context, it doesn't remove or +replace, **neither** does it create a new scope. What this means is that if one +applies this to multiple variables the compiler will find conflicts in what +variable to use: + +.. code-block:: nim + var a, b = "kill it" + using a + add(" with fire") + using b + add(" with water") + echo a + echo b + +When the compiler reaches the second ``add`` call, both ``a`` and ``b`` could +be used with the proc, so one gets ``Error: expression '(a|b)' has no type (or +is ambiguous)``. To solve this one would need to nest ``using`` with a +``block`` statement so as to control the reach of the ``using`` statement. + +If expression +------------- + +An `if expression` is almost like an if statement, but it is an expression. +Example: + +.. code-block:: nim + var y = if x > 8: 9 else: 10 + +An if expression always results in a value, so the ``else`` part is +required. ``Elif`` parts are also allowed. + +When expression +--------------- + +Just like an `if expression`, but corresponding to the when statement. + +Case expression +--------------- + +The `case expression` is again very similar to the case statement: + +.. code-block:: nim + var favoriteFood = case animal + of "dog": "bones" + of "cat": "mice" + elif animal.endsWith"whale": "plankton" + else: + echo "I'm not sure what to serve, but everybody loves ice cream" + "ice cream" + +As seen in the above example, the case expression can also introduce side +effects. When multiple statements are given for a branch, Nim will use +the last expression as the result value, much like in an `expr` template. + +Table constructor +----------------- + +A table constructor is syntactic sugar for an array constructor: + +.. code-block:: nim + {"key1": "value1", "key2", "key3": "value2"} + + # is the same as: + [("key1", "value1"), ("key2", "value2"), ("key3", "value2")] + + +The empty table can be written ``{:}`` (in contrast to the empty set +which is ``{}``) which is thus another way to write as the empty array +constructor ``[]``. This slightly unusal way of supporting tables +has lots of advantages: + +* The order of the (key,value)-pairs is preserved, thus it is easy to + support ordered dicts with for example ``{key: val}.newOrderedTable``. +* A table literal can be put into a ``const`` section and the compiler + can easily put it into the executable's data section just like it can + for arrays and the generated data section requires a minimal amount + of memory. +* Every table implementation is treated equal syntactically. +* Apart from the minimal syntactic sugar the language core does not need to + know about tables. + + +Type conversions +---------------- +Syntactically a `type conversion` is like a procedure call, but a +type name replaces the procedure name. A type conversion is always +safe in the sense that a failure to convert a type to another +results in an exception (if it cannot be determined statically). + + +Type casts +---------- +Example: + +.. code-block:: nim + cast[int](x) + +Type casts are a crude mechanism to interpret the bit pattern of +an expression as if it would be of another type. Type casts are +only needed for low-level programming and are inherently unsafe. + + +The addr operator +----------------- +The ``addr`` operator returns the address of an l-value. If the type of the +location is ``T``, the `addr` operator result is of the type ``ptr T``. An +address is always an untraced reference. Taking the address of an object that +resides on the stack is **unsafe**, as the pointer may live longer than the +object on the stack and can thus reference a non-existing object. One can get +the address of variables, but one can't use it on variables declared through +``let`` statements: + +.. code-block:: nim + + let t1 = "Hello" + var + t2 = t1 + t3 : pointer = addr(t2) + echo repr(addr(t2)) + # --> ref 0x7fff6b71b670 --> 0x10bb81050"Hello" + echo cast[ptr string](t3)[] + # --> Hello + # The following line doesn't compile: + echo repr(addr(t1)) + # Error: expression has no address diff --git a/doc/manual/syntax.txt b/doc/manual/syntax.txt new file mode 100644 index 000000000..0ae353edd --- /dev/null +++ b/doc/manual/syntax.txt @@ -0,0 +1,112 @@ +Syntax +====== + +This section lists Nim's standard syntax. How the parser handles +the indentation is already described in the `Lexical Analysis`_ section. + +Nim allows user-definable operators. +Binary operators have 10 different levels of precedence. + +Relevant character +------------------ + +An operator symbol's *relevant character* is its first +character unless the first character is ``\`` and its length is greater than 1 +then it is the second character. + +This rule allows to escape operator symbols with ``\`` and keeps the operator's +precedence and associativity; this is useful for meta programming. + + +Associativity +------------- + +Binary operators whose relevant character is ``^`` are right-associative, all +other binary operators are left-associative. + +Precedence +---------- + +Unary operators always bind stronger than any binary +operator: ``$a + b`` is ``($a) + b`` and not ``$(a + b)``. + +If an unary operator's relevant character is ``@`` it is a `sigil-like`:idx: +operator which binds stronger than a ``primarySuffix``: ``@x.abc`` is parsed +as ``(@x).abc`` whereas ``$x.abc`` is parsed as ``$(x.abc)``. + + +For binary operators that are not keywords the precedence is determined by the +following rules: + +If the operator ends with ``=`` and its relevant character is none of +``<``, ``>``, ``!``, ``=``, ``~``, ``?``, it is an *assignment operator* which +has the lowest precedence. + +Otherwise precedence is determined by the relevant character. + +================ =============================================== ================== =============== +Precedence level Operators Relevant character Terminal symbol +================ =============================================== ================== =============== + 9 (highest) ``$ ^`` OP9 + 8 ``* / div mod shl shr %`` ``* % \ /`` OP8 + 7 ``+ -`` ``+ ~ |`` OP7 + 6 ``&`` ``&`` OP6 + 5 ``..`` ``.`` OP5 + 4 ``== <= < >= > != in notin is isnot not of`` ``= < > !`` OP4 + 3 ``and`` OP3 + 2 ``or xor`` OP2 + 1 ``@ : ?`` OP1 + 0 (lowest) *assignment operator* (like ``+=``, ``*=``) OP0 +================ =============================================== ================== =============== + + +Strong spaces +------------- + +The number of spaces preceeding a non-keyword operator affects precedence +if the experimental parser directive ``#!strongSpaces`` is used. Indentation +is not used to determine the number of spaces. If 2 or more operators have the +same number of preceding spaces the precedence table applies, so ``1 + 3 * 4`` +is still parsed as ``1 + (3 * 4)``, but ``1+3 * 4`` is parsed as ``(1+3) * 4``: + +.. code-block:: nim + #! strongSpaces + if foo+4 * 4 == 8 and b&c | 9 ++ + bar: + echo "" + # is parsed as + if ((foo+4)*4 == 8) and (((b&c) | 9) ++ bar): echo "" + + +Furthermore whether an operator is used a prefix operator is affected by the +number of spaces: + +.. code-block:: nim + #! strongSpaces + echo $foo + # is parsed as + echo($foo) + +This also affects whether ``[]``, ``{}``, ``()`` are parsed as constructors +or as accessors: + +.. code-block:: nim + #! strongSpaces + echo (1,2) + # is parsed as + echo((1,2)) + +Only 0, 1, 2, 4 or 8 spaces are allowed to specify precedence and it is +enforced that infix operators have the same amount of spaces before and after +them. This rules does not apply when a newline follows after the operator, +then only the preceding spaces are considered. + + +Grammar +------- + +The grammar's start symbol is ``module``. + +.. include:: grammar.txt + :literal: + diff --git a/doc/manual/taint.txt b/doc/manual/taint.txt new file mode 100644 index 000000000..84f0c68b1 --- /dev/null +++ b/doc/manual/taint.txt @@ -0,0 +1,20 @@ +Taint mode +========== + +The Nim compiler and most parts of the standard library support +a taint mode. Input strings are declared with the `TaintedString`:idx: +string type declared in the ``system`` module. + +If the taint mode is turned on (via the ``--taintMode:on`` command line +option) it is a distinct string type which helps to detect input +validation errors: + +.. code-block:: nim + echo "your name: " + var name: TaintedString = stdin.readline + # it is safe here to output the name without any input validation, so + # we simply convert `name` to string to make the compiler happy: + echo "hi, ", name.string + +If the taint mode is turned off, ``TaintedString`` is simply an alias for +``string``. diff --git a/doc/manual/templates.txt b/doc/manual/templates.txt new file mode 100644 index 000000000..d63f61f54 --- /dev/null +++ b/doc/manual/templates.txt @@ -0,0 +1,404 @@ +Templates +========= + +A template is a simple form of a macro: It is a simple substitution +mechanism that operates on Nim's abstract syntax trees. It is processed in +the semantic pass of the compiler. + +The syntax to *invoke* a template is the same as calling a procedure. + +Example: + +.. code-block:: nim + template `!=` (a, b: expr): expr = + # this definition exists in the System module + not (a == b) + + assert(5 != 6) # the compiler rewrites that to: assert(not (5 == 6)) + +The ``!=``, ``>``, ``>=``, ``in``, ``notin``, ``isnot`` operators are in fact +templates: + +| ``a > b`` is transformed into ``b < a``. +| ``a in b`` is transformed into ``contains(b, a)``. +| ``notin`` and ``isnot`` have the obvious meanings. + +The "types" of templates can be the symbols ``expr`` (stands for *expression*), +``stmt`` (stands for *statement*) or ``typedesc`` (stands for *type +description*). These are "meta types", they can only be used in certain +contexts. Real types can be used too; this implies that expressions are +expected. + + +Ordinary vs immediate templates +------------------------------- + +There are two different kinds of templates: immediate templates and +ordinary templates. Ordinary templates take part in overloading resolution. As +such their arguments need to be type checked before the template is invoked. +So ordinary templates cannot receive undeclared identifiers: + +.. code-block:: nim + + template declareInt(x: expr) = + var x: int + + declareInt(x) # error: unknown identifier: 'x' + +An ``immediate`` template does not participate in overload resolution and so +its arguments are not checked for semantics before invocation. So they can +receive undeclared identifiers: + +.. code-block:: nim + + template declareInt(x: expr) {.immediate.} = + var x: int + + declareInt(x) # valid + + +Passing a code block to a template +---------------------------------- + +If there is a ``stmt`` parameter it should be the last in the template +declaration, because statements are passed to a template via a +special ``:`` syntax: + +.. code-block:: nim + + template withFile(f, fn, mode: expr, actions: stmt): stmt {.immediate.} = + var f: TFile + if open(f, fn, mode): + try: + actions + finally: + close(f) + else: + quit("cannot open: " & fn) + + withFile(txt, "ttempl3.txt", fmWrite): + txt.writeln("line 1") + txt.writeln("line 2") + +In the example the two ``writeln`` statements are bound to the ``actions`` +parameter. + + +Symbol binding in templates +--------------------------- + +A template is a `hygienic`:idx: macro and so opens a new scope. Most symbols are +bound from the definition scope of the template: + +.. code-block:: nim + # Module A + var + lastId = 0 + + template genId*: expr = + inc(lastId) + lastId + +.. code-block:: nim + # Module B + import A + + echo genId() # Works as 'lastId' has been bound in 'genId's defining scope + +As in generics symbol binding can be influenced via ``mixin`` or ``bind`` +statements. + + + +Identifier construction +----------------------- + +In templates identifiers can be constructed with the backticks notation: + +.. code-block:: nim + + template typedef(name: expr, typ: typedesc) {.immediate.} = + type + `T name`* {.inject.} = typ + `P name`* {.inject.} = ref `T name` + + typedef(myint, int) + var x: PMyInt + +In the example ``name`` is instantiated with ``myint``, so \`T name\` becomes +``Tmyint``. + + +Lookup rules for template parameters +------------------------------------ + +A parameter ``p`` in a template is even substituted in the expression ``x.p``. +Thus template arguments can be used as field names and a global symbol can be +shadowed by the same argument name even when fully qualified: + +.. code-block:: nim + # module 'm' + + type + TLev = enum + levA, levB + + var abclev = levB + + template tstLev(abclev: TLev) = + echo abclev, " ", m.abclev + + tstLev(levA) + # produces: 'levA levA' + +But the global symbol can properly be captured by a ``bind`` statement: + +.. code-block:: nim + # module 'm' + + type + TLev = enum + levA, levB + + var abclev = levB + + template tstLev(abclev: TLev) = + bind m.abclev + echo abclev, " ", m.abclev + + tstLev(levA) + # produces: 'levA levB' + + +Hygiene in templates +-------------------- + +Per default templates are `hygienic`:idx:\: Local identifiers declared in a +template cannot be accessed in the instantiation context: + +.. code-block:: nim + + template newException*(exceptn: typedesc, message: string): expr = + var + e: ref exceptn # e is implicitly gensym'ed here + new(e) + e.msg = message + e + + # so this works: + let e = "message" + raise newException(EIO, e) + + +Whether a symbol that is declared in a template is exposed to the instantiation +scope is controlled by the `inject`:idx: and `gensym`:idx: pragmas: gensym'ed +symbols are not exposed but inject'ed are. + +The default for symbols of entity ``type``, ``var``, ``let`` and ``const`` +is ``gensym`` and for ``proc``, ``iterator``, ``converter``, ``template``, +``macro`` is ``inject``. However, if the name of the entity is passed as a +template parameter, it is an inject'ed symbol: + +.. code-block:: nim + template withFile(f, fn, mode: expr, actions: stmt): stmt {.immediate.} = + block: + var f: TFile # since 'f' is a template param, it's injected implicitly + ... + + withFile(txt, "ttempl3.txt", fmWrite): + txt.writeln("line 1") + txt.writeln("line 2") + + +The ``inject`` and ``gensym`` pragmas are second class annotations; they have +no semantics outside of a template definition and cannot be abstracted over: + +.. code-block:: nim + {.pragma myInject: inject.} + + template t() = + var x {.myInject.}: int # does NOT work + + +To get rid of hygiene in templates, one can use the `dirty`:idx: pragma for +a template. ``inject`` and ``gensym`` have no effect in ``dirty`` templates. + + + +Macros +====== + +A macro is a special kind of low level template. Macros can be used +to implement `domain specific languages`:idx:. Like templates, macros come in +the 2 flavors *immediate* and *ordinary*. + +While macros enable advanced compile-time code transformations, they +cannot change Nim's syntax. However, this is no real restriction because +Nim's syntax is flexible enough anyway. + +To write macros, one needs to know how the Nim concrete syntax is converted +to an abstract syntax tree. + +There are two ways to invoke a macro: +(1) invoking a macro like a procedure call (`expression macros`) +(2) invoking a macro with the special ``macrostmt`` syntax (`statement macros`) + + +Expression Macros +----------------- + +The following example implements a powerful ``debug`` command that accepts a +variable number of arguments: + +.. code-block:: nim + # to work with Nim syntax trees, we need an API that is defined in the + # ``macros`` module: + import macros + + macro debug(n: varargs[expr]): stmt = + # `n` is a Nim AST that contains the whole macro invocation + # this macro returns a list of statements: + result = newNimNode(nnkStmtList, n) + # iterate over any argument that is passed to this macro: + for i in 0..n.len-1: + # add a call to the statement list that writes the expression; + # `toStrLit` converts an AST to its string representation: + add(result, newCall("write", newIdentNode("stdout"), toStrLit(n[i]))) + # add a call to the statement list that writes ": " + add(result, newCall("write", newIdentNode("stdout"), newStrLitNode(": "))) + # add a call to the statement list that writes the expressions value: + add(result, newCall("writeln", newIdentNode("stdout"), n[i])) + + var + a: array [0..10, int] + x = "some string" + a[0] = 42 + a[1] = 45 + + debug(a[0], a[1], x) + +The macro call expands to: + +.. code-block:: nim + write(stdout, "a[0]") + write(stdout, ": ") + writeln(stdout, a[0]) + + write(stdout, "a[1]") + write(stdout, ": ") + writeln(stdout, a[1]) + + write(stdout, "x") + write(stdout, ": ") + writeln(stdout, x) + + +Arguments that are passed to a ``varargs`` parameter are wrapped in an array +constructor expression. This is why ``debug`` iterates over all of ``n``'s +children. + + +BindSym +------- + +The above ``debug`` macro relies on the fact that ``write``, ``writeln`` and +``stdout`` are declared in the system module and thus visible in the +instantiating context. There is a way to use bound identifiers +(aka `symbols`:idx:) instead of using unbound identifiers. The ``bindSym`` +builtin can be used for that: + +.. code-block:: nim + import macros + + macro debug(n: varargs[expr]): stmt = + result = newNimNode(nnkStmtList, n) + for i in 0..n.len-1: + # we can bind symbols in scope via 'bindSym': + add(result, newCall(bindSym"write", bindSym"stdout", toStrLit(n[i]))) + add(result, newCall(bindSym"write", bindSym"stdout", newStrLitNode(": "))) + add(result, newCall(bindSym"writeln", bindSym"stdout", n[i])) + + var + a: array [0..10, int] + x = "some string" + a[0] = 42 + a[1] = 45 + + debug(a[0], a[1], x) + +The macro call expands to: + +.. code-block:: nim + write(stdout, "a[0]") + write(stdout, ": ") + writeln(stdout, a[0]) + + write(stdout, "a[1]") + write(stdout, ": ") + writeln(stdout, a[1]) + + write(stdout, "x") + write(stdout, ": ") + writeln(stdout, x) + +However, the symbols ``write``, ``writeln`` and ``stdout`` are already bound +and are not looked up again. As the example shows, ``bindSym`` does work with +overloaded symbols implicitly. + + +Statement Macros +---------------- + +Statement macros are defined just as expression macros. However, they are +invoked by an expression following a colon. + +The following example outlines a macro that generates a lexical analyzer from +regular expressions: + +.. code-block:: nim + import macros + + macro case_token(n: stmt): stmt = + # creates a lexical analyzer from regular expressions + # ... (implementation is an exercise for the reader :-) + discard + + case_token: # this colon tells the parser it is a macro statement + of r"[A-Za-z_]+[A-Za-z_0-9]*": + return tkIdentifier + of r"0-9+": + return tkInteger + of r"[\+\-\*\?]+": + return tkOperator + else: + return tkUnknown + + +**Style note**: For code readability, it is the best idea to use the least +powerful programming construct that still suffices. So the "check list" is: + +(1) Use an ordinary proc/iterator, if possible. +(2) Else: Use a generic proc/iterator, if possible. +(3) Else: Use a template, if possible. +(4) Else: Use a macro. + + +Macros as pragmas +----------------- + +Whole routines (procs, iterators etc.) can also be passed to a template or +a macro via the pragma notation: + +.. code-block:: nim + template m(s: stmt) = discard + + proc p() {.m.} = discard + +This is a simple syntactic transformation into: + +.. code-block:: nim + template m(s: stmt) = discard + + m: + proc p() = discard + diff --git a/doc/manual/threads.txt b/doc/manual/threads.txt new file mode 100644 index 000000000..d29cab6ca --- /dev/null +++ b/doc/manual/threads.txt @@ -0,0 +1,209 @@ +Threads +======= + +To enable thread support the ``--threads:on`` command line switch needs to +be used. The ``system`` module then contains several threading primitives. +See the `threads <threads.html>`_ and `channels <channels.html>`_ modules +for the low level thread API. There are also high level parallelism constructs +available. See `spawn`_ for further details. + +Nim's memory model for threads is quite different than that of other common +programming languages (C, Pascal, Java): Each thread has its own (garbage +collected) heap and sharing of memory is restricted to global variables. This +helps to prevent race conditions. GC efficiency is improved quite a lot, +because the GC never has to stop other threads and see what they reference. +Memory allocation requires no lock at all! This design easily scales to massive +multicore processors that are becoming the norm. + + +Thread pragma +------------- + +A proc that is executed as a new thread of execution should be marked by the +``thread`` pragma for reasons of readability. The compiler checks for +violations of the `no heap sharing restriction`:idx:\: This restriction implies +that it is invalid to construct a data structure that consists of memory +allocated from different (thread local) heaps. + +A thread proc is passed to ``createThread`` or ``spawn`` and invoked +indirectly; so the ``thread`` pragma implies ``procvar``. + + +GC safety +--------- + +We call a proc ``p`` `GC safe`:idx: when it doesn't access any global variable +that contains GC'ed memory (``string``, ``seq``, ``ref`` or a closure) either +directly or indirectly through a call to a GC unsafe proc. + +The `gcsafe`:idx: annotation can be used to mark a proc to be gcsafe, +otherwise this property is inferred by the compiler. Note that ``noSideEfect`` +implies ``gcsafe``. The only way to create a thread is via ``spawn`` or +``createThead``. ``spawn`` is usually the preferable method. Either way +the invoked proc must not use ``var`` parameters nor must any of its parameters +contain a ``ref`` or ``closure`` type. This enforces +the *no heap sharing restriction*. + +Routines that are imported from C are always assumed to be ``gcsafe``. +To enable the GC-safety checking the ``--threadAnalysis:on`` command line +switch must be used. This is a temporary workaround to ease the porting effort +from old code to the new threading model. In the future the thread analysis +will always be performed. + + +Future directions: + +- A shared GC'ed heap might be provided. + + +Threadvar pragma +---------------- + +A global variable can be marked with the ``threadvar`` pragma; it is +a `thread-local`:idx: variable then: + +.. code-block:: nim + var checkpoints* {.threadvar.}: seq[string] + +Due to implementation restrictions thread local variables cannot be +initialized within the ``var`` section. (Every thread local variable needs to +be replicated at thread creation.) + + +Threads and exceptions +---------------------- + +The interaction between threads and exceptions is simple: A *handled* exception +in one thread cannot affect any other thread. However, an *unhandled* +exception in one thread terminates the whole *process*! + + + +Parallel & Spawn +================ + +Nim has two flavors of parallelism: +1) `Structured`:idx: parallelism via the ``parallel`` statement. +2) `Unstructured`:idx: parallelism via the standalone ``spawn`` statement. + +Nim has a builtin thread pool that can be used for CPU intensive tasks. For +IO intensive tasks the ``async`` and ``await`` features should be +used instead. Both parallel and spawn need the `threadpool <threadpool.html>`_ +module to work. + +Somewhat confusingly, ``spawn`` is also used in the ``parallel`` statement +with slightly different semantics. ``spawn`` always takes a call expression of +the form ``f(a, ...)``. Let ``T`` be ``f``'s return type. If ``T`` is ``void`` +then ``spawn``'s return type is also ``void``. Within a ``parallel`` section +``spawn``'s return type is ``T``, otherwise it is ``FlowVar[T]``. + +The compiler can ensure the location in ``location = spawn f(...)`` is not +read prematurely within a ``parallel`` section and so there is no need for +the overhead of an indirection via ``FlowVar[T]`` to ensure correctness. + +**Note**: Currently exceptions are not propagated between ``spawn``ed tasks! + + +Spawn statement +--------------- + +`spawn`:idx: can be used to pass a task to the thread pool: + +.. code-block:: nim + import threadpool + + proc processLine(line: string) = + discard "do some heavy lifting here" + + for x in lines("myinput.txt"): + spawn processLine(x) + sync() + +For reasons of type safety and implementation simplicity the expression +that ``spawn`` takes is restricted: + +* It must be a call expression ``f(a, ...)``. +* ``f`` must be ``gcsafe``. +* ``f`` must not have the calling convention ``closure``. +* ``f``'s parameters may not be of type ``var``. + This means one has to use raw ``ptr``'s for data passing reminding the + programmer to be careful. +* ``ref`` parameters are deeply copied which is a subtle semantic change and + can cause performance problems but ensures memory safety. This deep copy + is performed via ``system.deepCopy`` and so can be overriden. +* For *safe* data exchange between ``f`` and the caller a global ``TChannel`` + needs to be used. However, since spawn can return a result, often no further + communication is required. + + +``spawn`` executes the passed expression on the thread pool and returns +a `data flow variable`:idx: ``FlowVar[T]`` that can be read from. The reading +with the ``^`` operator is **blocking**. However, one can use ``awaitAny`` to +wait on multiple flow variables at the same time: + +.. code-block:: nim + import threadpool, ... + + # wait until 2 out of 3 servers received the update: + proc main = + var responses = newSeq[RawFlowVar](3) + for i in 0..2: + responses[i] = spawn tellServer(Update, "key", "value") + var index = awaitAny(responses) + assert index >= 0 + responses.del(index) + discard awaitAny(responses) + +Data flow variables ensure that no data races +are possible. Due to technical limitations not every type ``T`` is possible in +a data flow variable: ``T`` has to be of the type ``ref``, ``string``, ``seq`` +or of a type that doesn't contain a type that is garbage collected. This +restriction will be removed in the future. + + + +Parallel statement +------------------ + +Example: + +.. code-block:: nim + # Compute PI in an inefficient way + import strutils, math, threadpool + + proc term(k: float): float = 4 * math.pow(-1, k) / (2*k + 1) + + proc pi(n: int): float = + var ch = newSeq[float](n+1) + parallel: + for k in 0..ch.high: + ch[k] = spawn term(float(k)) + for k in 0..ch.high: + result += ch[k] + + echo formatFloat(pi(5000)) + + +The parallel statement is the preferred mechanism to introduce parallelism +in a Nim program. A subset of the Nim language is valid within a +``parallel`` section. This subset is checked to be free of data races at +compile time. A sophisticated `disjoint checker`:idx: ensures that no data +races are possible even though shared memory is extensively supported! + +The subset is in fact the full language with the following +restrictions / changes: + +* ``spawn`` within a ``parallel`` section has special semantics. +* Every location of the form ``a[i]`` and ``a[i..j]`` and ``dest`` where + ``dest`` is part of the pattern ``dest = spawn f(...)`` has to be + provably disjoint. This is called the *disjoint check*. +* Every other complex location ``loc`` that is used in a spawned + proc (``spawn f(loc)``) has to be immutable for the duration of + the ``parallel`` section. This is called the *immutability check*. Currently + it is not specified what exactly "complex location" means. We need to make + this an optimization! +* Every array access has to be provably within bounds. This is called + the *bounds check*. +* Slices are optimized so that no copy is performed. This optimization is not + yet performed for ordinary slices outside of a ``parallel`` section. Slices + are also special in that they currently do not support negative indexes! diff --git a/doc/manual/trmacros.txt b/doc/manual/trmacros.txt new file mode 100644 index 000000000..715c9f850 --- /dev/null +++ b/doc/manual/trmacros.txt @@ -0,0 +1,361 @@ +Term rewriting macros +===================== + +Term rewriting macros are macros or templates that have not only +a *name* but also a *pattern* that is searched for after the semantic checking +phase of the compiler: This means they provide an easy way to enhance the +compilation pipeline with user defined optimizations: + +.. code-block:: nim + template optMul{`*`(a, 2)}(a: int): int = a+a + + let x = 3 + echo x * 2 + +The compiler now rewrites ``x * 2`` as ``x + x``. The code inside the +curlies is the pattern to match against. The operators ``*``, ``**``, +``|``, ``~`` have a special meaning in patterns if they are written in infix +notation, so to match verbatim against ``*`` the ordinary function call syntax +needs to be used. + + +Unfortunately optimizations are hard to get right and even the tiny example +is **wrong**: + +.. code-block:: nim + template optMul{`*`(a, 2)}(a: int): int = a+a + + proc f(): int = + echo "side effect!" + result = 55 + + echo f() * 2 + +We cannot duplicate 'a' if it denotes an expression that has a side effect! +Fortunately Nim supports side effect analysis: + +.. code-block:: nim + template optMul{`*`(a, 2)}(a: int{noSideEffect}): int = a+a + + proc f(): int = + echo "side effect!" + result = 55 + + echo f() * 2 # not optimized ;-) + +So what about ``2 * a``? We should tell the compiler ``*`` is commutative. We +cannot really do that however as the following code only swaps arguments +blindly: + +.. code-block:: nim + template mulIsCommutative{`*`(a, b)}(a, b: int): int = b*a + +What optimizers really need to do is a *canonicalization*: + +.. code-block:: nim + template canonMul{`*`(a, b)}(a: int{lit}, b: int): int = b*a + +The ``int{lit}`` parameter pattern matches against an expression of +type ``int``, but only if it's a literal. + + + +Parameter constraints +--------------------- + +The `parameter constraint`:idx: expression can use the operators ``|`` (or), +``&`` (and) and ``~`` (not) and the following predicates: + +=================== ===================================================== +Predicate Meaning +=================== ===================================================== +``atom`` The matching node has no children. +``lit`` The matching node is a literal like "abc", 12. +``sym`` The matching node must be a symbol (a bound + identifier). +``ident`` The matching node must be an identifier (an unbound + identifier). +``call`` The matching AST must be a call/apply expression. +``lvalue`` The matching AST must be an lvalue. +``sideeffect`` The matching AST must have a side effect. +``nosideeffect`` The matching AST must have no side effect. +``param`` A symbol which is a parameter. +``genericparam`` A symbol which is a generic parameter. +``module`` A symbol which is a module. +``type`` A symbol which is a type. +``var`` A symbol which is a variable. +``let`` A symbol which is a ``let`` variable. +``const`` A symbol which is a constant. +``result`` The special ``result`` variable. +``proc`` A symbol which is a proc. +``method`` A symbol which is a method. +``iterator`` A symbol which is an iterator. +``converter`` A symbol which is a converter. +``macro`` A symbol which is a macro. +``template`` A symbol which is a template. +``field`` A symbol which is a field in a tuple or an object. +``enumfield`` A symbol which is a field in an enumeration. +``forvar`` A for loop variable. +``label`` A label (used in ``block`` statements). +``nk*`` The matching AST must have the specified kind. + (Example: ``nkIfStmt`` denotes an ``if`` statement.) +``alias`` States that the marked parameter needs to alias + with *some* other parameter. +``noalias`` States that *every* other parameter must not alias + with the marked parameter. +=================== ===================================================== + +The ``alias`` and ``noalias`` predicates refer not only to the matching AST, +but also to every other bound parameter; syntactially they need to occur after +the ordinary AST predicates: + +.. code-block:: nim + template ex{a = b + c}(a: int{noalias}, b, c: int) = + # this transformation is only valid if 'b' and 'c' do not alias 'a': + a = b + inc a, c + + +Pattern operators +----------------- + +The operators ``*``, ``**``, ``|``, ``~`` have a special meaning in patterns +if they are written in infix notation. + + +The ``|`` operator +~~~~~~~~~~~~~~~~~~ + +The ``|`` operator if used as infix operator creates an ordered choice: + +.. code-block:: nim + template t{0|1}(): expr = 3 + let a = 1 + # outputs 3: + echo a + +The matching is performed after the compiler performed some optimizations like +constant folding, so the following does not work: + +.. code-block:: nim + template t{0|1}(): expr = 3 + # outputs 1: + echo 1 + +The reason is that the compiler already transformed the 1 into "1" for +the ``echo`` statement. However, a term rewriting macro should not change the +semantics anyway. In fact they can be deactived with the ``--patterns:off`` +command line option or temporarily with the ``patterns`` pragma. + + +The ``{}`` operator +~~~~~~~~~~~~~~~~~~~ + +A pattern expression can be bound to a pattern parameter via the ``expr{param}`` +notation: + +.. code-block:: nim + template t{(0|1|2){x}}(x: expr): expr = x+1 + let a = 1 + # outputs 2: + echo a + + +The ``~`` operator +~~~~~~~~~~~~~~~~~~ + +The ``~`` operator is the **not** operator in patterns: + +.. code-block:: nim + template t{x = (~x){y} and (~x){z}}(x, y, z: bool): stmt = + x = y + if x: x = z + + var + a = false + b = true + c = false + a = b and c + echo a + + +The ``*`` operator +~~~~~~~~~~~~~~~~~~ + +The ``*`` operator can *flatten* a nested binary expression like ``a & b & c`` +to ``&(a, b, c)``: + +.. code-block:: nim + var + calls = 0 + + proc `&&`(s: varargs[string]): string = + result = s[0] + for i in 1..len(s)-1: result.add s[i] + inc calls + + template optConc{ `&&` * a }(a: string): expr = &&a + + let space = " " + echo "my" && (space & "awe" && "some " ) && "concat" + + # check that it's been optimized properly: + doAssert calls == 1 + + +The second operator of `*` must be a parameter; it is used to gather all the +arguments. The expression ``"my" && (space & "awe" && "some " ) && "concat"`` +is passed to ``optConc`` in ``a`` as a special list (of kind ``nkArgList``) +which is flattened into a call expression; thus the invocation of ``optConc`` +produces: + +.. code-block:: nim + `&&`("my", space & "awe", "some ", "concat") + + +The ``**`` operator +~~~~~~~~~~~~~~~~~~~ + +The ``**`` is much like the ``*`` operator, except that it gathers not only +all the arguments, but also the matched operators in reverse polish notation: + +.. code-block:: nim + import macros + + type + TMatrix = object + dummy: int + + proc `*`(a, b: TMatrix): TMatrix = discard + proc `+`(a, b: TMatrix): TMatrix = discard + proc `-`(a, b: TMatrix): TMatrix = discard + proc `$`(a: TMatrix): string = result = $a.dummy + proc mat21(): TMatrix = + result.dummy = 21 + + macro optM{ (`+`|`-`|`*`) ** a }(a: TMatrix): expr = + echo treeRepr(a) + result = newCall(bindSym"mat21") + + var x, y, z: TMatrix + + echo x + y * z - x + +This passes the expression ``x + y * z - x`` to the ``optM`` macro as +an ``nnkArgList`` node containing:: + + Arglist + Sym "x" + Sym "y" + Sym "z" + Sym "*" + Sym "+" + Sym "x" + Sym "-" + +(Which is the reverse polish notation of ``x + y * z - x``.) + + +Parameters +---------- + +Parameters in a pattern are type checked in the matching process. If a +parameter is of the type ``varargs`` it is treated specially and it can match +0 or more arguments in the AST to be matched against: + +.. code-block:: nim + template optWrite{ + write(f, x) + ((write|writeln){w})(f, y) + }(x, y: varargs[expr], f: TFile, w: expr) = + w(f, x, y) + + + +Example: Partial evaluation +--------------------------- + +The following example shows how some simple partial evaluation can be +implemented with term rewriting: + +.. code-block:: nim + proc p(x, y: int; cond: bool): int = + result = if cond: x + y else: x - y + + template optP1{p(x, y, true)}(x, y: expr): expr = x + y + template optP2{p(x, y, false)}(x, y: expr): expr = x - y + + +Example: Hoisting +----------------- + +The following example shows how some form of hoisting can be implemented: + +.. code-block:: nim + import pegs + + template optPeg{peg(pattern)}(pattern: string{lit}): TPeg = + var gl {.global, gensym.} = peg(pattern) + gl + + for i in 0 .. 3: + echo match("(a b c)", peg"'(' @ ')'") + echo match("W_HI_Le", peg"\y 'while'") + +The ``optPeg`` template optimizes the case of a peg constructor with a string +literal, so that the pattern will only be parsed once at program startup and +stored in a global ``gl`` which is then re-used. This optimization is called +hoisting because it is comparable to classical loop hoisting. + + +AST based overloading +===================== + +Parameter constraints can also be used for ordinary routine parameters; these +constraints affect ordinary overloading resolution then: + +.. code-block:: nim + proc optLit(a: string{lit|`const`}) = + echo "string literal" + proc optLit(a: string) = + echo "no string literal" + + const + constant = "abc" + + var + variable = "xyz" + + optLit("literal") + optLit(constant) + optLit(variable) + +However, the constraints ``alias`` and ``noalias`` are not available in +ordinary routines. + + +Move optimization +----------------- + +The ``call`` constraint is particularly useful to implement a move +optimization for types that have copying semantics: + +.. code-block:: nim + proc `[]=`*(t: var TTable, key: string, val: string) = + ## puts a (key, value)-pair into `t`. The semantics of string require + ## a copy here: + let idx = findInsertionPosition(key) + t[idx] = key + t[idx] = val + + proc `[]=`*(t: var TTable, key: string{call}, val: string{call}) = + ## puts a (key, value)-pair into `t`. Optimized version that knows that + ## the strings are unique and thus don't need to be copied: + let idx = findInsertionPosition(key) + shallowCopy t[idx], key + shallowCopy t[idx], val + + var t: TTable + # overloading resolution ensures that the optimized []= is called here: + t[f()] = g() + diff --git a/doc/manual/type_bound_ops.txt b/doc/manual/type_bound_ops.txt new file mode 100644 index 000000000..64c6c325d --- /dev/null +++ b/doc/manual/type_bound_ops.txt @@ -0,0 +1,112 @@ +Type bound operations +===================== + +There are 3 operations that are bound to a type: + +1. Assignment +2. Destruction +3. Deep copying for communication between threads + +These operations can be *overriden* instead of *overloaded*. This means the +implementation is automatically lifted to structured types. For instance if type +``T`` has an overriden assignment operator ``=`` this operator is also used +for assignments of the type ``seq[T]``. Since these operations are bound to a +type they have to be bound to a nominal type for reasons of simplicity of +implementation: This means an overriden ``deepCopy`` for ``ref T`` is really +bound to ``T`` and not to ``ref T``. This also means that one cannot override +``deepCopy`` for both ``ptr T`` and ``ref T`` at the same time; instead a +helper distinct or object type has to be used for one pointer type. + + +operator `=` +------------ + +This operator is the assignment operator. Note that in the contexts +like ``let v = expr``, ``var v = expr``, ``parameter = defaultValue`` or for +parameter passing no assignment is performed. The ``override`` pragma is +optional for overriding ``=``. + +**Note**: Overriding of operator ``=`` is not yet implemented. + + +destructors +----------- + +A destructor must have a single parameter with a concrete type (the name of a +generic type is allowed too). The name of the destructor has to be ``destroy`` +and it need to be annotated with the ``override`` pragma. + +``destroy(v)`` will be automatically invoked for every local stack +variable ``v`` that goes out of scope. + +If a structured type features a field with destructable type and +the user has not provided an explicit implementation, a destructor for the +structured type will be automatically generated. Calls to any base class +destructors in both user-defined and generated destructors will be inserted. + +A destructor is attached to the type it destructs; expressions of this type +can then only be used in *destructible contexts* and as parameters: + +.. code-block:: nim + type + TMyObj = object + x, y: int + p: pointer + + proc destroy(o: var TMyObj) {.override.} = + if o.p != nil: dealloc o.p + + proc open: TMyObj = + result = TMyObj(x: 1, y: 2, p: alloc(3)) + + proc work(o: TMyObj) = + echo o.x + # No destructor invoked here for 'o' as 'o' is a parameter. + + proc main() = + # destructor automatically invoked at the end of the scope: + var x = open() + # valid: pass 'x' to some other proc: + work(x) + + # Error: usage of a type with a destructor in a non destructible context + echo open() + +A destructible context is currently only the following: + +1. The ``expr`` in ``var x = expr``. +2. The ``expr`` in ``let x = expr``. +3. The ``expr`` in ``return expr``. +4. The ``expr`` in ``result = expr`` where ``result`` is the special symbol + introduced by the compiler. + +These rules ensure that the construction is tied to a variable and can easily +be destructed at its scope exit. Later versions of the language will improve +the support of destructors. + +Be aware that destructors are not called for objects allocated with ``new``. +This may change in future versions of language, but for now the ``finalizer`` +parameter to ``new`` has to be used. + +**Note**: Destructors are still experimental and the spec might change +significantly in order to incorporate an escape analysis. + + +deepCopy +-------- + +``deepCopy`` is a builtin that is invoked whenever data is passed to +a ``spawn``'ed proc to ensure memory safety. The programmer can override its +behaviour for a specific ``ref`` or ``ptr`` type ``T``. (Later versions of the +language may weaken this restriction.) + +The signature has to be: + +.. code-block:: nim + proc deepCopy(x: T): T {.override.} + +This mechanism is used by most data structures that support shared memory like +channels to implement thread safe automatic memory management. + +The builtin ``deepCopy`` can even clone closures and their environments. See +the documentation of `spawn`_ for details. diff --git a/doc/manual/type_rel.txt b/doc/manual/type_rel.txt new file mode 100644 index 000000000..74539f907 --- /dev/null +++ b/doc/manual/type_rel.txt @@ -0,0 +1,191 @@ +Type relations +============== + +The following section defines several relations on types that are needed to +describe the type checking done by the compiler. + + +Type equality +------------- +Nim uses structural type equivalence for most types. Only for objects, +enumerations and distinct types name equivalence is used. The following +algorithm (in pseudo-code) determines type equality: + +.. code-block:: nim + proc typeEqualsAux(a, b: PType, + s: var set[PType * PType]): bool = + if (a,b) in s: return true + incl(s, (a,b)) + if a.kind == b.kind: + case a.kind + of int, intXX, float, floatXX, char, string, cstring, pointer, + bool, nil, void: + # leaf type: kinds identical; nothing more to check + result = true + of ref, ptr, var, set, seq, openarray: + result = typeEqualsAux(a.baseType, b.baseType, s) + of range: + result = typeEqualsAux(a.baseType, b.baseType, s) and + (a.rangeA == b.rangeA) and (a.rangeB == b.rangeB) + of array: + result = typeEqualsAux(a.baseType, b.baseType, s) and + typeEqualsAux(a.indexType, b.indexType, s) + of tuple: + if a.tupleLen == b.tupleLen: + for i in 0..a.tupleLen-1: + if not typeEqualsAux(a[i], b[i], s): return false + result = true + of object, enum, distinct: + result = a == b + of proc: + result = typeEqualsAux(a.parameterTuple, b.parameterTuple, s) and + typeEqualsAux(a.resultType, b.resultType, s) and + a.callingConvention == b.callingConvention + + proc typeEquals(a, b: PType): bool = + var s: set[PType * PType] = {} + result = typeEqualsAux(a, b, s) + +Since types are graphs which can have cycles, the above algorithm needs an +auxiliary set ``s`` to detect this case. + + +Type equality modulo type distinction +------------------------------------- + +The following algorithm (in pseudo-code) determines whether two types +are equal with no respect to ``distinct`` types. For brevity the cycle check +with an auxiliary set ``s`` is omitted: + +.. code-block:: nim + proc typeEqualsOrDistinct(a, b: PType): bool = + if a.kind == b.kind: + case a.kind + of int, intXX, float, floatXX, char, string, cstring, pointer, + bool, nil, void: + # leaf type: kinds identical; nothing more to check + result = true + of ref, ptr, var, set, seq, openarray: + result = typeEqualsOrDistinct(a.baseType, b.baseType) + of range: + result = typeEqualsOrDistinct(a.baseType, b.baseType) and + (a.rangeA == b.rangeA) and (a.rangeB == b.rangeB) + of array: + result = typeEqualsOrDistinct(a.baseType, b.baseType) and + typeEqualsOrDistinct(a.indexType, b.indexType) + of tuple: + if a.tupleLen == b.tupleLen: + for i in 0..a.tupleLen-1: + if not typeEqualsOrDistinct(a[i], b[i]): return false + result = true + of distinct: + result = typeEqualsOrDistinct(a.baseType, b.baseType) + of object, enum: + result = a == b + of proc: + result = typeEqualsOrDistinct(a.parameterTuple, b.parameterTuple) and + typeEqualsOrDistinct(a.resultType, b.resultType) and + a.callingConvention == b.callingConvention + elif a.kind == distinct: + result = typeEqualsOrDistinct(a.baseType, b) + elif b.kind == distinct: + result = typeEqualsOrDistinct(a, b.baseType) + + +Subtype relation +---------------- +If object ``a`` inherits from ``b``, ``a`` is a subtype of ``b``. This subtype +relation is extended to the types ``var``, ``ref``, ``ptr``: + +.. code-block:: nim + proc isSubtype(a, b: PType): bool = + if a.kind == b.kind: + case a.kind + of object: + var aa = a.baseType + while aa != nil and aa != b: aa = aa.baseType + result = aa == b + of var, ref, ptr: + result = isSubtype(a.baseType, b.baseType) + +.. XXX nil is a special value! + + +Convertible relation +-------------------- +A type ``a`` is **implicitly** convertible to type ``b`` iff the following +algorithm returns true: + +.. code-block:: nim + # XXX range types? + proc isImplicitlyConvertible(a, b: PType): bool = + case a.kind + of int: result = b in {int8, int16, int32, int64, uint, uint8, uint16, + uint32, uint64, float, float32, float64} + of int8: result = b in {int16, int32, int64, int} + of int16: result = b in {int32, int64, int} + of int32: result = b in {int64, int} + of uint: result = b in {uint32, uint64} + of uint8: result = b in {uint16, uint32, uint64} + of uint16: result = b in {uint32, uint64} + of uint32: result = b in {uint64} + of float: result = b in {float32, float64} + of float32: result = b in {float64, float} + of float64: result = b in {float32, float} + of seq: + result = b == openArray and typeEquals(a.baseType, b.baseType) + of array: + result = b == openArray and typeEquals(a.baseType, b.baseType) + if a.baseType == char and a.indexType.rangeA == 0: + result = b = cstring + of cstring, ptr: + result = b == pointer + of string: + result = b == cstring + +A type ``a`` is **explicitly** convertible to type ``b`` iff the following +algorithm returns true: + +.. code-block:: nim + proc isIntegralType(t: PType): bool = + result = isOrdinal(t) or t.kind in {float, float32, float64} + + proc isExplicitlyConvertible(a, b: PType): bool = + result = false + if isImplicitlyConvertible(a, b): return true + if typeEqualsOrDistinct(a, b): return true + if isIntegralType(a) and isIntegralType(b): return true + if isSubtype(a, b) or isSubtype(b, a): return true + +The convertible relation can be relaxed by a user-defined type +`converter`:idx:. + +.. code-block:: nim + converter toInt(x: char): int = result = ord(x) + + var + x: int + chr: char = 'a' + + # implicit conversion magic happens here + x = chr + echo x # => 97 + # you can use the explicit form too + x = chr.toInt + echo x # => 97 + +The type conversion ``T(a)`` is an L-value if ``a`` is an L-value and +``typeEqualsOrDistinct(T, type(a))`` holds. + + +Assignment compatibility +------------------------ + +An expression ``b`` can be assigned to an expression ``a`` iff ``a`` is an +`l-value` and ``isImplicitlyConvertible(b.typ, a.typ)`` holds. + + +Overloading resolution +---------------------- + +To be written. diff --git a/doc/manual/type_sections.txt b/doc/manual/type_sections.txt new file mode 100644 index 000000000..5413e896e --- /dev/null +++ b/doc/manual/type_sections.txt @@ -0,0 +1,23 @@ +Type sections +============= + +Example: + +.. code-block:: nim + type # example demonstrating mutually recursive types + PNode = ref TNode # a traced pointer to a TNode + TNode = object + le, ri: PNode # left and right subtrees + sym: ref TSym # leaves contain a reference to a TSym + + TSym = object # a symbol + name: string # the symbol's name + line: int # the line the symbol was declared in + code: PNode # the symbol's abstract syntax tree + +A type section begins with the ``type`` keyword. It contains multiple +type definitions. A type definition binds a type to a name. Type definitions +can be recursive or even mutually recursive. Mutually recursive types are only +possible within a single ``type`` section. Nominal types like ``objects`` +or ``enums`` can only be defined in a ``type`` section. + diff --git a/doc/manual/typedesc.txt b/doc/manual/typedesc.txt new file mode 100644 index 000000000..d1d8bc87a --- /dev/null +++ b/doc/manual/typedesc.txt @@ -0,0 +1,146 @@ +Special Types +============= + +static[T] +--------- + +**Note**: static[T] is still in development. + +As their name suggests, static params must be known at compile-time: + +.. code-block:: nim + + proc precompiledRegex(pattern: static[string]): TRegEx = + var res {.global.} = re(pattern) + return res + + precompiledRegex("/d+") # Replaces the call with a precompiled + # regex, stored in a global variable + + precompiledRegex(paramStr(1)) # Error, command-line options + # are not known at compile-time + + +For the purposes of code generation, all static params are treated as +generic params - the proc will be compiled separately for each unique +supplied value (or combination of values). + +Furthermore, the system module defines a `semistatic[T]` type than can be +used to declare procs accepting both static and run-time values, which can +optimize their body according to the supplied param using the `isStatic(p)` +predicate: + +.. code-block:: nim + + # The following proc will be compiled once for each unique static + # value and also once for the case handling all run-time values: + + proc re(pattern: semistatic[string]): TRegEx = + when isStatic(pattern): + result = precompiledRegex(pattern) + else: + result = compile(pattern) + +Static params can also appear in the signatures of generic types: + +.. code-block:: nim + + type + Matrix[M,N: static[int]; T: Number] = array[0..(M*N - 1), T] + # Note how `Number` is just a type constraint here, while + # `static[int]` requires us to supply a compile-time int value + + AffineTransform2D[T] = Matrix[3, 3, T] + AffineTransform3D[T] = Matrix[4, 4, T] + + var m1: AffineTransform3D[float] # OK + var m2: AffineTransform2D[string] # Error, `string` is not a `Number` + + +typedesc +-------- + +`typedesc` is a special type allowing one to treat types as compile-time values +(i.e. if types are compile-time values and all values have a type, then +typedesc must be their type). + +When used as a regular proc param, typedesc acts as a type class. The proc +will be instantiated for each unique type parameter and one can refer to the +instantiation type using the param name: + +.. code-block:: nim + + proc new(T: typedesc): ref T = + echo "allocating ", T.name + new(result) + + var n = TNode.new + var tree = new(TBinaryTree[int]) + +When multiple typedesc params are present, they act like a distinct type class +(i.e. they will bind freely to different types). To force a bind-once behavior +one can use a named alias or an explicit `typedesc` generic param: + +.. code-block:: nim + + # `type1` and `type2` are aliases for typedesc available from system.nim + proc acceptOnlyTypePairs(A, B: type1; C, D: type2) + proc acceptOnlyTypePairs[T: typedesc, U: typedesc](A, B: T; C, D: U) + +Once bound, typedesc params can appear in the rest of the proc signature: + +.. code-block:: nim + + template declareVariableWithType(T: typedesc, value: T) = + var x: T = value + + declareVariableWithType int, 42 + +When used with macros and .compileTime. procs on the other hand, the compiler +does not need to instantiate the code multiple times, because types then can be +manipulated using the unified internal symbol representation. In such context +typedesc acts as any other type. One can create variables, store typedesc +values inside containers and so on. For example, here is how one can create +a type-safe wrapper for the unsafe `printf` function from C: + +.. code-block:: nim + macro safePrintF(formatString: string{lit}, args: varargs[expr]): expr = + var i = 0 + for c in formatChars(formatString): + var expectedType = case c + of 'c': char + of 'd', 'i', 'x', 'X': int + of 'f', 'e', 'E', 'g', 'G': float + of 's': string + of 'p': pointer + else: EOutOfRange + + var actualType = args[i].getType + inc i + + if expectedType == EOutOfRange: + error c & " is not a valid format character" + elif expectedType != actualType: + error "type mismatch for argument ", i, ". expected type: ", + expectedType.name, ", actual type: ", actualType.name + + # keep the original callsite, but use cprintf instead + result = callsite() + result[0] = newIdentNode(!"cprintf") + + +Overload resolution can be further influenced by constraining the set of +types that will match the typedesc param: + +.. code-block:: nim + + template maxval(T: typedesc[int]): int = high(int) + template maxval(T: typedesc[float]): float = Inf + + var i = int.maxval + var f = float.maxval + var s = string.maxval # error, maxval is not implemented for string + +The constraint can be a concrete type or a type class. + + diff --git a/doc/manual/types.txt b/doc/manual/types.txt new file mode 100644 index 000000000..274177cfb --- /dev/null +++ b/doc/manual/types.txt @@ -0,0 +1,1153 @@ +Types +===== + +All expressions have a type which is known at compile time. Nim +is statically typed. One can declare new types, which is in essence defining +an identifier that can be used to denote this custom type. + +These are the major type classes: + +* ordinal types (consist of integer, bool, character, enumeration + (and subranges thereof) types) +* floating point types +* string type +* structured types +* reference (pointer) type +* procedural type +* generic type + + +Ordinal types +------------- +Ordinal types have the following characteristics: + +- Ordinal types are countable and ordered. This property allows + the operation of functions as ``inc``, ``ord``, ``dec`` on ordinal types to + be defined. +- Ordinal values have a smallest possible value. Trying to count further + down than the smallest value gives a checked runtime or static error. +- Ordinal values have a largest possible value. Trying to count further + than the largest value gives a checked runtime or static error. + +Integers, bool, characters and enumeration types (and subranges of these +types) belong to ordinal types. For reasons of simplicity of implementation +the types ``uint`` and ``uint64`` are not ordinal types. + + +Pre-defined integer types +------------------------- +These integer types are pre-defined: + +``int`` + the generic signed integer type; its size is platform dependent and has the + same size as a pointer. This type should be used in general. An integer + literal that has no type suffix is of this type. + +intXX + additional signed integer types of XX bits use this naming scheme + (example: int16 is a 16 bit wide integer). + The current implementation supports ``int8``, ``int16``, ``int32``, ``int64``. + Literals of these types have the suffix 'iXX. + +``uint`` + the generic `unsigned integer`:idx: type; its size is platform dependent and + has the same size as a pointer. An integer literal with the type + suffix ``'u`` is of this type. + +uintXX + additional signed integer types of XX bits use this naming scheme + (example: uint16 is a 16 bit wide unsigned integer). + The current implementation supports ``uint8``, ``uint16``, ``uint32``, + ``uint64``. Literals of these types have the suffix 'uXX. + Unsigned operations all wrap around; they cannot lead to over- or + underflow errors. + + +In addition to the usual arithmetic operators for signed and unsigned integers +(``+ - *`` etc.) there are also operators that formally work on *signed* +integers but treat their arguments as *unsigned*: They are mostly provided +for backwards compatibility with older versions of the language that lacked +unsigned integer types. These unsigned operations for signed integers use +the ``%`` suffix as convention: + + +====================== ====================================================== +operation meaning +====================== ====================================================== +``a +% b`` unsigned integer addition +``a -% b`` unsigned integer subtraction +``a *% b`` unsigned integer multiplication +``a /% b`` unsigned integer division +``a %% b`` unsigned integer modulo operation +``a <% b`` treat ``a`` and ``b`` as unsigned and compare +``a <=% b`` treat ``a`` and ``b`` as unsigned and compare +``ze(a)`` extends the bits of ``a`` with zeros until it has the + width of the ``int`` type +``toU8(a)`` treats ``a`` as unsigned and converts it to an + unsigned integer of 8 bits (but still the + ``int8`` type) +``toU16(a)`` treats ``a`` as unsigned and converts it to an + unsigned integer of 16 bits (but still the + ``int16`` type) +``toU32(a)`` treats ``a`` as unsigned and converts it to an + unsigned integer of 32 bits (but still the + ``int32`` type) +====================== ====================================================== + +`Automatic type conversion`:idx: is performed in expressions where different +kinds of integer types are used: the smaller type is converted to the larger. + +A `narrowing type conversion`:idx: converts a larger to a smaller type (for +example ``int32 -> int16``. A `widening type conversion`:idx: converts a +smaller type to a larger type (for example ``int16 -> int32``). In Nim only +widening type conversions are *implicit*: + +.. code-block:: nim + var myInt16 = 5i16 + var myInt: int + myInt16 + 34 # of type ``int16`` + myInt16 + myInt # of type ``int`` + myInt16 + 2i32 # of type ``int32`` + +However, ``int`` literals are implicitly convertible to a smaller integer type +if the literal's value fits this smaller type and such a conversion is less +expensive than other implicit conversions, so ``myInt16 + 34`` produces +an ``int16`` result. + +For further details, see `Convertible relation`_. + + +Subrange types +-------------- +A subrange type is a range of values from an ordinal type (the base +type). To define a subrange type, one must specify it's limiting values: the +lowest and highest value of the type: + +.. code-block:: nim + type + Subrange = range[0..5] + + +``Subrange`` is a subrange of an integer which can only hold the values 0 +to 5. Assigning any other value to a variable of type ``Subrange`` is a +checked runtime error (or static error if it can be statically +determined). Assignments from the base type to one of its subrange types +(and vice versa) are allowed. + +A subrange type has the same size as its base type (``int`` in the example). + +Nim requires `interval arithmetic`:idx: for subrange types over a set +of built-in operators that involve constants: ``x %% 3`` is of +type ``range[0..2]``. The following built-in operators for integers are +affected by this rule: ``-``, ``+``, ``*``, ``min``, ``max``, ``succ``, +``pred``, ``mod``, ``div``, ``%%``, ``and`` (bitwise ``and``). + +Bitwise ``and`` only produces a ``range`` if one of its operands is a +constant *x* so that (x+1) is a number of two. +(Bitwise ``and`` is then a ``%%`` operation.) + +This means that the following code is accepted: + +.. code-block:: nim + case (x and 3) + 7 + of 7: echo "A" + of 8: echo "B" + of 9: echo "C" + of 10: echo "D" + # note: no ``else`` required as (x and 3) + 7 has the type: range[7..10] + + +Pre-defined floating point types +-------------------------------- + +The following floating point types are pre-defined: + +``float`` + the generic floating point type; its size is platform dependent + (the compiler chooses the processor's fastest floating point type). + This type should be used in general. + +floatXX + an implementation may define additional floating point types of XX bits using + this naming scheme (example: float64 is a 64 bit wide float). The current + implementation supports ``float32`` and ``float64``. Literals of these types + have the suffix 'fXX. + + +Automatic type conversion in expressions with different kinds +of floating point types is performed: See `Convertible relation`_ for further +details. Arithmetic performed on floating point types follows the IEEE +standard. Integer types are not converted to floating point types automatically +and vice versa. + +The IEEE standard defines five types of floating-point exceptions: + +* Invalid: operations with mathematically invalid operands, + for example 0.0/0.0, sqrt(-1.0), and log(-37.8). +* Division by zero: divisor is zero and dividend is a finite nonzero number, + for example 1.0/0.0. +* Overflow: operation produces a result that exceeds the range of the exponent, + for example MAXDOUBLE+0.0000000000001e308. +* Underflow: operation produces a result that is too small to be represented + as a normal number, for example, MINDOUBLE * MINDOUBLE. +* Inexact: operation produces a result that cannot be represented with infinite + precision, for example, 2.0 / 3.0, log(1.1) and 0.1 in input. + +The IEEE exceptions are either ignored at runtime or mapped to the +Nim exceptions: `EFloatInvalidOp`:idx:, `EFloatDivByZero`:idx:, +`EFloatOverflow`:idx:, `EFloatUnderflow`:idx:, and `EFloatInexact`:idx:. +These exceptions inherit from the `EFloatingPoint`:idx: base class. + +Nim provides the pragmas `NaNChecks`:idx: and `InfChecks`:idx: to control +whether the IEEE exceptions are ignored or trap a Nim exception: + +.. code-block:: nim + {.NanChecks: on, InfChecks: on.} + var a = 1.0 + var b = 0.0 + echo b / b # raises EFloatInvalidOp + echo a / b # raises EFloatOverflow + +In the current implementation ``EFloatDivByZero`` and ``EFloatInexact`` are +never raised. ``EFloatOverflow`` is raised instead of ``EFloatDivByZero``. +There is also a `floatChecks`:idx: pragma that is a short-cut for the +combination of ``NaNChecks`` and ``InfChecks`` pragmas. ``floatChecks`` are +turned off as default. + +The only operations that are affected by the ``floatChecks`` pragma are +the ``+``, ``-``, ``*``, ``/`` operators for floating point types. + +An implementation should always use the maximum precision available to evaluate +floating pointer values at compile time; this means expressions like +``0.09'f32 + 0.01'f32 == 0.09'f64 + 0.01'f64`` are true. + + +Boolean type +------------ +The boolean type is named `bool`:idx: in Nim and can be one of the two +pre-defined values ``true`` and ``false``. Conditions in while, +if, elif, when statements need to be of type bool. + +This condition holds:: + + ord(false) == 0 and ord(true) == 1 + +The operators ``not, and, or, xor, <, <=, >, >=, !=, ==`` are defined +for the bool type. The ``and`` and ``or`` operators perform short-cut +evaluation. Example: + +.. code-block:: nim + + while p != nil and p.name != "xyz": + # p.name is not evaluated if p == nil + p = p.next + + +The size of the bool type is one byte. + + +Character type +-------------- +The character type is named ``char`` in Nim. Its size is one byte. +Thus it cannot represent an UTF-8 character, but a part of it. +The reason for this is efficiency: for the overwhelming majority of use-cases, +the resulting programs will still handle UTF-8 properly as UTF-8 was specially +designed for this. +Another reason is that Nim can support ``array[char, int]`` or +``set[char]`` efficiently as many algorithms rely on this feature. The +`TRune` type is used for Unicode characters, it can represent any Unicode +character. ``TRune`` is declared in the `unicode module <unicode.html>`_. + + + + +Enumeration types +----------------- +Enumeration types define a new type whose values consist of the ones +specified. The values are ordered. Example: + +.. code-block:: nim + + type + TDirection = enum + north, east, south, west + + +Now the following holds:: + + ord(north) == 0 + ord(east) == 1 + ord(south) == 2 + ord(west) == 3 + +Thus, north < east < south < west. The comparison operators can be used +with enumeration types. + +For better interfacing to other programming languages, the fields of enum +types can be assigned an explicit ordinal value. However, the ordinal values +have to be in ascending order. A field whose ordinal value is not +explicitly given is assigned the value of the previous field + 1. + +An explicit ordered enum can have *holes*: + +.. code-block:: nim + type + TTokenType = enum + a = 2, b = 4, c = 89 # holes are valid + +However, it is then not an ordinal anymore, so it is not possible to use these +enums as an index type for arrays. The procedures ``inc``, ``dec``, ``succ`` +and ``pred`` are not available for them either. + + +The compiler supports the built-in stringify operator ``$`` for enumerations. +The stringify's result can be controlled by explicitly giving the string +values to use: + +.. code-block:: nim + + type + TMyEnum = enum + valueA = (0, "my value A"), + valueB = "value B", + valueC = 2, + valueD = (3, "abc") + +As can be seen from the example, it is possible to both specify a field's +ordinal value and its string value by using a tuple. It is also +possible to only specify one of them. + +An enum can be marked with the ``pure`` pragma so that it's fields are not +added to the current scope, so they always need to be accessed +via ``TMyEnum.value``: + +.. code-block:: nim + + type + TMyEnum {.pure.} = enum + valueA, valueB, valueC, valueD + + echo valueA # error: Unknown identifier + echo TMyEnum.valueA # works + + +String type +----------- +All string literals are of the type ``string``. A string in Nim is very +similar to a sequence of characters. However, strings in Nim are both +zero-terminated and have a length field. One can retrieve the length with the +builtin ``len`` procedure; the length never counts the terminating zero. +The assignment operator for strings always copies the string. +The ``&`` operator concatenates strings. + +Strings are compared by their lexicographical order. All comparison operators +are available. Strings can be indexed like arrays (lower bound is 0). Unlike +arrays, they can be used in case statements: + +.. code-block:: nim + + case paramStr(i) + of "-v": incl(options, optVerbose) + of "-h", "-?": incl(options, optHelp) + else: write(stdout, "invalid command line option!\n") + +Per convention, all strings are UTF-8 strings, but this is not enforced. For +example, when reading strings from binary files, they are merely a sequence of +bytes. The index operation ``s[i]`` means the i-th *char* of ``s``, not the +i-th *unichar*. The iterator ``runes`` from the `unicode module +<unicode.html>`_ can be used for iteration over all Unicode characters. + + +CString type +------------ +The ``cstring`` type represents a pointer to a zero-terminated char array +compatible to the type ``char*`` in Ansi C. Its primary purpose lies in easy +interfacing with C. The index operation ``s[i]`` means the i-th *char* of +``s``; however no bounds checking for ``cstring`` is performed making the +index operation unsafe. + +A Nim ``string`` is implicitly convertible +to ``cstring`` for convenience. If a Nim string is passed to a C-style +variadic proc, it is implicitly converted to ``cstring`` too: + +.. code-block:: nim + proc printf(formatstr: cstring) {.importc: "printf", varargs, + header: "<stdio.h>".} + + printf("This works %s", "as expected") + +Even though the conversion is implicit, it is not *safe*: The garbage collector +does not consider a ``cstring`` to be a root and may collect the underlying +memory. However in practice this almost never happens as the GC considers +stack roots conservatively. One can use the builtin procs ``GC_ref`` and +``GC_unref`` to keep the string data alive for the rare cases where it does +not work. + +A `$` proc is defined for cstrings that returns a string. Thus to get a nim +string from a cstring: + +.. code-block:: nim + var str: string = "Hello!" + var cstr: cstring = s + var newstr: string = $cstr + + +Structured types +---------------- +A variable of a structured type can hold multiple values at the same +time. Structured types can be nested to unlimited levels. Arrays, sequences, +tuples, objects and sets belong to the structured types. + +Array and sequence types +------------------------ +Arrays are a homogeneous type, meaning that each element in the array +has the same type. Arrays always have a fixed length which is specified at +compile time (except for open arrays). They can be indexed by any ordinal type. +A parameter ``A`` may be an *open array*, in which case it is indexed by +integers from 0 to ``len(A)-1``. An array expression may be constructed by the +array constructor ``[]``. + +Sequences are similar to arrays but of dynamic length which may change +during runtime (like strings). Sequences are implemented as growable arrays, +allocating pieces of memory as items are added. A sequence ``S`` is always +indexed by integers from 0 to ``len(S)-1`` and its bounds are checked. +Sequences can be constructed by the array constructor ``[]`` in conjunction +with the array to sequence operator ``@``. Another way to allocate space for a +sequence is to call the built-in ``newSeq`` procedure. + +A sequence may be passed to a parameter that is of type *open array*. + +Example: + +.. code-block:: nim + + type + TIntArray = array[0..5, int] # an array that is indexed with 0..5 + TIntSeq = seq[int] # a sequence of integers + var + x: TIntArray + y: TIntSeq + x = [1, 2, 3, 4, 5, 6] # [] is the array constructor + y = @[1, 2, 3, 4, 5, 6] # the @ turns the array into a sequence + +The lower bound of an array or sequence may be received by the built-in proc +``low()``, the higher bound by ``high()``. The length may be +received by ``len()``. ``low()`` for a sequence or an open array always returns +0, as this is the first valid index. +One can append elements to a sequence with the ``add()`` proc or the ``&`` +operator, and remove (and get) the last element of a sequence with the +``pop()`` proc. + +The notation ``x[i]`` can be used to access the i-th element of ``x``. + +Arrays are always bounds checked (at compile-time or at runtime). These +checks can be disabled via pragmas or invoking the compiler with the +``--boundChecks:off`` command line switch. + + +Open arrays +----------- + +Often fixed size arrays turn out to be too inflexible; procedures should +be able to deal with arrays of different sizes. The `openarray`:idx: type +allows this; it can only be used for parameters. Openarrays are always +indexed with an ``int`` starting at position 0. The ``len``, ``low`` +and ``high`` operations are available for open arrays too. Any array with +a compatible base type can be passed to an openarray parameter, the index +type does not matter. In addition to arrays sequences can also be passed +to an open array parameter. + +The openarray type cannot be nested: multidimensional openarrays are not +supported because this is seldom needed and cannot be done efficiently. + + +Varargs +------- + +A ``varargs`` parameter is an openarray parameter that additionally +allows to pass a variable number of arguments to a procedure. The compiler +converts the list of arguments to an array implicitly: + +.. code-block:: nim + proc myWriteln(f: TFile, a: varargs[string]) = + for s in items(a): + write(f, s) + write(f, "\n") + + myWriteln(stdout, "abc", "def", "xyz") + # is transformed to: + myWriteln(stdout, ["abc", "def", "xyz"]) + +This transformation is only done if the varargs parameter is the +last parameter in the procedure header. It is also possible to perform +type conversions in this context: + +.. code-block:: nim + proc myWriteln(f: TFile, a: varargs[string, `$`]) = + for s in items(a): + write(f, s) + write(f, "\n") + + myWriteln(stdout, 123, "abc", 4.0) + # is transformed to: + myWriteln(stdout, [$123, $"def", $4.0]) + +In this example ``$`` is applied to any argument that is passed to the +parameter ``a``. (Note that ``$`` applied to strings is a nop.) + + + +Tuples and object types +----------------------- +A variable of a tuple or object type is a heterogeneous storage +container. +A tuple or object defines various named *fields* of a type. A tuple also +defines an *order* of the fields. Tuples are meant for heterogeneous storage +types with no overhead and few abstraction possibilities. The constructor ``()`` +can be used to construct tuples. The order of the fields in the constructor +must match the order of the tuple's definition. Different tuple-types are +*equivalent* if they specify the same fields of the same type in the same +order. The *names* of the fields also have to be identical but this might +change in a future version of the language. + +The assignment operator for tuples copies each component. +The default assignment operator for objects copies each component. Overloading +of the assignment operator for objects is not possible, but this will change +in future versions of the compiler. + +.. code-block:: nim + + type + TPerson = tuple[name: string, age: int] # type representing a person: + # a person consists of a name + # and an age + var + person: TPerson + person = (name: "Peter", age: 30) + # the same, but less readable: + person = ("Peter", 30) + +The implementation aligns the fields for best access performance. The alignment +is compatible with the way the C compiler does it. + +For consistency with ``object`` declarations, tuples in a ``type`` section +can also be defined with indentation instead of ``[]``: + +.. code-block:: nim + type + TPerson = tuple # type representing a person + name: string # a person consists of a name + age: natural # and an age + +Objects provide many features that tuples do not. Object provide inheritance +and information hiding. Objects have access to their type at runtime, so that +the ``of`` operator can be used to determine the object's type. + +.. code-block:: nim + type + TPerson {.inheritable.} = object + name*: string # the * means that `name` is accessible from other modules + age: int # no * means that the field is hidden + + TStudent = object of TPerson # a student is a person + id: int # with an id field + + var + student: TStudent + person: TPerson + assert(student of TStudent) # is true + +Object fields that should be visible from outside the defining module, have to +be marked by ``*``. In contrast to tuples, different object types are +never *equivalent*. Objects that have no ancestor are implicitly ``final`` +and thus have no hidden type field. One can use the ``inheritable`` pragma to +introduce new object roots apart from ``system.TObject``. + + +Object construction +------------------- + +Objects can also be created with an `object construction expression`:idx: that +has the syntax ``T(fieldA: valueA, fieldB: valueB, ...)`` where ``T`` is +an ``object`` type or a ``ref object`` type: + +.. code-block:: nim + var student = TStudent(name: "Anton", age: 5, id: 3) + +For a ``ref object`` type ``system.new`` is invoked implicitly. + + +Object variants +--------------- +Often an object hierarchy is overkill in certain situations where simple +variant types are needed. + +An example: + +.. code-block:: nim + + # This is an example how an abstract syntax tree could be modelled in Nim + type + TNodeKind = enum # the different node types + nkInt, # a leaf with an integer value + nkFloat, # a leaf with a float value + nkString, # a leaf with a string value + nkAdd, # an addition + nkSub, # a subtraction + nkIf # an if statement + PNode = ref TNode + TNode = object + case kind: TNodeKind # the ``kind`` field is the discriminator + of nkInt: intVal: int + of nkFloat: floatVal: float + of nkString: strVal: string + of nkAdd, nkSub: + leftOp, rightOp: PNode + of nkIf: + condition, thenPart, elsePart: PNode + + # create a new case object: + var n = PNode(kind: nkIf, condition: nil) + # accessing n.thenPart is valid because the ``nkIf`` branch is active: + n.thenPart = PNode(kind: nkFloat, floatVal: 2.0) + + # the following statement raises an `EInvalidField` exception, because + # n.kind's value does not fit and the ``nkString`` branch is not active: + n.strVal = "" + + # invalid: would change the active object branch: + n.kind = nkInt + + var x = PNode(kind: nkAdd, leftOp: PNode(kind: nkInt, intVal: 4), + rightOp: PNode(kind: nkInt, intVal: 2)) + # valid: does not change the active object branch: + x.kind = nkSub + +As can been seen from the example, an advantage to an object hierarchy is that +no casting between different object types is needed. Yet, access to invalid +object fields raises an exception. + +The syntax of ``case`` in an object declaration follows closely the syntax of +the ``case`` statement: The branches in a ``case`` section may be indented too. + +In the example the ``kind`` field is called the `discriminator`:idx:\: For +safety its address cannot be taken and assignments to it are restricted: The +new value must not lead to a change of the active object branch. For an object +branch switch ``system.reset`` has to be used. + + +Set type +-------- + +.. include:: sets_fragment.txt + +Reference and pointer types +--------------------------- +References (similar to pointers in other programming languages) are a +way to introduce many-to-one relationships. This means different references can +point to and modify the same location in memory (also called `aliasing`:idx:). + +Nim distinguishes between `traced`:idx: and `untraced`:idx: references. +Untraced references are also called *pointers*. Traced references point to +objects of a garbage collected heap, untraced references point to +manually allocated objects or to objects somewhere else in memory. Thus +untraced references are *unsafe*. However for certain low-level operations +(accessing the hardware) untraced references are unavoidable. + +Traced references are declared with the **ref** keyword, untraced references +are declared with the **ptr** keyword. + +An empty subscript ``[]`` notation can be used to derefer a reference, +the ``addr`` procedure returns the address of an item. An address is always +an untraced reference. +Thus the usage of ``addr`` is an *unsafe* feature. + +The ``.`` (access a tuple/object field operator) +and ``[]`` (array/string/sequence index operator) operators perform implicit +dereferencing operations for reference types: + +.. code-block:: nim + + type + PNode = ref TNode + TNode = object + le, ri: PNode + data: int + + var + n: PNode + new(n) + n.data = 9 + # no need to write n[].data; in fact n[].data is highly discouraged! + +As a syntactical extension ``object`` types can be anonymous if +declared in a type section via the ``ref object`` or ``ptr object`` notations. +This feature is useful if an object should only gain reference semantics: + +.. code-block:: nim + + type + Node = ref object + le, ri: Node + data: int + + +To allocate a new traced object, the built-in procedure ``new`` has to be used. +To deal with untraced memory, the procedures ``alloc``, ``dealloc`` and +``realloc`` can be used. The documentation of the system module contains +further information. + +If a reference points to *nothing*, it has the value ``nil``. + +Special care has to be taken if an untraced object contains traced objects like +traced references, strings or sequences: in order to free everything properly, +the built-in procedure ``GCunref`` has to be called before freeing the untraced +memory manually: + +.. code-block:: nim + type + TData = tuple[x, y: int, s: string] + + # allocate memory for TData on the heap: + var d = cast[ptr TData](alloc0(sizeof(TData))) + + # create a new string on the garbage collected heap: + d.s = "abc" + + # tell the GC that the string is not needed anymore: + GCunref(d.s) + + # free the memory: + dealloc(d) + +Without the ``GCunref`` call the memory allocated for the ``d.s`` string would +never be freed. The example also demonstrates two important features for low +level programming: the ``sizeof`` proc returns the size of a type or value +in bytes. The ``cast`` operator can circumvent the type system: the compiler +is forced to treat the result of the ``alloc0`` call (which returns an untyped +pointer) as if it would have the type ``ptr TData``. Casting should only be +done if it is unavoidable: it breaks type safety and bugs can lead to +mysterious crashes. + +**Note**: The example only works because the memory is initialized to zero +(``alloc0`` instead of ``alloc`` does this): ``d.s`` is thus initialized to +``nil`` which the string assignment can handle. One needs to know low level +details like this when mixing garbage collected data with unmanaged memory. + +.. XXX finalizers for traced objects + + +Not nil annotation +------------------ + +All types for that ``nil`` is a valid value can be annotated to +exclude ``nil`` as a valid value with the ``not nil`` annotation: + +.. code-block:: nim + type + PObject = ref TObj not nil + TProc = (proc (x, y: int)) not nil + + proc p(x: PObject) = + echo "not nil" + + # compiler catches this: + p(nil) + + # and also this: + var x: PObject + p(x) + +The compiler ensures that every code path initializes variables which contain +not nilable pointers. The details of this analysis are still to be specified +here. + + +Memory regions +-------------- + +The types ``ref`` and ``ptr`` can get an optional ``region`` annotation. +A region has to be an object type. + +Regions are very useful to separate user space and kernel memory in the +development of OS kernels: + +.. code-block:: nim + type + Kernel = object + Userspace = object + + var a: Kernel ptr Stat + var b: Userspace ptr Stat + + # the following does not compile as the pointer types are incompatible: + a = b + +As the example shows ``ptr`` can also be used as a binary +operator, ``region ptr T`` is a shortcut for ``ptr[region, T]``. + +In order to make generic code easier to write ``ptr T`` is a subtype +of ``ptr[R, T]`` for any ``R``. + +Furthermore the subtype relation of the region object types is lifted to +the pointer types: If ``A <: B`` then ``ptr[A, T] <: ptr[B, T]``. This can be +used to model subregions of memory. As a special typing rule ``ptr[R, T]`` is +not compatible to ``pointer`` to prevent the following from compiling: + +.. code-block:: nim + # from system + proc dealloc(p: pointer) + + # wrap some scripting language + type + PythonsHeap = object + PyObjectHeader = object + rc: int + typ: pointer + PyObject = ptr[PythonsHeap, PyObjectHeader] + + proc createPyObject(): PyObject {.importc: "...".} + proc destroyPyObject(x: PyObject) {.importc: "...".} + + var foo = createPyObject() + # type error here, how convenient: + dealloc(foo) + + +Future directions: + +* Memory regions might become available for ``string`` and ``seq`` too. +* Builtin regions like ``private``, ``global`` and ``local`` will + prove very useful for the upcoming OpenCL target. +* Builtin "regions" can model ``lent`` and ``unique`` pointers. + + + +Procedural type +--------------- +A procedural type is internally a pointer to a procedure. ``nil`` is +an allowed value for variables of a procedural type. Nim uses procedural +types to achieve `functional`:idx: programming techniques. + +Examples: + +.. code-block:: nim + + proc printItem(x: int) = ... + + proc forEach(c: proc (x: int) {.cdecl.}) = + ... + + forEach(printItem) # this will NOT compile because calling conventions differ + + +.. code-block:: nim + + type + TOnMouseMove = proc (x, y: int) {.closure.} + + proc onMouseMove(mouseX, mouseY: int) = + # has default calling convention + echo "x: ", mouseX, " y: ", mouseY + + proc setOnMouseMove(mouseMoveEvent: TOnMouseMove) = discard + + # ok, 'onMouseMove' has the default calling convention, which is compatible + # to 'closure': + setOnMouseMove(onMouseMove) + + +A subtle issue with procedural types is that the calling convention of the +procedure influences the type compatibility: procedural types are only +compatible if they have the same calling convention. As a special extension, +a procedure of the calling convention ``nimcall`` can be passed to a parameter +that expects a proc of the calling convention ``closure``. + +Nim supports these `calling conventions`:idx:\: + +`nimcall`:idx: + is the default convention used for a Nim **proc**. It is the + same as ``fastcall``, but only for C compilers that support ``fastcall``. + +`closure`:idx: + is the default calling convention for a **procedural type** that lacks + any pragma annotations. It indicates that the procedure has a hidden + implicit parameter (an *environment*). Proc vars that have the calling + convention ``closure`` take up two machine words: One for the proc pointer + and another one for the pointer to implicitly passed environment. + +`stdcall`:idx: + This the stdcall convention as specified by Microsoft. The generated C + procedure is declared with the ``__stdcall`` keyword. + +`cdecl`:idx: + The cdecl convention means that a procedure shall use the same convention + as the C compiler. Under windows the generated C procedure is declared with + the ``__cdecl`` keyword. + +`safecall`:idx: + This is the safecall convention as specified by Microsoft. The generated C + procedure is declared with the ``__safecall`` keyword. The word *safe* + refers to the fact that all hardware registers shall be pushed to the + hardware stack. + +`inline`:idx: + The inline convention means the the caller should not call the procedure, + but inline its code directly. Note that Nim does not inline, but leaves + this to the C compiler; it generates ``__inline`` procedures. This is + only a hint for the compiler: it may completely ignore it and + it may inline procedures that are not marked as ``inline``. + +`fastcall`:idx: + Fastcall means different things to different C compilers. One gets whatever + the C ``__fastcall`` means. + +`syscall`:idx: + The syscall convention is the same as ``__syscall`` in C. It is used for + interrupts. + +`noconv`:idx: + The generated C code will not have any explicit calling convention and thus + use the C compiler's default calling convention. This is needed because + Nim's default calling convention for procedures is ``fastcall`` to + improve speed. + +Most calling conventions exist only for the Windows 32-bit platform. + +Assigning/passing a procedure to a procedural variable is only allowed if one +of the following conditions hold: +1) The procedure that is accessed resides in the current module. +2) The procedure is marked with the ``procvar`` pragma (see `procvar pragma`_). +3) The procedure has a calling convention that differs from ``nimcall``. +4) The procedure is anonymous. + +The rules' purpose is to prevent the case that extending a non-``procvar`` +procedure with default parameters breaks client code. + +The default calling convention is ``nimcall``, unless it is an inner proc (a +proc inside of a proc). For an inner proc an analysis is performed whether it +accesses its environment. If it does so, it has the calling convention +``closure``, otherwise it has the calling convention ``nimcall``. + + +Distinct type +------------- + +A ``distinct`` type is new type derived from a `base type`:idx: that is +incompatible with its base type. In particular, it is an essential property +of a distinct type that it **does not** imply a subtype relation between it +and its base type. Explicit type conversions from a distinct type to its +base type and vice versa are allowed. + + +Modelling currencies +~~~~~~~~~~~~~~~~~~~~ + +A distinct type can be used to model different physical `units`:idx: with a +numerical base type, for example. The following example models currencies. + +Different currencies should not be mixed in monetary calculations. Distinct +types are a perfect tool to model different currencies: + +.. code-block:: nim + type + TDollar = distinct int + TEuro = distinct int + + var + d: TDollar + e: TEuro + + echo d + 12 + # Error: cannot add a number with no unit and a ``TDollar`` + +Unfortunately, ``d + 12.TDollar`` is not allowed either, +because ``+`` is defined for ``int`` (among others), not for ``TDollar``. So +a ``+`` for dollars needs to be defined: + +.. code-block:: + proc `+` (x, y: TDollar): TDollar = + result = TDollar(int(x) + int(y)) + +It does not make sense to multiply a dollar with a dollar, but with a +number without unit; and the same holds for division: + +.. code-block:: + proc `*` (x: TDollar, y: int): TDollar = + result = TDollar(int(x) * y) + + proc `*` (x: int, y: TDollar): TDollar = + result = TDollar(x * int(y)) + + proc `div` ... + +This quickly gets tedious. The implementations are trivial and the compiler +should not generate all this code only to optimize it away later - after all +``+`` for dollars should produce the same binary code as ``+`` for ints. +The pragma `borrow`:idx: has been designed to solve this problem; in principle +it generates the above trivial implementations: + +.. code-block:: nim + proc `*` (x: TDollar, y: int): TDollar {.borrow.} + proc `*` (x: int, y: TDollar): TDollar {.borrow.} + proc `div` (x: TDollar, y: int): TDollar {.borrow.} + +The ``borrow`` pragma makes the compiler use the same implementation as +the proc that deals with the distinct type's base type, so no code is +generated. + +But it seems all this boilerplate code needs to be repeated for the ``TEuro`` +currency. This can be solved with templates_. + +.. code-block:: nim + template additive(typ: typedesc): stmt = + proc `+` *(x, y: typ): typ {.borrow.} + proc `-` *(x, y: typ): typ {.borrow.} + + # unary operators: + proc `+` *(x: typ): typ {.borrow.} + proc `-` *(x: typ): typ {.borrow.} + + template multiplicative(typ, base: typedesc): stmt = + proc `*` *(x: typ, y: base): typ {.borrow.} + proc `*` *(x: base, y: typ): typ {.borrow.} + proc `div` *(x: typ, y: base): typ {.borrow.} + proc `mod` *(x: typ, y: base): typ {.borrow.} + + template comparable(typ: typedesc): stmt = + proc `<` * (x, y: typ): bool {.borrow.} + proc `<=` * (x, y: typ): bool {.borrow.} + proc `==` * (x, y: typ): bool {.borrow.} + + template defineCurrency(typ, base: expr): stmt = + type + typ* = distinct base + additive(typ) + multiplicative(typ, base) + comparable(typ) + + defineCurrency(TDollar, int) + defineCurrency(TEuro, int) + + +The borrow pragma can also be used to annotate the distinct type to allow +certain builtin operations to be lifted: + +.. code-block:: nim + type + Foo = object + a, b: int + s: string + + Bar {.borrow: `.`.} = distinct Foo + + var bb: ref Bar + new bb + # field access now valid + bb.a = 90 + bb.s = "abc" + +Currently only the dot accessor can be borrowed in this way. + + +Avoiding SQL injection attacks +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +An SQL statement that is passed from Nim to an SQL database might be +modelled as a string. However, using string templates and filling in the +values is vulnerable to the famous `SQL injection attack`:idx:\: + +.. code-block:: nim + import strutils + + proc query(db: TDbHandle, statement: string) = ... + + var + username: string + + db.query("SELECT FROM users WHERE name = '$1'" % username) + # Horrible security hole, but the compiler does not mind! + +This can be avoided by distinguishing strings that contain SQL from strings +that don't. Distinct types provide a means to introduce a new string type +``TSQL`` that is incompatible with ``string``: + +.. code-block:: nim + type + TSQL = distinct string + + proc query(db: TDbHandle, statement: TSQL) = ... + + var + username: string + + db.query("SELECT FROM users WHERE name = '$1'" % username) + # Error at compile time: `query` expects an SQL string! + + +It is an essential property of abstract types that they **do not** imply a +subtype relation between the abtract type and its base type. Explict type +conversions from ``string`` to ``TSQL`` are allowed: + +.. code-block:: nim + import strutils, sequtils + + proc properQuote(s: string): TSQL = + # quotes a string properly for an SQL statement + return TSQL(s) + + proc `%` (frmt: TSQL, values: openarray[string]): TSQL = + # quote each argument: + let v = values.mapIt(TSQL, properQuote(it)) + # we need a temporary type for the type conversion :-( + type TStrSeq = seq[string] + # call strutils.`%`: + result = TSQL(string(frmt) % TStrSeq(v)) + + db.query("SELECT FROM users WHERE name = '$1'".TSQL % [username]) + +Now we have compile-time checking against SQL injection attacks. Since +``"".TSQL`` is transformed to ``TSQL("")`` no new syntax is needed for nice +looking ``TSQL`` string literals. The hypothetical ``TSQL`` type actually +exists in the library as the `TSqlQuery type <db_sqlite.html#TSqlQuery>`_ of +modules like `db_sqlite <db_sqlite.html>`_. + + +Void type +--------- + +The ``void`` type denotes the absense of any type. Parameters of +type ``void`` are treated as non-existent, ``void`` as a return type means that +the procedure does not return a value: + +.. code-block:: nim + proc nothing(x, y: void): void = + echo "ha" + + nothing() # writes "ha" to stdout + +The ``void`` type is particularly useful for generic code: + +.. code-block:: nim + proc callProc[T](p: proc (x: T), x: T) = + when T is void: + p() + else: + p(x) + + proc intProc(x: int) = discard + proc emptyProc() = discard + + callProc[int](intProc, 12) + callProc[void](emptyProc) + +However, a ``void`` type cannot be inferred in generic code: + +.. code-block:: nim + callProc(emptyProc) + # Error: type mismatch: got (proc ()) + # but expected one of: + # callProc(p: proc (T), x: T) + +The ``void`` type is only valid for parameters and return types; other symbols +cannot have the type ``void``. diff --git a/doc/spawn.txt b/doc/spawn.txt index b2496285f..fb2f851c7 100644 --- a/doc/spawn.txt +++ b/doc/spawn.txt @@ -19,6 +19,36 @@ read prematurely within a ``parallel`` section and so there is no need for the overhead of an indirection via ``FlowVar[T]`` to ensure correctness. +Spawn statement +=============== + +A standalone ``spawn`` statement is a simple construct. It executes +the passed expression on the thread pool and returns a `data flow variable`:idx: +``FlowVar[T]`` that can be read from. The reading with the ``^`` operator is +**blocking**. However, one can use ``awaitAny`` to wait on multiple flow +variables at the same time: + +.. code-block:: nim + import threadpool, ... + + # wait until 2 out of 3 servers received the update: + proc main = + var responses = newSeq[RawFlowVar](3) + for i in 0..2: + responses[i] = spawn tellServer(Update, "key", "value") + var index = awaitAny(responses) + assert index >= 0 + responses.del(index) + discard awaitAny(responses) + +Data flow variables ensure that no data races +are possible. Due to technical limitations not every type ``T`` is possible in +a data flow variable: ``T`` has to be of the type ``ref``, ``string``, ``seq`` +or of a type that doesn't contain a type that is garbage collected. This +restriction will be removed in the future. + + + Parallel statement ================== @@ -64,35 +94,3 @@ restrictions / changes: * Slices are optimized so that no copy is performed. This optimization is not yet performed for ordinary slices outside of a ``parallel`` section. Slices are also special in that they currently do not support negative indexes! - - - - -Spawn statement -=============== - -A standalone ``spawn`` statement is a simple construct. It executes -the passed expression on the thread pool and returns a `data flow variable`:idx: -``FlowVar[T]`` that can be read from. The reading with the ``^`` operator is -**blocking**. However, one can use ``awaitAny`` to wait on multiple flow -variables at the same time: - -.. code-block:: nim - import threadpool, ... - - # wait until 2 out of 3 servers received the update: - proc main = - var responses = newSeq[RawFlowVar](3) - for i in 0..2: - responses[i] = spawn tellServer(Update, "key", "value") - var index = awaitAny(responses) - assert index >= 0 - responses.del(index) - discard awaitAny(responses) - -Like the ``parallel`` statement data flow variables ensure that no data races -are possible. Due to technical limitations not every type ``T`` is possible in -a data flow variable: ``T`` has to be of the type ``ref``, ``string``, ``seq`` -or of a type that doesn't contain a type that is garbage collected. This -restriction will be removed in the future. - diff --git a/doc/tut1.txt b/doc/tut1.txt index b90736bd0..2a836190c 100644 --- a/doc/tut1.txt +++ b/doc/tut1.txt @@ -65,8 +65,8 @@ done with spaces only, tabulators are not allowed. String literals are enclosed in double quotes. The ``var`` statement declares a new variable named ``name`` of type ``string`` with the value that is -returned by the `readLine <system.html#readLine,TFile>`_ procedure. Since the -compiler knows that `readLine <system.html#readLine,TFile>`_ returns a string, +returned by the `readLine <system.html#readLine,File>`_ procedure. Since the +compiler knows that `readLine <system.html#readLine,File>`_ returns a string, you can leave out the type in the declaration (this is called `local type inference`:idx:). So this will work too: @@ -77,7 +77,7 @@ Note that this is basically the only form of type inference that exists in Nim: it is a good compromise between brevity and readability. The "hello world" program contains several identifiers that are already known -to the compiler: ``echo``, `readLine <system.html#readLine,TFile>`_, etc. +to the compiler: ``echo``, `readLine <system.html#readLine,File>`_, etc. These built-ins are declared in the system_ module which is implicitly imported by any other module. @@ -526,7 +526,7 @@ Procedures ========== To define new commands like `echo <system.html#echo>`_ and `readLine -<system.html#readLine,TFile>`_ in the examples, the concept of a `procedure` +<system.html#readLine,File>`_ in the examples, the concept of a `procedure` is needed. (Some languages call them *methods* or *functions*.) In Nim new procedures are defined with the ``proc`` keyword: @@ -1269,7 +1269,7 @@ arguments to a procedure. The compiler converts the list of arguments to an array automatically: .. code-block:: nim - proc myWriteln(f: TFile, a: varargs[string]) = + proc myWriteln(f: File, a: varargs[string]) = for s in items(a): write(f, s) write(f, "\n") @@ -1283,7 +1283,7 @@ last parameter in the procedure header. It is also possible to perform type conversions in this context: .. code-block:: nim - proc myWriteln(f: TFile, a: varargs[string, `$`]) = + proc myWriteln(f: File, a: varargs[string, `$`]) = for s in items(a): write(f, s) write(f, "\n") diff --git a/doc/tut2.txt b/doc/tut2.txt index 0c54fa9a1..651c38838 100644 --- a/doc/tut2.txt +++ b/doc/tut2.txt @@ -130,7 +130,7 @@ The syntax for type conversions is ``destination_type(expression_to_convert)`` proc getID(x: TPerson): int = TStudent(x).id -The ``EInvalidObjectConversion`` exception is raised if ``x`` is not a +The ``InvalidObjectConversionError`` exception is raised if ``x`` is not a ``TStudent``. @@ -164,7 +164,7 @@ An example: condition, thenPart, elsePart: PNode var n = PNode(kind: nkFloat, floatVal: 1.0) - # the following statement raises an `EInvalidField` exception, because + # the following statement raises an `FieldError` exception, because # n.kind's value does not fit: n.strVal = "" @@ -346,9 +346,9 @@ Exceptions ========== In Nim exceptions are objects. By convention, exception types are -prefixed with an 'E', not 'T'. The `system <system.html>`_ module defines an +suffixed with 'Error'. The `system <system.html>`_ module defines an exception hierarchy that you might want to stick to. Exceptions derive from -E_Base, which provides the common interface. +``system.Exception``, which provides the common interface. Exceptions have to be allocated on the heap because their lifetime is unknown. The compiler will prevent you from raising an exception created on the stack. @@ -366,7 +366,7 @@ Raising an exception is done with the ``raise`` statement: .. code-block:: nim var - e: ref EOS + e: ref OSError new(e) e.msg = "the request to the OS failed" raise e @@ -376,7 +376,7 @@ is *re-raised*. For the purpose of avoiding repeating this common code pattern, the template ``newException`` in the ``system`` module can be used: .. code-block:: nim - raise newException(EOS, "the request to the OS failed") + raise newException(OSError, "the request to the OS failed") Try statement @@ -388,17 +388,17 @@ The ``try`` statement handles exceptions: # read the first two lines of a text file that should contain numbers # and tries to add them var - f: TFile + f: File if open(f, "numbers.txt"): try: let a = readLine(f) let b = readLine(f) echo "sum: ", parseInt(a) + parseInt(b) - except EOverflow: + except OverflowError: echo "overflow!" - except EInvalidValue: + except ValueError: echo "could not convert string to integer" - except EIO: + except IOError: echo "IO error!" except: echo "Unknown exception!" @@ -426,7 +426,7 @@ If you need to *access* the actual exception object or message inside an ``except`` branch you can use the `getCurrentException() <system.html#getCurrentException>`_ and `getCurrentExceptionMsg() <system.html#getCurrentExceptionMsg>`_ procs from the `system <system.html>`_ -module. Example: +module. Example: .. code-block:: nim try: @@ -441,9 +441,9 @@ module. Example: Exception hierarchy ------------------- -If you want to create your own exceptions you can inherit from E_Base, but you -can also inherit from one of the existing exceptions if they fit your purpose. -The exception tree is: +If you want to create your own exceptions you can inherit from ``system.Exception``, +but you can also inherit from one of the existing exceptions if they fit your +purpose. The exception tree is: .. include:: exception_hierarchy_fragment.txt @@ -456,12 +456,12 @@ Annotating procs with raised exceptions Through the use of the optional ``{.raises.}`` pragma you can specify that a proc is meant to raise a specific set of exceptions, or none at all. If the ``{.raises.}`` pragma is used, the compiler will verify that this is true. For -instance, if you specify that a proc raises ``EIO``, and at some point it (or -one of the procs it calls) starts raising a new exception the compiler will +instance, if you specify that a proc raises ``IOError``, and at some point it +(or one of the procs it calls) starts raising a new exception the compiler will prevent that proc from compiling. Usage example: .. code-block:: nim - proc complexProc() {.raises: [EIO, EArithmetic].} = + proc complexProc() {.raises: [IOError, ArithmeticError].} = ... proc simpleProc() {.raises: [].} = @@ -624,10 +624,10 @@ via a special ``:`` syntax: .. code-block:: nim - template withFile(f: expr, filename: string, mode: TFileMode, + template withFile(f: expr, filename: string, mode: FileMode, body: stmt): stmt {.immediate.} = let fn = filename - var f: TFile + var f: File if open(f, fn, mode): try: body @@ -767,7 +767,7 @@ use the following snippet of code as the starting point: import strutils, tables - proc readCfgAtRuntime(cfgFilename: string): TTable[string, string] = + proc readCfgAtRuntime(cfgFilename: string): Table[string, string] = let inputString = readFile(cfgFilename) var @@ -801,7 +801,7 @@ to be included along the program containing the license information:: licenseKey,M1Tl3PjBWO2CC48m The ``readCfgAtRuntime`` proc will open the given filename and return a -``TTable`` from the `tables module <tables.html>`_. The parsing of the file is +``Table`` from the `tables module <tables.html>`_. The parsing of the file is done (without much care for handling invalid data or corner cases) using the ``splitLines`` proc from the `strutils module <strutils.html>`_. There are many things which can fail; mind the purpose is explaining how to make this run at @@ -871,7 +871,7 @@ this limitation by using the ``slurp`` proc from the `system module <system.html>`_, which was precisely made for compilation time (just like ``gorge`` which executes an external program and captures its output). -The interesting thing is that our macro does not return a runtime ``TTable`` +The interesting thing is that our macro does not return a runtime ``Table`` object. Instead, it builds up Nim source code into the ``source`` variable. For each line of the configuration file a ``const`` variable will be generated. To avoid conflicts we prefix these variables with ``cfg``. In essence, what the diff --git a/koch.nim b/koch.nim index b7a32a1b4..a43701194 100644 --- a/koch.nim +++ b/koch.nim @@ -38,7 +38,7 @@ Options: --help, -h shows this help and quits Possible Commands: boot [options] bootstraps with given command line options - install [dir] installs to given directory + install [bindir] installs to given directory clean cleans Nimrod project; removes generated files web [options] generates the website csource [options] builds the C sources for installation diff --git a/todo.txt b/todo.txt index c43132cd5..b5d832fe0 100644 --- a/todo.txt +++ b/todo.txt @@ -1,6 +1,7 @@ version 0.10 ============ +- document the new concurrency system - Test nimfix on various babel packages - deprecate recursive tuples; tuple needs laxer type checking - string case should require an 'else' @@ -14,7 +15,6 @@ version 0.9.6 - split idetools into separate tool - split docgen into separate tool -- .benign pragma - scopes are still broken for generic instantiation! Concurrency |