Nim - This repository contains the Nim compiler, Nim's stdlib, tools, and documentation. (mirror)

diff options

Diffstat (limited to 'lib/pure/httpcore.nim')

0 files changed, 0 insertions, 0 deletions

========== Nim Manual ========== :Authors: Andreas Rumpf, Zahary Karadjov :Version: |nimversion| .. contents:: "Complexity" seems to be a lot like "energy": you can transfer it from the end 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 may need more precise wording. This manual is constantly evolving until the 1.0 release and is not to be considered as the final proper specification. 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 occurrences separated by its second argument; likewise ``^+`` means 1 or more occurrences: ``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 the, more easily comprehensible, 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. Whether 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 IndexError: 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! Multiline comments ------------------ Starting with version 0.13.0 of the language Nim supports multiline comments. They look like: .. code-block:: nim #[Comment here. Multiple lines are not a problem.]# Multiline comments support nesting: .. code-block:: nim #[ #[ Multiline comment in already commented out code. ]# proc p[T](x: T) = discard ]# Multiline documentation comments also exist and support nesting too: .. code-block:: nim proc foo = ##[Long documentation comment here. ]## 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. Identifier equality ------------------- Two identifiers are considered equal if the following algorithm returns true: .. code-block:: nim proc sameIdentifier(a, b: string): bool = a[0] == b[0] and a.replace("_", "").toLowerAscii == b.replace("_", "").toLowerAscii That means only the first letters are compared in a case sensitive manner. Other letters are compared case insensitively within the ASCII range and underscores are ignored. This rather unorthodox way to do identifier comparisons is called `partial case insensitivity`:idx: and has some advantages over the conventional case sensitivity: It allows programmers to mostly use their own preferred spelling style, be it humpStyle or snake_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. The exception with respect to the first letter allows common code like ``var foo: Foo`` to be parsed unambiguously. Historically, Nim was a fully `style-insensitive`:idx: language. This meant that it was not case-sensitive and underscores were ignored and there was not even a distinction between ``foo`` and ``Foo``. 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 ================== =================================================== ``\p`` platform specific newline: CRLF on Windows, LF on Unix ``\r``, ``\c`` `carriage return`:idx: ``\n``, ``\l`` `line feed`:idx: (often called `newline`: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: the platform dependent `newline`:idx: (``\p``) 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: ``\n``, ``\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 `Rune` type is used for Unicode characters, it can represent any Unicode character. ``Rune`` 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 = '0' 'o' 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' UINT_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 (['_'] digit)* [exponent]) |exponent) FLOAT32_SUFFIX = ('f' | 'F') ['32'] FLOAT32_LIT = HEX_LIT '\'' FLOAT32_SUFFIX | (FLOAT_LIT | DEC_LIT | OCT_LIT | BIN_LIT) ['\''] FLOAT32_SUFFIX FLOAT64_SUFFIX = ( ('f' | 'F') '64' ) | 'd' | 'D' FLOAT64_LIT = HEX_LIT '\'' FLOAT64_SUFFIX | (FLOAT_LIT | DEC_LIT | OCT_LIT | BIN_LIT) ['\''] FLOAT64_SUFFIX 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 an integer type, unless the literal contains a dot or ``E|e`` in which case it is of type ``float``. This integer type is ``int`` if the literal is in the range ``low(i32)..high(i32)``, otherwise it is ``int64``. 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 ``'f`` float32 ``'d`` float64 ``'f32`` float32 ``'f64`` float64 ``'f128`` float128 ================= ========================= 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. Literals are bounds checked so that they fit the datatype. Non base-10 literals are used mainly for flags and bit pattern representations, therefore bounds checking is done on bit width, not value range. If the literal fits in the bit width of the datatype, it is accepted. Hence: 0b10000000'u8 == 0x80'u8 == 128, but, 0b10000000'i8 == 0x80'i8 == -1 instead of causing an overflow error. Operators --------- Nim allows user defined 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 treated as 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 11 different levels of precedence. Associativity ------------- Binary operators whose first character is ``^`` are right-associative, all other binary operators are left-associative. .. code-block:: nim proc `^/`(x, y: float): float = # a right-associative division operator result = x / y echo 12 ^/ 4 ^/ 8 # 24.0 (4 / 8 = 0.5, then 12 / 0.5 = 24.0) echo 12 / 4 / 8 # 0.375 (12 / 4 = 3.0, then 3 / 8 = 0.375) Precedence ---------- Unary operators always bind stronger than any binary operator: ``$a + b`` is ``($a) + b`` and not ``$(a + b)``. If an unary operator's first character is ``@`` it is a `sigil-like`:idx: operator which binds stronger than a ``primarySuffix``: ``@x.abc`` is parsed as ``(@x).abc`` whereas ``$x.abc`` is parsed as ``$(x.abc)``. For binary operators that are not keywords the precedence is determined by the following rules: Operators ending in either ``->``, ``~>`` or ``=>`` are called `arrow like`:idx:, and have the lowest precedence of all operators. If the operator ends with ``=`` and its first character is none of ``<``, ``>``, ``!``, ``=``, ``~``, ``?``, it is an *assignment operator* which has the second lowest precedence. Otherwise precedence is determined by the first character. ================ =============================================== ================== =============== Precedence level Operators First character Terminal symbol ================ =============================================== ================== =============== 10 (highest) ``$ ^`` OP10 9 ``* / div mod shl shr %`` ``* % \ /`` OP9 8 ``+ -`` ``+ - ~ |`` OP8 7 ``&`` ``&`` OP7 6 ``..`` ``.`` OP6 5 ``== <= < >= > != in notin is isnot not of`` ``= < > !`` OP5 4 ``and`` OP4 3 ``or xor`` OP3 2 ``@ : ?`` OP2 1 *assignment operator* (like ``+=``, ``*=``) OP1 0 (lowest) *arrow like operator* (like ``->``, ``=>``) OP0 ================ =============================================== ================== =============== Whether an operator is used a prefix operator is also affected by preceding whitespace (this parsing change was introduced with version 0.13.0): .. code-block:: nim echo $foo # is parsed as echo($foo) Spacing also determines whether ``(a, b)`` is parsed as an the argument list of a call or whether it is parsed as a tuple constructor: .. code-block:: nim echo(1, 2) # pass 1 and 2 to echo .. code-block:: nim echo (1, 2) # pass the tuple (1, 2) to echo Grammar ------- The grammar's start symbol is ``module``. .. include:: grammar.txt :literal: Order of evaluation =================== Order of evaluation is strictly left-to-right, inside-out as it is typical for most others imperative programming languages: .. code-block:: nim :test: "nim c $1" var s = "" proc p(arg: int): int = s.add $arg result = arg discard p(p(1) + p(2)) doAssert s == "123" Assignments are not special, the left-hand-side expression is evaluated before the right-hand side: .. code-block:: nim :test: "nim c $1" var v = 0 proc getI(): int = result = v inc v var a, b: array[0..2, int] proc someCopy(a: var int; b: int) = a = b a[getI()] = getI() doAssert a == [1, 0, 0] v = 0 someCopy(b[getI()], getI()) doAssert b == [1, 0, 0] Rationale: Consistency with overloaded assignment or assignment-like operations, ``a = b`` can be read as ``performSomeCopy(a, b)``. 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 if it is in the range ``low(int32)..high(int32)`` otherwise the literal's type is ``int64``. 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 <#type-relations-convertible-relation>`_. Subrange types -------------- A subrange type is a range of values from an ordinal or floating point 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] PositiveFloat = range[0.0..Inf] ``Subrange`` is a subrange of an integer which can only hold the values 0 to 5. ``PositiveFloat`` defines a subrange of all positive floating point values. NaN does not belong to any subrange of floating point types. 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 Subrange example). Pre-defined floating point types -------------------------------- The following floating point types are pre-defined: ``float`` the generic floating point type; its size used to be platform dependent, but now it is always mapped to ``float64``. 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: `FloatInvalidOpError`:idx:, `FloatDivByZeroError`:idx:, `FloatOverflowError`:idx:, `FloatUnderflowError`:idx:, and `FloatInexactError`:idx:. These exceptions inherit from the `FloatingPointError`: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 FloatInvalidOpError echo a / b # raises FloatOverflowError In the current implementation ``FloatDivByZeroError`` and ``FloatInexactError`` are never raised. ``FloatOverflowError`` is raised instead of ``FloatDivByZeroError``. 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 `Rune` type is used for Unicode characters, it can represent any Unicode character. ``Rune`` 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 Direction = 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 TokenType = 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 MyEnum = 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 ``MyEnum.value``: .. code-block:: nim type MyEnum {.pure.} = enum valueA, valueB, valueC, valueD echo valueA # error: Unknown identifier echo MyEnum.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. Most native Nim types support conversion to strings with the special ``$`` proc. When calling the ``echo`` proc, for example, the built-in stringify operation for the parameter is called: .. code-block:: nim echo 3 # calls `$` for `int` Whenever a user creates a specialized object, implementation of this procedure provides for ``string`` representation. .. code-block:: nim type Person = object name: string age: int proc `$`(p: Person): string = # `$` always returns a string result = p.name & " is " & $p.age & # we *need* the `$` in front of p.age which # is natively an integer to convert it to # a string " years old." While ``$p.name`` can also be used, the ``$`` operation on a string does nothing. Note that we cannot rely on automatic conversion from an ``int`` to a ``string`` like we can for the ``echo`` proc. 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 meaning `compatible string` is the native representation of a string for the compilation backend. For the C backend 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 = str 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 ``[]``. The element type of this array expression is inferred from the type of the first element. All other elements need to be implicitly convertable to this type. 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 IntArray = array[0..5, int] # an array that is indexed with 0..5 IntSeq = seq[int] # a sequence of integers var x: IntArray y: IntSeq 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 let z = [1.0, 2, 3, 4] # the type of z is array[0..3, float] 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. .. code-block:: nim proc testOpenArray(x: openArray[int]) = echo repr(x) testOpenArray([1,2,3]) # array[] testOpenArray(@[1,2,3]) # seq[] 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: File, 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: File, 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 ``$`` applied to strings is a nop.) Note that an explicit array constructor passed to a ``varargs`` parameter is not wrapped in another implicit array construction: .. code-block:: nim proc takeV[T](a: varargs[T]) = discard takeV([123, 2, 1]) # takeV's T is "int", not "array of int" ``varargs[typed]`` is treated specially: It matches a variable list of arguments of arbitrary type but *always* constructs an implicit array. This is required so that the builtin ``echo`` proc does what is expected: .. code-block:: nim proc echo*(x: varargs[typed, `$`]) {...} echo @[1, 2, 3] # prints "@[1, 2, 3]" and not "123" 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. The assignment operator for tuples copies each component. The default assignment operator for objects copies each component. Overloading of the assignment operator is described in `type-bound-operations-operator`_. .. code-block:: nim type Person = tuple[name: string, age: int] # type representing a person: # a person consists of a name # and an age var person: Person person = (name: "Peter", age: 30) # the same, but less readable: person = ("Peter", 30) A tuple with one unnamed field can be constructed with the parentheses and a trailing comma: .. code-block:: nim proc echoUnaryTuple(a: (int,)) = echo a[0] echoUnaryTuple (1,) In fact, a trailing comma is allowed for every tuple construction. 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 Person = 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. The ``of`` operator is similar to the ``instanceof`` operator in Java. .. code-block:: nim type Person = object of RootObj name*: string # the * means that `name` is accessible from other modules age: int # no * means that the field is hidden Student = ref object of Person # a student is a person id: int # with an id field var student: Student person: Person assert(student of Student) # is true assert(student of Person) # also 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.RootObj``. 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 = Student(name: "Anton", age: 5, id: 3) Note that, unlike tuples, objects require the field names along with their values. 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 NodeKind = 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 Node = ref NodeObj NodeObj = object case kind: NodeKind # the ``kind`` field is the discriminator of nkInt: intVal: int of nkFloat: floatVal: float of nkString: strVal: string of nkAdd, nkSub: leftOp, rightOp: Node of nkIf: condition, thenPart, elsePart: Node # create a new case object: var n = Node(kind: nkIf, condition: nil) # accessing n.thenPart is valid because the ``nkIf`` branch is active: n.thenPart = Node(kind: nkFloat, floatVal: 2.0) # the following statement raises an `FieldError` 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 = Node(kind: nkAdd, leftOp: Node(kind: nkInt, intVal: 4), rightOp: Node(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. Also, when the fields of a particular branch are specified during object construction, the correct value for the discriminator must be supplied at compile-time. Package level objects --------------------- Every Nim module resides in a (nimble) package. An object type can be attached to the package it resides in. If that is done, the type can be referenced from other modules as an `incomplete`:idx: object type. This features allows to break up recursive type dependencies accross module boundaries. Incomplete object types are always passed ``byref`` and can only be used in pointer like contexts (``var/ref/ptr IncompleteObject``) in general since the compiler does not yet know the size of the object. To complete an incomplete object the ``package`` pragma has to be used. ``package`` implies ``byref``. As long as a type ``T`` is incomplete ``sizeof(T)`` or "runtime type information" for ``T`` is not available. Example: .. code-block:: nim # module A (in an arbitrary package) type Pack.SomeObject = object ## declare as incomplete object of package 'Pack' Triple = object a, b, c: ref SomeObject ## pointers to incomplete objects are allowed ## Incomplete objects can be used as parameters: proc myproc(x: SomeObject) = discard .. code-block:: nim # module B (in package "Pack") type SomeObject* {.package.} = object ## Use 'package' to complete the object s, t: string x, y: int 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. In general, a `ptr T` is implicitly convertible to the `pointer` type. 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 Node = ref NodeObj NodeObj = object le, ri: Node data: int var n: Node new(n) n.data = 9 # no need to write n[].data; in fact n[].data is highly discouraged! Automatic dereferencing is also performed for the first argument of a routine call. But currently this feature has to be only enabled via ``{.experimental: "implicitDeref".}``: .. code-block:: nim {.experimental: "implicitDeref".} proc depth(x: NodeObj): int = ... var n: Node new(n) echo n.depth # no need to write n[].depth either In order to simplify structural type checking, recursive tuples are not valid: .. code-block:: nim # invalid recursion type MyTuple = tuple[a: ref MyTuple] Likewise ``T = ref T`` is an invalid type. 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 Data = tuple[x, y: int, s: string] # allocate memory for Data on the heap: var d = cast[ptr Data](alloc0(sizeof(Data))) # 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 Data``. 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 binary zero 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 non nilable pointers. The details of this analysis are still to be specified here. 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 OnMouseMove = proc (x, y: int) {.closure.} proc onMouseMove(mouseX, mouseY: int) = # has default calling convention echo "x: ", mouseX, " y: ", mouseY proc setOnMouseMove(mouseMoveEvent: OnMouseMove) = 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. 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. See also ``distinctBase`` to get the reverse operation. 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 Dollar = distinct int Euro = distinct int var d: Dollar e: Euro echo d + 12 # Error: cannot add a number with no unit and a ``Dollar`` Unfortunately, ``d + 12.Dollar`` is not allowed either, because ``+`` is defined for ``int`` (among others), not for ``Dollar``. So a ``+`` for dollars needs to be defined: .. code-block:: proc `+` (x, y: Dollar): Dollar = result = Dollar(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: Dollar, y: int): Dollar = result = Dollar(int(x) * y) proc `*` (x: int, y: Dollar): Dollar = result = Dollar(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: Dollar, y: int): Dollar {.borrow.} proc `*` (x: int, y: Dollar): Dollar {.borrow.} proc `div` (x: Dollar, y: int): Dollar {.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 ``Euro`` currency. This can be solved with templates_. .. code-block:: nim template additive(typ: type) = 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: type) = 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: type) = proc `<` * (x, y: typ): bool {.borrow.} proc `<=` * (x, y: typ): bool {.borrow.} proc `==` * (x, y: typ): bool {.borrow.} template defineCurrency(typ, base: untyped) = type typ* = distinct base additive(typ) multiplicative(typ, base) comparable(typ) defineCurrency(Dollar, int) defineCurrency(Euro, 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: DbHandle, 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 ``SQL`` that is incompatible with ``string``: .. code-block:: nim type SQL = distinct string proc query(db: DbHandle, statement: SQL) = ... 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 abstract type and its base type. Explicit type conversions from ``string`` to ``SQL`` are allowed: .. code-block:: nim import strutils, sequtils proc properQuote(s: string): SQL = # quotes a string properly for an SQL statement return SQL(s) proc `%` (frmt: SQL, values: openarray[string]): SQL = # quote each argument: let v = values.mapIt(SQL, properQuote(it)) # we need a temporary type for the type conversion :-( type StrSeq = seq[string] # call strutils.`%`: result = SQL(string(frmt) % StrSeq(v)) db.query("SELECT FROM users WHERE name = '$1'".SQL % [username]) Now we have compile-time checking against SQL injection attacks. Since ``"".SQL`` is transformed to ``SQL("")`` no new syntax is needed for nice looking ``SQL`` string literals. The hypothetical ``SQL`` 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 absence 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``. Auto type --------- The ``auto`` type can only be used for return types and parameters. For return types it causes the compiler to infer the type from the routine body: .. code-block:: nim proc returnsInt(): auto = 1984 For parameters it currently creates implicitly generic routines: .. code-block:: nim proc foo(a, b: auto) = discard Is the same as: .. code-block:: nim proc foo[T1, T2](a: T1, b: T2) = discard However later versions of the language might change this to mean "infer the parameters' types from the body". Then the above ``foo`` would be rejected as the parameters' types can not be inferred from an empty ``discard`` statement. 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 HashSet[(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: HashSet[(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! Covariance ---------- Covariance in Nim can be introduced only though pointer-like types such as ``ptr`` and ``ref``. Sequence, Array and OpenArray types, instantiated with pointer-like types will be considered covariant if and only if they are also immutable. The introduction of a ``var`` modifier or additional ``ptr`` or ``ref`` indirections would result in invariant treatment of these types. ``proc`` types are currently always invariant, but future versions of Nim may relax this rule. User-defined generic types may also be covariant with respect to some of their parameters. By default, all generic params are considered invariant, but you may choose the apply the prefix modifier ``in`` to a parameter to make it contravariant or ``out`` to make it covariant: .. code-block:: nim type AnnotatedPtr[out T] = metadata: MyTypeInfo p: ref T RingBuffer[out T] = startPos: int data: seq[T] Action {.importcpp: "std::function<void ('0)>".} [in T] = object When the designated generic parameter is used to instantiate a pointer-like type as in the case of `AnnotatedPtr` above, the resulting generic type will also have pointer-like covariance: .. code-block:: nim type GuiWidget = object of RootObj Button = object of GuiWidget ComboBox = object of GuiWidget var widgetPtr: AnnotatedPtr[GuiWidget] buttonPtr: AnnotatedPtr[Button] ... proc drawWidget[T](x: AnnotatedPtr[GuiWidget]) = ... # you can call procs expecting base types by supplying a derived type drawWidget(buttonPtr) # and you can convert more-specific pointer types to more general ones widgetPtr = buttonPtr Just like with regular pointers, covariance will be enabled only for immutable values: .. code-block:: nim proc makeComboBox[T](x: var AnnotatedPtr[GuiWidget]) = x.p = new(ComboBox) makeComboBox(buttonPtr) # Error, AnnotatedPtr[Button] cannot be modified # to point to a ComboBox On the other hand, in the `RingBuffer` example above, the designated generic param is used to instantiate the non-pointer ``seq`` type, which means that the resulting generic type will have covariance that mimics an array or sequence (i.e. it will be covariant only when instantiated with ``ptr`` and ``ref`` types): .. code-block:: nim type Base = object of RootObj Derived = object of Base proc consumeBaseValues(b: RingBuffer[Base]) = ... var derivedValues: RingBuffer[Derived] consumeBaseValues(derivedValues) # Error, Base and Derived values may differ # in size proc consumeBasePointers(b: RingBuffer[ptr Base]) = ... var derivedPointers: RingBuffer[ptr Derived] consumeBaseValues(derivedPointers) # This is legal Please note that Nim will treat the user-defined pointer-like types as proper alternatives to the built-in pointer types. That is, types such as `seq[AnnotatedPtr[T]]` or `RingBuffer[AnnotatedPtr[T]]` will also be considered covariant and you can create new pointer-like types by instantiating other user-defined pointer-like types. The contravariant parameters introduced with the ``in`` modifier are currently useful only when interfacing with imported types having such semantics. 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 = if isSubtype(a, b) or isCovariant(a, b): return true 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 ====================== In a call ``p(args)`` the routine ``p`` that matches best is selected. If multiple routines match equally well, the ambiguity is reported at compiletime. Every arg in args needs to match. There are multiple different categories how an argument can match. Let ``f`` be the formal parameter's type and ``a`` the type of the argument. 1. Exact match: ``a`` and ``f`` are of the same type. 2. Literal match: ``a`` is an integer literal of value ``v`` and ``f`` is a signed or unsigned integer type and ``v`` is in ``f``'s range. Or: ``a`` is a floating point literal of value ``v`` and ``f`` is a floating point type and ``v`` is in ``f``'s range. 3. Generic match: ``f`` is a generic type and ``a`` matches, for instance ``a`` is ``int`` and ``f`` is a generic (constrained) parameter type (like in ``[T]`` or ``[T: int|char]``. 4. Subrange or subtype match: ``a`` is a ``range[T]`` and ``T`` matches ``f`` exactly. Or: ``a`` is a subtype of ``f``. 5. Integral conversion match: ``a`` is convertible to ``f`` and ``f`` and ``a`` is some integer or floating point type. 6. Conversion match: ``a`` is convertible to ``f``, possibly via a user defined ``converter``. These matching categories have a priority: An exact match is better than a literal match and that is better than a generic match etc. In the following ``count(p, m)`` counts the number of matches of the matching category ``m`` for the routine ``p``. A routine ``p`` matches better than a routine ``q`` if the following algorithm returns true:: for each matching category m in ["exact match", "literal match", "generic match", "subtype match", "integral match", "conversion match"]: if count(p, m) > count(q, m): return true elif count(p, m) == count(q, m): discard "continue with next category m" else: return false return "ambiguous" Some examples: .. code-block:: nim proc takesInt(x: int) = echo "int" proc takesInt[T](x: T) = echo "T" proc takesInt(x: int16) = echo "int16" takesInt(4) # "int" var x: int32 takesInt(x) # "T" var y: int16 takesInt(y) # "int16" var z: range[0..4] = 0 takesInt(z) # "T" If this algorithm returns "ambiguous" further disambiguation is performed: If the argument ``a`` matches both the parameter type ``f`` of ``p`` and ``g`` of ``q`` via a subtyping relation, the inheritance depth is taken into account: .. code-block:: nim type A = object of RootObj B = object of A C = object of B proc p(obj: A) = echo "A" proc p(obj: B) = echo "B" var c = C() # not ambiguous, calls 'B', not 'A' since B is a subtype of A # but not vice versa: p(c) proc pp(obj: A, obj2: B) = echo "A B" proc pp(obj: B, obj2: A) = echo "B A" # but this is ambiguous: pp(c, c) Likewise for generic matches the most specialized generic type (that still matches) is preferred: .. code-block:: nim proc gen[T](x: ref ref T) = echo "ref ref T" proc gen[T](x: ref T) = echo "ref T" proc gen[T](x: T) = echo "T" var ri: ref int gen(ri) # "ref T" Overloading based on 'var T' ---------------------------- If the formal parameter ``f`` is of type ``var T`` in addition to the ordinary type checking, the argument is checked to be an `l-value`:idx:. ``var T`` matches better than just ``T`` then. .. code-block:: nim proc sayHi(x: int): string = # matches a non-var int result = $x proc sayHi(x: var int): string = # matches a var int result = $(x + 10) proc sayHello(x: int) = var m = x # a mutable version of x echo sayHi(x) # matches the non-var version of sayHi echo sayHi(m) # matches the var version of sayHi sayHello(3) # 3 # 13 Automatic dereferencing ----------------------- If the `experimental mode <#pragmas-experimental-pragma>`_ is active and no other match is found, the first argument ``a`` is dereferenced automatically if it's a pointer type and overloading resolution is tried with ``a[]`` instead. Automatic self insertions ------------------------- **Note**: The ``.this`` pragma is deprecated and should not be used anymore. Starting with version 0.14 of the language, Nim supports ``field`` as a shortcut for ``self.field`` comparable to the `this`:idx: keyword in Java or C++. This feature has to be explicitly enabled via a ``{.this: self.}`` statement pragma. This pragma is active for the rest of the module: .. code-block:: nim type Parent = object of RootObj parentField: int Child = object of Parent childField: int {.this: self.} proc sumFields(self: Child): int = result = parentField + childField # is rewritten to: # result = self.parentField + self.childField Instead of ``self`` any other identifier can be used too, but ``{.this: self.}`` will become the default directive for the whole language eventually. In addition to fields, routine applications are also rewritten, but only if no other interpretation of the call is possible: .. code-block:: nim proc test(self: Child) = echo childField, " ", sumFields() # is rewritten to: echo self.childField, " ", sumFields(self) # but NOT rewritten to: echo self, self.childField, " ", sumFields(self) Lazy type resolution for untyped -------------------------------- **Note**: An `unresolved`:idx: expression is an expression for which no symbol lookups and no type checking have been performed. Since templates and macros that are not declared as ``immediate`` participate in overloading resolution it's essential to have a way to pass unresolved expressions to a template or macro. This is what the meta-type ``untyped`` accomplishes: .. code-block:: nim template rem(x: untyped) = discard rem unresolvedExpression(undeclaredIdentifier) A parameter of type ``untyped`` always matches any argument (as long as there is any argument passed to it). But one has to watch out because other overloads might trigger the argument's resolution: .. code-block:: nim template rem(x: untyped) = discard proc rem[T](x: T) = discard # undeclared identifier: 'unresolvedExpression' rem unresolvedExpression(undeclaredIdentifier) ``untyped`` and ``varargs[untyped]`` are the only metatype that are lazy in this sense, the other metatypes ``typed`` and ``type`` are not lazy. Varargs matching ---------------- See `Varargs <#types-varargs>`_. 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 indented. 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 Void context ------------ In a list of statements every expression except the last one needs to have the type ``void``. In addition to this rule an assignment to the builtin ``result`` symbol also triggers a mandatory ``void`` context for the subsequent expressions: .. code-block:: nim proc invalid*(): string = result = "foo" "invalid" # Error: value of type 'string' has to be discarded .. code-block:: nim proc valid*(): string = let x = 317 "valid" 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 ``@[]`` string ``""`` 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 for the object and all of its fields. 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 MyObject = object {.requiresInit.} proc p() = # the following is valid: var x: MyObject 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 same as that 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. Tuple unpacking --------------- In a ``var`` or ``let`` statement tuple unpacking can be performed. The special identifier ``_`` can be used to ignore some parts of the tuple: .. code-block:: nim proc returnsTuple(): (int, int, int) = (4, 2, 3) let (x, _, z) = returnsTuple() 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 <#pragmas-nosideeffect-pragma>`_ for details) and if ``X`` is a (possibly empty) list of compile-time computable arguments. Constants cannot be of type ``ptr``, ``ref`` or ``var``, 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 next statement. In ``if`` statements new scopes begin immediately after the ``if``/``elif``/``else`` keywords and ends after the corresponding *then* block. For visualization purposes the scopes have been enclosed in ``{| |}`` in the following example: .. 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: {| echo "m not declared here" |} 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. To suppress the static error an ``else`` part with an empty ``discard`` statement should be used. For non ordinal types it is not possible to list every possible value and so these always require an ``else`` part. As case statements perform compile-time exhaustiveness checks, the value in every ``of`` branch must be known at compile time. This fact is also exploited to generate more performant code. 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. When nimvm statement -------------------- ``nimvm`` is a special symbol, that may be used as expression of ``when nimvm`` statement to differentiate execution path between runtime and compile time. Example: .. code-block:: nim proc someProcThatMayRunInCompileTime(): bool = when nimvm: # This code runs in compile time result = true else: # This code runs in runtime result = false const ctValue = someProcThatMayRunInCompileTime() let rtValue = someProcThatMayRunInCompileTime() assert(ctValue == true) assert(rtValue == false) ``when nimvm`` statement must meet the following requirements: * Its expression must always be ``nimvm``. More complex expressions are not allowed. * It must not contain ``elif`` branches. * It must contain ``else`` branch. * Code in branches must not affect semantics of the code that follows the ``when nimvm`` statement. E.g. it must not define symbols that are used in the following code. 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:" var pw = readLine(stdin) while pw != "12345": echo "Wrong password! Next try:" 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 --------------- The using statement provides syntactic convenience in modules where the same parameter names and types are used over and over. Instead of: .. code-block:: nim proc foo(c: Context; n: Node) = ... proc bar(c: Context; n: Node, counter: int) = ... proc baz(c: Context; n: Node) = ... One can tell the compiler about the convention that a parameter of name ``c`` should default to type ``Context``, ``n`` should default to ``Node`` etc.: .. code-block:: nim using c: Context n: Node counter: int proc foo(c, n) = ... proc bar(c, n, counter) = ... proc baz(c, n) = ... The ``using`` section uses the same indentation based grouping syntax as a ``var`` or ``let`` section. Note that ``using`` is not applied for ``template`` since untyped template parameters default to the type ``system.untyped``. 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 unusual 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). Ordinary procs are often preferred over type conversions in Nim: For instance, ``$`` is the ``toString`` operator by convention and ``toFloat`` and ``toInt`` can be used to convert from floating point to integer or vice versa. 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. A procedure declaration consists of an identifier, zero or more formal parameters, a return value type and a block of code. Formal parameters are declared as a list of identifiers separated by either comma or semicolon. A parameter is given a type by ``: typename``. The type applies to all parameters immediately before it, until either the beginning of the parameter list, a semicolon separator or an already typed parameter, is reached. The semicolon can be used to make separation of types and subsequent identifiers more distinct. .. code-block:: nim # Using only commas proc foo(a, b: int, c, d: bool): int # Using semicolon for visual distinction proc foo(a, b: int; c, d: bool): int # Will fail: a is untyped since ';' stops type propagation. proc foo(a; b: int; c, d: bool): int A parameter may be declared with a default value which is used if the caller does not provide a value for the argument. .. code-block:: nim # b is optional with 47 as its default value proc foo(a: int, b: int = 47): int Parameters can be declared mutable and so allow the proc to modify those arguments, by using the type modifier `var`. .. code-block:: nim # "returning" a value to the caller through the 2nd argument # Notice that the function uses no actual return value at all (ie void) proc foo(inp: int, outp: var int) = outp = inp + 47 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 determines which proc 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' # (x=0, y=1, s="abc", c='\t', b=false) A procedure may call itself recursively. `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) Export marker ------------- If a declared symbol is marked with an `asterisk`:idx: it is exported from the current module: .. code-block:: nim proc exportedEcho*(s: string) = echo s proc `*`*(a: string; b: int): string = result = newStringOfCap(a.len * b) for i in 1..b: result.add a var exportedVar*: int const exportedConst* = 78 type ExportedType* = object exportedField*: int 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.writeLine("Hallo") # the same as writeLine(stdout, "Hallo") Another way to look at the method call syntax is that it provides the missing postfix notation. The method call syntax conflicts with explicit generic instantiations: ``p[T](x)`` cannot be written as ``x.p[T]`` because ``x.p[T]`` is always parsed as ``(x.p)[T]``. See also: `Limitations of the method call syntax <#templates-limitations-of-the-method-call-syntax>`_. The ``[: ]`` notation has been designed to mitigate this issue: ``x.p[:T]`` is rewritten by the parser to ``p[T](x)``, ``x.p[:T](y)`` is rewritten to ``p[T](x, y)``. Note that ``[: ]`` has no AST representation, the rewrite is performed directly in the parsing step. 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 # Module asocket type Socket* = ref object of RootObj host: int # cannot be accessed from the outside of the module proc `host=`*(s: var Socket, value: int) {.inline.} = ## setter of hostAddr. ## This accesses the 'host' field and is not a recursive call to ## ``host=`` because the builtin dot access is preferred if it is ## avaliable: s.host = value proc host*(s: Socket): int {.inline.} = ## getter of hostAddr ## This accesses the 'host' field and is not a recursive call to ## ``host`` because the builtin dot access is preferred if it is ## avaliable: s.host .. code-block:: nim # module B import asocket var s: Socket new s s.host = 34 # same as `host=`(s, 34) Command invocation syntax ------------------------- Routines can be invoked without the ``()`` if the call is syntactically 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. Creating closures in loops ~~~~~~~~~~~~~~~~~~~~~~~~~~ Since closures capture local variables by reference it is often not wanted behavior inside loop bodies. See `closureScope <system.html#closureScope>`_ for details on how to change this behavior. 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", "Kyiv"] 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. Func ---- The ``func`` keyword introduces a shortcut for a `noSideEffect`:idx: proc. .. code-block:: nim func binarySearch[T](a: openArray[T]; elem: T): int Is short for: .. code-block:: nim proc binarySearch[T](a: openArray[T]; elem: T): int {.noSideEffect.} 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 # In macros, the do notation is often used for quasi-quoting macroResults.add quote do: if not `ex`: echo `info`, ": Check failed: ", `expString` ``do`` is written after the parentheses enclosing the regular proc params. The proc expression represented by the do block is appended to them. In calls using the command syntax, the do block will bind to the immediately preceeding expression, transforming it in a call. ``do`` with parentheses is an anonymous ``proc``; however a ``do`` without parentheses is just a block of code. The ``do`` notation can be used to pass multiple blocks to a macro: .. code-block:: nim macro performWithUndo(task, undo: untyped) = ... performWithUndo do: # multiple-line block of code # to perform the task do: # code to undo it Nonoverloadable builtins ------------------------ The following builtin procs cannot be overloaded for reasons of implementation simplicity (they require specialized semantic checking):: declared, defined, definedInScope, compiles, sizeOf, is, shallowCopy, getAst, astToStr, spawn, procCall 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. From this list the following should not be written in dot notation ``x.f`` since ``x`` cannot be type checked before it gets passed to ``f``:: declared, defined, definedInScope, compiles, getAst, astToStr 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 **Note**: ``var`` parameters are never necessary for efficient parameter passing. Since non-var parameters cannot be modified the compiler is always free to pass arguments by reference if it considers it can speed up execution. 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. .. include:: manual/var_t_return.rst Future directions ~~~~~~~~~~~~~~~~~ Later versions of Nim can be more precise about the borrowing rule with a syntax like: .. code-block:: nim proc foo(other: Y; container: var X): var T from container Here ``var T from container`` explicitly exposes that the location is deviated from the second parameter (called 'container' in this case). The syntax ``var T from p`` specifies a type ``varTy[T, 2]`` which is incompatible with ``varTy[T, 1]``. 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. For dynamic dispatch to work on an object it should be a reference type as well. .. code-block:: nim type Expression = ref object of RootObj ## abstract base class for an expression Literal = ref object of Expression x: int PlusExpr = ref object of Expression a, b: Expression method eval(e: Expression): int {.base.} = # override this base method quit "to override!" method eval(e: Literal): int = return e.x method eval(e: PlusExpr): int = # watch out: relies on dynamic binding result = eval(e.a) + eval(e.b) proc newLit(x: int): Literal = new(result) result.x = x proc newPlus(a, b: Expression): PlusExpr = 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. As can be seen in the example, base methods have to be annotated with the `base`:idx: pragma. The ``base`` pragma also acts as a reminder for the programmer that a base method ``m`` is used as the foundation to determine all the effects that a call to ``m`` might cause. In a multi-method all parameters that have an object type are used for the dispatching: .. code-block:: nim type Thing = ref object of RootObj Unit = ref object of Thing x: int method collide(a, b: Thing) {.base, inline.} = quit "to override!" method collide(a: Thing, b: Unit) {.inline.} = echo "1" method collide(a: Unit, b: Thing) {.inline.} = echo "2" var a, b: Unit new a new b 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 ``Unit, Thing`` is preferred over ``Thing, Unit``. **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 overloads 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) and ends iteration. 4. Neither inline nor closure iterators can be recursive. 5. Closure iterators are not supported by the js backend. Iterators that are neither marked ``{.closure.}`` nor ``{.inline.}`` explicitly default to being inline, but 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 Task = 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[Task]) = 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. Note that ``system.finished`` is error prone to use because it only returns ``true`` one iteration after the iterator has finished: .. code-block:: nim iterator mycount(a, b: int): int {.closure.} = var x = a while x <= b: yield x inc x var c = mycount # instantiate the iterator while not finished(c): echo c(1, 3) # Produces 1 2 3 0 Instead this code has to be used: .. code-block:: nim var c = mycount # instantiate the iterator while true: let value = c(1, 3) if finished(c): break # and discard 'value'! echo value It helps to think that the iterator actually returns a pair ``(value, done)`` and ``finished`` is used to access the hidden ``done`` field. 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 iterators must always produce values that will be consumed in a for loop, the compiler will implicitly 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. Converters ========== A converter is like an ordinary proc except that it enhances the "implicitly convertible" type relation (see `Convertible relation`_): .. code-block:: nim # bad style ahead: Nim is not C. converter toBool(x: int): bool = x != 0 if 4: echo "compiles" A converter can also be explicitly invoked for improved readability. Note that implicit converter chaining is not supported: If there is a converter from type A to type B and from type B to type C the implicit conversion from A to C is not provided. Type sections ============= Example: .. code-block:: nim type # example demonstrating mutually recursive types Node = ref object # an object managed by the garbage collector (ref) le, ri: Node # left and right subtrees sym: ref Sym # leaves contain a reference to a Sym Sym = object # a symbol name: string # the symbol's name line: int # the line the symbol was declared in code: Node # 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: 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). Try expression -------------- Try can also be used as an expression; the type of the ``try`` branch then needs to fit the types of ``except`` branches, but the type of the ``finally`` branch always has to be ``void``: .. code-block:: nim let x = try: parseInt("133a") except: -1 finally: echo "hi" To prevent confusing code there is a parsing limitation; if the ``try`` follows a ``(`` it has to be written as a one liner: .. code-block:: nim let x = (try: parseInt("133a") except: -1) Except clauses -------------- Within an ``except`` clause, it is possible to use ``getCurrentException`` to retrieve the exception that has been raised: .. code-block:: nim try: # ... except IOError: let e = getCurrentException() # Now use "e" Note that ``getCurrentException`` always returns a ``ref Exception`` type. If a variable of the proper type is needed (in the example above, ``IOError``), one must convert it explicitly: .. code-block:: nim try: # ... except IOError: let e = (ref IOError)(getCurrentException()) # "e" is now of the proper type However, this is seldom needed. The most common case is to extract an error message from ``e``, and for such situations it is enough to use ``getCurrentExceptionMsg``: .. code-block:: nim try: # ... except IOError: echo "I/O error: " & getCurrentExceptionMsg() Defer statement --------------- Instead of a ``try finally`` statement a ``defer`` statement can be used. Any statements following the ``defer`` in the current block will be considered to be in an implicit try block: .. code-block:: nim :test: "nim c $1" proc main = var f = open("numbers.txt") defer: close(f) f.write "abc" f.write "def" Is rewritten to: .. code-block:: nim :test: "nim c $1" proc main = var f = open("numbers.txt") try: f.write "abc" f.write "def" finally: close(f) Top level ``defer`` statements are not supported since it's unclear what such a statement should refer to. 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. Exception hierarchy ------------------- The exception tree is defined in the `system <system.html>`_ module. Every exception inherits from ``system.Exception``. Exceptions that indicate programming bugs inherit from ``system.Defect`` (which is a subtype of ``Exception``) and are stricly speaking not catchable as they can also be mapped to an operation that terminates the whole process. Exceptions that indicate any other runtime error that can be caught inherit from ``system.CatchableError`` (which is a subtype of ``Exception``). Imported exceptions ------------------- It is possible to raise/catch imported C++ exceptions. Types imported using `importcpp` can be raised or caught. Exceptions are raised by value and caught by reference. Example: .. code-block:: nim type std_exception {.importcpp: "std::exception", header: "<exception>".} = object proc what(s: std_exception): cstring {.importcpp: "((char *)#.what())".} try: raise std_exception() except std_exception as ex: echo ex.what() 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 :test: "nim c $1" 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 :test: "nim c $1" :status: 1 type Callback = proc (s: string) {.raises: [IOError].} var c: Callback 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 within 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 :test: "nim c $1" :status: 1 type IO = object ## input/output effect proc readLine(): string {.tags: [IO].} = discard 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. 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 :test: "nim c $1" type BinaryTree*[T] = ref object # BinaryTree is a generic type with # generic param ``T`` le, ri: BinaryTree[T] # left and right subtrees; may be nil data: T # the data stored in a node proc newNode*[T](data: T): BinaryTree[T] = # constructor for a node result = BinaryTree[T](le: nil, ri: nil, data: data) proc add*[T](root: var BinaryTree[T], n: BinaryTree[T]) = # insert a node into the tree if root == nil: root = n else: var it = root while it != nil: # compare the data items; uses the generic ``cmp`` proc # that works for any type that has a ``==`` and ``<`` operator var c = cmp(it.data, n.data) 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 proc add*[T](root: var BinaryTree[T], data: T) = # convenience proc: add(root, newNode(data)) iterator preorder*[T](root: BinaryTree[T]): T = # Preorder traversal of a binary tree. # Since recursive iterators are not yet implemented, # this uses an explicit stack (which is more efficient anyway): var stack: seq[BinaryTree[T]] = @[root] while stack.len > 0: var n = stack.pop() while n != nil: yield n.data add(stack, n.ri) # push right subtree onto the stack n = n.le # and follow the left pointer var root: BinaryTree[string] # instantiate a BinaryTree with ``string`` add(root, newNode("hello")) # instantiates ``newNode`` and ``add`` add(root, "world") # instantiates the second ``add`` proc for str in preorder(root): stdout.writeLine(str) The ``T`` is called a `generic type parameter`:idx: or a `type variable`:idx:. 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 Table[Key, Value] = object keys: seq[Key] values: seq[Value] when not (Key is string): # empty value for strings used for optimization deletedKeys: seq[bool] 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 ``any`` distinct auto (see below) ================== =================================================== 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 RecordType = tuple or object proc printFields(rec: RecordType) = 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. We call such type classes `bind once`:idx: types. 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. Such type classes are called `bind many`:idx: types. 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 Matrix[T, Rows, Columns] = object ... proc `[]`(m: Matrix, row, col: int): Matrix.T = m.data[col * high(Matrix.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 Matrix[Ordinal] # Any Matrix 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. Concepts -------- **Note**: Concepts are still in development. Concepts, also known as "user-defined type classes", are used to specify an arbitrary set of requirements that the matched type must satisfy. Concepts are written in the following form: .. code-block:: nim type Comparable = concept x, y (x < y) is bool Stack[T] = concept s, var v s.pop() is T v.push(T) s.len is Ordinal for value in s: value is T The concept is a match if: a) all of the expressions within the body can be compiled for the tested type b) all statically evaluable boolean expressions in the body must be true The identifiers following the ``concept`` keyword represent instances of the currently matched type. You can apply any of the standard type modifiers such as ``var``, ``ref``, ``ptr`` and ``static`` to denote a more specific type of instance. You can also apply the `type` modifier to create a named instance of the type itself: .. code-block:: nim type MyConcept = concept x, var v, ref r, ptr p, static s, type T ... Within the concept body, types can appear in positions where ordinary values and parameters are expected. This provides a more convenient way to check for the presence of callable symbols with specific signatures: .. code-block:: nim type OutputStream = concept var s s.write(string) In order to check for symbols accepting ``type`` params, you must prefix the type with the explicit ``type`` modifier. The named instance of the type, following the ``concept`` keyword is also considered to have the explicit modifier and will be matched only as a type. .. code-block:: nim type # Let's imagine a user-defined casting framework with operators # such as `val.to(string)` and `val.to(JSonValue)`. We can test # for these with the following concept: MyCastables = concept x x.to(type string) x.to(type JSonValue) # Let's define a couple of concepts, known from Algebra: AdditiveMonoid* = concept x, y, type T x + y is T T.zero is T # require a proc such as `int.zero` or 'Position.zero' AdditiveGroup* = concept x, y, type T x is AdditiveMonoid -x is T x - y is T 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 concept body, it's also possible to describe usage protocols that do not reveal implementation details. Much like generics, concepts are instantiated exactly once for each tested type and any static code included within the body is executed only once. Concept diagnostics ------------------- By default, the compiler will report the matching errors in concepts only when no other overload can be selected and a normal compilation error is produced. When you need to understand why the compiler is not matching a particular concept and, as a result, a wrong overload is selected, you can apply the ``explain`` pragma to either the concept body or a particular call-site. .. code-block:: nim type MyConcept {.explain.} = concept ... overloadedProc(x, y, z) {.explain.} This will provide Hints in the compiler output either every time the concept is not matched or only on the particular call-site. Generic concepts and type binding rules --------------------------------------- The concept types can be parametric just like the regular generic types: .. code-block:: nim ### matrixalgo.nim import typetraits type AnyMatrix*[R, C: static int; T] = concept m, var mvar, type M M.ValueType is T M.Rows == R M.Cols == C m[int, int] is T mvar[int, int] = T type TransposedType = stripGenericParams(M)[C, R, T] AnySquareMatrix*[N: static int, T] = AnyMatrix[N, N, T] AnyTransform3D* = AnyMatrix[4, 4, float] proc transposed*(m: AnyMatrix): m.TransposedType = for r in 0 ..< m.R: for c in 0 ..< m.C: result[r, c] = m[c, r] proc determinant*(m: AnySquareMatrix): int = ... proc setPerspectiveProjection*(m: AnyTransform3D) = ... -------------- ### matrix.nim type Matrix*[M, N: static int; T] = object data: array[M*N, T] proc `[]`*(M: Matrix; m, n: int): M.T = M.data[m * M.N + n] proc `[]=`*(M: var Matrix; m, n: int; v: M.T) = M.data[m * M.N + n] = v # Adapt the Matrix type to the concept's requirements template Rows*(M: type Matrix): expr = M.M template Cols*(M: type Matrix): expr = M.N template ValueType*(M: type Matrix): type = M.T ------------- ### usage.nim import matrix, matrixalgo var m: Matrix[3, 3, int] projectionMatrix: Matrix[4, 4, float] echo m.transposed.determinant setPerspectiveProjection projectionMatrix When the concept type is matched against a concrete type, the unbound type parameters are inferred from the body of the concept in a way that closely resembles the way generic parameters of callable symbols are inferred on call sites. Unbound types can appear both as params to calls such as `s.push(T)` and on the right-hand side of the ``is`` operator in cases such as `x.pop is T` and `x.data is seq[T]`. Unbound static params will be inferred from expressions involving the `==` operator and also when types dependent on them are being matched: .. code-block:: nim type MatrixReducer[M, N: static int; T] = concept x x.reduce(SquareMatrix[N, T]) is array[M, int] The Nim compiler includes a simple linear equation solver, allowing it to infer static params in some situations where integer arithmetic is involved. Just like in regular type classes, Nim discriminates between ``bind once`` and ``bind many`` types when matching the concept. You can add the ``distinct`` modifier to any of the otherwise inferable types to get a type that will be matched without permanently inferring it. This may be useful when you need to match several procs accepting the same wide class of types: .. code-block:: nim type Enumerable[T] = concept e for v in e: v is T type MyConcept = concept o # this could be inferred to a type such as Enumerable[int] o.foo is distinct Enumerable # this could be inferred to a different type such as Enumerable[float] o.bar is distinct Enumerable # it's also possible to give an alias name to a `bind many` type class type Enum = distinct Enumerable o.baz is Enum On the other hand, using ``bind once`` types allows you to test for equivalent types used in multiple signatures, without actually requiring any concrete types, thus allowing you to encode implementation-defined types: .. code-block:: nim type MyConcept = concept x type T1 = auto x.foo(T1) x.bar(T1) # both procs must accept the same type type T2 = seq[SomeNumber] x.alpha(T2) x.omega(T2) # both procs must accept the same type # and it must be a numeric sequence As seen in the previous examples, you can refer to generic concepts such as `Enumerable[T]` just by their short name. Much like the regular generic types, the concept will be automatically instantiated with the bind once auto type in the place of each missing generic param. Please note that generic concepts such as `Enumerable[T]` can be matched against concrete types such as `string`. Nim doesn't require the concept type to have the same number of parameters as the type being matched. If you wish to express a requirement towards the generic parameters of the matched type, you can use a type mapping operator such as `genericHead` or `stripGenericParams` within the body of the concept to obtain the uninstantiated version of the type, which you can then try to instantiate in any required way. For example, here is how one might define the classic `Functor` concept from Haskell and then demonstrate that Nim's `Option[T]` type is an instance of it: .. code-block:: nim :test: "nim c $1" import future, typetraits type Functor[A] = concept f type MatchedGenericType = genericHead(f.type) # `f` will be a value of a type such as `Option[T]` # `MatchedGenericType` will become the `Option` type f.val is A # The Functor should provide a way to obtain # a value stored inside it type T = auto map(f, A -> T) is MatchedGenericType[T] # And it should provide a way to map one instance of # the Functor to a instance of a different type, given # a suitable `map` operation for the enclosed values import options echo Option[int] is Functor # prints true Concept derived values ---------------------- All top level constants or types appearing within the concept body are accessible through the dot operator in procs where the concept was successfully matched to a concrete type: .. code-block:: nim type DateTime = concept t1, t2, type T const Min = T.MinDate T.Now is T t1 < t2 is bool type TimeSpan = type(t1 - t2) TimeSpan * int is TimeSpan TimeSpan + TimeSpan is TimeSpan t1 + TimeSpan is T proc eventsJitter(events: Enumerable[DateTime]): float = var # this variable will have the inferred TimeSpan type for # the concrete Date-like value the proc was called with: averageInterval: DateTime.TimeSpan deviation: float ... Concept refinement ------------------ When the matched type within a concept is directly tested against a different concept, we say that the outer concept is a refinement of the inner concept and thus it is more-specific. When both concepts are matched in a call during overload resolution, Nim will assign a higher precedence to the most specific one. As an alternative way of defining concept refinements, you can use the object inheritance syntax involving the ``of`` keyword: .. code-block:: nim type Graph = concept g, type G of EqualyComparable, Copyable type VertexType = G.VertexType EdgeType = G.EdgeType VertexType is Copyable EdgeType is Copyable var v: VertexType e: EdgeType IncidendeGraph = concept of Graph # symbols such as variables and types from the refined # concept are automatically in scope: g.source(e) is VertexType g.target(e) is VertexType g.outgoingEdges(v) is Enumerable[EdgeType] BidirectionalGraph = concept g, type G # The following will also turn the concept into a refinement when it # comes to overload resolution, but it doesn't provide the convenient # symbol inheritance g is IncidendeGraph g.incomingEdges(G.VertexType) is Enumerable[G.EdgeType] proc f(g: IncidendeGraph) proc f(g: BidirectionalGraph) # this one will be preferred if we pass a type # matching the BidirectionalGraph concept .. Converter type classes ---------------------- Concepts can also be used to convert a whole range of types to a single type or a small set of simpler types. This is achieved with a `return` statement within the concept body: .. code-block:: nim type Stringable = concept x $x is string return $x StringRefValue[CharType] = object base: ptr CharType len: int StringRef = concept x # the following would be an overloaded proc for cstring, string, seq and # other user-defined types, returning either a StringRefValue[char] or # StringRefValue[wchar] return makeStringRefValue(x) # the varargs param will here be converted to an array of StringRefValues # the proc will have only two instantiations for the two character types proc log(format: static string, varargs[StringRef]) # this proc will allow char and wchar values to be mixed in # the same call at the cost of additional instantiations # the varargs param will be converted to a tuple proc log(format: static string, varargs[distinct StringRef]) .. VTable types ------------ Concepts allow Nim to define a great number of algorithms, using only static polymorphism and without erasing any type information or sacrificing any execution speed. But when polymorphic collections of objects are required, the user must use one of the provided type erasure techniques - either common base types or VTable types. VTable types are represented as "fat pointers" storing a reference to an object together with a reference to a table of procs implementing a set of required operations (the so called vtable). In contrast to other programming languages, the vtable in Nim is stored externally to the object, allowing you to create multiple different vtable views for the same object. Thus, the polymorphism in Nim is unbounded - any type can implement an unlimited number of protocols or interfaces not originally envisioned by the type's author. Any concept type can be turned into a VTable type by using the ``vtref`` or the ``vtptr`` compiler magics. Under the hood, these magics generate a converter type class, which converts the regular instances of the matching types to the corresponding VTable type. .. code-block:: nim type IntEnumerable = vtref Enumerable[int] MyObject = object enumerables: seq[IntEnumerable] streams: seq[OutputStream.vtref] proc addEnumerable(o: var MyObject, e: IntEnumerable) = o.enumerables.add e proc addStream(o: var MyObject, e: OutputStream.vtref) = o.streams.add e The procs that will be included in the vtable are derived from the concept body and include all proc calls for which all param types were specified as concrete types. All such calls should include exactly one param of the type matched against the concept (not necessarily in the first position), which will be considered the value bound to the vtable. Overloads will be created for all captured procs, accepting the vtable type in the position of the captured underlying object. Under these rules, it's possible to obtain a vtable type for a concept with unbound type parameters or one instantiated with metatypes (type classes), but it will include a smaller number of captured procs. A completely empty vtable will be reported as an error. The ``vtref`` magic produces types which can be bound to ``ref`` types and the ``vtptr`` magic produced types bound to ``ptr`` types. Symbol lookup in generics ------------------------- Open and Closed symbols ~~~~~~~~~~~~~~~~~~~~~~~ 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 :test: "nim c $1" type Index = distinct int proc `==` (a, b: Index): bool {.borrow.} var a = (0, 0.Index) var b = (0, 0.Index) 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 ``Index`` type is defined *after* the ``==`` for tuples; yet the example compiles as the instantiation takes the currently defined symbols into account too. Mixin statement --------------- A symbol can be forced to be open by a `mixin`:idx: declaration: .. code-block:: nim :test: "nim c $1" 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*: untyped = 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: untyped): untyped = # 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 ``untyped``, ``typed`` or ``type``. These are "meta types", they can only be used in certain contexts. Regular types can be used too; this implies that ``typed`` expressions are expected. Typed vs untyped parameters --------------------------- An ``untyped`` parameter means that symbol lookups and type resolution is not performed before the expression is passed to the template. This means that for example *undeclared* identifiers can be passed to the template: .. code-block:: nim :test: "nim c $1" template declareInt(x: untyped) = var x: int declareInt(x) # valid x = 3 .. code-block:: nim :test: "nim c $1" :status: 1 template declareInt(x: typed) = var x: int declareInt(x) # invalid, because x has not been declared and so has no type A template where every parameter is ``untyped`` is called an `immediate`:idx: template. For historical reasons templates can be explicitly annotated with an ``immediate`` pragma and then these templates do not take part in overloading resolution and the parameters' types are *ignored* by the compiler. Explicit immediate templates are now deprecated. **Note**: For historical reasons ``stmt`` is an alias for ``typed`` and ``expr`` an alias for ``untyped``, but new code should use the newer, clearer names. Passing a code block to a template ---------------------------------- You can pass a block of statements as the last argument to a template following the special ``:`` syntax: .. code-block:: nim :test: "nim c $1" template withFile(f, fn, mode, actions: untyped): untyped = var f: File if open(f, fn, mode): try: actions finally: close(f) else: quit("cannot open: " & fn) withFile(txt, "ttempl3.txt", fmWrite): # special colon txt.writeLine("line 1") txt.writeLine("line 2") In the example, the two ``writeLine`` statements are bound to the ``actions`` parameter. Usually to pass a block of code to a template the parameter that accepts the block needs to be of type ``untyped``. Because symbol lookups are then delayed until template instantiation time: .. code-block:: nim :test: "nim c $1" :status: 1 template t(body: typed) = block: body t: var i = 1 echo i t: var i = 2 # fails with 'attempt to redeclare i' echo i The above code fails with the mysterious error message that ``i`` has already been declared. The reason for this is that the ``var i = ...`` bodies need to be type-checked before they are passed to the ``body`` parameter and type checking in Nim implies symbol lookups. For the symbol lookups to succeed ``i`` needs to be added to the current (i.e. outer) scope. After type checking these additions to the symbol table are not rolled back (for better or worse). The same code works with ``untyped`` as the passed body is not required to be type-checked: .. code-block:: nim :test: "nim c $1" template t(body: untyped) = block: body t: var i = 1 echo i t: var i = 2 # compiles echo i Varargs of untyped ------------------ In addition to the ``untyped`` meta-type that prevents type checking there is also ``varargs[untyped]`` so that not even the number of parameters is fixed: .. code-block:: nim :test: "nim c $1" template hideIdentifiers(x: varargs[untyped]) = discard hideIdentifiers(undeclared1, undeclared2) However, since a template cannot iterate over varargs, this feature is generally much more useful for macros. **Note**: For historical reasons ``varargs[expr]`` is not equivalent to ``varargs[untyped]``. 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*: untyped = 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 :test: "nim c $1" template typedef(name: untyped, typ: type) = 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 Lev = enum levA, levB var abclev = levB template tstLev(abclev: Lev) = 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 Lev = enum levA, levB var abclev = levB template tstLev(abclev: Lev) = 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 :test: "nim c $1" template newException*(exceptn: type, message: string): untyped = var e: ref exceptn # e is implicitly gensym'ed here new(e) e.msg = message e # so this works: let e = "message" raise newException(IoError, 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: untyped, actions: untyped): untyped = block: var f: File # since 'f' is a template param, it's injected implicitly ... withFile(txt, "ttempl3.txt", fmWrite): txt.writeLine("line 1") txt.writeLine("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. Limitations of the method call syntax ------------------------------------- The expression ``x`` in ``x.f`` needs to be semantically checked (that means symbol lookup and type checking) before it can be decided that it needs to be rewritten to ``f(x)``. Therefore the dot syntax has some limitations when it is used to invoke templates/macros: .. code-block:: nim :test: "nim c $1" :status: 1 template declareVar(name: untyped) = const name {.inject.} = 45 # Doesn't compile: unknownIdentifier.declareVar Another common example is this: .. code-block:: nim :test: "nim c $1" :status: 1 from sequtils import toSeq iterator something: string = yield "Hello" yield "World" var info = something().toSeq The problem here is that the compiler already decided that ``something()`` as an iterator is not callable in this context before ``toSeq`` gets its chance to convert it into a sequence. Macros ====== A macro is a special function that is executed at compile-time. Normally the input for a macro is an abstract syntax tree (AST) of the code that is passed to it. The macro can then do transformations on it and return the transformed AST. The transformed AST is then passed to the compiler as if the macro invocation would have been replaced by its result in the source code. This can be used to implement `domain specific languages`:idx:. 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 AST. 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 :test: "nim c $1" # to work with Nim syntax trees, we need an API that is defined in the # ``macros`` module: import macros macro debug(args: varargs[untyped]): untyped = # `args` is a collection of `NimNode` values that each contain the # AST for an argument of the macro. A macro always has to # return a `NimNode`. A node of kind `nnkStmtList` is suitable for # this use case. result = nnkStmtList.newTree() # iterate over any argument that is passed to this macro: for n in args: # add a call to the statement list that writes the expression; # `toStrLit` converts an AST to its string representation: result.add newCall("write", newIdentNode("stdout"), newLit(n.repr)) # add a call to the statement list that writes ": " result.add newCall("write", newIdentNode("stdout"), newLit(": ")) # add a call to the statement list that writes the expressions value: result.add newCall("writeLine", newIdentNode("stdout"), n) 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, ": ") writeLine(stdout, a[0]) write(stdout, "a[1]") write(stdout, ": ") writeLine(stdout, a[1]) write(stdout, "x") write(stdout, ": ") writeLine(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``, ``writeLine`` 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 :test: "nim c $1" import macros macro debug(n: varargs[typed]): untyped = result = newNimNode(nnkStmtList, n) for x in n: # we can bind symbols in scope via 'bindSym': add(result, newCall(bindSym"write", bindSym"stdout", toStrLit(x))) add(result, newCall(bindSym"write", bindSym"stdout", newStrLitNode(": "))) add(result, newCall(bindSym"writeLine", bindSym"stdout", x)) 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, ": ") writeLine(stdout, a[0]) write(stdout, "a[1]") write(stdout, ": ") writeLine(stdout, a[1]) write(stdout, "x") write(stdout, ": ") writeLine(stdout, x) However, the symbols ``write``, ``writeLine`` 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: untyped): untyped = # 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: untyped) = discard proc p() {.m.} = discard This is a simple syntactic transformation into: .. code-block:: nim template m(s: untyped) = discard m: proc p() = discard For loop macros --------------- A macro that takes as its only input parameter an expression of the special type ``system.ForLoopStmt`` can rewrite the entirety of a ``for`` loop: .. code-block:: nim :test: "nim c $1" import macros {.experimental: "forLoopMacros".} macro enumerate(x: ForLoopStmt): untyped = expectKind x, nnkForStmt # we strip off the first for loop variable and use # it as an integer counter: result = newStmtList() result.add newVarStmt(x[0], newLit(0)) var body = x[^1] if body.kind != nnkStmtList: body = newTree(nnkStmtList, body) body.add newCall(bindSym"inc", x[0]) var newFor = newTree(nnkForStmt) for i in 1..x.len-3: newFor.add x[i] # transform enumerate(X) to 'X' newFor.add x[^2][1] newFor.add body result.add newFor for a, b in enumerate(items([1, 2, 3])): echo a, " ", b for a2, b2 in enumerate([1, 2, 3, 5]): echo a2, " ", b2 Currently for loop macros must be enabled explicitly via ``{.experimental: "forLoopMacros".}``. Case statement macros --------------------- A macro that needs to be called `match`:idx: can be used to rewrite ``case`` statements in order to implement `pattern matching`:idx: for certain types. The following example implements a simplistic form of pattern matching for tuples, leveraging the existing equality operator for tuples (as provided in ``system.==``): .. code-block:: nim :test: "nim c $1" {.experimental: "caseStmtMacros".} import macros macro match(n: tuple): untyped = result = newTree(nnkIfStmt) let selector = n[0] for i in 1 ..< n.len: let it = n[i] case it.kind of nnkElse, nnkElifBranch, nnkElifExpr, nnkElseExpr: result.add it of nnkOfBranch: for j in 0..it.len-2: let cond = newCall("==", selector, it[j]) result.add newTree(nnkElifBranch, cond, it[^1]) else: error "'match' cannot handle this node", it echo repr result case ("foo", 78) of ("foo", 78): echo "yes" of ("bar", 88): echo "no" else: discard Currently case statement macros must be enabled explicitly via ``{.experimental: "caseStmtMacros".}``. ``match`` macros are subject to overload resolution. First the ``case``'s selector expression is used to determine which ``match`` macro to call. To this macro is then the complete ``case`` statement body is passed and the macro is evaluated. In other words, the macro needs to transform the full ``case`` statement but only the statement's selector expression is used to determine which ``macro`` to call. Special Types ============= static[T] --------- **Note**: static[T] is still in development. As their name suggests, static parameters must be known at compile-time: .. code-block:: nim proc precompiledRegex(pattern: static string): RegEx = 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). 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` Please note that ``static T`` is just a syntactic convenience for the underlying generic type ``static[T]``. The type param can be omitted to obtain the type class of all values known at compile-time. A more specific type class can be created by instantiating ``static`` with another type class. You can force the evaluation of a certain expression at compile-time by coercing it to a corresponding ``static`` type: .. code-block:: nim import math echo static(fac(5)), " ", static[bool](16.isPowerOfTwo) The complier will report any failure to evaluate the expression or a possible type mismatch error. type[T] ------- In many contexts, Nim allows you to treat the names of types as regular values. These values exists only during the compilation phase, but since all values must have a type, ``type`` is considered their special type. ``type`` acts like a generic type. For instance, the type of the symbol ``int`` is ``type[int]``. Just like with regular generic types, when the generic param is ommited, ``type`` denotes the type class of all types. As a syntactic convenience, you can also use ``type`` as a modifier. ``type int`` is considered the same as ``type[int]``. Procs featuring ``type`` params are considered implicitly generic. They will be instantiated for each unique combination of supplied types and within the body of the proc, the name of each param will refer to the bound concrete type: .. code-block:: nim proc new(T: type): ref T = echo "allocating ", T.name new(result) var n = Node.new var tree = new(BinaryTree[int]) When multiple type params are present, they will bind freely to different types. To force a bind-once behavior one can use an explicit generic param: .. code-block:: nim proc acceptOnlyTypePairs[T, U](A, B: type[T]; C, D: type[U]) Once bound, type params can appear in the rest of the proc signature: .. code-block:: nim :test: "nim c $1" template declareVariableWithType(T: type, value: T) = var x: T = value declareVariableWithType int, 42 Overload resolution can be further influenced by constraining the set of types that will match the type param: .. code-block:: nim :test: "nim c $1" template maxval(T: type int): int = high(int) template maxval(T: type float): float = Inf var i = int.maxval var f = float.maxval when false: var s = string.maxval # error, maxval is not implemented for string The constraint can be a concrete type or a type class. type operator ------------- You can obtain the type of a given expression by constructing a ``type`` value from it (in many other languages this is known as the `typeof`:idx: operator): .. code-block:: nim var x = 0 var y: type(x) # y has type int You may add a constraint to the resulting type to trigger a compile-time error if the expression doesn't have the expected type: .. code-block:: nim var x = 0 var y: type[object](x) # Error: type mismatch: got <int> but expected 'object' 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) Special Operators ================= dot operators ------------- **Note**: Dot operators are still experimental and so need to be enabled via ``{.experimental: "dotOperators".}``. 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 passed to an ``untyped`` 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 template `.` (js: PJsonNode, field: untyped): JSON = js[astToStr(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 *overridden* instead of *overloaded*. This means the implementation is automatically lifted to structured types. For instance if type ``T`` has an overridden 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 overridden ``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 ``result = expr``, ``parameter = defaultValue`` or for parameter passing no assignment is performed. For a type ``T`` that has an overloaded assignment operator ``var v = T()`` is rewritten to ``var v: T; v = T()``; in other words ``var`` and ``let`` contexts do count as assignments. The assignment operator needs to be attached to an object or distinct type ``T``. Its signature has to be ``(var T, T)``. Example: .. code-block:: nim type Concrete = object a, b: string proc `=`(d: var Concrete; src: Concrete) = shallowCopy(d.a, src.a) shallowCopy(d.b, src.b) echo "Concrete '=' called" var x, y: array[0..2, Concrete] var cA, cB: Concrete var cATup, cBTup: tuple[x: int, ha: Concrete] x = y cA = cB cATup = cBTup 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``. ``=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 MyObj = object x, y: int p: pointer proc `=destroy`(o: var MyObj) = if o.p != nil: dealloc o.p proc open: MyObj = result = MyObj(x: 1, y: 2, p: alloc(3)) proc work(o: MyObj) = 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`:idx: 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 This mechanism will be 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 ;-) You can make one overload matching with a constraint and one without, and the one with a constraint will have precedence, and so you can handle both cases differently. 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. =================== ===================================================== Predicates that share their name with a keyword have to be escaped with backticks: `` `const` ``. The ``alias`` and ``noalias`` predicates refer not only to the matching AST, but also to every other bound parameter; syntactically 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}(): untyped = 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}(): untyped = 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 deactivated 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: untyped): untyped = 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) = 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): untyped = &&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 Matrix = object dummy: int proc `*`(a, b: Matrix): Matrix = discard proc `+`(a, b: Matrix): Matrix = discard proc `-`(a, b: Matrix): Matrix = discard proc `$`(a: Matrix): string = result = $a.dummy proc mat21(): Matrix = result.dummy = 21 macro optM{ (`+`|`-`|`*`) ** a }(a: Matrix): untyped = echo treeRepr(a) result = newCall(bindSym"mat21") var x, y, z: Matrix 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|writeLine){w})(f, y) }(x, y: varargs[untyped], f: File, w: untyped) = 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: untyped): untyped = x + y template optP2{p(x, y, false)}(x, y: untyped): untyped = 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}): Peg = 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 Table, 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 = key t[idx].val = val proc `[]=`*(t: var Table, 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, key shallowCopy t[idx].val, val var t: Table # 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. A valid module name can only be a valid Nim identifier (and thus its filename is ``identifier.nim``). 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`` list to prevent some symbols to be imported: .. code-block:: nim :test: "nim c $1" :status: 1 import strutils except `%`, toUpperAscii # doesn't work then: echo "$1" % "abc".toUpperAscii It is not checked that the ``except`` list is really exported from the module. This feature allows to compile against an older version of the module that does not export these identifiers. Include statement ~~~~~~~~~~~~~~~~~ The ``include`` statement does something fundamentally different than importing a module: it merely includes the contents of a file. The ``include`` statement is useful to split up a large module into several files: .. code-block:: nim include fileA, fileB, fileC 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"`` can be used to refer to a module in subdirectories: .. code-block:: nim import 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.toUpperAscii("abc") Likewise the following does not make sense as the name is ``strutils`` already: .. code-block:: nim import lib/pure/strutils as strutils Collective imports from a directory ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The syntax ``import dir / [moduleA, moduleB]`` can be used to import multiple modules from the same directory. Path names are syntactically either Nim identifiers or string literals. If the path name is not a valid Nim identifier it needs to be a string literal: .. code-block:: nim import "gfx/3d/somemodule" # in quotes because '3d' is not a valid Nim identifier Pseudo import/include paths ~~~~~~~~~~~~~~~~~~~~~~~~~~~ A directory can also be a so called "pseudo directory". They can be used to avoid ambiguity when there are multiple modules with the same path. There are two pseudo directories: 1. ``std``: The ``std`` pseudo directory is the abstract location of Nim's standard library. For example, the syntax ``import std / strutils`` is used to unambiguously refer to the standard library's ``strutils`` module. 2. ``pkg``: The ``pkg`` pseudo directory is used to unambiguously refer to a Nimble package. However, for technical details that lie outside of the scope of this document its semantics are: *Use the search path to look for module name but ignore the standard library locations*. In other words, it is the opposite of ``std``. From import statement ~~~~~~~~~~~~~~~~~~~~~ After the ``from`` statement a module name follows followed by an ``import`` to list the symbols one likes to use without explicit full qualification: .. code-block:: nim :test: "nim c $1" from strutils import `%` echo "$1" % "abc" # always possible: full qualification: echo strutils.replace("abc", "a", "z") It's also possible to use ``from module import nil`` if one wants to import the module but wants to enforce fully qualified access to every symbol in ``module``. Export statement ~~~~~~~~~~~~~~~~ An ``export`` statement can be used for symbol forwarding so that client modules don't need to import a module's dependencies: .. code-block:: nim # module B type MyObject* = object .. code-block:: nim # module A import B export B.MyObject proc `$`*(x: MyObject): string = "my object" .. code-block:: nim # module C import A # B.MyObject has been imported implicitly here: var x: MyObject echo $x When the exported symbol is another module, all of its definitions will be forwarded. You can use an ``except`` list to exclude some of the symbols. 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 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. deprecated pragma ----------------- The deprecated pragma is used to mark a symbol as deprecated: .. code-block:: nim proc p() {.deprecated.} var x {.deprecated.}: char This pragma can also take in an optional warning string to relay to developers. .. code-block:: nim proc thing(x: bool) {.deprecated: "See arguments of otherThing()".} It can also be used as a statement, in that case it takes a list of *renamings*. .. code-block:: nim type File = object Stream = ref object {.deprecated: [TFile: File, PStream: Stream].} 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``. ``func`` is syntactic sugar for a proc with no side effects: .. code-block:: nim func `+` (x, y: int): int compileTime pragma ------------------ The ``compileTime`` pragma is used to mark a proc or variable to be used at compile time only. No code will be generated for it. Compile time procs are useful as helpers for macros. Since version 0.12.0 of the language, a proc that uses ``system.NimNode`` within its parameter types is implicitly declared ``compileTime``: .. code-block:: nim proc astHelper(n: NimNode): NimNode = result = n Is the same as: .. code-block:: nim proc astHelper(n: NimNode): NimNode {.compileTime.} = result = n 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 Node = ref NodeObj NodeObj {.acyclic.} = object left, right: Node data: string Or if we directly use a ref object: .. code-block:: nim type Node = ref object {.acyclic.} left, right: Node data: string In the example a tree structure is declared with the ``Node`` 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 Node = acyclic ref NodeObj NodeObj = object left, right: Node data: string final pragma ------------ The ``final`` pragma can be used for an object type to specify that it cannot be inherited from. Note that inheritance is only available for objects that inherit from an existing object (via the ``object of SuperType`` syntax) or that have been marked as ``inheritable``. 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 NodeKind = enum nkLeaf, nkInner Node {.shallow.} = object case kind: NodeKind of nkLeaf: strVal: string of nkInner: children: seq[Node] 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: untyped, 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 ---------------- The immediate pragma is obsolete. See `Typed vs untyped parameters`_. 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 a 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. .. 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, whose 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 expressions 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. used pragma ----------- Nim produces a warning for symbols that are not exported and not used either. The ``used`` pragma can be attached to a symbol to suppress this warning. This is particularly useful when the symbol was generated by a macro: .. code-block:: nim template implementArithOps(T) = proc echoAdd(a, b: T) {.used.} = echo a + b proc echoSub(a, b: T) {.used.} = echo a - b # no warning produced for the unused 'echoSub' implementArithOps(int) echoAdd 3, 5 experimental pragma ------------------- The ``experimental`` pragma enables experimental language features. Depending on the concrete feature this means that the feature is either considered too unstable for an otherwise stable release or that the future of the feature is uncertain (it may be removed any time). Example: .. code-block:: nim {.experimental: "parallel".} proc useParallel() = parallel: for i in 0..4: echo "echo in parallel" As a top level statement, the experimental pragma enables a feature for the rest of the module it's enabled in. This is problematic for macro and generic instantiations that cross a module scope. Currently these usages have to be put into a ``.push/pop`` environment: .. code-block:: nim # client.nim proc useParallel*[T](unused: T) = # use a generic T here to show the problem. {.push experimental: "parallel".} parallel: for i in 0..4: echo "echo in parallel" {.pop.} .. code-block:: nim import client useParallel(1) Implementation Specific Pragmas =============================== This section describes additional pragmas that the current Nim implementation supports but which should not be seen as part of the language specification. Bitsize pragma -------------- The ``bitsize`` pragma is for object field members. It declares the field as a bitfield in C/C++. .. code-block:: Nim type mybitfield = object flag {.bitsize:1.}: cuint generates: .. code-block:: C struct mybitfield { unsigned int flag:1; }; Volatile pragma --------------- The ``volatile`` pragma is for variables only. It declares the variable as ``volatile``, whatever that means in C/C++ (its semantics are not well defined in C/C++). **Note**: This pragma will not exist for the LLVM backend. NoDecl pragma ------------- The ``noDecl`` pragma can be applied to almost any symbol (variable, proc, type, etc.) and is sometimes useful for interoperability with C: It tells Nim that it should not generate a declaration for the symbol in the C code. For example: .. code-block:: Nim var EACCES {.importc, noDecl.}: cint # pretend EACCES was a variable, as # Nim does not know its value However, the ``header`` pragma is often the better alternative. **Note**: This will not work for the LLVM backend. Header pragma ------------- The ``header`` pragma is very similar to the ``noDecl`` pragma: It can be applied to almost any symbol and specifies that it should not be declared and instead the generated code should contain an ``#include``: .. code-block:: Nim type PFile {.importc: "FILE*", header: "<stdio.h>".} = distinct pointer # import C's FILE* type; Nim will treat it as a new pointer type The ``header`` pragma always expects a string constant. The string contant contains the header file: As usual for C, a system header file is enclosed in angle brackets: ``<>``. If no angle brackets are given, Nim encloses the header file in ``""`` in the generated C code. **Note**: This will not work for the LLVM backend. IncompleteStruct pragma ----------------------- The ``incompleteStruct`` pragma tells the compiler to not use the underlying C ``struct`` in a ``sizeof`` expression: .. code-block:: Nim type DIR* {.importc: "DIR", header: "<dirent.h>", pure, incompleteStruct.} = object Compile pragma -------------- The ``compile`` pragma can be used to compile and link a C/C++ source file with the project: .. code-block:: Nim {.compile: "myfile.cpp".} **Note**: Nim computes a SHA1 checksum and only recompiles the file if it has changed. You can use the ``-f`` command line option to force recompilation of the file. Link pragma ----------- The ``link`` pragma can be used to link an additional file with the project: .. code-block:: Nim {.link: "myfile.o".} PassC pragma ------------ The ``passC`` pragma can be used to pass additional parameters to the C compiler like you would using the commandline switch ``--passC``: .. code-block:: Nim {.passC: "-Wall -Werror".} Note that you can use ``gorge`` from the `system module <system.html>`_ to embed parameters from an external command at compile time: .. code-block:: Nim {.passC: gorge("pkg-config --cflags sdl").} PassL pragma ------------ The ``passL`` pragma can be used to pass additional parameters to the linker like you would using the commandline switch ``--passL``: .. code-block:: Nim {.passL: "-lSDLmain -lSDL".} Note that you can use ``gorge`` from the `system module <system.html>`_ to embed parameters from an external command at compile time: .. code-block:: Nim {.passL: gorge("pkg-config --libs sdl").} Emit pragma ----------- The ``emit`` pragma can be used to directly affect the output of the compiler's code generator. So it makes your code unportable to other code generators/backends. Its usage is highly discouraged! However, it can be extremely useful for interfacing with `C++`:idx: or `Objective C`:idx: code. Example: .. code-block:: Nim {.emit: """ static int cvariable = 420; """.} {.push stackTrace:off.} proc embedsC() = var nimVar = 89 # access Nim symbols within an emit section outside of string literals: {.emit: ["""fprintf(stdout, "%d\n", cvariable + (int)""", nimVar, ");"].} {.pop.} embedsC() For backwards compatibility, if the argument to the ``emit`` statement is a single string literal, Nim symbols can be referred to via backticks. This usage is however deprecated. For a toplevel emit statement the section where in the generated C/C++ file the code should be emitted can be influenced via the prefixes ``/*TYPESECTION*/`` or ``/*VARSECTION*/`` or ``/*INCLUDESECTION*/``: .. code-block:: Nim {.emit: """/*TYPESECTION*/ struct Vector3 { public: Vector3(): x(5) {} Vector3(float x_): x(x_) {} float x; }; """.} type Vector3 {.importcpp: "Vector3", nodecl} = object x: cfloat proc constructVector3(a: cfloat): Vector3 {.importcpp: "Vector3(@)", nodecl} ImportCpp pragma ---------------- **Note**: `c2nim <c2nim.html>`_ can parse a large subset of C++ and knows about the ``importcpp`` pragma pattern language. It is not necessary to know all the details described here. Similar to the `importc pragma for C <#foreign-function-interface-importc-pragma>`_, the ``importcpp`` pragma can be used to import `C++`:idx: methods or C++ symbols in general. The generated code then uses the C++ method calling syntax: ``obj->method(arg)``. In combination with the ``header`` and ``emit`` pragmas this allows *sloppy* interfacing with libraries written in C++: .. code-block:: Nim # Horrible example of how to interface with a C++ engine ... ;-) {.link: "/usr/lib/libIrrlicht.so".} {.emit: """ using namespace irr; using namespace core; using namespace scene; using namespace video; using namespace io; using namespace gui; """.} const irr = "<irrlicht/irrlicht.h>" type IrrlichtDeviceObj {.header: irr, importcpp: "IrrlichtDevice".} = object IrrlichtDevice = ptr IrrlichtDeviceObj proc createDevice(): IrrlichtDevice {. header: irr, importcpp: "createDevice(@)".} proc run(device: IrrlichtDevice): bool {. header: irr, importcpp: "#.run(@)".} The compiler needs to be told to generate C++ (command ``cpp``) for this to work. The conditional symbol ``cpp`` is defined when the compiler emits C++ code. Namespaces ~~~~~~~~~~ The *sloppy interfacing* example uses ``.emit`` to produce ``using namespace`` declarations. It is usually much better to instead refer to the imported name via the ``namespace::identifier`` notation: .. code-block:: nim type IrrlichtDeviceObj {.header: irr, importcpp: "irr::IrrlichtDevice".} = object Importcpp for enums ~~~~~~~~~~~~~~~~~~~ When ``importcpp`` is applied to an enum type the numerical enum values are annotated with the C++ enum type, like in this example: ``((TheCppEnum)(3))``. (This turned out to be the simplest way to implement it.) Importcpp for procs ~~~~~~~~~~~~~~~~~~~ Note that the ``importcpp`` variant for procs uses a somewhat cryptic pattern language for maximum flexibility: - A hash ``#`` symbol is replaced by the first or next argument. - A dot following the hash ``#.`` indicates that the call should use C++'s dot or arrow notation. - An at symbol ``@`` is replaced by the remaining arguments, separated by commas. For example: .. code-block:: nim proc cppMethod(this: CppObj, a, b, c: cint) {.importcpp: "#.CppMethod(@)".} var x: ptr CppObj cppMethod(x[], 1, 2, 3) Produces: .. code-block:: C x->CppMethod(1, 2, 3) As a special rule to keep backwards compatibility with older versions of the ``importcpp`` pragma, if there is no special pattern character (any of ``# ' @``) at all, C++'s dot or arrow notation is assumed, so the above example can also be written as: .. code-block:: nim proc cppMethod(this: CppObj, a, b, c: cint) {.importcpp: "CppMethod".} Note that the pattern language naturally also covers C++'s operator overloading capabilities: .. code-block:: nim proc vectorAddition(a, b: Vec3): Vec3 {.importcpp: "# + #".} proc dictLookup(a: Dict, k: Key): Value {.importcpp: "#[#]".} - An apostrophe ``'`` followed by an integer ``i`` in the range 0..9 is replaced by the i'th parameter *type*. The 0th position is the result type. This can be used to pass types to C++ function templates. Between the ``'`` and the digit an asterisk can be used to get to the base type of the type. (So it "takes away a star" from the type; ``T*`` becomes ``T``.) Two stars can be used to get to the element type of the element type etc. For example: .. code-block:: nim type Input {.importcpp: "System::Input".} = object proc getSubsystem*[T](): ptr T {.importcpp: "SystemManager::getSubsystem<'*0>()", nodecl.} let x: ptr Input = getSubsystem[Input]() Produces: .. code-block:: C x = SystemManager::getSubsystem<System::Input>() - ``#@`` is a special case to support a ``cnew`` operation. It is required so that the call expression is inlined directly, without going through a temporary location. This is only required to circumvent a limitation of the current code generator. For example C++'s ``new`` operator can be "imported" like this: .. code-block:: nim proc cnew*[T](x: T): ptr T {.importcpp: "(new '*0#@)", nodecl.} # constructor of 'Foo': proc constructFoo(a, b: cint): Foo {.importcpp: "Foo(@)".} let x = cnew constructFoo(3, 4) Produces: .. code-block:: C x = new Foo(3, 4) However, depending on the use case ``new Foo`` can also be wrapped like this instead: .. code-block:: nim proc newFoo(a, b: cint): ptr Foo {.importcpp: "new Foo(@)".} let x = newFoo(3, 4) Wrapping constructors ~~~~~~~~~~~~~~~~~~~~~ Sometimes a C++ class has a private copy constructor and so code like ``Class c = Class(1,2);`` must not be generated but instead ``Class c(1,2);``. For this purpose the Nim proc that wraps a C++ constructor needs to be annotated with the `constructor`:idx: pragma. This pragma also helps to generate faster C++ code since construction then doesn't invoke the copy constructor: .. code-block:: nim # a better constructor of 'Foo': proc constructFoo(a, b: cint): Foo {.importcpp: "Foo(@)", constructor.} Wrapping destructors ~~~~~~~~~~~~~~~~~~~~ Since Nim generates C++ directly, any destructor is called implicitly by the C++ compiler at the scope exits. This means that often one can get away with not wrapping the destructor at all! However when it needs to be invoked explicitly, it needs to be wrapped. The pattern language provides everything that is required: .. code-block:: nim proc destroyFoo(this: var Foo) {.importcpp: "#.~Foo()".} Importcpp for objects ~~~~~~~~~~~~~~~~~~~~~ Generic ``importcpp``'ed objects are mapped to C++ templates. This means that you can import C++'s templates rather easily without the need for a pattern language for object types: .. code-block:: nim type StdMap {.importcpp: "std::map", header: "<map>".} [K, V] = object proc `[]=`[K, V](this: var StdMap[K, V]; key: K; val: V) {. importcpp: "#[#] = #", header: "<map>".} var x: StdMap[cint, cdouble] x[6] = 91.4 Produces: .. code-block:: C std::map<int, double> x; x[6] = 91.4; - If more precise control is needed, the apostrophe ``'`` can be used in the supplied pattern to denote the concrete type parameters of the generic type. See the usage of the apostrophe operator in proc patterns for more details. .. code-block:: nim type VectorIterator {.importcpp: "std::vector<'0>::iterator".} [T] = object var x: VectorIterator[cint] Produces: .. code-block:: C std::vector<int>::iterator x; ImportObjC pragma ----------------- Similar to the `importc pragma for C <#foreign-function-interface-importc-pragma>`_, the ``importobjc`` pragma can be used to import `Objective C`:idx: methods. The generated code then uses the Objective C method calling syntax: ``[obj method param1: arg]``. In addition with the ``header`` and ``emit`` pragmas this allows *sloppy* interfacing with libraries written in Objective C: .. code-block:: Nim # horrible example of how to interface with GNUStep ... {.passL: "-lobjc".} {.emit: """ #include <objc/Object.h> @interface Greeter:Object { } - (void)greet:(long)x y:(long)dummy; @end #include <stdio.h> @implementation Greeter - (void)greet:(long)x y:(long)dummy { printf("Hello, World!\n"); } @end #include <stdlib.h> """.} type Id {.importc: "id", header: "<objc/Object.h>", final.} = distinct int proc newGreeter: Id {.importobjc: "Greeter new", nodecl.} proc greet(self: Id, x, y: int) {.importobjc: "greet", nodecl.} proc free(self: Id) {.importobjc: "free", nodecl.} var g = newGreeter() g.greet(12, 34) g.free() The compiler needs to be told to generate Objective C (command ``objc``) for this to work. The conditional symbol ``objc`` is defined when the compiler emits Objective C code. CodegenDecl pragma ------------------ The ``codegenDecl`` pragma can be used to directly influence Nim's code generator. It receives a format string that determines how the variable or proc is declared in the generated code. For variables $1 in the format string represents the type of the variable and $2 is the name of the variable. The following Nim code: .. code-block:: nim var a {.codegenDecl: "$# progmem $#".}: int will generate this C code: .. code-block:: c int progmem a For procedures $1 is the return type of the procedure, $2 is the name of the procedure and $3 is the parameter list. The following nim code: .. code-block:: nim proc myinterrupt() {.codegenDecl: "__interrupt $# $#$#".} = echo "realistic interrupt handler" will generate this code: .. code-block:: c __interrupt void myinterrupt() InjectStmt pragma ----------------- The ``injectStmt`` pragma can be used to inject a statement before every other statement in the current module. It is only supposed to be used for debugging: .. code-block:: nim {.injectStmt: gcInvariants().} # ... complex code here that produces crashes ... compile time define pragmas --------------------------- The pragmas listed here can be used to optionally accept values from the -d/--define option at compile time. The implementation currently provides the following possible options (various others may be added later). ================= ============================================ pragma description ================= ============================================ `intdefine`:idx: Reads in a build-time define as an integer `strdefine`:idx: Reads in a build-time define as a string ================= ============================================ .. code-block:: nim const FooBar {.intdefine.}: int = 5 echo FooBar :: nim c -d:FooBar=42 foobar.c In the above example, providing the -d flag causes the symbol ``FooBar`` to be overwritten at compile time, printing out 42. If the ``-d:FooBar=42`` were to be omitted, the default value of 5 would be used. Custom annotations ------------------ It is possible to define custom typed pragmas. Custom pragmas do not effect code generation directly, but their presence can be detected by macros. Custom pragmas are defined using templates annotated with pragma ``pragma``: .. code-block:: nim template dbTable(name: string, table_space: string = "") {.pragma.} template dbKey(name: string = "", primary_key: bool = false) {.pragma.} template dbForeignKey(t: type) {.pragma.} template dbIgnore {.pragma.} Consider stylized example of possible Object Relation Mapping (ORM) implementation: .. code-block:: nim const tblspace {.strdefine.} = "dev" # switch for dev, test and prod environments type User {.dbTable("users", tblspace).} = object id {.dbKey(primary_key = true).}: int name {.dbKey"full_name".}: string is_cached {.dbIgnore.}: bool age: int UserProfile {.dbTable("profiles", tblspace).} = object id {.dbKey(primary_key = true).}: int user_id {.dbForeignKey: User.}: int read_access: bool write_access: bool admin_acess: bool In this example custom pragmas are used to describe how Nim objects are mapped to the schema of the relational database. Custom pragmas can have zero or more arguments. In order to pass multiple arguments use one of template call syntaxes. All arguments are typed and follow standard overload resolution rules for templates. Therefore, it is possible to have default values for arguments, pass by name, varargs, etc. Custom pragmas can be used in all locations where ordinary pragmas can be specified. It is possible to annotate procs, templates, type and variable definitions, statements, etc. Macros module includes helpers which can be used to simplify custom pragma access `hasCustomPragma`, `getCustomPragmaVal`. Please consult macros module documentation for details. These macros are no magic, they don't do anything you cannot do yourself by walking AST object representation. More examples with custom pragmas: - Better serialization/deserialization control: .. code-block:: nim type MyObj = object a {.dontSerialize.}: int b {.defaultDeserialize: 5.}: int c {.serializationKey: "_c".}: string - Adopting type for gui inspector in a game engine: .. code-block:: nim type MyComponent = object position {.editable, animatable.}: Vector3 alpha {.editRange: [0.0..1.0], animatable.}: float32 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 do provide the same feature under the same name. Also, if one is interfacing with C++ the `ImportCpp pragma <manual.html#implementation-specific-pragmas-importcpp-pragma>`_ and interfacing with Objective-C the `ImportObjC pragma <manual.html#implementation-specific-pragmas-importobjc-pragma>`_ can be used. The string literal passed to ``importc`` can be a format string: .. code-block:: Nim proc p(s: cstring) {.importc: "prefix$1".} In the example the external name of ``p`` is set to ``prefixp``. Only ``$1`` is available and a literal dollar sign must be written as ``$$``. Exportc pragma -------------- The ``exportc`` pragma provides a means to export a type, a variable, or a procedure to C. Enums and constants can't be exported. The optional argument is a string containing the C identifier. If the argument is missing, the C name is the Nim identifier *exactly as spelled*: .. code-block:: Nim proc callme(formatstr: cstring) {.exportc: "callMe", varargs.} Note that this pragma is somewhat of a misnomer: Other backends do provide the same feature under the same name. The string literal passed to ``exportc`` can be a format string: .. code-block:: Nim proc p(s: string) {.exportc: "prefix$1".} = echo s In the example the external name of ``p`` is set to ``prefixp``. Only ``$1`` is available and a literal dollar sign must be written as ``$$``. 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``. Only ``$1`` is available and a literal dollar sign must be written as ``$$``. 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 Vector {.bycopy.} = 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 to implement customized flexibly sized arrays. Additionally an unchecked array is translated into a C array of undetermined size: .. code-block:: nim type ArrayPart{.unchecked.} = array[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 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 overridden 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. This pragma only has an effect for the code generation on the Windows target, so when this pragma is forgotten and the dynamic library is only tested on Mac and/or Linux, there won't be an error. On Windows this pragma adds ``__declspec(dllexport)`` to the function declaration. 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 <#parallel-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 ``noSideEffect`` implies ``gcsafe``. The only way to create a thread is via ``spawn`` or ``createThread``. ``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 disable the GC-safety checking the ``--threadAnalysis:off`` command line switch can be used. This is a temporary workaround to ease the porting effort from old code to the new threading model. To override the compiler's gcsafety analysis a ``{.gcsafe.}`` pragma block can be used: .. code-block:: nim var someGlobal: string = "some string here" perThread {.threadvar.}: string proc setPerThread() = {.gcsafe.}: deepCopy(perThread, someGlobal) Future directions: - A shared GC'ed heap might be provided. Threadvar pragma ---------------- A variable can be marked with the ``threadvar`` pragma, which makes it a `thread-local`:idx: variable; Additionally, this implies all the effects of the ``global`` pragma. .. 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`` otherwise it is ``FlowVar[T]``. Within a ``parallel`` section sometimes the ``FlowVar[T]`` is eliminated to ``T``. This happens when ``T`` does not contain any GC'ed memory. 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 overridden. * 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 ``blockUntilAny`` 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[FlowVarBase](3) for i in 0..2: responses[i] = spawn tellServer(Update, "key", "value") var index = blockUntilAny(responses) assert index >= 0 responses.del(index) discard blockUntilAny(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 is not hard to work-around in practice. Parallel statement ------------------ Example: .. code-block:: nim :test: "nim c --threads:on $1" # Compute PI in an inefficient way import strutils, math, threadpool {.experimental: "parallel".} 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. Guards and locks ================ Apart from ``spawn`` and ``parallel`` Nim also provides all the common low level concurrency mechanisms like locks, atomic intrinsics 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 ~~~~~~~~~~~~~~~~~~~~~~~~~~~ Object fields and global variables can be annotated via a ``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: untyped) = 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): untyped = {.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 <#guards-and-locks-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].}: ... Here is how a typical multilock statement can be implemented in Nim. Note how the runtime check is required to ensure a global ordering for two locks ``a`` and ``b`` of the same lock level: .. code-block:: nim template multilock(a, b: ptr TLock; body: untyped) = 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). The ``locks`` pragma can also take the special value ``"unknown"``. This is useful in the context of dynamic method dispatching. In the following example, the compiler can infer a lock level of 0 for the ``base`` case. However, one of the overloaded methods calls a procvar which is potentially locking. Thus, the lock level of calling ``g.testMethod`` cannot be inferred statically, leading to compiler warnings. By using ``{.locks: "unknown".}``, the base method can be marked explicitly as having unknown lock level as well: .. code-block:: nim type SomeBase* = ref object of RootObj type SomeDerived* = ref object of SomeBase memberProc*: proc () method testMethod(g: SomeBase) {.base, locks: "unknown".} = discard method testMethod(g: SomeDerived) = if g.memberProc != nil: g.memberProc() 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``.