============= Nimrod Manual ============= :Author: Andreas Rumpf :Version: |nimrodversion| .. 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 Nimrod's features need more precise wording. This manual will evolve into a proper specification some day. This document describes the lexis, the syntax, and the semantics of Nimrod. 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*; an alternative spelling for optional parts is ``[a]``. The ``|`` symbol is used to mark alternatives and has the lowest precedence. Parentheses may be used to group elements. 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] Other parts of Nimrod - like scoping rules or runtime semantics are only described in an informal manner. The reason is that formal semantics are difficult to write and understand. However, there is only one Nimrod implementation, so one may consider it as the formal specification; especially since the compiler's code is pretty clean (well, some parts of it). Definitions =========== A Nimrod 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. 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*. However, the implementation provides a means to disable these runtime checks. See the section pragmas_ for details. 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 Nimrod 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 ----------- Nimrod'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 terminals ``IND`` (indentation), ``DED`` (dedentation) and ``SAD`` (same indentation) are generated by the scanner, denoting an indentation. These terminals are only generated for lines that are not empty. The parser and the scanner communicate over a stack which indentation terminal should be generated: the stack consists of integers counting the spaces. The stack is initialized with a zero on its top. The scanner reads from the stack: If the current indentation token consists of more spaces than the entry at the top of the stack, a ``IND`` token is generated, else if it consists of the same number of spaces, a ``SAD`` token is generated. If it consists of fewer spaces, a ``DED`` token is generated for any item on the stack that is greater than the current. These items are later popped from the stack by the parser. At the end of the file, a ``DED`` token is generated for each number remaining on the stack that is larger than zero. Because the grammar contains some optional ``IND`` tokens, the scanner cannot push new indentation levels. This has to be done by the parser. The symbol ``indPush`` indicates that an ``IND`` token is expected; the current number of leading spaces is pushed onto the stack by the parser. The symbol ``indPop`` denotes that the parser pops an item from the indentation stack. No token is consumed by ``indPop``. Comments -------- `Comments`:idx: 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 which is aligned to the preceding one, it does not start a new comment: .. code-block:: nimrod i = 0 # This is a single comment over multiple lines belonging to the # assignment statement. The scanner merges these two pieces. # This is a new comment belonging to the current block, but to no particular # statement. i = i + 1 # This a new comment that is NOT echo(i) # continued here, because this comment refers to the echo statement Comments are tokens; they are only allowed at certain places in the input file as they belong to the syntax tree! This feature enables perfect source-to-source transformations (such as pretty-printing) and superior documentation generators. A nice side-effect is that the human reader of the code always knows exactly which code snippet the comment refers to. Identifiers & Keywords ---------------------- `Identifiers`:idx: in Nimrod 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) )* The following `keywords`:idx: are reserved and cannot be used as identifiers: .. code-block:: nimrod :file: keywords.txt Some keywords are unused; they are reserved for future developments of the language. Nimrod is a `style-insensitive`:idx: language. This means that it is not case-sensitive and even underscores are ignored: **type** is a reserved word, and so is **TYPE** or **T_Y_P_E**. The idea behind this is that this allows programmers to use their own preferred spelling style and libraries written by different programmers cannot use incompatible conventions. A Nimrod-aware editor or IDE can show the identifiers as preferred. Another advantage is that it frees the programmer from remembering the exact spelling of an identifier. String literals --------------- Terminal symbol in the grammar: ``STR_LIT``. `String literals`:idx: can be delimited by matching double quotes, and can contain the following `escape sequences`:idx:\ : ================== =================================================== Escape sequence Meaning ================== =================================================== ``\n`` `newline`:idx: ``\r``, ``\c`` `carriage return`:idx: ``\l`` `line feed`:idx: ``\f`` `form feed`:idx: ``\t`` `tabulator`:idx: ``\v`` `vertical tabulator`:idx: ``\\`` `backslash`:idx: ``\"`` `quotation mark`:idx: ``\'`` `apostrophe`:idx: ``\`` '0'..'9'+ `character with decimal value d`:idx:; all decimal digits directly following are used for the character ``\a`` `alert`:idx: ``\b`` `backspace`:idx: ``\e`` `escape`:idx: `[ESC]`:idx: ``\x`` HH `character with hex value HH`:idx:; exactly two hex digits are allowed ================== =================================================== Strings in Nimrod 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 immediately followed by a newline, the newline is not included in the string. The ending of the string literal is defined by the pattern ``"""[^"]``, so this: .. code-block:: nimrod """"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`:idx: 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:: nimrod 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:: nimrod 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`:idx:. 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 Nimrod (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: ``\n`` is not allowed as it may be wider than one character (often it is the pair CR/LF for example). 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 Nimrod can thus support ``array[char, int]`` or ``set[char]`` efficiently as many algorithms rely on this feature. Numerical constants ------------------- `Numerical constants`:idx: are of a single type and have the form:: hexdigit ::= digit | 'A'..'F' | 'a'..'f' octdigit ::= '0'..'7' bindigit ::= '0'..'1' INT_LIT ::= digit ( ['_'] digit )* | '0' ('x' | 'X' ) hexdigit ( ['_'] hexdigit )* | '0o' octdigit ( ['_'] octdigit )* | '0' ('b' | 'B' ) bindigit ( ['_'] bindigit )* 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' exponent ::= ('e' | 'E' ) ['+' | '-'] digit ( ['_'] digit )* FLOAT_LIT ::= digit (['_'] digit)* ('.' (['_'] digit)* [exponent] |exponent) FLOAT32_LIT ::= ( FLOAT_LIT | INT_LIT ) '\'' ('f' | 'F') '32' FLOAT64_LIT ::= ( FLOAT_LIT | INT_LIT ) '\'' ('f' | 'F') '64' As can be seen in the productions, numerical constants can contain underscores for readability. Integer and floating point literals may be given in decimal (no prefix), binary (prefix ``0b``), octal (prefix ``0o``) and hexadecimal (prefix ``0x``) notation. There exists a literal for each numerical type that is defined. The suffix starting with an apostrophe ('\'') is called a `type suffix`:idx:. Literals without a type suffix are of the type ``int``, unless the literal contains a dot or ``E|e`` in which case it is of type ``float``. The type suffixes are: ================= ========================= Type Suffix Resulting type of literal ================= ========================= ``'i8`` int8 ``'i16`` int16 ``'i32`` int32 ``'i64`` int64 ``'f32`` float32 ``'f64`` float64 ================= ========================= Floating point literals may also be in binary, octal or hexadecimal notation: ``0B0_10001110100_0000101001000111101011101111111011000101001101001001'f64`` is approximately 1.72826e35 according to the IEEE floating point standard. Operators --------- In Nimrod one can define his own operators. An `operator`:idx: is any combination of the following characters:: = + - * / < > @ $ ~ & % | ! ? ^ . : \ These keywords are also operators: ``and or not xor shl shr div mod in notin is isnot``. `=`:tok:, `:`:tok:, `::`:tok: are not available as general operators; they are used for other notational purposes. ``*:`` is as a special case the two tokens `*`:tok: and `:`:tok: (to support ``var v*: T``). Other tokens ------------ The following strings denote other tokens:: ` ( ) { } [ ] , ; [. .] {. .} (. .) The `slice`:idx: operator `..`:tok: takes precedence over other tokens that contain a dot: `{..}`:tok: are the three tokens `{`:tok:, `..`:tok:, `}`:tok: and not the two tokens `{.`:tok:, `.}`:tok:. Syntax ====== This section lists Nimrod's standard syntax in ENBF. How the parser receives indentation tokens is already described in the `Lexical Analysis`_ section. Nimrod allows user-definable operators. Binary operators have 9 different levels of precedence. For user-defined operators, the precedence depends on the first character the operator consists of. All binary operators are left-associative, except binary operators starting with (or only consisting of) ``^``. ================ ============================================== ================== =============== Precedence level Operators First characters Terminal symbol ================ ============================================== ================== =============== 9 (highest) ``$ ^`` OP9 8 ``* / div mod shl shr %`` ``* % \ /`` OP8 7 ``+ -`` ``+ ~ |`` OP7 6 ``&`` ``&`` OP6 5 ``..`` ``.`` OP5 4 ``== <= < >= > != in not_in is isnot not`` ``= < > !`` OP4 3 ``and`` OP3 2 ``or xor`` OP2 1 (lowest) `` @ : ? `` OP1 ================ ============================================== ================== =============== The grammar's start symbol is ``module``. .. include:: grammar.txt :literal: Semantics ========= Constants --------- `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. Nimrod contains a sophisticated compile-time evaluator, so procedures which have no side-effect can be used in constant expressions too: .. code-block:: nimrod import strutils const constEval = contains("abc", 'b') # computed at compile time! Types ----- All expressions have a `type`:idx: which is known at compile time. Nimrod 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`:idx: 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. Pre-defined integer types ~~~~~~~~~~~~~~~~~~~~~~~~~ These integer types are pre-defined: ``int`` the generic signed integer type; its size is platform dependent (the compiler chooses the processor's fastest integer type). This type should be used in general. An integer literal that has no type suffix is of this type. intXX additional signed integer types of XX bits use this naming scheme (example: int16 is a 16 bit wide integer). The current implementation supports ``int8``, ``int16``, ``int32``, ``int64``. Literals of these types have the suffix 'iXX. There are no `unsigned integer`:idx: types, only `unsigned operations`:idx: that treat their arguments as unsigned. Unsigned operations all wrap around; they cannot lead to over- or underflow errors. Unsigned operations 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. For further details, see `Convertible relation`_. Pre-defined floating point types ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The following floating point types are pre-defined: ``float`` the generic floating point type; its size is platform dependent (the compiler chooses the processor's fastest floating point type). This type should be used in general. floatXX an implementation may define additional floating point types of XX bits using this naming scheme (example: float64 is a 64 bit wide float). The current implementation supports ``float32`` and ``float64``. Literals of these types have the suffix 'fXX. Automatic type conversion in expressions with different kinds of floating point types is performed: See `Convertible relation`_ for further details. Arithmetic performed on floating point types follows the IEEE standard. Integer types are not converted to floating point types automatically and vice versa. The IEEE standard defines five types of floating-point exceptions: * Invalid: operations with mathematically invalid operands, for example 0.0/0.0, sqrt(-1.0), and log(-37.8). * Division by zero: divisor is zero and dividend is a finite nonzero number, for example 1.0/0.0. * Overflow: operation produces a result that exceeds the range of the exponent, for example MAXDOUBLE+0.0000000000001e308. * Underflow: operation produces a result that is too small to be represented as a normal number, for example, MINDOUBLE * MINDOUBLE. * Inexact: operation produces a result that cannot be represented with infinite precision, for example, 2.0 / 3.0, log(1.1) and 0.1 in input. The IEEE exceptions are either ignored at runtime or mapped to the Nimrod exceptions: `EFloatInvalidOp`:idx:, `EFloatDivByZero`:idx:, `EFloatOverflow`:idx:, `EFloatUnderflow`:idx:, and `EFloatInexact`:idx:. These exceptions inherit from the `EFloatingPoint`:idx: base class. Nimrod provides the pragmas `NaNChecks`:idx: and `InfChecks`:idx: to control whether the IEEE exceptions are ignored or trap a Nimrod exception: .. code-block:: nimrod {.NanChecks: on, InfChecks: on.} var a = 1.0 var b = 0.0 echo b / b # raises EFloatInvalidOp echo a / b # raises EFloatOverflow In the current implementation ``EFloatDivByZero`` and ``EFloatInexact`` are never raised. ``EFloatOverflow`` is raised instead of ``EFloatDivByZero``. There is also a `floatChecks`:idx: pragma that is a short-cut for the combination of ``NaNChecks`` and ``InfChecks`` pragmas. ``floatChecks`` are turned off as default. The only operations that are affected by the ``floatChecks`` pragma are the ``+``, ``-``, ``*``, ``/`` operators for floating point types. Boolean type ~~~~~~~~~~~~ The `boolean`:idx: type is named ``bool`` in Nimrod 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:: nimrod 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`:idx: is named ``char`` in Nimrod. 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 Nimrod can support ``array[char, int]`` or ``set[char]`` efficiently as many algorithms rely on this feature. The `TRune` type is used for Unicode characters, it can represent any Unicode character. ``TRune`` is declared in the ``unicode`` module. Enumeration types ~~~~~~~~~~~~~~~~~ `Enumeration`:idx: types define a new type whose values consist of the ones specified. The values are ordered. Example: .. code-block:: nimrod type TDirection = enum north, east, south, west Now the following holds:: ord(north) == 0 ord(east) == 1 ord(south) == 2 ord(west) == 3 Thus, north < east < south < west. The comparison operators can be used with enumeration types. For better interfacing to other programming languages, the fields of enum types can be assigned an explicit ordinal value. However, the ordinal values have to be in ascending order. A field whose ordinal value is not explicitly given is assigned the value of the previous field + 1. An explicit ordered enum can have *holes*: .. code-block:: nimrod type TTokenType = enum a = 2, b = 4, c = 89 # holes are valid However, it is then not an ordinal anymore, so it is not possible to use these enums as an index type for arrays. The procedures ``inc``, ``dec``, ``succ`` and ``pred`` are not available for them either. The compiler supports the built-in stringify operator ``$`` for enumerations. The stringify's result can be controlled by specifying the string values to use explicitely: .. code-block:: nimrod type TMyEnum = enum valueA = (0, "my value A"), valueB = "value B", valueC = 2, valueD = (3, "abc") As can be seen from the example, it is possible to both specify a field's ordinal value and its string value by using a tuple construction. It is also possible to only specify one of them. Subrange types ~~~~~~~~~~~~~~ A `subrange`:idx: type is a range of values from an ordinal type (the base type). To define a subrange type, one must specify it's limiting values: the lowest and highest value of the type: .. code-block:: nimrod type TSubrange = range[0..5] ``TSubrange`` is a subrange of an integer which can only hold the values 0 to 5. Assigning any other value to a variable of type ``TSubrange`` is a checked runtime error (or static error if it can be statically determined). Assignments from the base type to one of its subrange types (and vice versa) are allowed. A subrange type has the same size as its base type (``int`` in the example). String type ~~~~~~~~~~~ All string literals are of the type `string`:idx:. A string in Nimrod is very similar to a sequence of characters. However, strings in Nimrod are both zero-terminated and have a length field. One can retrieve the length with the builtin ``len`` procedure; the length never counts the terminating zero. The assignment operator for strings always copies the string. The ``&`` operator concatenates strings. Strings are compared by their lexicographical order. All comparison operators are available. Strings can be indexed like arrays (lower bound is 0). Unlike arrays, they can be used in case statements: .. code-block:: nimrod 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 can be used for iteration over all Unicode characters. Structured types ~~~~~~~~~~~~~~~~ A variable of a `structured type`:idx: 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`:idx: are a homogeneous type, meaning that each element in the array has the same type. Arrays always have a fixed length which is specified at compile time (except for open arrays). They can be indexed by any ordinal type. A parameter ``A`` may be an *open array*, in which case it is indexed by integers from 0 to ``len(A)-1``. An array expression may be constructed by the array constructor ``[]``. `Sequences`:idx: are similar to arrays but of dynamic length which may change during runtime (like strings). 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:: nimrod type TIntArray = array[0..5, int] # an array that is indexed with 0..5 TIntSeq = seq[int] # a sequence of integers var x: TIntArray y: TIntSeq x = [1, 2, 3, 4, 5, 6] # [] is the array constructor y = @[1, 2, 3, 4, 5, 6] # the @ turns the array into a sequence The lower bound of an array or sequence may be received by the built-in proc ``low()``, the higher bound by ``high()``. The length may be received by ``len()``. ``low()`` for a sequence or an open array always returns 0, as this is the first valid index. One can append elements to a sequence with the ``add()`` proc or the ``&`` operator, and remove (and get) the last element of a sequence with the ``pop()`` proc. The notation ``x[i]`` can be used to access the i-th element of ``x``. Arrays are always bounds checked (at compile-time or at runtime). These checks can be disabled via pragmas or invoking the compiler with the ``--boundChecks:off`` command line switch. An open array is also a means to implement passing a variable number of arguments to a procedure. The compiler converts the list of arguments to an array automatically: .. code-block:: nimrod proc myWriteln(f: TFile, a: openarray[string]) = for s in items(a): write(f, s) write(f, "\n") myWriteln(stdout, "abc", "def", "xyz") # is transformed by the compiler to: myWriteln(stdout, ["abc", "def", "xyz"]) This transformation is only done if the openarray parameter is the last parameter in the procedure header. The current implementation does not support nested open arrays. Tuples and object types ~~~~~~~~~~~~~~~~~~~~~~~ A variable of a `tuple`:idx: or `object`:idx: 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 assignment operator for tuples copies each component. The default assignment operator for objects copies each component. Overloading of the assignment operator for objects is not possible, but this may change in future versions of the compiler. .. code-block:: nimrod type TPerson = tuple[name: string, age: int] # type representing a person # a person consists of a name # and an age var person: TPerson person = (name: "Peter", age: 30) # the same, but less readable: person = ("Peter", 30) The implementation aligns the fields for best access performance. The alignment is compatible with the way the C compiler does it. 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 ``is`` operator can be used to determine the object's type. .. code-block:: nimrod type TPerson = object name*: string # the * means that `name` is accessible from other modules age: int # no * means that the field is hidden TStudent = object of TPerson # a student is a person id: int # with an id field var student: TStudent person: TPerson assert(student is TStudent) # is true Object fields that should be visible from outside the defining module, have to be marked by ``*``. In contrast to tuples, different object types are never *equivalent*. Object variants ~~~~~~~~~~~~~~~ Often an object hierarchy is overkill in certain situations where simple `variant`:idx: types are needed. An example: .. code-block:: nimrod # This is an example how an abstract syntax tree could be modelled in Nimrod type TNodeKind = enum # the different node types nkInt, # a leaf with an integer value nkFloat, # a leaf with a float value nkString, # a leaf with a string value nkAdd, # an addition nkSub, # a subtraction nkIf # an if statement PNode = ref TNode TNode = object case kind: TNodeKind # the ``kind`` field is the discriminator of nkInt: intVal: int of nkFloat: floatVal: float of nkString: strVal: string of nkAdd, nkSub: leftOp, rightOp: PNode of nkIf: condition, thenPart, elsePart: PNode var n: PNode new(n) # creates a new node n.kind = nkFloat n.floatVal = 0.0 # valid, because ``n.kind==nkFloat``, so that it fits # the following statement raises an `EInvalidField` exception, because # n.kind's value does not fit: n.strVal = "" 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. Set type ~~~~~~~~ The `set type`:idx: models the mathematical notion of a set. The set's basetype can only be an ordinal type. The reason is that sets are implemented as high performance bit vectors. Sets can be constructed via the set constructor: ``{}`` is the empty set. The empty set is type compatible with any special set type. The constructor can also be used to include elements (and ranges of elements) in the set: .. code-block:: nimrod {'a'..'z', '0'..'9'} # This constructs a set that contains the # letters from 'a' to 'z' and the digits # from '0' to '9' These operations are supported by sets: ================== ======================================================== operation meaning ================== ======================================================== ``A + B`` union of two sets ``A * B`` intersection of two sets ``A - B`` difference of two sets (A without B's elements) ``A == B`` set equality ``A <= B`` subset relation (A is subset of B or equal to B) ``A < B`` strong subset relation (A is a real subset of B) ``e in A`` set membership (A contains element e) ``A -+- B`` symmetric set difference (= (A - B) + (B - A)) ``card(A)`` the cardinality of A (number of elements in A) ``incl(A, elem)`` same as A = A + {elem} ``excl(A, elem)`` same as A = A - {elem} ================== ======================================================== Reference and pointer types ~~~~~~~~~~~~~~~~~~~~~~~~~~~ References (similar to `pointers`:idx: 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. Nimrod distinguishes between `traced`:idx: and `untraced`:idx: references. Untraced references are also called *pointers*. Traced references point to objects of a garbage collected heap, untraced references point to manually allocated objects or to objects somewhere else in memory. Thus untraced references are *unsafe*. However for certain low-level operations (accessing the hardware) untraced references are unavoidable. Traced references are declared with the **ref** keyword, untraced references are declared with the **ptr** keyword. An empty subscript ``[]`` notation can be used to derefer a reference, the ``addr`` procedure returns the address of an item. An address is always an untraced reference. Thus the usage of ``addr`` is an *unsafe* feature. The ``.`` (access a tuple/object field operator) and ``[]`` (array/string/sequence index operator) operators perform implicit dereferencing operations for reference types: .. code-block:: nimrod type PNode = ref TNode TNode = object le, ri: PNode data: int var n: PNode new(n) n.data = 9 # no need to write n[].data; in fact n[].data is highly discouraged! 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:: nimrod type TData = tuple[x, y: int, s: string] # allocate memory for TData on the heap: var d = cast[ptr TData](alloc0(sizeof(TData))) # create a new string on the garbage collected heap: d.s = "abc" # tell the GC that the string is not needed anymore: GCunref(d.s) # free the memory: dealloc(d) Without the ``GCunref`` call the memory allocated for the ``d.s`` string would never be freed. The example also demonstrates two important features for low level programming: the ``sizeof`` proc returns the size of a type or value in bytes. The ``cast`` operator can circumvent the type system: the compiler is forced to treat the result of the ``alloc0`` call (which returns an untyped pointer) as if it would have the type ``ptr TData``. Casting should only be done if it is unavoidable: it breaks type safety and bugs can lead to mysterious crashes. **Note**: The example only works because the memory is initialized with zero (``alloc0`` instead of ``alloc`` does this): ``d.s`` is thus initialized to ``nil`` which the string assignment can handle. You need to know low level details like this when mixing garbage collected data with unmanaged memory. .. XXX finalizers for traced objects Procedural type ~~~~~~~~~~~~~~~ A `procedural type`:idx: is internally a pointer to a procedure. ``nil`` is an allowed value for variables of a procedural type. Nimrod uses procedural types to achieve `functional`:idx: programming techniques. Example: .. code-block:: nimrod type TCallback = proc (x: int) {.cdecl.} proc printItem(x: Int) = ... proc forEach(c: TCallback) = ... forEach(printItem) # this will NOT work because calling conventions differ 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. Nimrod supports these `calling conventions`:idx:, which are all incompatible to each other: `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 Nimrod does not inline, but leaves this to the C compiler. Thus 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. `nimcall`:idx: Nimcall is the default convention used for Nimrod procedures. It is the same as ``fastcall``, but only for C compilers that support ``fastcall``. `closure`:idx: indicates that the procedure expects a context, a closure that needs to be passed to the procedure. The calling convention ``nimcall`` is compatible to ``closure``. `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 Nimrod's default calling convention for procedures is ``fastcall`` to improve speed. Most calling conventions exist only for the Windows 32-bit platform. Assigning/passing a procedure to a procedural variable is only allowed if one of the following conditions hold: 1) The procedure that is accessed resists in the current module. 2) The procedure is marked with the ``procvar`` pragma (see `procvar pragma`_). 3) The procedure has a calling convention that differs from ``nimcall``. 4) The procedure is anonymous. The rules' purpose is to prevent the case that extending a non-``procvar`` procedure with default parameters breaks client code. 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. 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:: nimrod type TDollar = distinct int TEuro = distinct int var d: TDollar e: TEuro echo d + 12 # Error: cannot add a number with no unit and a ``TDollar`` Unfortunately, ``d + 12.TDollar`` is not allowed either, because ``+`` is defined for ``int`` (among others), not for ``TDollar``. So a ``+`` for dollars needs to be defined: .. code-block:: proc `+` (x, y: TDollar): TDollar = result = TDollar(int(x) + int(y)) It does not make sense to multiply a dollar with a dollar, but with a number without unit; and the same holds for division: .. code-block:: proc `*` (x: TDollar, y: int): TDollar = result = TDollar(int(x) * y) proc `*` (x: int, y: TDollar): TDollar = result = TDollar(x * int(y)) proc `div` ... This quickly gets tedious. The implementations are trivial and the compiler should not generate all this code only to optimize it away later - after all ``+`` for dollars should produce the same binary code as ``+`` for ints. The pragma ``borrow`` has been designed to solve this problem; in principle it generates the above trivial implementations: .. code-block:: nimrod proc `*` (x: TDollar, y: int): TDollar {.borrow.} proc `*` (x: int, y: TDollar): TDollar {.borrow.} proc `div` (x: TDollar, y: int): TDollar {.borrow.} The ``borrow`` pragma makes the compiler use the same implementation as the proc that deals with the distinct type's base type, so no code is generated. But it seems all this boilerplate code needs to be repeated for the ``TEuro`` currency. This can be solved with templates_. .. code-block:: nimrod template Additive(typ: typeDesc): stmt = proc `+` *(x, y: typ): typ {.borrow.} proc `-` *(x, y: typ): typ {.borrow.} # unary operators: proc `+` *(x: typ): typ {.borrow.} proc `-` *(x: typ): typ {.borrow.} template Multiplicative(typ, base: typeDesc): stmt = proc `*` *(x: typ, y: base): typ {.borrow.} proc `*` *(x: base, y: typ): typ {.borrow.} proc `div` *(x: typ, y: base): typ {.borrow.} proc `mod` *(x: typ, y: base): typ {.borrow.} template Comparable(typ: typeDesc): stmt = proc `<` * (x, y: typ): bool {.borrow.} proc `<=` * (x, y: typ): bool {.borrow.} proc `==` * (x, y: typ): bool {.borrow.} template DefineCurrency(typ, base: expr): stmt = type typ* = distinct base Additive(typ) Multiplicative(typ, base) Comparable(typ) DefineCurrency(TDollar, int) DefineCurrency(TEuro, int) Type relations -------------- The following section defines several relations on types that are needed to describe the type checking done by the compiler. Type equality ~~~~~~~~~~~~~ Nimrod 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:: nimrod proc typeEqualsAux(a, b: PType, s: var set[PType * PType]): bool = if (a,b) in s: return true incl(s, (a,b)) if a.kind == b.kind: case a.kind of int, intXX, float, floatXX, char, string, cstring, pointer, bool, nil: # leaf type: kinds identical; nothing more to check result = true of ref, ptr, var, set, seq, openarray: result = typeEqualsAux(a.baseType, b.baseType, s) of range: result = typeEqualsAux(a.baseType, b.baseType, s) and (a.rangeA == b.rangeA) and (a.rangeB == b.rangeB) of array: result = typeEqualsAux(a.baseType, b.baseType, s) and typeEqualsAux(a.indexType, b.indexType, s) of tuple: if a.tupleLen == b.tupleLen: for i in 0..a.tupleLen-1: if not typeEqualsAux(a[i], b[i], s): return false result = true of object, enum, distinct: result = a == b of proc: result = typeEqualsAux(a.parameterTuple, b.parameterTuple, s) and typeEqualsAux(a.resultType, b.resultType, s) and a.callingConvention == b.callingConvention proc typeEquals(a, b: PType): bool = var s: set[PType * PType] = {} result = typeEqualsAux(a, b, s) Since types are graphs which can have cycles, the above algorithm needs an auxiliary set ``s`` to detect this case. 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:: nimrod proc isSubtype(a, b: PType): bool = if a.kind == b.kind: case a.kind of object: var aa = a.baseType while aa != nil and aa != b: aa = aa.baseType result = aa == b of var, ref, ptr: result = isSubtype(a.baseType, b.baseType) .. XXX nil is a special value! Convertible relation ~~~~~~~~~~~~~~~~~~~~ A type ``a`` is **implicitly** convertible to type ``b`` iff the following algorithm returns true: .. code-block:: nimrod # XXX range types? proc isImplicitlyConvertible(a, b: PType): bool = case a.kind of proc: if b.kind == proc: var x = a.parameterTuple var y = b.parameterTuple if x.tupleLen == y.tupleLen: for i in 0.. x.tupleLen-1: if not isSubtype(x[i], y[i]): return false result = isSubType(b.resultType, a.resultType) of int8: result = b.kind in {int16, int32, int64, int} of int16: result = b.kind in {int32, int64, int} of int32: result = b.kind in {int64, int} of float: result = b.kind in {float32, float64} of float32: result = b.kind in {float64, float} of float64: result = b.kind in {float32, float} of seq: result = b.kind == openArray and typeEquals(a.baseType, b.baseType) of array: result = b.kind == openArray and typeEquals(a.baseType, b.baseType) if a.baseType == char and a.indexType.rangeA == 0: result = b.kind = cstring of cstring, ptr: result = b.kind == pointer of string: result = b.kind == cstring A type ``a`` is **explicitly** convertible to type ``b`` iff the following algorithm returns true: .. code-block:: nimrod proc isIntegralType(t: PType): bool = result = isOrdinal(t) or t.kind in {float, float32, float64} proc isExplicitlyConvertible(a, b: PType): bool = if isImplicitlyConvertible(a, b): return true if isIntegralType(a) and isIntegralType(b): return true if isSubtype(a, b) or isSubtype(b, a): return true if a.kind == distinct and typeEquals(a.baseType, b): return true if b.kind == distinct and typeEquals(b.baseType, a): return true return false You can, however, define your own implicit converters: .. code-block:: nimrod 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 Assignment compatibility ~~~~~~~~~~~~~~~~~~~~~~~~ An expression ``b`` can be assigned to an expression ``a`` iff ``a`` is an `l-value` and ``isImplicitlyConvertible(b.typ, a.typ)`` holds. Overloading resolution ~~~~~~~~~~~~~~~~~~~~~~ To be written. Statements and expressions -------------------------- Nimrod uses the common statement/expression paradigm: `Statements`:idx: do not produce a value in contrast to expressions. Call expressions are statements. If the called procedure returns a value, it is not a valid statement as statements do not produce values. To evaluate an expression for side-effects and throw its value away, one can use the ``discard`` statement. Statements are separated into `simple statements`:idx: and `complex statements`:idx:. Simple statements are statements that cannot contain other statements like assignments, calls or the ``return`` statement; complex statements can contain other statements. To avoid the `dangling else problem`:idx:, complex statements always have to be intended:: simpleStmt ::= returnStmt | yieldStmt | discardStmt | raiseStmt | breakStmt | continueStmt | pragma | importStmt | fromStmt | includeStmt | exprStmt complexStmt ::= ifStmt | whileStmt | caseStmt | tryStmt | forStmt | blockStmt | asmStmt | procDecl | iteratorDecl | macroDecl | templateDecl | constSection | typeSection | whenStmt | varSection Discard statement ~~~~~~~~~~~~~~~~~ Syntax:: discardStmt ::= 'discard' expr Example: .. code-block:: nimrod discard proc_call("arg1", "arg2") # discard the return value of `proc_call` The `discard`:idx: statement evaluates its expression for side-effects and throws the expression's resulting value away. If the expression has no side-effects, this generates a static error. Ignoring the return value of a procedure without using a discard statement is a static error too. Var statement ~~~~~~~~~~~~~ Syntax:: colonOrEquals ::= ':' typeDesc ['=' expr] | '=' expr varField ::= symbol ['*'] [pragma] varPart ::= symbol (comma symbol)* [comma] colonOrEquals [COMMENT | IND COMMENT] varSection ::= 'var' (varPart | indPush (COMMENT|varPart) (SAD (COMMENT|varPart))* DED indPop) `Var`:idx: 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:: nimrod var a: int = 0 x, y, z: int If an initializer is given the type can be omitted: the variable is then of the same type as the initializing expression. Variables are always initialized with a default value if there is no initializing expression. The default value depends on the type and is always a zero in binary. ============================ ============================================== Type default value ============================ ============================================== any integer type 0 any float 0.0 char '\\0' bool false ref or pointer type nil procedural type nil sequence nil (*not* ``@[]``) string nil (*not* "") tuple[x: A, y: B, ...] (default(A), default(B), ...) (analogous for objects) array[0..., T] [default(T), ...] range[T] default(T); this may be out of the valid range T = enum cast[T](0); this may be an invalid value ============================ ============================================== Const section ~~~~~~~~~~~~~ Syntax:: colonAndEquals ::= [':' typeDesc] '=' expr constDecl ::= symbol ['*'] [pragma] colonAndEquals [COMMENT | IND COMMENT] | COMMENT constSection ::= 'const' indPush constDecl (SAD constDecl)* DED indPop Example: .. code-block:: nimrod const MyFilename = "/home/my/file.txt" debugMode: bool = false The `const`:idx: section declares symbolic constants. A symbolic constant is a name for a constant expression. Symbolic constants only allow read-access. If statement ~~~~~~~~~~~~ Syntax:: ifStmt ::= 'if' expr ':' stmt ('elif' expr ':' stmt)* ['else' ':' stmt] Example: .. code-block:: nimrod 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`:idx: statement is a simple way to make a branch in the control flow: The expression after the keyword ``if`` is evaluated, if it is true the corresponding statements after the ``:`` are executed. Otherwise the expression after the ``elif`` is evaluated (if there is an ``elif`` branch), if it is true the corresponding statements after the ``:`` are executed. This goes on until the last ``elif``. If all conditions fail, the ``else`` part is executed. If there is no ``else`` part, execution continues with the statement after the ``if`` statement. Case statement ~~~~~~~~~~~~~~ Syntax:: caseStmt ::= 'case' expr [':'] ('of' sliceExprList ':' stmt)* ('elif' expr ':' stmt)* ['else' ':' stmt] Example: .. code-block:: nimrod 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`:idx: 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. If the expression is not of an ordinal type, and no ``else`` part is given, control passes after the ``case`` statement. To suppress the static error in the ordinal case an ``else`` part with a ``nil`` statement can be used. When statement ~~~~~~~~~~~~~~ Syntax:: whenStmt ::= 'when' expr ':' stmt ('elif' expr ':' stmt)* ['else' ':' stmt] Example: .. code-block:: nimrod 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`:idx: statement is almost identical to the ``if`` statement with some exceptions: * Each ``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 ``expr`` 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. Raise statement ~~~~~~~~~~~~~~~ Syntax:: raiseStmt ::= 'raise' [expr] Example: .. code-block:: nimrod raise newEOS("operating system failed") Apart from built-in operations like array indexing, memory allocation, etc. the ``raise`` statement is the only way to raise an exception. .. XXX document this better! If no exception name is given, the current exception is `re-raised`:idx:. The `ENoExceptionToReraise`:idx: exception is raised if there is no exception to re-raise. It follows that the ``raise`` statement *always* raises an exception. Try statement ~~~~~~~~~~~~~ Syntax:: qualifiedIdent ::= symbol ['.' symbol] exceptList ::= [qualifiedIdent (comma qualifiedIdent)* [comma]] tryStmt ::= 'try' ':' stmt ('except' exceptList ':' stmt)* ['finally' ':' stmt] Example: .. code-block:: nimrod # read the first two lines of a text file that should contain numbers # and tries to add them var f: TFile if open(f, "numbers.txt"): try: var a = readLine(f) var b = readLine(f) echo("sum: " & $(parseInt(a) + parseInt(b))) except EOverflow: echo("overflow!") except EInvalidValue: echo("could not convert string to integer") except EIO: echo("IO error!") except: echo("Unknown exception!") finally: close(f) The statements after the `try`:idx: are executed in sequential order unless an exception ``e`` is raised. If the exception type of ``e`` matches any of the list ``exceptlist`` 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 in no list. 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). Return statement ~~~~~~~~~~~~~~~~ Syntax:: returnStmt ::= 'return' [expr] Example: .. code-block:: nimrod return 40+2 The `return`:idx: 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:: nimrod 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:: nimrod proc returnZero(): int = # implicitly returns 0 Yield statement ~~~~~~~~~~~~~~~ Syntax:: yieldStmt ::= 'yield' expr Example: .. code-block:: nimrod yield (1, 2, 3) The `yield`:idx: 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 ~~~~~~~~~~~~~~~ Syntax:: blockStmt ::= 'block' [symbol] ':' stmt Example: .. code-block:: nimrod 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`:idx:. 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 ~~~~~~~~~~~~~~~ Syntax:: breakStmt ::= 'break' [symbol] Example: .. code-block:: nimrod break The `break`:idx: 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 ~~~~~~~~~~~~~~~ Syntax:: whileStmt ::= 'while' expr ':' stmt Example: .. code-block:: nimrod echo("Please tell me your password: \n") var pw = readLine(stdin) while pw != "12345": echo("Wrong password! Next try: \n") pw = readLine(stdin) The `while`:idx: 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 ~~~~~~~~~~~~~~~~~~ Syntax:: continueStmt ::= 'continue' A `continue`:idx: 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:: nimrod while expr1: stmt1 continue stmt2 Is equivalent to: .. code-block:: nimrod while expr1: block myBlockName: stmt1 break myBlockName stmt2 Assembler statement ~~~~~~~~~~~~~~~~~~~ Syntax:: asmStmt ::= 'asm' [pragma] (STR_LIT | RSTR_LIT | TRIPLESTR_LIT) The direct embedding of `assembler`:idx: code into Nimrod code is supported by the unsafe ``asm`` statement. Identifiers in the assembler code that refer to Nimrod identifiers shall be enclosed in a special character which can be specified in the statement's pragmas. The default special character is ``'`'``: .. code-block:: nimrod proc addInt(a, b: int): int {.pure.} = # a in eax, and b in edx asm """ mov eax, `a` add eax, `b` jno theEnd call `raiseOverflow` theEnd: """ If expression ~~~~~~~~~~~~~ An `if expression` is almost like an if statement, but it is an expression. Example: .. code-block:: nimrod 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 (but unlikely to be good style). Table constructor ~~~~~~~~~~~~~~~~~ A `table constructor`:idx: is syntactic sugar for an array constructor: .. code-block:: nimrod {"key1": "value1", "key2": "value2"} # is the same as: [("key1", "value1"), ("key2", "value2")] The empty table can be written ``{:}`` (in contrast to the empty set which is ``{}``) which is thus another way to write as the empty array constructor ``[]``. This slightly unusal way of supporting tables has lots of advantages: * The order of the (key,value)-pairs is preserved, thus it is easy to support ordered dicts with for example ``{key: val}.newOrderedTable``. * A table literal can be put into a ``const`` section and the compiler can easily put it into the executable's data section just like it can for arrays and the generated data section requires a minimal amount of memory. * Every table implementation is treated equal syntactically. * Apart from the minimal syntactic sugar the language core does not need to know about tables. Type conversions ~~~~~~~~~~~~~~~~ Syntactically a `type conversion` is like a procedure call, but a type name replaces the procedure name. A type conversion is always safe in the sense that a failure to convert a type to another results in an exception (if it cannot be determined statically). Type casts ~~~~~~~~~~ Example: .. code-block:: nimrod 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``. 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. Procedures ~~~~~~~~~~ What most programming languages call `methods`:idx: or `functions`:idx: are called `procedures`:idx: in Nimrod (which is the correct terminology). A procedure declaration defines an identifier and associates it with a block of code. A procedure may call itself recursively. A parameter may be given a default value that is used if the caller does not provide a value for this parameter. The syntax is:: param ::= symbol (comma symbol)* (':' typeDesc ['=' expr] | '=' expr) paramList ::= ['(' [param (comma param)*] [SAD] ')'] [':' typeDesc] genericParam ::= symbol [':' typeDesc] ['=' expr] genericParams ::= '[' genericParam (comma genericParam)* [SAD] ']' procDecl ::= 'proc' symbol ['*'] [genericParams] paramList [pragma] ['=' stmt] If the ``= stmt`` part is missing, it is a `forward`:idx: declaration. If the proc returns a value, the procedure body can access an implicitly declared variable named `result`:idx: that represents the return value. Procs can be overloaded. The overloading resolution algorithm tries to find the proc that is the best match for the arguments. Example: .. code-block:: nimrod 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:: nimrod proc callme(x, y: int, s: string = "", c: char, b: bool = false) = ... # call with positional arguments # parameter bindings: callme(0, 1, "abc", '\t', true) # (x=0, y=1, s="abc", c='\t', b=true) # call with named and positional arguments: callme(y=1, x=0, "abd", '\t') # (x=0, y=1, s="abd", c='\t', b=false) # call with named arguments (order is not relevant): callme(c='\t', y=1, x=0) # (x=0, y=1, s="", c='\t', b=false) # call as a command statement: no () needed: callme 0, 1, "abc", '\t' A procedure cannot modify its parameters (unless the parameters have the type `var`). `Operators`:idx: are procedures with a special operator symbol as identifier: .. code-block:: nimrod proc `$` (x: int): string = # converts an integer to a string; this is a prefix operator. return 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:: nimrod proc `*+` (a, b, c: int): int = # Multiply and add return a * b + c assert `*+`(3, 4, 6) == `*`(a, `+`(b, c)) Var parameters ~~~~~~~~~~~~~~ The type of a parameter may be prefixed with the ``var`` keyword: .. code-block:: nimrod 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:: nimrod 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:: nimrod proc divmod(a, b: int): tuple[res, remainder: int] = return (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:: nimrod var (x, y) = divmod(8, 5) # tuple unpacking assert x == 1 assert y == 3 Overloading of the subscript operator ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The ``[]`` subscript operator for arrays/openarrays/sequences can be overloaded. Overloading support is only possible if the first parameter has no type that already supports the built-in ``[]`` notation. Currently the compiler does not check this. XXX Multiple indexes Multi-methods ~~~~~~~~~~~~~ Procedures always use static dispatch. `Multi-methods`:idx: use dynamic dispatch. .. code-block:: nimrod type TExpr = object ## abstract base class for an expression TLiteral = object of TExpr x: int TPlusExpr = object of TExpr a, b: ref TExpr method eval(e: ref TExpr): int = # override this base method quit "to override!" method eval(e: ref TLiteral): int = return e.x method eval(e: ref TPlusExpr): int = # watch out: relies on dynamic binding return eval(e.a) + eval(e.b) proc newLit(x: int): ref TLiteral = new(result) result.x = x proc newPlus(a, b: ref TExpr): ref TPlusExpr = new(result) result.a = a result.b = b echo eval(newPlus(newPlus(newLit(1), newLit(2)), newLit(4))) In the example the constructors ``newLit`` and ``newPlus`` are procs because they should use static binding, but ``eval`` is a method because it requires dynamic binding. In a multi-method all parameters that have an object type are used for the dispatching: .. code-block:: nimrod type TThing = object TUnit = object of TThing x: int method collide(a, b: TThing) {.inline.} = quit "to override!" method collide(a: TThing, b: TUnit) {.inline.} = echo "1" method collide(a: TUnit, b: TThing) {.inline.} = echo "2" var a, b: TUnit collide(a, b) # output: 2 Invocation of a multi-method cannot be ambiguous: collide 2 is preferred over collide 1 because the resolution works from left to right. In the example ``TUnit, TThing`` is prefered over ``TThing, TUnit``. **Performance note**: Nimrod 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 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Syntax:: forStmt ::= 'for' symbol (comma symbol)* [comma] 'in' expr ['..' expr] ':' stmt param ::= symbol (comma symbol)* [comma] ':' typeDesc paramList ::= ['(' [param (comma param)* [comma]] ')'] [':' typeDesc] genericParam ::= symbol [':' typeDesc] genericParams ::= '[' genericParam (comma genericParam)* [comma] ']' iteratorDecl ::= 'iterator' symbol ['*'] [genericParams] paramList [pragma] ['=' stmt] 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 (``x`` in the example) - 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 is always 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:: nimrod # 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:: nimrod var i = 0 while i < len(a): var ch = a[i] echo(ch) inc(i) The current implementation always inlines the iterator code leading to zero overhead for the abstraction. But this may increase the code size. Later versions of the compiler will only inline iterators which have the calling convention ``inline``. If the iterator yields a tuple, there have to 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. Type sections ~~~~~~~~~~~~~ Syntax:: typeDef ::= typeDesc | objectDef | enumDef genericParam ::= symbol [':' typeDesc] genericParams ::= '[' genericParam (comma genericParam)* [comma] ']' typeDecl ::= COMMENT | symbol ['*'] [genericParams] ['=' typeDef] [COMMENT|IND COMMENT] typeSection ::= 'type' indPush typeDecl (SAD typeDecl)* DED indPop Example: .. code-block:: nimrod type # example demonstrates mutually recursive types PNode = ref TNode # a traced pointer to a TNode TNode = object le, ri: PNode # left and right subtrees sym: ref TSym # leaves contain a reference to a TSym TSym = object # a symbol name: string # the symbol's name line: int # the line the symbol was declared in code: PNode # the symbol's abstract syntax tree A `type`:idx: 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. Generics ~~~~~~~~ Example: .. code-block:: nimrod type TBinaryTree[T] = object # TBinaryTree is a generic type with # with generic param ``T`` le, ri: ref TBinaryTree[T] # left and right subtrees; may be nil data: T # the data stored in a node PBinaryTree[T] = ref TBinaryTree[T] # a shorthand for notational convenience proc newNode[T](data: T): PBinaryTree[T] = # constructor for a node new(result) result.data = data proc add[T](root: var PBinaryTree[T], n: PBinaryTree[T]) = if root == nil: root = n else: var it = root while it != nil: var c = cmp(it.data, n.data) # compare the data items; uses # the generic ``cmp`` proc that works for # any type that has a ``==`` and ``<`` # operator if c < 0: if it.le == nil: it.le = n return it = it.le else: if it.ri == nil: it.ri = n return it = it.ri iterator inorder[T](root: PBinaryTree[T]): T = # inorder traversal of a binary tree # recursive iterators are not yet implemented, so this does not work in # the current compiler! if root.le != nil: yield inorder(root.le) yield root.data if root.ri != nil: yield inorder(root.ri) var root: PBinaryTree[string] # instantiate a PBinaryTree with the type string add(root, newNode("hallo")) # instantiates generic procs ``newNode`` and add(root, newNode("world")) # ``add`` for str in inorder(root): writeln(stdout, str) `Generics`:idx: are Nimrod'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. Templates ~~~~~~~~~ A `template`:idx: is a simple form of a macro: It is a simple substitution mechanism that operates on Nimrod'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:: nimrod template `!=` (a, b: expr): expr = # this definition exists in the System module not (a == b) assert(5 != 6) # the compiler rewrites that to: assert(not (5 == 6)) The ``!=``, ``>``, ``>=``, ``in``, ``notin``, ``isnot`` operators are in fact templates: | ``a > b`` is transformed into ``b < a``. | ``a in b`` is transformed into ``contains(b, a)``. | ``notin`` and ``isnot`` have the obvious meanings. The "types" of templates can be the symbols ``expr`` (stands for *expression*), ``stmt`` (stands for *statement*) or ``typedesc`` (stands for *type description*). These are no real types, they just help the compiler parsing. Real types can be used too; this implies that expressions are expected. However, for parameter type checking the arguments are semantically checked before being passed to the template. Other arguments are not semantically checked before being passed to the template. The template body does not open a new scope. To open a new scope a ``block`` statement can be used: .. code-block:: nimrod template declareInScope(x: expr, t: typeDesc): stmt = var x: t template declareInNewScope(x: expr, t: typeDesc): stmt = # open a new scope: block: var x: t declareInScope(a, int) a = 42 # works, `a` is known here declareInNewScope(b, int) b = 42 # does not work, `b` is unknown If there is a ``stmt`` parameter it should be the last in the template declaration, because statements are passed to a template via a special ``:`` syntax: .. code-block:: nimrod template withFile(f, fn, mode: expr, actions: stmt): stmt = block: var f: TFile if open(f, fn, mode): try: actions finally: close(f) else: quit("cannot open: " & fn) withFile(txt, "ttempl3.txt", fmWrite): txt.writeln("line 1") txt.writeln("line 2") In the example the two ``writeln`` statements are bound to the ``actions`` parameter. **Note:** Symbol binding rules for templates might change! Symbol binding within templates happens after template instantation: .. code-block:: nimrod # Module A var lastId = 0 template genId*: expr = inc(lastId) lastId .. code-block:: nimrod # Module B import A echo genId() # Error: undeclared identifier: 'lastId' Exporting a template is a often a leaky abstraction. However, to compensate for this case, the ``bind`` operator can be used: All identifiers within a ``bind`` context are bound early (i.e. when the template is parsed). The affected identifiers are then always bound early even if the other occurences are in no ``bind`` context: .. code-block:: nimrod # Module A var lastId = 0 template genId*: expr = inc(bind lastId) lastId .. code-block:: nimrod # Module B import A echo genId() # Works **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 ------ `Macros`:idx: are the most powerful feature of Nimrod. They can be used to implement `domain specific languages`:idx:. While macros enable advanced compile-time code transformations, they cannot change Nimrod's syntax. However, this is no real restriction because Nimrod's syntax is flexible enough anyway. To write macros, one needs to know how the Nimrod concrete syntax is converted to an abstract syntax tree. There are two ways to invoke a macro: (1) invoking a macro like a procedure call (`expression macros`) (2) invoking a macro with the special ``macrostmt`` syntax (`statement macros`) Expression Macros ~~~~~~~~~~~~~~~~~ The following example implements a powerful ``debug`` command that accepts a variable number of arguments: .. code-block:: nimrod # to work with Nimrod syntax trees, we need an API that is defined in the # ``macros`` module: import macros macro debug(n: expr): stmt = # `n` is a Nimrod AST that contains the whole macro invocation # this macro returns a list of statements: result = newNimNode(nnkStmtList, n) # iterate over any argument that is passed to this macro: for i in 1..n.len-1: # add a call to the statement list that writes the expression; # `toStrLit` converts an AST to its string representation: add(result, newCall("write", newIdentNode("stdout"), toStrLit(n[i]))) # add a call to the statement list that writes ": " add(result, newCall("write", newIdentNode("stdout"), newStrLitNode(": "))) # add a call to the statement list that writes the expressions value: add(result, newCall("writeln", newIdentNode("stdout"), n[i])) var a: array [0..10, int] x = "some string" a[0] = 42 a[1] = 45 debug(a[0], a[1], x) The macro call expands to: .. code-block:: nimrod write(stdout, "a[0]") write(stdout, ": ") writeln(stdout, a[0]) write(stdout, "a[1]") write(stdout, ": ") writeln(stdout, a[1]) write(stdout, "x") write(stdout, ": ") writeln(stdout, x) Statement Macros ~~~~~~~~~~~~~~~~ Statement macros are defined just as expression macros. However, they are invoked by an expression following a colon:: exprStmt ::= lowestExpr ['=' expr | [expr (comma expr)* [comma]] [macroStmt]] macroStmt ::= ':' [stmt] ('of' [sliceExprList] ':' stmt | 'elif' expr ':' stmt | 'except' exceptList ':' stmt )* ['else' ':' stmt] The following example outlines a macro that generates a lexical analyzer from regular expressions: .. code-block:: nimrod import macros macro case_token(n: stmt): stmt = # creates a lexical analyzer from regular expressions # ... (implementation is an exercise for the reader :-) nil 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 Modules ------- Nimrod supports splitting a program into pieces by a `module`:idx: concept. Each module needs to be in its own file and has its own `namespace`:idx:. Modules enable `information hiding`:idx: and `separate compilation`:idx:. A module may gain access to symbols of another module by the `import`:idx: statement. `Recursive module dependencies`:idx: are allowed, but slightly subtle. Only top-level symbols that are marked with an asterisk (``*``) are exported. The algorithm for compiling modules is: - compile the whole module as usual, following import statements recursively - if there is a cycle only import the already parsed symbols (that are exported); if an unknown identifier occurs then abort This is best illustrated by an example: .. code-block:: nimrod # 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:: nimrod # 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 return x + 1 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`:idx: of the identifier. The exact scope of an identifier depends on the way it was declared. Block scope ~~~~~~~~~~~ The *scope* of a variable declared in the declaration part of a block is valid from the point of declaration until the end of the block. If a block contains a second block, in which the identifier is redeclared, then inside this block, the second declaration will be valid. Upon leaving the inner block, the first declaration is valid again. An identifier cannot be redefined in the same block, except if valid for procedure or iterator overloading purposes. Tuple or object scope ~~~~~~~~~~~~~~~~~~~~~ The field identifiers inside a tuple or object definition are valid in the following places: * To the end of the tuple/object definition. * Field designators of a variable of the given tuple/object type. * In all descendant types of the object type. Module scope ~~~~~~~~~~~~ All identifiers of a module are valid from the point of declaration until the end of the module. Identifiers from indirectly dependent modules are *not* available. The `system`:idx: module is automatically imported in every other module. If a module imports an identifier by two different modules, each occurrence of the identifier has to be qualified, unless it is an overloaded procedure or iterator in which case the overloading resolution takes place: .. code-block:: nimrod # Module A var x*: string .. code-block:: nimrod # Module B var x*: int .. code-block:: nimrod # 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 Messages ======== The Nimrod 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 ======= Syntax:: colonExpr ::= expr [':' expr] colonExprList ::= [colonExpr (comma colonExpr)* [comma]] pragma ::= '{.' optInd (colonExpr [comma])* [SAD] ('.}' | '}') Pragmas are Nimrod's method to give the compiler additional information / commands without introducing a massive number of new keywords. Pragmas are processed on the fly during semantic checking. Pragmas are enclosed in the special ``{.`` and ``.}`` curly brackets. Pragmas are also often used as a first implementation to play with a language feature before a nicer syntax to access the feature becomes available. noSideEffect pragma ------------------- The `noSideEffect`:idx: 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. **Future directions**: ``func`` may become a keyword and syntactic sugar for a proc with no side effects: .. code-block:: nimrod func `+` (x, y: int): int procvar pragma -------------- The `procvar`:idx: pragma is used to mark a proc that it can be passed to a procedural variable. compileTime pragma ------------------ The `compileTime`:idx: pragma is used to mark a proc to be used at compile time only. No code will be generated for it. Compile time procs are useful as helpers for macros. noReturn pragma --------------- The `noreturn`:idx: pragma is used to mark a proc that it never returns. Acyclic pragma -------------- The `acyclic`:idx: 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:: nimrod type PNode = ref TNode TNode {.acyclic, final.} = object left, right: PNode data: string In the example a tree structure is declared with the ``TNode`` type. Note that the type definition is recursive and the GC has to assume that objects of this type may form a cyclic graph. The ``acyclic`` pragma passes the information that this cannot happen to the GC. If the programmer uses the ``acyclic`` pragma for data types that are in reality cyclic, the GC may leak memory, but nothing worse happens. **Future directions**: The ``acyclic`` pragma may become a property of a ``ref`` type: .. code-block:: nimrod type PNode = acyclic ref TNode TNode = object left, right: PNode data: string Final pragma ------------ The `final`:idx: pragma can be used for an object type to specify that it cannot be inherited from. shallow pragma -------------- The `shallow`:idx: 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 Nimrod require deep copying of sequences and strings. This can be expensive, especially if sequences are used to build a tree structure: .. code-block:: nimrod type TNodeKind = enum nkLeaf, nkInner TNode {.final, shallow.} = object case kind: TNodeKind of nkLeaf: strVal: string of nkInner: children: seq[TNode] Pure pragma ----------- The `pure`:idx: pragma serves two completely different purposes: 1. To mark a procedure that Nimrod should not generate any exit statements like ``return result;`` in the generated code. This is useful for procs that only consist of an assembler statement. 2. To mark an object type so that its type field should be omitted. This is necessary for binary compatibility with other compiled languages. error pragma ------------ The `error`:idx: pragma is used to make the compiler output an error message with the given content. Compilation currently aborts after an error, but this may be changed in later versions. fatal pragma ------------ The `fatal`:idx: 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. warning pragma -------------- The `warning`:idx: pragma is used to make the compiler output a warning message with the given content. Compilation continues after the warning. hint pragma ----------- The `hint`:idx: pragma is used to make the compiler output a hint message with the given content. Compilation continues after the hint. linearScanEnd pragma -------------------- The `linearScanEnd`:idx: pragma can be used to tell the compiler how to compile a Nimrod `case`:idx: statement. Syntactially it has to be used as a statement: .. code-block:: nimrod 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. unroll pragma ------------- The `unroll`:idx: pragma can be used to tell the compiler that it should unroll a `for`:idx: or `while`:idx: loop for runtime efficiency: .. code-block:: nimrod 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. compilation option pragmas -------------------------- The listed pragmas here can be used to override the code generation options for a section of code. 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. callconv cdecl|... Specifies the default calling convention for all procedures (and procedure types) that follow. =============== =============== ============================================ Example: .. code-block:: nimrod {.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:: nimrod {.push checks: off.} # compile this section without runtime checks as it is # speed critical # ... some code ... {.pop.} # restore old settings Register pragma --------------- The `register`:idx: pragma is for variables only. It declares the variable as ``register``, giving the compiler a hint that the variable should be placed in a hardware register for faster access. C compilers usually ignore this though and for good reasons: Often they do a better job without it anyway. In highly specific cases (a dispatch loop of an bytecode interpreter for example) it may provide benefits, though. DeadCodeElim pragma ------------------- The `deadCodeElim`:idx: pragma only applies to whole modules: It tells the compiler to activate (or deactivate) dead code elimination for the module the pragma appers in. The ``--deadCodeElim:on`` command line switch has the same effect as marking every module with ``{.deadCodeElim:on}``. However, for some modules such as the GTK wrapper it makes sense to *always* turn on dead code elimination - no matter if it is globally active or not. Example: .. code-block:: nimrod {.deadCodeElim: on.} Pragma pragma ------------- The `pragma`:idx: pragma can be used to declare user defined pragmas. This is useful because Nimrod'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:: nimrod when appType == "lib": {.pragma: rtl, exportc, dynlib, cdecl.} else: {.pragma: rtl, importc, dynlib: "client.dll", cdecl.} proc p*(a, b: int): int {.rtl.} = return 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 -------------------------- Nimrod 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:: Nimrod {.warning[LineTooLong]: off.} # turn off warning about too long lines This is often better than disabling all warnings at once. Foreign function interface ========================== Nimrod's `FFI`:idx: (foreign function interface) is extensive and only the parts that scale to other future backends (like the LLVM/EcmaScript backends) are documented here. Importc pragma -------------- The `importc`:idx: 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 Nimrod identifier *exactly as spelled*: .. code-block:: proc printf(formatstr: cstring) {.importc: "printf", varargs.} Note that this pragma is somewhat of a misnomer: Other backends will provide the same feature under the same name. Exportc pragma -------------- The `exportc`:idx: pragma provides a means to export a type, a variable, or a procedure to C. The optional argument is a string containing the C identifier. If the argument is missing, the C name is the Nimrod identifier *exactly as spelled*: .. code-block:: Nimrod proc callme(formatstr: cstring) {.exportc: "callMe", varargs.} Note that this pragma is somewhat of a misnomer: Other backends will provide the same feature under the same name. Varargs pragma -------------- The `varargs`:idx: pragma can be applied to procedures only (and procedure types). It tells Nimrod that the proc can take a variable number of parameters after the last specified parameter. Nimrod string values will be converted to C strings automatically: .. code-block:: Nimrod proc printf(formatstr: cstring) {.nodecl, varargs.} printf("hallo %s", "world") # "world" will be passed as C string Dynlib pragma for import ------------------------ With the `dynlib`:idx: pragma a procedure 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:: Nimrod 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:: nimrod 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:: nimrod 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. 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:: Nimrod 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.