# Nim Manual

Authors: Andreas Rumpf, Zahary Karadjov 0.19.9

"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

Note: This document is a draft! Several of Nim's features may need more precise wording. This manual is constantly evolving into a proper specification.

Note: The experimental features of Nim are covered here.

This document describes the lexis, the syntax, and the semantics of the Nim language.

To learn how to compile Nim programs and generate documentation see Compiler User Guide and DocGen Tools Guide.

The language constructs are explained using an extended BNF, in which (a)* means 0 or more a's, a+ means 1 or more a's, and (a)? means an optional a. Parentheses may be used to group elements.

& is the lookahead operator; &a means that an a is expected but not consumed. It will be consumed in the following rule.

The |, / symbols are used to mark alternatives and have the lowest precedence. / is the ordered choice that requires the parser to try the alternatives in the given order. / is often used to ensure the grammar is not ambiguous.

Non-terminals start with a lowercase letter, abstract terminal symbols are in UPPERCASE. Verbatim terminal symbols (including keywords) are quoted with '. An example:

ifStmt = 'if' expr ':' stmts ('elif' expr ':' stmts)* ('else' stmts)?

The binary ^* operator is used as a shorthand for 0 or more occurrences separated by its second argument; likewise ^+ means 1 or more occurrences: a ^+ b is short for a (b a)* and a ^* b is short for (a (b a)*)?. Example:

arrayConstructor = '[' expr ^* ',' ']'

Other parts of Nim, like scoping rules or runtime semantics, are described informally.

# Definitions

Nim code specifies a computation that acts on a memory consisting of components called locations. A variable is basically a name for a location. Each variable and location is of a certain type. The variable's type is called static type, the location's type is called dynamic type. 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 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 of the declaration. Scopes can be nested. The meaning of an identifier is determined by the smallest enclosing scope in which the identifier is declared unless overloading resolution rules suggest otherwise.

An expression specifies a computation that produces a value or location. Expressions that produce locations are called l-values. An l-value can denote either a location or the value the location contains, depending on the context.

A Nim program consists of one or more text source files containing Nim code. It is processed by a Nim compiler into an executable. The nature of this executable depends on the compiler implementation; it may, for example, be a native binary or JavaScript source code.

In a typical Nim program, most of the code is compiled into the executable. However, some of the code may be executed at compile time. This can include constant expressions, macro definitions, and Nim procedures used by macro definitions. Most of the Nim language is supported at compile time, but there are some restrictions -- see Restrictions on Compile-Time Execution for details. We use the term runtime to cover both compile-time execution and code execution in the executable.

The compiler parses Nim source code into an internal data structure called the abstract syntax tree (AST). Then, before executing the code or compiling it into the executable, it transforms the AST through semantic analysis. This adds semantic information such as expression types, identifier meanings, and in some cases expression values. An error detected during semantic analysis is called a static error. Errors described in this manual are static errors when not otherwise specified.

A checked runtime error is an error that the implementation detects and reports at runtime. The method for reporting such errors is via raising exceptions or dying with a fatal error. However, the implementation provides a means to disable these runtime checks. See the section pragmas for details.

Whether a checked runtime error results in an exception or in a fatal error is implementation specific. Thus the following program is invalid; even though the code purports to catch the IndexError from an out-of-bounds array access, the compiler may instead choose to allow the program to die with a fatal error.

var a: array[0..1, char]
let i = 5
try:
a[i] = 'N'
except IndexError:
echo "invalid index"

An unchecked runtime error 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 language features are used and if no runtime checks are disabled.

A constant expression is an expression whose value can be computed during semantic analysis of the code in which it appears. It is never an l-value and never has side effects. Constant expressions are not limited to the capabilities of semantic analysis, such as constant folding; they can use all Nim language features that are supported for compile-time execution. Since constant expressions can be used as an input to semantic analysis (such as for defining array bounds), this flexibility requires the compiler to interleave semantic analysis and compile-time code execution.

It is mostly accurate to picture semantic analysis proceeding top to bottom and left to right in the source code, with compile-time code execution interleaved when necessary to compute values that are required for subsequent semantic analysis. We will see much later in this document that macro invocation not only requires this interleaving, but also creates a situation where semantic analyis does not entirely proceed top to bottom and left to right.

# Lexical Analysis

## Encoding

All Nim source files are in the UTF-8 encoding (or its ASCII subset). Other encodings are not supported. Any of the standard platform line termination sequences can be used - the Unix form using ASCII LF (linefeed), the Windows form using the ASCII sequence CR LF (return followed by linefeed), or the old Macintosh form using the ASCII CR (return) character. All of these forms can be used equally, regardless of platform.

## Indentation

Nim's standard grammar describes an indentation sensitive language. This means that all the control structures are recognized by indentation. Indentation consists only of spaces; tabulators are not allowed.

The indentation handling is implemented as follows: The lexer annotates the following token with the preceding number of spaces; indentation is not a separate token. This trick allows parsing of Nim with only 1 token of lookahead.

The parser uses a stack of indentation levels: the stack consists of integers counting the spaces. The indentation information is queried at strategic places in the parser but ignored otherwise: The pseudo terminal IND{>} denotes an indentation that consists of more spaces than the entry at the top of the stack; IND{=} an indentation that has the same number of spaces. DED is another pseudo terminal that describes the action of popping a value from the stack, IND{>} then implies to push onto the stack.

With this notation we can now easily define the core of the grammar: A block of statements (simplified example):

ifStmt = 'if' expr ':' stmt
(IND{=} 'elif' expr ':' stmt)*
(IND{=} 'else' ':' stmt)?

simpleStmt = ifStmt / ...

stmt = IND{>} stmt ^+ IND{=} DED  # list of statements
/ simpleStmt                 # or a simple statement

Comments start anywhere outside a string or character literal with the hash character #. Comments consist of a concatenation of comment pieces. A comment piece starts with # and runs until the end of the line. The end of line characters belong to the piece. If the next line only consists of a comment piece with no other tokens between it and the preceding one, it does not start a new comment:

i = 0     # This is a single comment over multiple lines.
# The scanner merges these two pieces.
# The comment continues here.

Documentation comments are comments that start with two ##. Documentation comments are tokens; they are only allowed at certain places in the input file as they belong to the syntax tree!

Starting with version 0.13.0 of the language Nim supports multiline comments. They look like:

#[Comment here.
Multiple lines
are not a problem.]#

#[  #[ Multiline comment in already
commented out code. ]#
]#

Multiline documentation comments also exist and support nesting too:

proc foo =
##[Long documentation comment
here.
]##

## Identifiers & Keywords

Identifiers in Nim can be any string of letters, digits and underscores, beginning with a letter. Two immediate following underscores __ are not allowed:

letter ::= 'A'..'Z' | 'a'..'z' | '\x80'..'\xff'
digit ::= '0'..'9'
IDENTIFIER ::= letter ( ['_'] (letter | digit) )*

Currently any Unicode character with an ordinal value > 127 (non ASCII) is classified as a letter and may thus be part of an identifier but later versions of the language may assign some Unicode characters to belong to the operator characters instead.

The following keywords are reserved and cannot be used as identifiers:

addr and as asm
bind block break
case cast concept const continue converter
elif else end enum except export
finally for from func
if import in include interface is isnot iterator
let
macro method mixin mod
nil not notin
object of or out
proc ptr
raise ref return
shl shr static
template try tuple type
using
var
when while
xor
yield


Some keywords are unused; they are reserved for future developments of the language.

## Identifier equality

Two identifiers are considered equal if the following algorithm returns true:

proc sameIdentifier(a, b: string): bool =
a[0] == b[0] and
a.replace("_", "").toLowerAscii == b.replace("_", "").toLowerAscii

That means only the first letters are compared in a case sensitive manner. Other letters are compared case insensitively within the ASCII range and underscores are ignored.

This rather unorthodox way to do identifier comparisons is called partial case insensitivity and has some advantages over the conventional case sensitivity:

It allows programmers to mostly use their own preferred spelling style, be it humpStyle or snake_style, and libraries written by different programmers cannot use incompatible conventions. A Nim-aware editor or IDE can show the identifiers as preferred. Another advantage is that it frees the programmer from remembering the exact spelling of an identifier. The exception with respect to the first letter allows common code like var foo: Foo to be parsed unambiguously.

Note that this rule also applies to keywords, meaning that notin is the same as notIn and not_in (all-lowercase version (notin, isnot) is the preferred way of writing keywords).

Historically, Nim was a fully style-insensitive language. This meant that it was not case-sensitive and underscores were ignored and there was not even a distinction between foo and Foo.

## String literals

Terminal symbol in the grammar: STR_LIT.

String literals can be delimited by matching double quotes, and can contain the following escape sequences:

Escape sequenceMeaning
\pplatform specific newline: CRLF on Windows, LF on Unix
\r, \ccarriage return
\n, \lline feed (often called newline)
\fform feed
\ttabulator
\vvertical tabulator
\\backslash
\"quotation mark
\'apostrophe
\ '0'..'9'+character with decimal value d; all decimal digits directly following are used for the character
\bbackspace
\eescape [ESC]
\x HHcharacter with hex value HH; exactly two hex digits are allowed
\u HHHHunicode codepoint with hex value HHHH; exactly four hex digits are allowed
\u {H+}unicode codepoint; all hex digits enclosed in {} are used for the codepoint

Strings in Nim may contain any 8-bit value, even embedded zeros. However some operations may interpret the first binary zero as a terminator.

## Triple quoted string literals

Terminal symbol in the grammar: TRIPLESTR_LIT.

String literals can also be delimited by three double quotes """ ... """. Literals in this form may run for several lines, may contain " and do not interpret any escape sequences. For convenience, when the opening """ is followed by a newline (there may be whitespace between the opening """ and the newline), the newline (and the preceding whitespace) is not included in the string. The ending of the string literal is defined by the pattern """[^"], so this:

""""long string within quotes""""

Produces:

"long string within quotes"

## Raw string literals

Terminal symbol in the grammar: RSTR_LIT.

There are also raw string literals that are preceded with the letter r (or R) and are delimited by matching double quotes (just like ordinary string literals) and do not interpret the escape sequences. This is especially convenient for regular expressions or Windows paths:

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:

r"a""b"

Produces:

a"b

r"""" is not possible with this notation, because the three leading quotes introduce a triple quoted string literal. r""" is the same as """ since triple quoted string literals do not interpret escape sequences either.

## Generalized raw string literals

Terminal symbols in the grammar: GENERALIZED_STR_LIT, GENERALIZED_TRIPLESTR_LIT.

The construct identifier"string literal" (without whitespace between the identifier and the opening quotation mark) is a generalized raw string literal. It is a shortcut for the construct identifier(r"string literal"), so it denotes a procedure call with a raw string literal as its only argument. Generalized raw string literals are especially convenient for embedding mini languages directly into Nim (for example regular expressions).

The construct identifier"""string literal""" exists too. It is a shortcut for identifier("""string literal""").

## Character literals

Character literals are enclosed in single quotes '' and can contain the same escape sequences as strings - with one exception: the platform dependent newline (\p) is not allowed as it may be wider than one character (often it is the pair CR/LF for example). Here are the valid escape sequences for character literals:

Escape sequenceMeaning
\r, \ccarriage return
\n, \lline feed
\fform feed
\ttabulator
\vvertical tabulator
\\backslash
\"quotation mark
\'apostrophe
\ '0'..'9'+character with decimal value d; all decimal digits directly following are used for the character
\bbackspace
\eescape [ESC]
\x HHcharacter with hex value HH; exactly two hex digits are allowed

A character is not an Unicode character but a single byte. The reason for this is efficiency: for the overwhelming majority of use-cases, the resulting programs will still handle UTF-8 properly as UTF-8 was specially designed for this. Another reason is that Nim can thus support array[char, int] or set[char] efficiently as many algorithms rely on this feature. The Rune type is used for Unicode characters, it can represent any Unicode character. Rune is declared in the unicode module.

## Numerical constants

Numerical constants are of a single type and have the form:

hexdigit = digit | 'A'..'F' | 'a'..'f'
octdigit = '0'..'7'
bindigit = '0'..'1'
HEX_LIT = '0' ('x' | 'X' ) hexdigit ( ['_'] hexdigit )*
DEC_LIT = digit ( ['_'] digit )*
OCT_LIT = '0' 'o' octdigit ( ['_'] octdigit )*
BIN_LIT = '0' ('b' | 'B' ) bindigit ( ['_'] bindigit )*

INT_LIT = HEX_LIT
| DEC_LIT
| OCT_LIT
| BIN_LIT

INT8_LIT = INT_LIT ['\''] ('i' | 'I') '8'
INT16_LIT = INT_LIT ['\''] ('i' | 'I') '16'
INT32_LIT = INT_LIT ['\''] ('i' | 'I') '32'
INT64_LIT = INT_LIT ['\''] ('i' | 'I') '64'

UINT_LIT = INT_LIT ['\''] ('u' | 'U')
UINT8_LIT = INT_LIT ['\''] ('u' | 'U') '8'
UINT16_LIT = INT_LIT ['\''] ('u' | 'U') '16'
UINT32_LIT = INT_LIT ['\''] ('u' | 'U') '32'
UINT64_LIT = INT_LIT ['\''] ('u' | 'U') '64'

exponent = ('e' | 'E' ) ['+' | '-'] digit ( ['_'] digit )*
FLOAT_LIT = digit (['_'] digit)* (('.' digit (['_'] digit)* [exponent]) |exponent)
FLOAT32_SUFFIX = ('f' | 'F') ['32']
FLOAT32_LIT = HEX_LIT '\'' FLOAT32_SUFFIX
| (FLOAT_LIT | DEC_LIT | OCT_LIT | BIN_LIT) ['\''] FLOAT32_SUFFIX
FLOAT64_SUFFIX = ( ('f' | 'F') '64' ) | 'd' | 'D'
FLOAT64_LIT = HEX_LIT '\'' FLOAT64_SUFFIX
| (FLOAT_LIT | DEC_LIT | OCT_LIT | BIN_LIT) ['\''] FLOAT64_SUFFIX

As can be seen in the productions, numerical constants can contain underscores for readability. Integer and floating point literals may be given in decimal (no prefix), binary (prefix 0b), octal (prefix 0o) and hexadecimal (prefix 0x) notation.

There exists a literal for each numerical type that is defined. The suffix starting with an apostrophe (''') is called a type suffix. Literals without a type suffix are of an integer type, unless the literal contains a dot or E|e in which case it is of type float. This integer type is int if the literal is in the range low(i32)..high(i32), otherwise it is int64. For notational convenience the apostrophe of a type suffix is optional if it is not ambiguous (only hexadecimal floating point literals with a type suffix can be ambiguous).

The type suffixes are:

Type SuffixResulting type of literal
'i8int8
'i16int16
'i32int32
'i64int64
'uuint
'u8uint8
'u16uint16
'u32uint32
'u64uint64
'ffloat32
'dfloat64
'f32float32
'f64float64

Floating point literals may also be in binary, octal or hexadecimal notation: 0B0_10001110100_0000101001000111101011101111111011000101001101001001'f64 is approximately 1.72826e35 according to the IEEE floating point standard.

Literals are bounds checked so that they fit the datatype. Non base-10 literals are used mainly for flags and bit pattern representations, therefore bounds checking is done on bit width, not value range. If the literal fits in the bit width of the datatype, it is accepted. Hence: 0b10000000'u8 == 0x80'u8 == 128, but, 0b10000000'i8 == 0x80'i8 == -1 instead of causing an overflow error.

## Operators

Nim allows user defined operators. An operator is any combination of the following characters:

=     +     -     *     /     <     >
@     $~ & % | ! ? ^ . : \ These keywords are also operators: and or not xor shl shr div mod in notin is isnot of. . =, :, :: are not available as general operators; they are used for other notational purposes. *: is as a special case treated as the two tokens * and : (to support var v*: T). The not keyword is always a unary operator, a not b is parsed as a(not b), not as (a) not (b). ## Other tokens The following strings denote other tokens:  ( ) { } [ ] , ; [. .] {. .} (. .) [: The slice operator .. takes precedence over other tokens that contain a dot: {..} are the three tokens {, .., } and not the two tokens {., .}. # Syntax This section lists Nim's standard syntax. How the parser handles the indentation is already described in the Lexical Analysis section. Nim allows user-definable operators. Binary operators have 11 different levels of precedence. ## Associativity Binary operators whose first character is ^ are right-associative, all other binary operators are left-associative. proc ^/(x, y: float): float = # a right-associative division operator result = x / y echo 12 ^/ 4 ^/ 8 # 24.0 (4 / 8 = 0.5, then 12 / 0.5 = 24.0) echo 12 / 4 / 8 # 0.375 (12 / 4 = 3.0, then 3 / 8 = 0.375) ## Precedence Unary operators always bind stronger than any binary operator:$a + b is ($a) + b and not$(a + b).

If an unary operator's first character is @ it is a sigil-like operator which binds stronger than a primarySuffix: @x.abc is parsed as (@x).abc whereas $x.abc is parsed as$(x.abc).

For binary operators that are not keywords the precedence is determined by the following rules:

Operators ending in either ->, ~> or => are called arrow like, and have the lowest precedence of all operators.

If the operator ends with = and its first character is none of <, >, !, =, ~, ?, it is an assignment operator which has the second lowest precedence.

Otherwise precedence is determined by the first character.

Precedence levelOperatorsFirst characterTerminal symbol
10 (highest)$^OP10 9* / div mod shl shr %* % \ /OP9 8+ -+ - ~ |OP8 7&&OP7 6...OP6 5== <= < >= > != in notin is isnot not of= < > !OP5 4andOP4 3or xorOP3 2@ : ?OP2 1assignment operator (like +=, *=)OP1 0 (lowest)arrow like operator (like ->, =>)OP0 Whether an operator is used a prefix operator is also affected by preceding whitespace (this parsing change was introduced with version 0.13.0): echo$foo
# is parsed as
echo($foo) Spacing also determines whether (a, b) is parsed as an the argument list of a call or whether it is parsed as a tuple constructor: echo(1, 2) # pass 1 and 2 to echo echo (1, 2) # pass the tuple (1, 2) to echo ## Grammar The grammar's start symbol is module. # This file is generated by compiler/parser.nim. module = stmt ^* (';' / IND{=}) comma = ',' COMMENT? semicolon = ';' COMMENT? colon = ':' COMMENT? colcom = ':' COMMENT? operator = OP0 | OP1 | OP2 | OP3 | OP4 | OP5 | OP6 | OP7 | OP8 | OP9 | 'or' | 'xor' | 'and' | 'is' | 'isnot' | 'in' | 'notin' | 'of' | 'div' | 'mod' | 'shl' | 'shr' | 'not' | 'static' | '..' prefixOperator = operator optInd = COMMENT? IND? optPar = (IND{>} | IND{=})? simpleExpr = arrowExpr (OP0 optInd arrowExpr)* pragma? arrowExpr = assignExpr (OP1 optInd assignExpr)* assignExpr = orExpr (OP2 optInd orExpr)* orExpr = andExpr (OP3 optInd andExpr)* andExpr = cmpExpr (OP4 optInd cmpExpr)* cmpExpr = sliceExpr (OP5 optInd sliceExpr)* sliceExpr = ampExpr (OP6 optInd ampExpr)* ampExpr = plusExpr (OP7 optInd plusExpr)* plusExpr = mulExpr (OP8 optInd mulExpr)* mulExpr = dollarExpr (OP9 optInd dollarExpr)* dollarExpr = primary (OP10 optInd primary)* symbol = '' (KEYW|IDENT|literal|(operator|'('|')'|'['|']'|'{'|'}'|'=')+)+ '' | IDENT | KEYW exprColonEqExpr = expr (':'|'=' expr)? exprList = expr ^+ comma exprColonEqExprList = exprColonEqExpr (comma exprColonEqExpr)* (comma)? dotExpr = expr '.' optInd (symbol | '[:' exprList ']') explicitGenericInstantiation = '[:' exprList ']' ( '(' exprColonEqExpr ')' )? qualifiedIdent = symbol ('.' optInd symbol)? setOrTableConstr = '{' ((exprColonEqExpr comma)* | ':' ) '}' castExpr = 'cast' '[' optInd typeDesc optPar ']' '(' optInd expr optPar ')' parKeyw = 'discard' | 'include' | 'if' | 'while' | 'case' | 'try' | 'finally' | 'except' | 'for' | 'block' | 'const' | 'let' | 'when' | 'var' | 'mixin' par = '(' optInd ( &parKeyw complexOrSimpleStmt ^+ ';' | ';' complexOrSimpleStmt ^+ ';' | pragmaStmt | simpleExpr ( ('=' expr (';' complexOrSimpleStmt ^+ ';' )? ) | (':' expr (',' exprColonEqExpr ^+ ',' )? ) ) ) optPar ')' literal = | INT_LIT | INT8_LIT | INT16_LIT | INT32_LIT | INT64_LIT | UINT_LIT | UINT8_LIT | UINT16_LIT | UINT32_LIT | UINT64_LIT | FLOAT_LIT | FLOAT32_LIT | FLOAT64_LIT | STR_LIT | RSTR_LIT | TRIPLESTR_LIT | CHAR_LIT | NIL generalizedLit = GENERALIZED_STR_LIT | GENERALIZED_TRIPLESTR_LIT identOrLiteral = generalizedLit | symbol | literal | par | arrayConstr | setOrTableConstr | castExpr tupleConstr = '(' optInd (exprColonEqExpr comma?)* optPar ')' arrayConstr = '[' optInd (exprColonEqExpr comma?)* optPar ']' primarySuffix = '(' (exprColonEqExpr comma?)* ')' doBlocks? | doBlocks | '.' optInd symbol generalizedLit? | '[' optInd indexExprList optPar ']' | '{' optInd indexExprList optPar '}' | &( ''|IDENT|literal|'cast'|'addr'|'type') expr # command syntax condExpr = expr colcom expr optInd ('elif' expr colcom expr optInd)* 'else' colcom expr ifExpr = 'if' condExpr whenExpr = 'when' condExpr pragma = '{.' optInd (exprColonExpr comma?)* optPar ('.}' | '}') identVis = symbol opr? # postfix position identVisDot = symbol '.' optInd symbol opr? identWithPragma = identVis pragma? identWithPragmaDot = identVisDot pragma? declColonEquals = identWithPragma (comma identWithPragma)* comma? (':' optInd typeDesc)? ('=' optInd expr)? identColonEquals = ident (comma ident)* comma? (':' optInd typeDesc)? ('=' optInd expr)?) inlTupleDecl = 'tuple' [' optInd (identColonEquals (comma/semicolon)?)* optPar ']' extTupleDecl = 'tuple' COMMENT? (IND{>} identColonEquals (IND{=} identColonEquals)*)? tupleClass = 'tuple' paramList = '(' declColonEquals ^* (comma/semicolon) ')' paramListArrow = paramList? ('->' optInd typeDesc)? paramListColon = paramList? (':' optInd typeDesc)? doBlock = 'do' paramListArrow pragmas? colcom stmt procExpr = 'proc' paramListColon pragmas? ('=' COMMENT? stmt)? distinct = 'distinct' optInd typeDesc forStmt = 'for' (identWithPragma ^+ comma) 'in' expr colcom stmt forExpr = forStmt expr = (blockExpr | ifExpr | whenExpr | caseExpr | forExpr | tryExpr) / simpleExpr typeKeyw = 'var' | 'out' | 'ref' | 'ptr' | 'shared' | 'tuple' | 'proc' | 'iterator' | 'distinct' | 'object' | 'enum' primary = typeKeyw typeDescK / prefixOperator* identOrLiteral primarySuffix* / 'bind' primary typeDesc = simpleExpr typeDefAux = simpleExpr | 'concept' typeClass postExprBlocks = ':' stmt? ( IND{=} doBlock | IND{=} 'of' exprList ':' stmt | IND{=} 'elif' expr ':' stmt | IND{=} 'except' exprList ':' stmt | IND{=} 'else' ':' stmt )* exprStmt = simpleExpr (( '=' optInd expr colonBody? ) / ( expr ^+ comma doBlocks / macroColon ))? importStmt = 'import' optInd expr ((comma expr)* / 'except' optInd (expr ^+ comma)) includeStmt = 'include' optInd expr ^+ comma fromStmt = 'from' moduleName 'import' optInd expr (comma expr)* returnStmt = 'return' optInd expr? raiseStmt = 'raise' optInd expr? yieldStmt = 'yield' optInd expr? discardStmt = 'discard' optInd expr? breakStmt = 'break' optInd expr? continueStmt = 'break' optInd expr? condStmt = expr colcom stmt COMMENT? (IND{=} 'elif' expr colcom stmt)* (IND{=} 'else' colcom stmt)? ifStmt = 'if' condStmt whenStmt = 'when' condStmt whileStmt = 'while' expr colcom stmt ofBranch = 'of' exprList colcom stmt ofBranches = ofBranch (IND{=} ofBranch)* (IND{=} 'elif' expr colcom stmt)* (IND{=} 'else' colcom stmt)? caseStmt = 'case' expr ':'? COMMENT? (IND{>} ofBranches DED | IND{=} ofBranches) tryStmt = 'try' colcom stmt &(IND{=}? 'except'|'finally') (IND{=}? 'except' exprList colcom stmt)* (IND{=}? 'finally' colcom stmt)? tryExpr = 'try' colcom stmt &(optInd 'except'|'finally') (optInd 'except' exprList colcom stmt)* (optInd 'finally' colcom stmt)? exceptBlock = 'except' colcom stmt blockStmt = 'block' symbol? colcom stmt blockExpr = 'block' symbol? colcom stmt staticStmt = 'static' colcom stmt deferStmt = 'defer' colcom stmt asmStmt = 'asm' pragma? (STR_LIT | RSTR_LIT | TRIPLESTR_LIT) genericParam = symbol (comma symbol)* (colon expr)? ('=' optInd expr)? genericParamList = '[' optInd genericParam ^* (comma/semicolon) optPar ']' pattern = '{' stmt '}' indAndComment = (IND{>} COMMENT)? | COMMENT? routine = optInd identVis pattern? genericParamList? paramListColon pragma? ('=' COMMENT? stmt)? indAndComment commentStmt = COMMENT section(p) = COMMENT? p / (IND{>} (p / COMMENT)^+IND{=} DED) constant = identWithPragma (colon typeDesc)? '=' optInd expr indAndComment enum = 'enum' optInd (symbol optInd ('=' optInd expr COMMENT?)? comma?)+ objectWhen = 'when' expr colcom objectPart COMMENT? ('elif' expr colcom objectPart COMMENT?)* ('else' colcom objectPart COMMENT?)? objectBranch = 'of' exprList colcom objectPart objectBranches = objectBranch (IND{=} objectBranch)* (IND{=} 'elif' expr colcom objectPart)* (IND{=} 'else' colcom objectPart)? objectCase = 'case' identWithPragma ':' typeDesc ':'? COMMENT? (IND{>} objectBranches DED | IND{=} objectBranches) objectPart = IND{>} objectPart^+IND{=} DED / objectWhen / objectCase / 'nil' / 'discard' / declColonEquals object = 'object' pragma? ('of' typeDesc)? COMMENT? objectPart typeClassParam = ('var' | 'out')? symbol typeClass = typeClassParam ^* ',' (pragma)? ('of' typeDesc ^* ',')? &IND{>} stmt typeDef = identWithPragmaDot genericParamList? '=' optInd typeDefAux indAndComment? varTuple = '(' optInd identWithPragma ^+ comma optPar ')' '=' optInd expr colonBody = colcom stmt doBlocks? variable = (varTuple / identColonEquals) colonBody? indAndComment bindStmt = 'bind' optInd qualifiedIdent ^+ comma mixinStmt = 'mixin' optInd qualifiedIdent ^+ comma pragmaStmt = pragma (':' COMMENT? stmt)? simpleStmt = ((returnStmt | raiseStmt | yieldStmt | discardStmt | breakStmt | continueStmt | pragmaStmt | importStmt | exportStmt | fromStmt | includeStmt | commentStmt) / exprStmt) COMMENT? complexOrSimpleStmt = (ifStmt | whenStmt | whileStmt | tryStmt | forStmt | blockStmt | staticStmt | deferStmt | asmStmt | 'proc' routine | 'method' routine | 'iterator' routine | 'macro' routine | 'template' routine | 'converter' routine | 'type' section(typeDef) | 'const' section(constant) | ('let' | 'var' | 'using') section(variable) | bindStmt | mixinStmt) / simpleStmt stmt = (IND{>} complexOrSimpleStmt^+(IND{=} / ';') DED) / simpleStmt ^+ ';'  # Order of evaluation Order of evaluation is strictly left-to-right, inside-out as it is typical for most others imperative programming languages: var s = "" proc p(arg: int): int = s.add$arg
result = arg

doAssert s == "123"

Assignments are not special, the left-hand-side expression is evaluated before the right-hand side:

var v = 0
proc getI(): int =
result = v
inc v

var a, b: array[0..2, int]

proc someCopy(a: var int; b: int) = a = b

a[getI()] = getI()

doAssert a == [1, 0, 0]

v = 0
someCopy(b[getI()], getI())

doAssert b == [1, 0, 0]

Rationale: Consistency with overloaded assignment or assignment-like operations, a = b can be read as performSomeCopy(a, b).

# Constants and Constant Expressions

A constant is a symbol that is bound to the value of a constant expression. Constant expressions are restricted to depend only on the following categories of values and operations, because these are either built into the language or declared and evaluated before semantic analysis of the constant expression:

• literals
• built-in operators
• previously declared constants and compile-time variables
• previously declared macros and templates
• previously declared procedures that have no side effects beyond possibly modifying compile-time variables

A constant expression can contain code blocks that may internally use all Nim features supported at compile time (as detailed in the next section below). Within such a code block, it is possible to declare variables and then later read and update them, or declare variables and pass them to procedures that modify them. However, the code in such a block must still adhere to the retrictions listed above for referencing values and operations outside the block.

The ability to access and modify compile-time variables adds flexibility to constant expressions that may be surprising to those coming from other statically typed languages. For example, the following code echoes the beginning of the Fibonacci series at compile time. (This is a demonstration of flexibility in defining constants, not a recommended style for solving this problem!)

import strformat

var fib_n {.compileTime.}: int
var fib_prev {.compileTime.}: int
var fib_prev_prev {.compileTime.}: int

proc next_fib(): int =
result = if fib_n < 2:
fib_n
else:
fib_prev_prev + fib_prev
inc(fib_n)
fib_prev_prev = fib_prev
fib_prev = result

const f0 = next_fib()
const f1 = next_fib()

const display_fib = block:
const f2 = next_fib()
var result = fmt"Fibonacci sequence: {f0}, {f1}, {f2}"
for i in 3..12:
result

static:
echo display_fib

# Restrictions on Compile-Time Execution

Nim code that will be executed at compile time cannot use the following language features:

• methods
• closure iterators
• the cast operator
• reference (pointer) types
• the FFI

Some or all of these restrictions are likely to be lifted over time.

# Types

All expressions have a type which is known during semantic analysis. Nim is statically typed. One can declare new types, which is in essence defining an identifier that can be used to denote this custom type.

These are the major type classes:

• ordinal types (consist of integer, bool, character, enumeration (and subranges thereof) types)
• floating point types
• string type
• structured types
• reference (pointer) type
• procedural type
• generic type

## Ordinal types

Ordinal types have the following characteristics:

• Ordinal types are countable and ordered. This property allows the operation of functions as inc, ord, dec on ordinal types to be defined.
• Ordinal values have a smallest possible value. Trying to count further down than the smallest value gives a checked runtime or static error.
• Ordinal values have a largest possible value. Trying to count further than the largest value gives a checked runtime or static error.

Integers, bool, characters and enumeration types (and subranges of these types) belong to ordinal types. For reasons of simplicity of implementation the types uint and uint64 are not ordinal types. (This will be changed in later versions of the language.)

A distinct type is an ordinal type if its base type is an ordinal type.

## Pre-defined integer types

These integer types are pre-defined:

int
the generic signed integer type; its size is platform dependent and has the same size as a pointer. This type should be used in general. An integer literal that has no type suffix is of this type if it is in the range low(int32)..high(int32) otherwise the literal's type is int64.
intXX
additional signed integer types of XX bits use this naming scheme (example: int16 is a 16 bit wide integer). The current implementation supports int8, int16, int32, int64. Literals of these types have the suffix 'iXX.
uint
the generic unsigned integer type; its size is platform dependent and has the same size as a pointer. An integer literal with the type suffix 'u is of this type.
uintXX
additional unsigned integer types of XX bits use this naming scheme (example: uint16 is a 16 bit wide unsigned integer). The current implementation supports uint8, uint16, uint32, uint64. Literals of these types have the suffix 'uXX. Unsigned operations all wrap around; they cannot lead to over- or underflow errors.

In addition to the usual arithmetic operators for signed and unsigned integers (+ - * etc.) there are also operators that formally work on signed integers but treat their arguments as unsigned: They are mostly provided for backwards compatibility with older versions of the language that lacked unsigned integer types. These unsigned operations for signed integers use the % suffix as convention:

operationmeaning
a -% bunsigned integer subtraction
a *% bunsigned integer multiplication
a /% bunsigned integer division
a %% bunsigned integer modulo operation
a <% btreat a and b as unsigned and compare
a <=% btreat 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 is performed in expressions where different kinds of integer types are used: the smaller type is converted to the larger.

A narrowing type conversion converts a larger to a smaller type (for example int32 -> int16. A widening type conversion converts a smaller type to a larger type (for example int16 -> int32). In Nim only widening type conversions are implicit:

var myInt16 = 5i16
var myInt: int
myInt16 + 34     # of type int16
myInt16 + myInt  # of type int
myInt16 + 2i32   # of type int32

However, int literals are implicitly convertible to a smaller integer type if the literal's value fits this smaller type and such a conversion is less expensive than other implicit conversions, so myInt16 + 34 produces an int16 result.

For further details, see Convertible relation.

## Subrange types

A subrange type is a range of values from an ordinal or floating point type (the base type). To define a subrange type, one must specify its limiting values -- the lowest and highest value of the type. For example:

type
Subrange = range[0..5]
PositiveFloat = range[0.0..Inf]

Subrange is a subrange of an integer which can only hold the values 0 to 5. PositiveFloat defines a subrange of all positive floating point values. NaN does not belong to any subrange of floating point types. Assigning any other value to a variable of type Subrange is a checked runtime error (or static error if it can be determined during semantic analysis). Assignments from the base type to one of its subrange types (and vice versa) are allowed.

A subrange type has the same size as its base type (int in the Subrange example).

## Pre-defined floating point types

The following floating point types are pre-defined:

float
the generic floating point type; its size used to be platform dependent, but now it is always mapped to float64. This type should be used in general.
floatXX
an implementation may define additional floating point types of XX bits using this naming scheme (example: float64 is a 64 bit wide float). The current implementation supports float32 and float64. Literals of these types have the suffix 'fXX.

Automatic type conversion in expressions with different kinds of floating point types is performed: See Convertible relation for further details. Arithmetic performed on floating point types follows the IEEE standard. Integer types are not converted to floating point types automatically and vice versa.

The IEEE standard defines five types of floating-point exceptions:

• Invalid: operations with mathematically invalid operands, for example 0.0/0.0, sqrt(-1.0), and log(-37.8).
• Division by zero: divisor is zero and dividend is a finite nonzero number, for example 1.0/0.0.
• Overflow: operation produces a result that exceeds the range of the exponent, for example MAXDOUBLE+0.0000000000001e308.
• Underflow: operation produces a result that is too small to be represented as a normal number, for example, MINDOUBLE * MINDOUBLE.
• Inexact: operation produces a result that cannot be represented with infinite precision, for example, 2.0 / 3.0, log(1.1) and 0.1 in input.

The IEEE exceptions are either ignored during execution or mapped to the Nim exceptions: FloatInvalidOpError, FloatDivByZeroError, FloatOverflowError, FloatUnderflowError, and FloatInexactError. These exceptions inherit from the FloatingPointError base class.

Nim provides the pragmas nanChecks and infChecks to control whether the IEEE exceptions are ignored or trap a Nim exception:

{.nanChecks: on, infChecks: on.}
var a = 1.0
var b = 0.0
echo b / b # raises FloatInvalidOpError
echo a / b # raises FloatOverflowError

In the current implementation FloatDivByZeroError and FloatInexactError are never raised. FloatOverflowError is raised instead of FloatDivByZeroError. There is also a floatChecks pragma that is a short-cut for the combination of nanChecks and infChecks pragmas. floatChecks are turned off as default.

The only operations that are affected by the floatChecks pragma are the +, -, *, / operators for floating point types.

An implementation should always use the maximum precision available to evaluate floating pointer values during semantic analysis; this means expressions like 0.09'f32 + 0.01'f32 == 0.09'f64 + 0.01'f64 that are evaluating during constant folding are true.

## Boolean type

The boolean type is named bool in Nim and can be one of the two pre-defined values true and false. Conditions in while, if, elif, when-statements need to be of type bool.

This condition holds:

ord(false) == 0 and ord(true) == 1

The operators not, and, or, xor, <, <=, >, >=, !=, == are defined for the bool type. The and and or operators perform short-cut evaluation. Example:

while p != nil and p.name != "xyz":
# p.name is not evaluated if p == nil
p = p.next

The size of the bool type is one byte.

## Character type

The character type is named char in Nim. Its size is one byte. Thus it cannot represent an UTF-8 character, but a part of it. The reason for this is efficiency: for the overwhelming majority of use-cases, the resulting programs will still handle UTF-8 properly as UTF-8 was specially designed for this. Another reason is that Nim can support array[char, int] or set[char] efficiently as many algorithms rely on this feature. The Rune type is used for Unicode characters, it can represent any Unicode character. Rune is declared in the unicode module.

## Enumeration types

Enumeration types define a new type whose values consist of the ones specified. The values are ordered. Example:

type
Direction = enum
north, east, south, west

Now the following holds:

ord(north) == 0
ord(east) == 1
ord(south) == 2
ord(west) == 3

# Also allowed:
ord(Direction.west) == 3

Thus, north < east < south < west. The comparison operators can be used with enumeration types. Instead of north etc, the enum value can also be qualified with the enum type that it resides in, Direction.north.

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:

type
TokenType = enum
a = 2, b = 4, c = 89 # holes are valid

However, it is then not an ordinal anymore, so it is not possible to use these enums as an index type for arrays. The procedures inc, dec, succ and pred are not available for them either.

echo 3 # calls $ for int Whenever a user creates a specialized object, implementation of this procedure provides for string representation. type Person = object name: string age: int proc $(p: Person): string = # $ always returns a string result = p.name & " is " &$p.age & # we *need* the $ in front of p.age which # is natively an integer to convert it to # a string " years old." While$p.name can also be used, the $operation on a string does nothing. Note that we cannot rely on automatic conversion from an int to a string like we can for the echo proc. Strings are compared by their lexicographical order. All comparison operators are available. Strings can be indexed like arrays (lower bound is 0). Unlike arrays, they can be used in case statements: case paramStr(i) of "-v": incl(options, optVerbose) of "-h", "-?": incl(options, optHelp) else: write(stdout, "invalid command line option!\n") Per convention, all strings are UTF-8 strings, but this is not enforced. For example, when reading strings from binary files, they are merely a sequence of bytes. The index operation s[i] means the i-th char of s, not the i-th unichar. The iterator runes from the unicode module can be used for iteration over all Unicode characters. ## cstring type The cstring type meaning compatible string is the native representation of a string for the compilation backend. For the C backend the cstring type represents a pointer to a zero-terminated char array compatible to the type char* in Ansi C. Its primary purpose lies in easy interfacing with C. The index operation s[i] means the i-th char of s; however no bounds checking for cstring is performed making the index operation unsafe. A Nim string is implicitly convertible to cstring for convenience. If a Nim string is passed to a C-style variadic proc, it is implicitly converted to cstring too: proc printf(formatstr: cstring) {.importc: "printf", varargs, header: "<stdio.h>".} printf("This works %s", "as expected") Even though the conversion is implicit, it is not safe: The garbage collector does not consider a cstring to be a root and may collect the underlying memory. However in practice this almost never happens as the GC considers stack roots conservatively. One can use the builtin procs GC_ref and GC_unref to keep the string data alive for the rare cases where it does not work. A$ proc is defined for cstrings that returns a string. Thus to get a nim string from a cstring:

var str: string = "Hello!"
var cstr: cstring = str
var newstr: string = $cstr ## Structured types A variable of a structured type can hold multiple values at the same time. Structured types can be nested to unlimited levels. Arrays, sequences, tuples, objects and sets belong to the structured types. ## Array and sequence types Arrays are a homogeneous type, meaning that each element in the array has the same type. Arrays always have a fixed length specified as a constant expression (except for open arrays). They can be indexed by any ordinal type. A parameter A may be an open array, in which case it is indexed by integers from 0 to len(A)-1. An array expression may be constructed by the array constructor []. The element type of this array expression is inferred from the type of the first element. All other elements need to be implicitly convertable to this type. Sequences are similar to arrays but of dynamic length which may change during runtime (like strings). Sequences are implemented as growable arrays, allocating pieces of memory as items are added. A sequence S is always indexed by integers from 0 to len(S)-1 and its bounds are checked. Sequences can be constructed by the array constructor [] in conjunction with the array to sequence operator @. Another way to allocate space for a sequence is to call the built-in newSeq procedure. A sequence may be passed to a parameter that is of type open array. Example: type IntArray = array[0..5, int] # an array that is indexed with 0..5 IntSeq = seq[int] # a sequence of integers var x: IntArray y: IntSeq x = [1, 2, 3, 4, 5, 6] # [] is the array constructor y = @[1, 2, 3, 4, 5, 6] # the @ turns the array into a sequence let z = [1.0, 2, 3, 4] # the type of z is array[0..3, float] The lower bound of an array or sequence may be received by the built-in proc low(), the higher bound by high(). The length may be received by len(). low() for a sequence or an open array always returns 0, as this is the first valid index. One can append elements to a sequence with the add() proc or the & operator, and remove (and get) the last element of a sequence with the pop() proc. The notation x[i] can be used to access the i-th element of x. Arrays are always bounds checked (statically or at runtime). These checks can be disabled via pragmas or invoking the compiler with the --boundChecks:off command line switch. ## Open arrays Often fixed size arrays turn out to be too inflexible; procedures should be able to deal with arrays of different sizes. The openarray type allows this; it can only be used for parameters. Openarrays are always indexed with an int starting at position 0. The len, low and high operations are available for open arrays too. Any array with a compatible base type can be passed to an openarray parameter, the index type does not matter. In addition to arrays sequences can also be passed to an open array parameter. The openarray type cannot be nested: multidimensional openarrays are not supported because this is seldom needed and cannot be done efficiently. proc testOpenArray(x: openArray[int]) = echo repr(x) testOpenArray([1,2,3]) # array[] testOpenArray(@[1,2,3]) # seq[] ## Varargs A varargs parameter is an openarray parameter that additionally allows to pass a variable number of arguments to a procedure. The compiler converts the list of arguments to an array implicitly: proc myWriteln(f: File, a: varargs[string]) = for s in items(a): write(f, s) write(f, "\n") myWriteln(stdout, "abc", "def", "xyz") # is transformed to: myWriteln(stdout, ["abc", "def", "xyz"]) This transformation is only done if the varargs parameter is the last parameter in the procedure header. It is also possible to perform type conversions in this context: proc myWriteln(f: File, a: varargs[string, $]) =
for s in items(a):
write(f, s)
write(f, "\n")

myWriteln(stdout, 123, "abc", 4.0)
# is transformed to:
myWriteln(stdout, [$123,$"def", $4.0]) In this example$ is applied to any argument that is passed to the parameter a. (Note that $applied to strings is a nop.) Note that an explicit array constructor passed to a varargs parameter is not wrapped in another implicit array construction: proc takeV[T](a: varargs[T]) = discard takeV([123, 2, 1]) # takeV's T is "int", not "array of int" varargs[typed] is treated specially: It matches a variable list of arguments of arbitrary type but always constructs an implicit array. This is required so that the builtin echo proc does what is expected: proc echo*(x: varargs[typed, $]) {...}

echo @[1, 2, 3]
# prints "@[1, 2, 3]" and not "123"

## Unchecked arrays

The UncheckedArray[T] type is a special kind of array where its bounds are not checked. This is often useful to implement customized flexibly sized arrays. Additionally an unchecked array is translated into a C array of undetermined size:

type
MySeq = object
len, cap: int
data: UncheckedArray[int]

Produces roughly this C code:

typedef struct {
NI len;
NI cap;
NI data[];
} MySeq;

The base type of the unchecked array may not contain any GC'ed memory but this is currently not checked.

Future directions: GC'ed memory should be allowed in unchecked arrays and there should be an explicit annotation of how the GC is to determine the runtime size of the array.

## Tuples and object types

A variable of a tuple or object type is a heterogeneous storage container. A tuple or object defines various named fields of a type. A tuple also defines an order of the fields. Tuples are meant for heterogeneous storage types with no overhead and few abstraction possibilities. The constructor () can be used to construct tuples. The order of the fields in the constructor must match the order of the tuple's definition. Different tuple-types are equivalent if they specify the same fields of the same type in the same order. The names of the fields also have to be identical.

The assignment operator for tuples copies each component. The default assignment operator for objects copies each component. Overloading of the assignment operator is described in type-bound-operations-operator.

type
Person = tuple[name: string, age: int] # type representing a person:
# a person consists of a name
# and an age
var
person: Person
person = (name: "Peter", age: 30)
# the same, but less readable:
person = ("Peter", 30)

A tuple with one unnamed field can be constructed with the parentheses and a trailing comma:

proc echoUnaryTuple(a: (int,)) =
echo a[0]

echoUnaryTuple (1,)

In fact, a trailing comma is allowed for every tuple construction.

The implementation aligns the fields for best access performance. The alignment is compatible with the way the C compiler does it.

For consistency with object declarations, tuples in a type section can also be defined with indentation instead of []:

type
Person = tuple   # type representing a person
name: string   # a person consists of a name
age: natural   # and an age

Objects provide many features that tuples do not. Object provide inheritance and information hiding. Objects have access to their type during at runtime, so that the of operator can be used to determine the object's type. The of operator is similar to the instanceof operator in Java.

type
Person = object of RootObj
name*: string   # the * means that name is accessible from other modules
age: int        # no * means that the field is hidden

Student = ref object of Person # a student is a person
id: int                      # with an id field

var
student: Student
person: Person
assert(student of Student) # is true
assert(student of Person) # also true

Object fields that should be visible from outside the defining module, have to be marked by *. In contrast to tuples, different object types are never equivalent. Objects that have no ancestor are implicitly final and thus have no hidden type field. One can use the inheritable pragma to introduce new object roots apart from system.RootObj.

## Object construction

Objects can also be created with an object construction expression that has the syntax T(fieldA: valueA, fieldB: valueB, ...) where T is an object type or a ref object type:

var student = Student(name: "Anton", age: 5, id: 3)

Note that, unlike tuples, objects require the field names along with their values. For a ref object type system.new is invoked implicitly.

## Object variants

Often an object hierarchy is overkill in certain situations where simple variant types are needed.

An example:

# This is an example how an abstract syntax tree could be modelled in Nim
type
NodeKind = enum  # the different node types
nkInt,          # a leaf with an integer value
nkFloat,        # a leaf with a float value
nkString,       # a leaf with a string value
nkSub,          # a subtraction
nkIf            # an if statement
Node = ref NodeObj
NodeObj = object
case kind: NodeKind  # the kind field is the discriminator
of nkInt: intVal: int
of nkFloat: floatVal: float
of nkString: strVal: string
leftOp, rightOp: Node
of nkIf:
condition, thenPart, elsePart: Node

# create a new case object:
var n = Node(kind: nkIf, condition: nil)
# accessing n.thenPart is valid because the nkIf branch is active:
n.thenPart = Node(kind: nkFloat, floatVal: 2.0)

# the following statement raises an FieldError exception, because
# n.kind's value does not fit and the nkString branch is not active:
n.strVal = ""

# invalid: would change the active object branch:
n.kind = nkInt

var x = Node(kind: nkAdd, leftOp: Node(kind: nkInt, intVal: 4),
rightOp: Node(kind: nkInt, intVal: 2))
# valid: does not change the active object branch:
x.kind = nkSub

As can been seen from the example, an advantage to an object hierarchy is that no casting between different object types is needed. Yet, access to invalid object fields raises an exception.

The syntax of case in an object declaration follows closely the syntax of the case statement: The branches in a case section may be indented too.

In the example the kind field is called the discriminator: For safety its address cannot be taken and assignments to it are restricted: The new value must not lead to a change of the active object branch. For an object branch switch system.reset has to be used. Also, when the fields of a particular branch are specified during object construction, the corresponding discriminator value must be specified as a constant expression.

As a special rule, the discriminator kind can also be bounded using a case statement. If possible values of the discriminator variable in a case statement branch are a subset of discriminator values for the selected object branch, the initialization is considered valid. This analysis only works for immutable discriminators of an ordinal type and disregards elif branches.

A small example:

let unknownKind = nkSub

# invalid: unsafe initialization because the kind field is not statically known:
var y = Node(kind: unknownKind, strVal: "y")

var z = Node()
case unknownKind
# valid: possible values of this branch are a subset of nkAdd/nkSub object branch:
z = Node(kind: unknownKind, leftOp: Node(), rightOp: Node())
else:
echo "ignoring: ", unknownKind

## Set type

The set type models the mathematical notion of a set. The set's basetype can only be an ordinal type of a certain size, namely:
• int8-int16
• uint8/byte-uint16
• char
• enum

or equivalent. For signed integers the set's base type is defined to be in the range 0 .. MaxSetElements-1 where MaxSetElements is currently always 2^16.

The reason is that sets are implemented as high performance bit vectors. Attempting to declare a set with a larger type will result in an error:

var s: set[int64] # Error: set is too large

Sets can be constructed via the set constructor: {} is the empty set. The empty set is type compatible with any concrete set type. The constructor can also be used to include elements (and ranges of elements):

type
CharSet = set[char]
var
x: CharSet
x = {'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:

operationmeaning
A + Bunion of two sets
A * Bintersection of two sets
A - Bdifference of two sets (A without B's elements)
A == Bset equality
A <= Bsubset relation (A is subset of B or equal to B)
A < Bstrong subset relation (A is a real subset of B)
e in Aset membership (A contains element e)
e notin AA does not contain element e
contains(A, e)A contains element e
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}

### Bit fields

Sets are often used to define a type for the flags of a procedure. This is a cleaner (and type safe) solution than defining integer constants that have to be or'ed together.

Enum, sets and casting can be used together as in:

type
MyFlag* {.size: sizeof(cint).} = enum
A
B
C
D
MyFlags = set[MyFlag]

proc toNum(f: MyFlags): int = cast[cint](f)
proc toFlags(v: int): MyFlags = cast[MyFlags](v)

assert toNum({}) == 0
assert toNum({A}) == 1
assert toNum({D}) == 8
assert toNum({A, C}) == 5
assert toFlags(0) == {}
assert toFlags(7) == {A, B, C}

Note how the set turns enum values into powers of 2.

If using enums and sets with C, use distinct cint.

## Reference and pointer types

References (similar to pointers in other programming languages) are a way to introduce many-to-one relationships. This means different references can point to and modify the same location in memory (also called aliasing).

Nim distinguishes between traced and untraced references. Untraced references are also called pointers. Traced references point to objects of a garbage collected heap, untraced references point to manually allocated objects or to objects somewhere else in memory. Thus untraced references are unsafe. However for certain low-level operations (accessing the hardware) untraced references are unavoidable.

Traced references are declared with the ref keyword, untraced references are declared with the ptr keyword. In general, a ptr T is implicitly convertible to the pointer type.

An empty subscript [] notation can be used to derefer a reference, the addr procedure returns the address of an item. An address is always an untraced reference. Thus the usage of addr is an unsafe feature.

The . (access a tuple/object field operator) and [] (array/string/sequence index operator) operators perform implicit dereferencing operations for reference types:

type
Node = ref NodeObj
NodeObj = object
le, ri: Node
data: int

var
n: Node
new(n)
n.data = 9
# no need to write n[].data; in fact n[].data is highly discouraged!

Automatic dereferencing is also performed for the first argument of a routine call. But currently this feature has to be only enabled via {.experimental: "implicitDeref".}:

{.experimental: "implicitDeref".}

proc depth(x: NodeObj): int = ...

var
n: Node
new(n)
echo n.depth
# no need to write n[].depth either

In order to simplify structural type checking, recursive tuples are not valid:

# invalid recursion
type MyTuple = tuple[a: ref MyTuple]

Likewise T = ref T is an invalid type.

As a syntactical extension object types can be anonymous if declared in a type section via the ref object or ptr object notations. This feature is useful if an object should only gain reference semantics:

type
Node = ref object
le, ri: Node
data: int

To allocate a new traced object, the built-in procedure new has to be used. To deal with untraced memory, the procedures alloc, dealloc and realloc can be used. The documentation of the system module contains further information.

If a reference points to nothing, it has the value nil.

Special care has to be taken if an untraced object contains traced objects like traced references, strings or sequences: in order to free everything properly, the built-in procedure GCunref has to be called before freeing the untraced memory manually:

type
Data = tuple[x, y: int, s: string]

# allocate memory for Data on the heap:
var d = cast[ptr Data](alloc0(sizeof(Data)))

# create a new string on the garbage collected heap:
d.s = "abc"

# tell the GC that the string is not needed anymore:
GCunref(d.s)

# free the memory:
dealloc(d)

Without the GCunref call the memory allocated for the d.s string would never be freed. The example also demonstrates two important features for low level programming: the sizeof proc returns the size of a type or value in bytes. The cast operator can circumvent the type system: the compiler is forced to treat the result of the alloc0 call (which returns an untyped pointer) as if it would have the type ptr Data. Casting should only be done if it is unavoidable: it breaks type safety and bugs can lead to mysterious crashes.

Note: The example only works because the memory is initialized to zero (alloc0 instead of alloc does this): d.s is thus initialized to binary zero which the string assignment can handle. One needs to know low level details like this when mixing garbage collected data with unmanaged memory.

## Not nil annotation

All types for which nil is a valid value can be annotated to exclude nil as a valid value with the not nil annotation:

type
PObject = ref TObj not nil
TProc = (proc (x, y: int)) not nil

proc p(x: PObject) =
echo "not nil"

# compiler catches this:
p(nil)

# and also this:
var x: PObject
p(x)

The compiler ensures that every code path initializes variables which contain non nilable pointers. The details of this analysis are still to be specified here.

## Procedural type

A procedural type is internally a pointer to a procedure. nil is an allowed value for variables of a procedural type. Nim uses procedural types to achieve functional programming techniques.

Examples:

proc printItem(x: int) = ...

proc forEach(c: proc (x: int) {.cdecl.}) =
...

forEach(printItem)  # this will NOT compile because calling conventions differ
type
OnMouseMove = proc (x, y: int) {.closure.}

proc onMouseMove(mouseX, mouseY: int) =
# has default calling convention
echo "x: ", mouseX, " y: ", mouseY

# ok, 'onMouseMove' has the default calling convention, which is compatible
# to 'closure':
setOnMouseMove(onMouseMove)

A subtle issue with procedural types is that the calling convention of the procedure influences the type compatibility: procedural types are only compatible if they have the same calling convention. As a special extension, a procedure of the calling convention nimcall can be passed to a parameter that expects a proc of the calling convention closure.

Nim supports these calling conventions:

nimcall
is the default convention used for a Nim proc. It is the same as fastcall, but only for C compilers that support fastcall.
closure
is the default calling convention for a procedural type that lacks any pragma annotations. It indicates that the procedure has a hidden implicit parameter (an environment). Proc vars that have the calling convention closure take up two machine words: One for the proc pointer and another one for the pointer to implicitly passed environment.
stdcall
This is the stdcall convention as specified by Microsoft. The generated C procedure is declared with the __stdcall keyword.
cdecl
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
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
The inline convention means the the caller should not call the procedure, but inline its code directly. Note that Nim does not inline, but leaves this to the C compiler; it generates __inline procedures. This is only a hint for the compiler: it may completely ignore it and it may inline procedures that are not marked as inline.
fastcall
Fastcall means different things to different C compilers. One gets whatever the C __fastcall means.
syscall
The syscall convention is the same as __syscall in C. It is used for interrupts.
noconv
The generated C code will not have any explicit calling convention and thus use the C compiler's default calling convention. This is needed because Nim's default calling convention for procedures is fastcall to improve speed.

Most calling conventions exist only for the Windows 32-bit platform.

The default calling convention is nimcall, unless it is an inner proc (a proc inside of a proc). For an inner proc an analysis is performed whether it accesses its environment. If it does so, it has the calling convention closure, otherwise it has the calling convention nimcall.

## Distinct type

A distinct type is new type derived from a base type that is incompatible with its base type. In particular, it is an essential property of a distinct type that it does not imply a subtype relation between it and its base type. Explicit type conversions from a distinct type to its base type and vice versa are allowed. See also distinctBase to get the reverse operation.

A distinct type is an ordinal type if its base type is an ordinal type.

### Modelling currencies

A distinct type can be used to model different physical units 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:

type
Dollar = distinct int
Euro = distinct int

var
d: Dollar
e: Euro

echo d + 12
# Error: cannot add a number with no unit and a Dollar

Unfortunately, d + 12.Dollar is not allowed either, because + is defined for int (among others), not for Dollar. So a + for dollars needs to be defined:

proc + (x, y: Dollar): Dollar =
result = Dollar(int(x) + int(y))

It does not make sense to multiply a dollar with a dollar, but with a number without unit; and the same holds for division:

proc * (x: Dollar, y: int): Dollar =
result = Dollar(int(x) * y)

proc * (x: int, y: Dollar): Dollar =
result = Dollar(x * int(y))

proc div ...

This quickly gets tedious. The implementations are trivial and the compiler should not generate all this code only to optimize it away later - after all + for dollars should produce the same binary code as + for ints. The pragma borrow has been designed to solve this problem; in principle it generates the above trivial implementations:

proc * (x: Dollar, y: int): Dollar {.borrow.}
proc * (x: int, y: Dollar): Dollar {.borrow.}
proc div (x: Dollar, y: int): Dollar {.borrow.}

The borrow pragma makes the compiler use the same implementation as the proc that deals with the distinct type's base type, so no code is generated.

But it seems all this boilerplate code needs to be repeated for the Euro currency. This can be solved with templates.

template additive(typ: typedesc) =
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) =
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) =
proc < * (x, y: typ): bool {.borrow.}
proc <= * (x, y: typ): bool {.borrow.}
proc == * (x, y: typ): bool {.borrow.}

template defineCurrency(typ, base: untyped) =
type
typ* = distinct base
multiplicative(typ, base)
comparable(typ)

defineCurrency(Dollar, int)
defineCurrency(Euro, int)

The borrow pragma can also be used to annotate the distinct type to allow certain builtin operations to be lifted:

type
Foo = object
a, b: int
s: string

Bar {.borrow: ..} = distinct Foo

var bb: ref Bar
new bb
# field access now valid
bb.a = 90
bb.s = "abc"

Currently only the dot accessor can be borrowed in this way.

### Avoiding SQL injection attacks

An SQL statement that is passed from Nim to an SQL database might be modelled as a string. However, using string templates and filling in the values is vulnerable to the famous SQL injection attack:

import strutils

proc query(db: DbHandle, statement: string) = ...

var

db.query("SELECT FROM users WHERE name = '$1'" % username) # Horrible security hole, but the compiler does not mind! This can be avoided by distinguishing strings that contain SQL from strings that don't. Distinct types provide a means to introduce a new string type SQL that is incompatible with string: type SQL = distinct string proc query(db: DbHandle, statement: SQL) = ... var username: string db.query("SELECT FROM users WHERE name = '$1'" % username)
# Static error: query expects an SQL string!

It is an essential property of abstract types that they do not imply a subtype relation between the abstract type and its base type. Explicit type conversions from string to SQL are allowed:

import strutils, sequtils

proc properQuote(s: string): SQL =
# quotes a string properly for an SQL statement
return SQL(s)

proc % (frmt: SQL, values: openarray[string]): SQL =
# quote each argument:
let v = values.mapIt(SQL, properQuote(it))
# we need a temporary type for the type conversion :-(
type StrSeq = seq[string]
# call strutils.%:
result = SQL(string(frmt) % StrSeq(v))

db.query("SELECT FROM users WHERE name = '$1'".SQL % [username]) Now we have compile-time checking against SQL injection attacks. Since "".SQL is transformed to SQL("") no new syntax is needed for nice looking SQL string literals. The hypothetical SQL type actually exists in the library as the TSqlQuery type of modules like db_sqlite. ## Auto type The auto type can only be used for return types and parameters. For return types it causes the compiler to infer the type from the routine body: proc returnsInt(): auto = 1984 For parameters it currently creates implicitly generic routines: proc foo(a, b: auto) = discard Is the same as: proc foo[T1, T2](a: T1, b: T2) = discard However later versions of the language might change this to mean "infer the parameters' types from the body". Then the above foo would be rejected as the parameters' types can not be inferred from an empty discard statement. # Type relations The following section defines several relations on types that are needed to describe the type checking done by the compiler. ## Type equality Nim uses structural type equivalence for most types. Only for objects, enumerations and distinct types name equivalence is used. The following algorithm, in pseudo-code, determines type equality: proc typeEqualsAux(a, b: PType, s: var HashSet[(PType, PType)]): bool = if (a,b) in s: return true incl(s, (a,b)) if a.kind == b.kind: case a.kind of int, intXX, float, floatXX, char, string, cstring, pointer, bool, nil, void: # leaf type: kinds identical; nothing more to check result = true of ref, ptr, var, set, seq, openarray: result = typeEqualsAux(a.baseType, b.baseType, s) of range: result = typeEqualsAux(a.baseType, b.baseType, s) and (a.rangeA == b.rangeA) and (a.rangeB == b.rangeB) of array: result = typeEqualsAux(a.baseType, b.baseType, s) and typeEqualsAux(a.indexType, b.indexType, s) of tuple: if a.tupleLen == b.tupleLen: for i in 0..a.tupleLen-1: if not typeEqualsAux(a[i], b[i], s): return false result = true of object, enum, distinct: result = a == b of proc: result = typeEqualsAux(a.parameterTuple, b.parameterTuple, s) and typeEqualsAux(a.resultType, b.resultType, s) and a.callingConvention == b.callingConvention proc typeEquals(a, b: PType): bool = var s: HashSet[(PType, PType)] = {} result = typeEqualsAux(a, b, s) Since types are graphs which can have cycles, the above algorithm needs an auxiliary set s to detect this case. ## Type equality modulo type distinction The following algorithm (in pseudo-code) determines whether two types are equal with no respect to distinct types. For brevity the cycle check with an auxiliary set s is omitted: proc typeEqualsOrDistinct(a, b: PType): bool = if a.kind == b.kind: case a.kind of int, intXX, float, floatXX, char, string, cstring, pointer, bool, nil, void: # leaf type: kinds identical; nothing more to check result = true of ref, ptr, var, set, seq, openarray: result = typeEqualsOrDistinct(a.baseType, b.baseType) of range: result = typeEqualsOrDistinct(a.baseType, b.baseType) and (a.rangeA == b.rangeA) and (a.rangeB == b.rangeB) of array: result = typeEqualsOrDistinct(a.baseType, b.baseType) and typeEqualsOrDistinct(a.indexType, b.indexType) of tuple: if a.tupleLen == b.tupleLen: for i in 0..a.tupleLen-1: if not typeEqualsOrDistinct(a[i], b[i]): return false result = true of distinct: result = typeEqualsOrDistinct(a.baseType, b.baseType) of object, enum: result = a == b of proc: result = typeEqualsOrDistinct(a.parameterTuple, b.parameterTuple) and typeEqualsOrDistinct(a.resultType, b.resultType) and a.callingConvention == b.callingConvention elif a.kind == distinct: result = typeEqualsOrDistinct(a.baseType, b) elif b.kind == distinct: result = typeEqualsOrDistinct(a, b.baseType) ## Subtype relation If object a inherits from b, a is a subtype of b. This subtype relation is extended to the types var, ref, ptr: 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) ## Convertible relation A type a is implicitly convertible to type b iff the following algorithm returns true: proc isImplicitlyConvertible(a, b: PType): bool = if isSubtype(a, b) or isCovariant(a, b): return true case a.kind of int: result = b in {int8, int16, int32, int64, uint, uint8, uint16, uint32, uint64, float, float32, float64} of int8: result = b in {int16, int32, int64, int} of int16: result = b in {int32, int64, int} of int32: result = b in {int64, int} of uint: result = b in {uint32, uint64} of uint8: result = b in {uint16, uint32, uint64} of uint16: result = b in {uint32, uint64} of uint32: result = b in {uint64} of float: result = b in {float32, float64} of float32: result = b in {float64, float} of float64: result = b in {float32, float} of seq: result = b == openArray and typeEquals(a.baseType, b.baseType) of array: result = b == openArray and typeEquals(a.baseType, b.baseType) if a.baseType == char and a.indexType.rangeA == 0: result = b == cstring of cstring, ptr: result = b == pointer of string: result = b == cstring Implicit conversions are also performed for Nim's range type constructor. Let a0, b0 of type T. Let A = range[a0..b0] be the argument's type, F the formal parameter's type. Then an implicit conversion from A to F exists if a0 >= low(F) and b0 <= high(F) and both T and F are signed integers or if both are unsigned integers. A type a is explicitly convertible to type b iff the following algorithm returns true: proc isIntegralType(t: PType): bool = result = isOrdinal(t) or t.kind in {float, float32, float64} proc isExplicitlyConvertible(a, b: PType): bool = result = false if isImplicitlyConvertible(a, b): return true if typeEqualsOrDistinct(a, b): return true if isIntegralType(a) and isIntegralType(b): return true if isSubtype(a, b) or isSubtype(b, a): return true The convertible relation can be relaxed by a user-defined type converter. converter toInt(x: char): int = result = ord(x) var x: int chr: char = 'a' # implicit conversion magic happens here x = chr echo x # => 97 # you can use the explicit form too x = chr.toInt echo x # => 97 The type conversion T(a) is an L-value if a is an L-value and typeEqualsOrDistinct(T, type(a)) holds. ## Assignment compatibility An expression b can be assigned to an expression a iff a is an l-value and isImplicitlyConvertible(b.typ, a.typ) holds. # Overloading resolution In a call p(args) the routine p that matches best is selected. If multiple routines match equally well, the ambiguity is reported during semantic analysis. Every arg in args needs to match. There are multiple different categories how an argument can match. Let f be the formal parameter's type and a the type of the argument. 1. Exact match: a and f are of the same type. 2. Literal match: a is an integer literal of value v and f is a signed or unsigned integer type and v is in f's range. Or: a is a floating point literal of value v and f is a floating point type and v is in f's range. 3. Generic match: f is a generic type and a matches, for instance a is int and f is a generic (constrained) parameter type (like in [T] or [T: int|char]. 4. Subrange or subtype match: a is a range[T] and T matches f exactly. Or: a is a subtype of f. 5. Integral conversion match: a is convertible to f and f and a is some integer or floating point type. 6. Conversion match: a is convertible to f, possibly via a user defined converter. These matching categories have a priority: An exact match is better than a literal match and that is better than a generic match etc. In the following count(p, m) counts the number of matches of the matching category m for the routine p. A routine p matches better than a routine q if the following algorithm returns true: for each matching category m in ["exact match", "literal match", "generic match", "subtype match", "integral match", "conversion match"]: if count(p, m) > count(q, m): return true elif count(p, m) == count(q, m): discard "continue with next category m" else: return false return "ambiguous" Some examples: proc takesInt(x: int) = echo "int" proc takesInt[T](x: T) = echo "T" proc takesInt(x: int16) = echo "int16" takesInt(4) # "int" var x: int32 takesInt(x) # "T" var y: int16 takesInt(y) # "int16" var z: range[0..4] = 0 takesInt(z) # "T" If this algorithm returns "ambiguous" further disambiguation is performed: If the argument a matches both the parameter type f of p and g of q via a subtyping relation, the inheritance depth is taken into account: type A = object of RootObj B = object of A C = object of B proc p(obj: A) = echo "A" proc p(obj: B) = echo "B" var c = C() # not ambiguous, calls 'B', not 'A' since B is a subtype of A # but not vice versa: p(c) proc pp(obj: A, obj2: B) = echo "A B" proc pp(obj: B, obj2: A) = echo "B A" # but this is ambiguous: pp(c, c) Likewise for generic matches the most specialized generic type (that still matches) is preferred: proc gen[T](x: ref ref T) = echo "ref ref T" proc gen[T](x: ref T) = echo "ref T" proc gen[T](x: T) = echo "T" var ri: ref int gen(ri) # "ref T" ## Overloading based on 'var T' If the formal parameter f is of type var T in addition to the ordinary type checking, the argument is checked to be an l-value. var T matches better than just T then. proc sayHi(x: int): string = # matches a non-var int result =$x
proc sayHi(x: var int): string =
# matches a var int

## Type casts

Example:

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 returns the address of an l-value. If the type of the location is T, the addr operator result is of the type ptr T. An address is always an untraced reference. Taking the address of an object that resides on the stack is unsafe, as the pointer may live longer than the object on the stack and can thus reference a non-existing object. One can get the address of variables, but one can't use it on variables declared through let statements:

let t1 = "Hello"
var
t2 = t1
# --> ref 0x7fff6b71b670 --> 0x10bb81050"Hello"
echo cast[ptr string](t3)[]
# --> Hello
# The following line doesn't compile:
# Error: expression has no address

For easier interoperability with other compiled languages such as C, retrieving the address of a let variable, a parameter or a for loop variable, the unsafeAddr operation can be used:

let myArray = [1, 2, 3]
foreignProcThatTakesAnAddr(unsafeAddr myArray)

# Procedures

What most programming languages call methods or functions are called procedures in Nim. A procedure declaration consists of an identifier, zero or more formal parameters, a return value type and a block of code. Formal parameters are declared as a list of identifiers separated by either comma or semicolon. A parameter is given a type by : typename. The type applies to all parameters immediately before it, until either the beginning of the parameter list, a semicolon separator or an already typed parameter, is reached. The semicolon can be used to make separation of types and subsequent identifiers more distinct.

# Using only commas
proc foo(a, b: int, c, d: bool): int

# Using semicolon for visual distinction
proc foo(a, b: int; c, d: bool): int

# Will fail: a is untyped since ';' stops type propagation.
proc foo(a; b: int; c, d: bool): int

A parameter may be declared with a default value which is used if the caller does not provide a value for the argument.

# b is optional with 47 as its default value
proc foo(a: int, b: int = 47): int

Parameters can be declared mutable and so allow the proc to modify those arguments, by using the type modifier var.

# "returning" a value to the caller through the 2nd argument
# Notice that the function uses no actual return value at all (ie void)
proc foo(inp: int, outp: var int) =
outp = inp + 47

If the proc declaration has no body, it is a forward declaration. If the proc returns a value, the procedure body can access an implicitly declared variable named result that represents the return value. Procs can be overloaded. The overloading resolution algorithm determines which proc is the best match for the arguments. Example:

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:

proc callme(x, y: int, s: string = "", c: char, b: bool = false) = ...

# call with positional arguments      # parameter bindings:
callme(0, 1, "abc", '\t', true)       # (x=0, y=1, s="abc", c='\t', b=true)
# call with named and positional arguments:
callme(y=1, x=0, "abd", '\t')         # (x=0, y=1, s="abd", c='\t', b=false)
# call with named arguments (order is not relevant):
callme(c='\t', y=1, x=0)              # (x=0, y=1, s="", c='\t', b=false)
# call as a command statement: no () needed:
callme 0, 1, "abc", '\t'              # (x=0, y=1, s="abc", c='\t', b=false)

A procedure may call itself recursively.

Operators are procedures with a special operator symbol as identifier:

proc $ (x: int): string = # converts an integer to a string; this is a prefix operator. result = intToStr(x) Operators with one parameter are prefix operators, operators with two parameters are infix operators. (However, the parser distinguishes these from the operator's position within an expression.) There is no way to declare postfix operators: all postfix operators are built-in and handled by the grammar explicitly. Any operator can be called like an ordinary proc with the 'opr' notation. (Thus an operator can have more than two parameters): proc *+ (a, b, c: int): int = # Multiply and add result = a * b + c assert *+(3, 4, 6) == +(*(a, b), c) ## Export marker If a declared symbol is marked with an asterisk it is exported from the current module: proc exportedEcho*(s: string) = echo s proc **(a: string; b: int): string = result = newStringOfCap(a.len * b) for i in 1..b: result.add a var exportedVar*: int const exportedConst* = 78 type ExportedType* = object exportedField*: int ## Method call syntax For object oriented programming, the syntax obj.method(args) can be used instead of method(obj, args). The parentheses can be omitted if there are no remaining arguments: obj.len (instead of len(obj)). This method call syntax is not restricted to objects, it can be used to supply any type of first argument for procedures: echo "abc".len # is the same as echo len "abc" echo "abc".toUpper() echo {'a', 'b', 'c'}.card stdout.writeLine("Hallo") # the same as writeLine(stdout, "Hallo") Another way to look at the method call syntax is that it provides the missing postfix notation. The method call syntax conflicts with explicit generic instantiations: p[T](x) cannot be written as x.p[T] because x.p[T] is always parsed as (x.p)[T]. See also: Limitations of the method call syntax. The [: ] notation has been designed to mitigate this issue: x.p[:T] is rewritten by the parser to p[T](x), x.p[:T](y) is rewritten to p[T](x, y). Note that [: ] has no AST representation, the rewrite is performed directly in the parsing step. ## Properties Nim has no need for get-properties: Ordinary get-procedures that are called with the method call syntax achieve the same. But setting a value is different; for this a special setter syntax is needed: # Module asocket type Socket* = ref object of RootObj host: int # cannot be accessed from the outside of the module proc host=*(s: var Socket, value: int) {.inline.} = ## setter of hostAddr. ## This accesses the 'host' field and is not a recursive call to ## host= because the builtin dot access is preferred if it is ## avaliable: s.host = value proc host*(s: Socket): int {.inline.} = ## getter of hostAddr ## This accesses the 'host' field and is not a recursive call to ## host because the builtin dot access is preferred if it is ## avaliable: s.host # module B import asocket var s: Socket new s s.host = 34 # same as host=(s, 34) ## Command invocation syntax Routines can be invoked without the () if the call is syntactically a statement. This command invocation syntax also works for expressions, but then only a single argument may follow. This restriction means echo f 1, f 2 is parsed as echo(f(1), f(2)) and not as echo(f(1, f(2))). The method call syntax may be used to provide one more argument in this case: proc optarg(x: int, y: int = 0): int = x + y proc singlearg(x: int): int = 20*x echo optarg 1, " ", singlearg 2 # prints "1 40" let fail = optarg 1, optarg 8 # Wrong. Too many arguments for a command call let x = optarg(1, optarg 8) # traditional procedure call with 2 arguments let y = 1.optarg optarg 8 # same thing as above, w/o the parenthesis assert x == y The command invocation syntax also can't have complex expressions as arguments. For example: (anonymous procs), if, case or try. Function calls with no arguments still needs () to distinguish between a call and the function itself as a first class value. ## Closures Procedures can appear at the top level in a module as well as inside other scopes, in which case they are called nested procs. A nested proc can access local variables from its enclosing scope and if it does so it becomes a closure. Any captured variables are stored in a hidden additional argument to the closure (its environment) and they are accessed by reference by both the closure and its enclosing scope (i.e. any modifications made to them are visible in both places). The closure environment may be allocated on the heap or on the stack if the compiler determines that this would be safe. ### Creating closures in loops Since closures capture local variables by reference it is often not wanted behavior inside loop bodies. See closureScope for details on how to change this behavior. ## Anonymous Procs Procs can also be treated as expressions, in which case it's allowed to omit the proc's name. var cities = @["Frankfurt", "Tokyo", "New York", "Kyiv"] cities.sort(proc (x,y: string): int = cmp(x.len, y.len)) Procs as expressions can appear both as nested procs and inside top level executable code. ## Func The func keyword introduces a shortcut for a noSideEffect proc. func binarySearch[T](a: openArray[T]; elem: T): int Is short for: proc binarySearch[T](a: openArray[T]; elem: T): int {.noSideEffect.} ## Nonoverloadable builtins The following builtin procs cannot be overloaded for reasons of implementation simplicity (they require specialized semantic checking): declared, defined, definedInScope, compiles, sizeOf, is, shallowCopy, getAst, astToStr, spawn, procCall Thus they act more like keywords than like ordinary identifiers; unlike a keyword however, a redefinition may shadow the definition in the system module. From this list the following should not be written in dot notation x.f since x cannot be type checked before it gets passed to f: declared, defined, definedInScope, compiles, getAst, astToStr ## Var parameters The type of a parameter may be prefixed with the var keyword: 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: 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: proc divmod(a, b: int): tuple[res, remainder: int] = (a div b, a mod b) var t = divmod(8, 5) assert t.res == 1 assert t.remainder == 3 One can use tuple unpacking to access the tuple's fields: var (x, y) = divmod(8, 5) # tuple unpacking assert x == 1 assert y == 3 Note: var parameters are never necessary for efficient parameter passing. Since non-var parameters cannot be modified the compiler is always free to pass arguments by reference if it considers it can speed up execution. ## Var return type A proc, converter or iterator may return a var type which means that the returned value is an l-value and can be modified by the caller: var g = 0 proc writeAccessToG(): var int = result = g writeAccessToG() = 6 assert g == 6 It is a static error if the implicitly introduced pointer could be used to access a location beyond its lifetime: proc writeAccessToG(): var int = var g = 0 result = g # Error! For iterators, a component of a tuple return type can have a var type too: iterator mpairs(a: var seq[string]): tuple[key: int, val: var string] = for i in 0..a.high: yield (i, a[i]) In the standard library every name of a routine that returns a var type starts with the prefix m per convention. Memory safety for returning by var T is ensured by a simple borrowing rule: If result does not refer to a location pointing to the heap (that is in result = X the X involves a ptr or ref access) then it has to be deviated by the routine's first parameter: proc forward[T](x: var T): var T = result = x # ok, deviated from the first parameter. proc p(param: var int): var int = var x: int # we know 'forward' provides a view into the location deviated by # its first argument 'x'. result = forward(x) # Error: location is derived from x # which is not p's first parameter and lives # on the stack. In other words, the lifetime of what result points to is attached to the lifetime of the first parameter and that is enough knowledge to verify memory safety at the callsite. ### Future directions Later versions of Nim can be more precise about the borrowing rule with a syntax like: proc foo(other: Y; container: var X): var T from container Here var T from container explicitly exposes that the location is deviated from the second parameter (called 'container' in this case). The syntax var T from p specifies a type varTy[T, 2] which is incompatible with varTy[T, 1]. ## Overloading of the subscript operator The [] subscript operator for arrays/openarrays/sequences can be overloaded. # Multi-methods Note: Starting from Nim 0.20, to use multi-methods one must explicitly pass --multimethods:on when compiling. Procedures always use static dispatch. Multi-methods use dynamic dispatch. For dynamic dispatch to work on an object it should be a reference type. type Expression = ref object of RootObj ## abstract base class for an expression Literal = ref object of Expression x: int PlusExpr = ref object of Expression a, b: Expression method eval(e: Expression): int {.base.} = # override this base method quit "to override!" method eval(e: Literal): int = return e.x method eval(e: PlusExpr): int = # watch out: relies on dynamic binding result = eval(e.a) + eval(e.b) proc newLit(x: int): Literal = new(result) result.x = x proc newPlus(a, b: Expression): PlusExpr = new(result) result.a = a result.b = b echo eval(newPlus(newPlus(newLit(1), newLit(2)), newLit(4))) In the example the constructors newLit and newPlus are procs because they should use static binding, but eval is a method because it requires dynamic binding. As can be seen in the example, base methods have to be annotated with the base pragma. The base pragma also acts as a reminder for the programmer that a base method m is used as the foundation to determine all the effects that a call to m might cause. Note: Compile-time execution is not (yet) supported for methods. Note: Starting from Nim 0.20, generic methods are deprecated. ## Inhibit dynamic method resolution via procCall Dynamic method resolution can be inhibited via the builtin system.procCall. This is somewhat comparable to the super keyword that traditional OOP languages offer. type Thing = ref object of RootObj Unit = ref object of Thing x: int method m(a: Thing) {.base.} = echo "base" method m(a: Unit) = # Call the base method: procCall m(Thing(a)) echo "1" # Iterators and the for statement The for statement is an abstract mechanism to iterate over the elements of a container. It relies on an iterator to do so. Like while statements, for statements open an implicit block, so that they can be left with a break statement. The for loop declares iteration variables - their scope reaches until the end of the loop body. The iteration variables' types are inferred by the return type of the iterator. An iterator is similar to a procedure, except that it can be called in the context of a for loop. Iterators provide a way to specify the iteration over an abstract type. A key role in the execution of a for loop plays the yield statement in the called iterator. Whenever a yield statement is reached the data is bound to the for loop variables and control continues in the body of the for loop. The iterator's local variables and execution state are automatically saved between calls. Example: # 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: var i = 0 while i < len(a): var ch = a[i] echo ch inc(i) If the iterator yields a tuple, there can be as many iteration variables as there are components in the tuple. The i'th iteration variable's type is the type of the i'th component. In other words, implicit tuple unpacking in a for loop context is supported. ## Implict items/pairs invocations If the for loop expression e does not denote an iterator and the for loop has exactly 1 variable, the for loop expression is rewritten to items(e); ie. an items iterator is implicitly invoked: for x in [1,2,3]: echo x If the for loop has exactly 2 variables, a pairs iterator is implicitly invoked. Symbol lookup of the identifiers items/pairs is performed after the rewriting step, so that all overloads of items/pairs are taken into account. ## First class iterators There are 2 kinds of iterators in Nim: inline and closure iterators. An inline iterator is an iterator that's always inlined by the compiler leading to zero overhead for the abstraction, but may result in a heavy increase in code size. Caution: the body of a for loop over an inline iterator is inlined into each yield statement appearing in the iterator code, so ideally the code should be refactored to contain a single yield when possible to avoid code bloat. Inline iterators are second class citizens; They can be passed as parameters only to other inlining code facilities like templates, macros and other inline iterators. In contrast to that, a closure iterator can be passed around more freely: iterator count0(): int {.closure.} = yield 0 iterator count2(): int {.closure.} = var x = 1 yield x inc x yield x proc invoke(iter: iterator(): int {.closure.}) = for x in iter(): echo x invoke(count0) invoke(count2) Closure iterators and inline iterators have some restrictions: 1. For now, a closure iterator cannot be executed at compile time. 2. return is allowed in a closure iterator but not in an inline iterator (but rarely useful) and ends the iteration. 3. Neither inline nor closure iterators can be recursive. 4. Neither inline nor closure iterators have the special result variable. 5. Closure iterators are not supported by the js backend. Iterators that are neither marked {.closure.} nor {.inline.} explicitly default to being inline, but this may change in future versions of the implementation. The iterator type is always of the calling convention closure implicitly; the following example shows how to use iterators to implement a collaborative tasking system: # simple tasking: type Task = iterator (ticker: int) iterator a1(ticker: int) {.closure.} = echo "a1: A" yield echo "a1: B" yield echo "a1: C" yield echo "a1: D" iterator a2(ticker: int) {.closure.} = echo "a2: A" yield echo "a2: B" yield echo "a2: C" proc runTasks(t: varargs[Task]) = var ticker = 0 while true: let x = t[ticker mod t.len] if finished(x): break x(ticker) inc ticker runTasks(a1, a2) The builtin system.finished can be used to determine if an iterator has finished its operation; no exception is raised on an attempt to invoke an iterator that has already finished its work. Note that system.finished is error prone to use because it only returns true one iteration after the iterator has finished: iterator mycount(a, b: int): int {.closure.} = var x = a while x <= b: yield x inc x var c = mycount # instantiate the iterator while not finished(c): echo c(1, 3) # Produces 1 2 3 0 Instead this code has to be used: var c = mycount # instantiate the iterator while true: let value = c(1, 3) if finished(c): break # and discard 'value'! echo value It helps to think that the iterator actually returns a pair (value, done) and finished is used to access the hidden done field. Closure iterators are resumable functions and so one has to provide the arguments to every call. To get around this limitation one can capture parameters of an outer factory proc: proc mycount(a, b: int): iterator (): int = result = iterator (): int = var x = a while x <= b: yield x inc x let foo = mycount(1, 4) for f in foo(): echo f # Converters A converter is like an ordinary proc except that it enhances the "implicitly convertible" type relation (see Convertible relation): # bad style ahead: Nim is not C. converter toBool(x: int): bool = x != 0 if 4: echo "compiles" A converter can also be explicitly invoked for improved readability. Note that implicit converter chaining is not supported: If there is a converter from type A to type B and from type B to type C the implicit conversion from A to C is not provided. # Type sections Example: type # example demonstrating mutually recursive types Node = ref object # an object managed by the garbage collector (ref) le, ri: Node # left and right subtrees sym: ref Sym # leaves contain a reference to a Sym Sym = object # a symbol name: string # the symbol's name line: int # the line the symbol was declared in code: Node # the symbol's abstract syntax tree A type section begins with the type keyword. It contains multiple type definitions. A type definition binds a type to a name. Type definitions can be recursive or even mutually recursive. Mutually recursive types are only possible within a single type section. Nominal types like objects or enums can only be defined in a type section. # Exception handling ## Try statement Example: # read the first two lines of a text file that should contain numbers # and tries to add them var f: File if open(f, "numbers.txt"): try: var a = readLine(f) var b = readLine(f) echo "sum: " &$(parseInt(a) + parseInt(b))
except OverflowError:
echo "overflow!"
except ValueError:
echo "could not convert string to integer"
except IOError:
echo "IO error!"
except:
echo "Unknown exception!"
finally:
close(f)

The statements after the try are executed in sequential order unless an exception e is raised. If the exception type of e matches any listed in an except clause the corresponding statements are executed. The statements following the except clauses are called exception handlers.

The empty except clause is executed if there is an exception that is not listed otherwise. It is similar to an else clause in if statements.

If there is a finally clause, it is always executed after the exception handlers.

The exception is consumed in an exception handler. However, an exception handler may raise another exception. If the exception is not handled, it is propagated through the call stack. This means that often the rest of the procedure - that is not within a finally clause - is not executed (if an exception occurs).

## Try expression

Try can also be used as an expression; the type of the try branch then needs to fit the types of except branches, but the type of the finally branch always has to be void:

let x = try: parseInt("133a")
except: -1
finally: echo "hi"

To prevent confusing code there is a parsing limitation; if the try follows a ( it has to be written as a one liner:

let x = (try: parseInt("133a") except: -1)

## Except clauses

Within an except clause, it is possible to use getCurrentException to retrieve the exception that has been raised:

try:
# ...
except IOError:
let e = getCurrentException()
# Now use "e"

Note that getCurrentException always returns a ref Exception type. If a variable of the proper type is needed (in the example above, IOError), one must convert it explicitly:

try:
# ...
except IOError:
let e = (ref IOError)(getCurrentException())
# "e" is now of the proper type

However, this is seldom needed. The most common case is to extract an error message from e, and for such situations it is enough to use getCurrentExceptionMsg:

try:
# ...
except IOError:
echo "I/O error: " & getCurrentExceptionMsg()

## Defer statement

Instead of a try finally statement a defer statement can be used.

Any statements following the defer in the current block will be considered to be in an implicit try block:

proc main =
var f = open("numbers.txt")
defer: close(f)
f.write "abc"
f.write "def"

Is rewritten to:

proc main =
var f = open("numbers.txt")
try:
f.write "abc"
f.write "def"
finally:
close(f)

Top level defer statements are not supported since it's unclear what such a statement should refer to.

## Raise statement

Example:

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.

If no exception name is given, the current exception is re-raised. The ReraiseError exception is raised if there is no exception to re-raise. It follows that the raise statement always raises an exception.

## Exception hierarchy

The exception tree is defined in the system module. Every exception inherits from system.Exception. Exceptions that indicate programming bugs inherit from system.Defect (which is a subtype of Exception) and are stricly speaking not catchable as they can also be mapped to an operation that terminates the whole process. Exceptions that indicate any other runtime error that can be caught inherit from system.CatchableError (which is a subtype of Exception).

## Imported exceptions

It is possible to raise/catch imported C++ exceptions. Types imported using importcpp can be raised or caught. Exceptions are raised by value and caught by reference. Example:

type
std_exception {.importcpp: "std::exception", header: "<exception>".} = object

proc what(s: std_exception): cstring {.importcpp: "((char *)#.what())".}

try:
raise std_exception()
except std_exception as ex:
echo ex.what()

# Effect system

## Exception tracking

Nim supports exception tracking. The raises pragma can be used to explicitly define which exceptions a proc/iterator/method/converter is allowed to raise. The compiler verifies this:

proc p(what: bool) {.raises: [IOError, OSError].} =
if what: raise newException(IOError, "IO")
else: raise newException(OSError, "OS")

An empty raises list (raises: []) means that no exception may be raised:

proc p(): bool {.raises: [].} =
try:
unsafeCall()
result = true
except:
result = false

A raises list can also be attached to a proc type. This affects type compatibility:

type
Callback = proc (s: string) {.raises: [IOError].}
var
c: Callback

proc p(x: string) =
raise newException(OSError, "OS")

c = p # type error

For a routine p the compiler uses inference rules to determine the set of possibly raised exceptions; the algorithm operates on p's call graph:

1. Every indirect call via some proc type T is assumed to raise system.Exception (the base type of the exception hierarchy) and thus any exception unless T has an explicit raises list. However if the call is of the form f(...) where f is a parameter of the currently analysed routine it is ignored. The call is optimistically assumed to have no effect. Rule 2 compensates for this case.
2. Every expression of some proc type within a call that is not a call itself (and not nil) is assumed to be called indirectly somehow and thus its raises list is added to p's raises list.
3. Every call to a proc q which has an unknown body (due to a forward declaration or an importc pragma) is assumed to raise system.Exception unless q has an explicit raises list.
4. Every call to a method m is assumed to raise system.Exception unless m has an explicit raises list.
5. For every other call the analysis can determine an exact raises list.
6. For determining a raises list, the raise and try statements of p are taken into consideration.

Rules 1-2 ensure the following works:

proc noRaise(x: proc()) {.raises: [].} =
# unknown call that might raise anything, but valid:
x()

proc doRaise() {.raises: [IOError].} =
raise newException(IOError, "IO")

proc use() {.raises: [].} =
# doesn't compile! Can raise IOError!
noRaise(doRaise)

So in many cases a callback does not cause the compiler to be overly conservative in its effect analysis.

## Tag tracking

The exception tracking is part of Nim's effect system. Raising an exception is an effect. Other effects can also be defined. A user defined effect is a means to tag a routine and to perform checks against this tag:

type IO = object ## input/output effect

# the compiler prevents this:
let x = readLine()

A tag has to be a type name. A tags list - like a raises list - can also be attached to a proc type. This affects type compatibility.

The inference for tag tracking is analogous to the inference for exception tracking.

## Effects pragma

The effects pragma has been designed to assist the programmer with the effects analysis. It is a statement that makes the compiler output all inferred effects up to the effects's position:

proc p(what: bool) =
if what:
raise newException(IOError, "IO")
{.effects.}
else:
raise newException(OSError, "OS")

The compiler produces a hint message that IOError can be raised. OSError is not listed as it cannot be raised in the branch the effects pragma appears in.

# Generics

Generics are Nim's means to parametrize procs, iterators or types with type parameters. Depending on context, the brackets are used either to introduce type parameters or to instantiate a generic proc, iterator or type.

The following example shows a generic binary tree can be modelled:

type
BinaryTree*[T] = ref object # BinaryTree is a generic type with
# generic param T
le, ri: BinaryTree[T]     # left and right subtrees; may be nil
data: T                   # the data stored in a node

proc newNode*[T](data: T): BinaryTree[T] =
# constructor for a node
result = BinaryTree[T](le: nil, ri: nil, data: data)

proc add*[T](root: var BinaryTree[T], n: BinaryTree[T]) =
# insert a node into the tree
if root == nil:
root = n
else:
var it = root
while it != nil:
# compare the data items; uses the generic cmp proc
# that works for any type that has a == and < operator
var c = cmp(it.data, n.data)
if c < 0:
if it.le == nil:
it.le = n
return
it = it.le
else:
if it.ri == nil:
it.ri = n
return
it = it.ri

proc add*[T](root: var BinaryTree[T], data: T) =
# convenience proc:

iterator preorder*[T](root: BinaryTree[T]): T =
# Preorder traversal of a binary tree.
# Since recursive iterators are not yet implemented,
# this uses an explicit stack (which is more efficient anyway):
var stack: seq[BinaryTree[T]] = @[root]
while stack.len > 0:
var n = stack.pop()
while n != nil:
yield n.data
add(stack, n.ri)  # push right subtree onto the stack
n = n.le          # and follow the left pointer

var
root: BinaryTree[string] # instantiate a BinaryTree with string
add(root, newNode("hello")) # instantiates newNode and add
add(root, "world")          # instantiates the second add proc
for str in preorder(root):
stdout.writeLine(str)

The T is called a generic type parameter or a type variable.

## Is operator

The is operator is evaluated during semantic analysis to check for type equivalence. It is therefore very useful for type specialization within generic code:

type
Table[Key, Value] = object
keys: seq[Key]
values: seq[Value]
when not (Key is string): # empty value for strings used for optimization
deletedKeys: seq[bool]

## Type Classes

A type class is a special pseudo-type that can be used to match against types in the context of overload resolution or the is operator. Nim supports the following built-in type classes:

type classmatches
objectany object type
tupleany tuple type
enumany enumeration
procany proc type
refany ref type
ptrany ptr type
varany var type
distinctany distinct type
arrayany array type
setany set type
seqany seq type
autoany type
anydistinct auto (see below)

Furthermore, every generic type automatically creates a type class of the same name that will match any instantiation of the generic type.

Type classes can be combined using the standard boolean operators to form more complex type classes:

# create a type class that will match all tuple and object types
type RecordType = tuple or object

proc printFields(rec: RecordType) =
for key, value in fieldPairs(rec):
echo key, " = ", value

Procedures utilizing type classes in such manner are considered to be implicitly generic. They will be instantiated once for each unique combination of param types used within the program.

Nim also allows for type classes and regular types to be specified as type constraints of the generic type parameter:

proc onlyIntOrString[T: int|string](x, y: T) = discard

onlyIntOrString(450, 616) # valid
onlyIntOrString(5.0, 0.0) # type mismatch
onlyIntOrString("xy", 50) # invalid as 'T' cannot be both at the same time

By default, during overload resolution each named type class will bind to exactly one concrete type. We call such type classes bind once types. Here is an example taken directly from the system module to illustrate this:

proc ==*(x, y: tuple): bool =
## requires x and y to be of the same tuple type
## generic == operator for tuples that is lifted from the components
## of x and y.
result = true
for a, b in fields(x, y):
if a != b: result = false

Alternatively, the distinct type modifier can be applied to the type class to allow each param matching the type class to bind to a different type. Such type classes are called bind many types.

Procs written with the implicitly generic style will often need to refer to the type parameters of the matched generic type. They can be easily accessed using the dot syntax:

type Matrix[T, Rows, Columns] = object
...

proc [](m: Matrix, row, col: int): Matrix.T =
m.data[col * high(Matrix.Columns) + row]

Alternatively, the type operator can be used over the proc params for similar effect when anonymous or distinct type classes are used.

When a generic type is instantiated with a type class instead of a concrete type, this results in another more specific type class:

seq[ref object]  # Any sequence storing references to any object type

type T1 = auto
proc foo(s: seq[T1], e: T1)
# seq[T1] is the same as just seq, but T1 will be allowed to bind
# to a single type, while the signature is being matched

Matrix[Ordinal] # Any Matrix instantiation using integer values

As seen in the previous example, in such instantiations, it's not necessary to supply all type parameters of the generic type, because any missing ones will be inferred to have the equivalent of the any type class and thus they will match anything without discrimination.

## Generic inference restrictions

The types var T and typedesc[T] cannot be inferred in a generic instantiation. The following is not allowed:

proc g[T](f: proc(x: T); x: T) =
f(x)

proc c(y: int) = echo y
proc v(y: var int) =
y += 100
var i: int

# allowed: infers 'T' to be of type 'int'
g(c, 42)

# not valid: 'T' is not inferred to be of type 'var int'
g(v, i)

# also not allowed: explict instantiation via 'var int'
g[var int](v, i)

## Symbol lookup in generics

### Open and Closed symbols

The symbol binding rules in generics are slightly subtle: There are "open" and "closed" symbols. A "closed" symbol cannot be re-bound in the instantiation context, an "open" symbol can. Per default overloaded symbols are open and every other symbol is closed.

Open symbols are looked up in two different contexts: Both the context at definition and the context at instantiation are considered:

type
Index = distinct int

proc == (a, b: Index): bool {.borrow.}

var a = (0, 0.Index)
var b = (0, 0.Index)

echo a == b # works!

In the example the generic == for tuples (as defined in the system module) uses the == operators of the tuple's components. However, the == for the Index type is defined after the == for tuples; yet the example compiles as the instantiation takes the currently defined symbols into account too.

## Mixin statement

A symbol can be forced to be open by a mixin declaration:

proc create*[T](): ref T =
# there is no overloaded 'init' here, so we need to state that it's an
# open symbol explicitly:
mixin init
new result
init result

mixin statements only make sense in templates and generics.

## Bind statement

The bind statement is the counterpart to the mixin statement. It can be used to explicitly declare identifiers that should be bound early (i.e. the identifiers should be looked up in the scope of the template/generic definition):

# Module A
var
lastId = 0

template genId*: untyped =
bind lastId
inc(lastId)
lastId
# Module B
import A

echo genId()

But a bind is rarely useful because symbol binding from the definition scope is the default.

bind statements only make sense in templates and generics.

# Templates

A template is a simple form of a macro: It is a simple substitution mechanism that operates on Nim's abstract syntax trees. It is processed in the semantic pass of the compiler.

The syntax to invoke a template is the same as calling a procedure.

Example:

template != (a, b: untyped): untyped =
# this definition exists in the System module
not (a == b)

assert(5 != 6) # the compiler rewrites that to: assert(not (5 == 6))

The !=, >, >=, in, notin, isnot operators are in fact templates:

a > b is transformed into b < a.
a in b is transformed into contains(b, a).
notin and isnot have the obvious meanings.

The "types" of templates can be the symbols untyped, typed or typedesc. These are "meta types", they can only be used in certain contexts. Regular types can be used too; this implies that typed expressions are expected.

## Typed vs untyped parameters

An untyped parameter means that symbol lookups and type resolution is not performed before the expression is passed to the template. This means that for example undeclared identifiers can be passed to the template:

template declareInt(x: untyped) =
var x: int

declareInt(x) # valid
x = 3
template declareInt(x: typed) =
var x: int

declareInt(x) # invalid, because x has not been declared and so has no type

A template where every parameter is untyped is called an immediate template. For historical reasons templates can be explicitly annotated with an immediate pragma and then these templates do not take part in overloading resolution and the parameters' types are ignored by the compiler. Explicit immediate templates are now deprecated.

Note: For historical reasons stmt was an alias for typed and expr was an alias for untyped, but they are removed.

## Passing a code block to a template

You can pass a block of statements as the last argument to a template following the special : syntax:

template withFile(f, fn, mode, actions: untyped): untyped =
var f: File
if open(f, fn, mode):
try:
actions
finally:
close(f)
else:
quit("cannot open: " & fn)

withFile(txt, "ttempl3.txt", fmWrite):  # special colon
txt.writeLine("line 1")
txt.writeLine("line 2")

In the example, the two writeLine statements are bound to the actions parameter.

Usually to pass a block of code to a template the parameter that accepts the block needs to be of type untyped. Because symbol lookups are then delayed until template instantiation time:

template t(body: typed) =
block:
body

t:
var i = 1
echo i

t:
var i = 2  # fails with 'attempt to redeclare i'
echo i

The above code fails with the mysterious error message that i has already been declared. The reason for this is that the var i = ... bodies need to be type-checked before they are passed to the body parameter and type checking in Nim implies symbol lookups. For the symbol lookups to succeed i needs to be added to the current (i.e. outer) scope. After type checking these additions to the symbol table are not rolled back (for better or worse). The same code works with untyped as the passed body is not required to be type-checked:

template t(body: untyped) =
block:
body

t:
var i = 1
echo i

t:
var i = 2  # compiles
echo i

## Varargs of untyped

In addition to the untyped meta-type that prevents type checking there is also varargs[untyped] so that not even the number of parameters is fixed:

template hideIdentifiers(x: varargs[untyped]) = discard

hideIdentifiers(undeclared1, undeclared2)

However, since a template cannot iterate over varargs, this feature is generally much more useful for macros.

## Symbol binding in templates

A template is a hygienic macro and so opens a new scope. Most symbols are bound from the definition scope of the template:

# Module A
var
lastId = 0

template genId*: untyped =
inc(lastId)
lastId
# Module B
import A

echo genId() # Works as 'lastId' has been bound in 'genId's defining scope

As in generics symbol binding can be influenced via mixin or bind statements.

## Identifier construction

In templates identifiers can be constructed with the backticks notation:

template typedef(name: untyped, typ: typedesc) =
type
T name* {.inject.} = typ
P name* {.inject.} = ref T name

typedef(myint, int)
var x: PMyInt

In the example name is instantiated with myint, so T name becomes Tmyint.

## Lookup rules for template parameters

A parameter p in a template is even substituted in the expression x.p. Thus template arguments can be used as field names and a global symbol can be shadowed by the same argument name even when fully qualified:

# module 'm'

type
Lev = enum
levA, levB

var abclev = levB

template tstLev(abclev: Lev) =
echo abclev, " ", m.abclev

tstLev(levA)
# produces: 'levA levA'

But the global symbol can properly be captured by a bind statement:

# module 'm'

type
Lev = enum
levA, levB

var abclev = levB

template tstLev(abclev: Lev) =
bind m.abclev
echo abclev, " ", m.abclev

tstLev(levA)
# produces: 'levA levB'

## Hygiene in templates

Per default templates are hygienic: Local identifiers declared in a template cannot be accessed in the instantiation context:

template newException*(exceptn: typedesc, message: string): untyped =
var
e: ref exceptn  # e is implicitly gensym'ed here
new(e)
e.msg = message
e

# so this works:
let e = "message"
raise newException(IoError, e)

Whether a symbol that is declared in a template is exposed to the instantiation scope is controlled by the inject and gensym pragmas: gensym'ed symbols are not exposed but inject'ed are.

The default for symbols of entity type, var, let and const is gensym and for proc, iterator, converter, template, macro is inject. However, if the name of the entity is passed as a template parameter, it is an inject'ed symbol:

template withFile(f, fn, mode: untyped, actions: untyped): untyped =
block:
var f: File  # since 'f' is a template param, it's injected implicitly
...

withFile(txt, "ttempl3.txt", fmWrite):
txt.writeLine("line 1")
txt.writeLine("line 2")

The inject and gensym pragmas are second class annotations; they have no semantics outside of a template definition and cannot be abstracted over:

{.pragma myInject: inject.}

template t() =
var x {.myInject.}: int # does NOT work

To get rid of hygiene in templates, one can use the dirty pragma for a template. inject and gensym have no effect in dirty templates.

## Limitations of the method call syntax

The expression x in x.f needs to be semantically checked (that means symbol lookup and type checking) before it can be decided that it needs to be rewritten to f(x). Therefore the dot syntax has some limitations when it is used to invoke templates/macros:

template declareVar(name: untyped) =
const name {.inject.} = 45

# Doesn't compile:
unknownIdentifier.declareVar

Another common example is this:

from sequtils import toSeq

iterator something: string =
yield "Hello"
yield "World"

var info = something().toSeq

The problem here is that the compiler already decided that something() as an iterator is not callable in this context before toSeq gets its chance to convert it into a sequence.

# Macros

A macro is a special function that is executed at compile time. Normally the input for a macro is an abstract syntax tree (AST) of the code that is passed to it. The macro can then do transformations on it and return the transformed AST. This can be used to add custom language features and implement domain specific languages.

Macro invocation is a case where semantic analyis does not entirely proceed top to bottom and left to right. Instead, semantic analysis happens at least twice:

• Semantic analysis recognizes and resolves the macro invocation.
• The compiler executes the macro body (which may invoke other procs).
• It replaces the AST of the macro invocation with the AST returned by the macro.
• It repeats semantic analysis of that region of the code.
• If the AST returned by the macro contains other macro invocations, this process iterates.

While macros enable advanced compile-time code transformations, they cannot change Nim's syntax. However, this is no real restriction because Nim's syntax is flexible enough anyway.

To write macros, one needs to know how the Nim concrete syntax is converted to an AST.

There are two ways to invoke a macro:

1. invoking a macro like a procedure call (expression macros)
2. invoking a macro with the special macrostmt syntax (statement macros)

## Expression Macros

The following example implements a powerful debug command that accepts a variable number of arguments:

# to work with Nim syntax trees, we need an API that is defined in the
# macros module:
import macros

macro debug(args: varargs[untyped]): untyped =
# args is a collection of NimNode values that each contain the
# AST for an argument of the macro. A macro always has to
# return a NimNode. A node of kind nnkStmtList is suitable for
# this use case.
result = nnkStmtList.newTree()
# iterate over any argument that is passed to this macro:
for n in args:
# add a call to the statement list that writes the expression;
# toStrLit converts an AST to its string representation:
# add a call to the statement list that writes ": "
# add a call to the statement list that writes the expressions value:

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:

write(stdout, "a[0]")
write(stdout, ": ")
writeLine(stdout, a[0])

write(stdout, "a[1]")
write(stdout, ": ")
writeLine(stdout, a[1])

write(stdout, "x")
write(stdout, ": ")
writeLine(stdout, x)

Arguments that are passed to a varargs parameter are wrapped in an array constructor expression. This is why debug iterates over all of n's children.

## BindSym

The above debug macro relies on the fact that write, writeLine and stdout are declared in the system module and thus visible in the instantiating context. There is a way to use bound identifiers (aka symbols) instead of using unbound identifiers. The bindSym builtin can be used for that:

import macros

macro debug(n: varargs[typed]): untyped =
result = newNimNode(nnkStmtList, n)
for x in n:
# we can bind symbols in scope via 'bindSym':

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:

write(stdout, "a[0]")
write(stdout, ": ")
writeLine(stdout, a[0])

write(stdout, "a[1]")
write(stdout, ": ")
writeLine(stdout, a[1])

write(stdout, "x")
write(stdout, ": ")
writeLine(stdout, x)

However, the symbols write, writeLine and stdout are already bound and are not looked up again. As the example shows, bindSym does work with overloaded symbols implicitly.

## Statement Macros

Statement macros are defined just as expression macros. However, they are invoked by an expression following a colon.

The following example outlines a macro that generates a lexical analyzer from regular expressions:

import macros

macro case_token(n: untyped): untyped =
# creates a lexical analyzer from regular expressions
# ... (implementation is an exercise for the reader :-)

case_token: # this colon tells the parser it is a macro statement
of r"[A-Za-z_]+[A-Za-z_0-9]*":
return tkIdentifier
of r"0-9+":
return tkInteger
of r"[\+\-\*\?]+":
return tkOperator
else:
return tkUnknown

Style note: For code readability, it is the best idea to use the least powerful programming construct that still suffices. So the "check list" is:

1. Use an ordinary proc/iterator, if possible.
2. Else: Use a generic proc/iterator, if possible.
3. Else: Use a template, if possible.
4. Else: Use a macro.

## Macros as pragmas

Whole routines (procs, iterators etc.) can also be passed to a template or a macro via the pragma notation:

template m(s: untyped) = discard

proc p() {.m.} = discard

This is a simple syntactic transformation into:

template m(s: untyped) = discard

m:
proc p() = discard

## For loop macros

A macro that takes as its only input parameter an expression of the special type system.ForLoopStmt can rewrite the entirety of a for loop:

import macros
{.experimental: "forLoopMacros".}

macro enumerate(x: ForLoopStmt): untyped =
expectKind x, nnkForStmt
# we strip off the first for loop variable and use
# it as an integer counter:
result = newStmtList()
var body = x[^1]
if body.kind != nnkStmtList:
body = newTree(nnkStmtList, body)
var newFor = newTree(nnkForStmt)
for i in 1..x.len-3:
# transform enumerate(X) to 'X'
# now wrap the whole macro in a block to create a new scope
result = quote do:
block: result

for a, b in enumerate(items([1, 2, 3])):
echo a, " ", b

# without wrapping the macro in a block, we'd need to choose different
# names for a and b here to avoid redefinition errors
for a, b in enumerate([1, 2, 3, 5]):
echo a, " ", b

Currently for loop macros must be enabled explicitly via {.experimental: "forLoopMacros".}.

## Case statement macros

A macro that needs to be called match can be used to rewrite case statements in order to implement pattern matching for certain types. The following example implements a simplistic form of pattern matching for tuples, leveraging the existing equality operator for tuples (as provided in system.==):

{.experimental: "caseStmtMacros".}

import macros

macro match(n: tuple): untyped =
result = newTree(nnkIfStmt)
let selector = n[0]
for i in 1 ..< n.len:
let it = n[i]
case it.kind
of nnkElse, nnkElifBranch, nnkElifExpr, nnkElseExpr:
of nnkOfBranch:
for j in 0..it.len-2:
let cond = newCall("==", selector, it[j])
else:
error "'match' cannot handle this node", it
echo repr result

case ("foo", 78)
of ("foo", 78): echo "yes"
of ("bar", 88): echo "no"
else: discard

Currently case statement macros must be enabled explicitly via {.experimental: "caseStmtMacros".}.

match macros are subject to overload resolution. First the case's selector expression is used to determine which match macro to call. To this macro is then passed the complete case statement body and the macro is evaluated.

In other words, the macro needs to transform the full case statement but only the statement's selector expression is used to determine which macro to call.

# Special Types

## static[T]

As their name suggests, static parameters must be constant expressions:

proc precompiledRegex(pattern: static string): RegEx =
var res {.global.} = re(pattern)
return res

precompiledRegex("/d+") # Replaces the call with a precompiled
# regex, stored in a global variable

precompiledRegex(paramStr(1)) # Error, command-line options
# are not constant expressions

For the purposes of code generation, all static params are treated as generic params - the proc will be compiled separately for each unique supplied value (or combination of values).

Static params can also appear in the signatures of generic types:

type
Matrix[M,N: static int; T: Number] = array[0..(M*N - 1), T]
# Note how Number is just a type constraint here, while
# static int requires us to supply an int value

AffineTransform2D[T] = Matrix[3, 3, T]
AffineTransform3D[T] = Matrix[4, 4, T]

var m1: AffineTransform3D[float]  # OK
var m2: AffineTransform2D[string] # Error, string is not a Number

Please note that static T is just a syntactic convenience for the underlying generic type static[T]. The type param can be omitted to obtain the type class of all constant expressions. A more specific type class can be created by instantiating static with another type class.

You can force an expression to be evaluated at compile time as a constant expression by coercing it to a corresponding static type:

import math

echo static(fac(5)), " ", static[bool](16.isPowerOfTwo)

The complier will report any failure to evaluate the expression or a possible type mismatch error.

## typedesc[T]

In many contexts, Nim allows you to treat the names of types as regular values. These values exists only during the compilation phase, but since all values must have a type, typedesc is considered their special type.

typedesc acts like a generic type. For instance, the type of the symbol int is typedesc[int]. Just like with regular generic types, when the generic param is ommited, typedesc denotes the type class of all types. As a syntactic convenience, you can also use typedesc as a modifier.

Procs featuring typedesc params are considered implicitly generic. They will be instantiated for each unique combination of supplied types and within the body of the proc, the name of each param will refer to the bound concrete type:

proc new(T: typedesc): ref T =
echo "allocating ", T.name
new(result)

var n = Node.new
var tree = new(BinaryTree[int])

When multiple type params are present, they will bind freely to different types. To force a bind-once behavior one can use an explicit generic param:

proc acceptOnlyTypePairs[T, U](A, B: typedesc[T]; C, D: typedesc[U])

Once bound, type params can appear in the rest of the proc signature:

template declareVariableWithType(T: typedesc, value: T) =
var x: T = value

declareVariableWithType int, 42

Overload resolution can be further influenced by constraining the set of types that will match the type param:

template maxval(T: typedesc[int]): int = high(int)
template maxval(T: typedesc[float]): float = Inf

var i = int.maxval
var f = float.maxval
when false:
var s = string.maxval # error, maxval is not implemented for string

The constraint can be a concrete type or a type class.

## typeof operator

Note: typeof(x) can also be written as type(x) but type(x) is discouraged.

You can obtain the type of a given expression by constructing a typeof value from it (in many other languages this is known as the typeof operator):

var x = 0
var y: typeof(x) # y has type int

If typeof is used to determine the result type of a proc/iterator/converter call c(X) (where X stands for a possibly empty list of arguments), the interpretation where c is an iterator is preferred over the other interpretations, but this behavior can be changed by passing typeOfProc as the second argument to typeof:

iterator split(s: string): string = discard
proc split(s: string): seq[string] = discard

# since an iterator is the preferred interpretation, y has the type string:
assert typeof("a b c".split) is string

assert typeof("a b c".split, typeOfProc) is seq[string]

# Modules

Nim supports splitting a program into pieces by a module concept. Each module needs to be in its own file and has its own namespace. Modules enable information hiding and separate compilation. A module may gain access to symbols of another module by the import statement. Recursive module dependencies are allowed, but slightly subtle. Only top-level symbols that are marked with an asterisk (*) are exported. A valid module name can only be a valid Nim identifier (and thus its filename is identifier.nim).

The algorithm for compiling modules is:

• compile the whole module as usual, following import statements recursively
• if there is a cycle only import the already parsed symbols (that are exported); if an unknown identifier occurs then abort

This is best illustrated by an example:

# 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()
# Module B
import A  # A is not parsed here! Only the already known symbols
# of A are imported.

proc p*(x: A.T1): A.T1 =
# this works because the compiler has already
# added T1 to A's interface symbol table
result = x + 1

### Import statement

After the import statement a list of module names can follow or a single module name followed by an except list to prevent some symbols to be imported:

import strutils except %, toUpperAscii

# doesn't work then:
echo "$1" % "abc".toUpperAscii It is not checked that the except list is really exported from the module. This feature allows to compile against an older version of the module that does not export these identifiers. ### Include statement The include statement does something fundamentally different than importing a module: it merely includes the contents of a file. The include statement is useful to split up a large module into several files: include fileA, fileB, fileC ### Module names in imports A module alias can be introduced via the as keyword: import strutils as su, sequtils as qu echo su.format("$1", "lalelu")

The original module name is then not accessible. The notations path/to/module or "path/to/module" can be used to refer to a module in subdirectories:

import lib/pure/os, "lib/pure/times"

Note that the module name is still strutils and not lib/pure/strutils and so one cannot do:

import lib/pure/strutils
echo lib/pure/strutils.toUpperAscii("abc")

Likewise the following does not make sense as the name is strutils already:

import lib/pure/strutils as strutils

### Collective imports from a directory

The syntax import dir / [moduleA, moduleB] can be used to import multiple modules from the same directory.

Path names are syntactically either Nim identifiers or string literals. If the path name is not a valid Nim identifier it needs to be a string literal:

import "gfx/3d/somemodule" # in quotes because '3d' is not a valid Nim identifier

### Pseudo import/include paths

A directory can also be a so called "pseudo directory". They can be used to avoid ambiguity when there are multiple modules with the same path.

There are two pseudo directories:

1. std: The std pseudo directory is the abstract location of Nim's standard library. For example, the syntax import std / strutils is used to unambiguously refer to the standard library's strutils module.

2. pkg: The pkg pseudo directory is used to unambiguously refer to a Nimble package. However, for technical details that lie outside of the scope of this document its semantics are: Use the search path to look for module name but ignore the standard library locations. In other words, it is the opposite of std.

### From import statement

After the from statement a module name follows followed by an import to list the symbols one likes to use without explicit full qualification:

from strutils import %

echo "$1" % "abc" # always possible: full qualification: echo strutils.replace("abc", "a", "z") It's also possible to use from module import nil if one wants to import the module but wants to enforce fully qualified access to every symbol in module. ### Export statement An export statement can be used for symbol forwarding so that client modules don't need to import a module's dependencies: # module B type MyObject* = object # module A import B export B.MyObject proc $*(x: MyObject): string = "my object"
# module C
import A

# B.MyObject has been imported implicitly here:
var x: MyObject
echo $x When the exported symbol is another module, all of its definitions will be forwarded. You can use an except list to exclude some of the symbols. Notice that when exporting, you need to specify only the module name: import foo/bar/baz export baz ## Scope rules Identifiers are valid from the point of their declaration until the end of the block in which the declaration occurred. The range where the identifier is known is the scope of the identifier. The exact scope of an identifier depends on the way it was declared. ### Block scope The scope of a variable declared in the declaration part of a block is valid from the point of declaration until the end of the block. If a block contains a second block, in which the identifier is redeclared, then inside this block, the second declaration will be valid. Upon leaving the inner block, the first declaration is valid again. An identifier cannot be redefined in the same block, except if valid for procedure or iterator overloading purposes. ### Tuple or object scope The field identifiers inside a tuple or object definition are valid in the following places: • To the end of the tuple/object definition. • Field designators of a variable of the given tuple/object type. • In all descendant types of the object type. ### Module scope All identifiers of a module are valid from the point of declaration until the end of the module. Identifiers from indirectly dependent modules are not available. The system module is automatically imported in every module. If a module imports an identifier by two different modules, each occurrence of the identifier has to be qualified, unless it is an overloaded procedure or iterator in which case the overloading resolution takes place: # Module A var x*: string # Module B var x*: int # 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 ### Code reordering Note: Code reordering is experimental and must be enabled via the {.experimental.} pragma. The code reordering feature can implicitly rearrange procedure, template, and macro definitions along with variable declarations and initializations at the top level scope so that, to a large extent, a programmer should not have to worry about ordering definitions correctly or be forced to use forward declarations to preface definitions inside a module. Example: {.experimental: "codeReordering".} proc foo(x: int) = bar(x) proc bar(x: int) = echo(x) foo(10) Variables can also be reordered as well. Variables that are initialized (i.e. variables that have their declaration and assignment combined in a single statement) can have their entire initialization statement reordered. Be wary of what code is executed at the top level: {.experimental: "codeReordering".} proc a() = echo(foo) var foo = 5 a() # outputs: "5" It is important to note that reordering only works for symbols at top level scope. Therefore, the following will fail to compile: {.experimental: "codeReordering".} proc a() = b() proc b() = echo("Hello!") a() # Compiler Messages The Nim compiler emits different kinds of messages: hint, warning, and error messages. An error message is emitted if the compiler encounters any static error. # Pragmas Pragmas are Nim's method to give the compiler additional information / commands without introducing a massive number of new keywords. Pragmas are processed on the fly during semantic checking. Pragmas are enclosed in the special {. and .} curly brackets. Pragmas are also often used as a first implementation to play with a language feature before a nicer syntax to access the feature becomes available. ## deprecated pragma The deprecated pragma is used to mark a symbol as deprecated: proc p() {.deprecated.} var x {.deprecated.}: char This pragma can also take in an optional warning string to relay to developers. proc thing(x: bool) {.deprecated: "use thong instead".} ## noSideEffect pragma The noSideEffect pragma is used to mark a proc/iterator to have no side effects. This means that the proc/iterator only changes locations that are reachable from its parameters and the return value only depends on the arguments. If none of its parameters have the type var T or ref T or ptr T this means no locations are modified. It is a static error to mark a proc/iterator to have no side effect if the compiler cannot verify this. As a special semantic rule, the built-in debugEcho pretends to be free of side effects, so that it can be used for debugging routines marked as noSideEffect. func is syntactic sugar for a proc with no side effects: func + (x, y: int): int To override the compiler's side effect analysis a {.noSideEffect.} pragma block can be used: func f() = {.noSideEffect.}: echo "test" ## compileTime pragma The compileTime pragma is used to mark a proc or variable to be used only during compile-time execution. No code will be generated for it. Compile-time procs are useful as helpers for macros. Since version 0.12.0 of the language, a proc that uses system.NimNode within its parameter types is implicitly declared compileTime: proc astHelper(n: NimNode): NimNode = result = n Is the same as: proc astHelper(n: NimNode): NimNode {.compileTime.} = result = n ## noReturn pragma The noreturn pragma is used to mark a proc that never returns. ## acyclic pragma The acyclic pragma applies to type declarations. It is deprecated and ignored. ## final pragma The final pragma can be used for an object type to specify that it cannot be inherited from. Note that inheritance is only available for objects that inherit from an existing object (via the object of SuperType syntax) or that have been marked as inheritable. ## shallow pragma The shallow pragma affects the semantics of a type: The compiler is allowed to make a shallow copy. This can cause serious semantic issues and break memory safety! However, it can speed up assignments considerably, because the semantics of Nim require deep copying of sequences and strings. This can be expensive, especially if sequences are used to build a tree structure: type NodeKind = enum nkLeaf, nkInner Node {.shallow.} = object case kind: NodeKind of nkLeaf: strVal: string of nkInner: children: seq[Node] ## pure pragma An object type can be marked with the pure pragma so that its type field which is used for runtime type identification is omitted. This used to be necessary for binary compatibility with other compiled languages. An enum type can be marked as pure. Then access of its fields always requires full qualification. ## asmNoStackFrame pragma A proc can be marked with the asmNoStackFrame pragma to tell the compiler it should not generate a stack frame for the proc. There are also no exit statements like return result; generated and the generated C function is declared as __declspec(naked) or __attribute__((naked)) (depending on the used C compiler). Note: This pragma should only be used by procs which consist solely of assembler statements. ## error pragma The error pragma is used to make the compiler output an error message with the given content. Compilation does not necessarily abort after an error though. The error pragma can also be used to annotate a symbol (like an iterator or proc). The usage of the symbol then triggers a static error. This is especially useful to rule out that some operation is valid due to overloading and type conversions: ## check that underlying int values are compared and not the pointers: proc ==(x, y: ptr int): bool {.error.} ## fatal pragma The fatal pragma is used to make the compiler output an error message with the given content. In contrast to the error pragma, compilation is guaranteed to be aborted by this pragma. Example: when not defined(objc): {.fatal: "Compile this program with the objc command!".} ## warning pragma The warning pragma is used to make the compiler output a warning message with the given content. Compilation continues after the warning. ## hint pragma The hint pragma is used to make the compiler output a hint message with the given content. Compilation continues after the hint. ## line pragma The line pragma can be used to affect line information of the annotated statement as seen in stack backtraces: template myassert*(cond: untyped, msg = "") = if not cond: # change run-time line information of the 'raise' statement: {.line: instantiationInfo().}: raise newException(EAssertionFailed, msg) If the line pragma is used with a parameter, the parameter needs be a tuple[filename: string, line: int]. If it is used without a parameter, system.InstantiationInfo() is used. ## linearScanEnd pragma The linearScanEnd pragma can be used to tell the compiler how to compile a Nim case statement. Syntactically it has to be used as a statement: case myInt of 0: echo "most common case" of 1: {.linearScanEnd.} echo "second most common case" of 2: echo "unlikely: use branch table" else: echo "unlikely too: use branch table for ", myInt In the example, the case branches 0 and 1 are much more common than the other cases. Therefore the generated assembler code should test for these values first, so that the CPU's branch predictor has a good chance to succeed (avoiding an expensive CPU pipeline stall). The other cases might be put into a jump table for O(1) overhead, but at the cost of a (very likely) pipeline stall. The linearScanEnd pragma should be put into the last branch that should be tested against via linear scanning. If put into the last branch of the whole case statement, the whole case statement uses linear scanning. ## computedGoto pragma The computedGoto pragma can be used to tell the compiler how to compile a Nim case in a while true statement. Syntactically it has to be used as a statement inside the loop: type MyEnum = enum enumA, enumB, enumC, enumD, enumE proc vm() = var instructions: array[0..100, MyEnum] instructions[2] = enumC instructions[3] = enumD instructions[4] = enumA instructions[5] = enumD instructions[6] = enumC instructions[7] = enumA instructions[8] = enumB instructions[12] = enumE var pc = 0 while true: {.computedGoto.} let instr = instructions[pc] case instr of enumA: echo "yeah A" of enumC, enumD: echo "yeah CD" of enumB: echo "yeah B" of enumE: break inc(pc) vm() As the example shows computedGoto is mostly useful for interpreters. If the underlying backend (C compiler) does not support the computed goto extension the pragma is simply ignored. ## unroll pragma The unroll pragma can be used to tell the compiler that it should unroll a for or while loop for execution efficiency: proc searchChar(s: string, c: char): int = for i in 0 .. s.high: {.unroll: 4.} if s[i] == c: return i result = -1 In the above example, the search loop is unrolled by a factor 4. The unroll factor can be left out too; the compiler then chooses an appropriate unroll factor. Note: Currently the compiler recognizes but ignores this pragma. ## immediate pragma The immediate pragma is obsolete. See Typed vs untyped parameters. ## compilation option pragmas The listed pragmas here can be used to override the code generation options for a proc/method/converter. The implementation currently provides the following possible options (various others may be added later). pragmaallowed valuesdescription checkson|off Turns the code generation for all runtime checks on or off. boundCheckson|off Turns the code generation for array bound checks on or off. overflowCheckson|off Turns the code generation for over- or underflow checks on or off. nilCheckson|off Turns the code generation for nil pointer checks on or off. assertionson|off Turns the code generation for assertions on or off. warningson|off Turns the warning messages of the compiler on or off. hintson|off Turns the hint messages of the compiler on or off. optimizationnone|speed|size Optimize the code for speed or size, or disable optimization. patternson|off Turns the term rewriting templates/macros on or off. callconvcdecl|...Specifies the default calling convention for all procedures (and procedure types) that follow. Example: {.checks: off, optimization: speed.} # compile without runtime checks and optimize for speed ## push and pop pragmas The push/pop pragmas are very similar to the option directive, but are used to override the settings temporarily. Example: {.push checks: off.} # compile this section without runtime checks as it is # speed critical # ... some code ... {.pop.} # restore old settings ## register pragma The register pragma is for variables only. It declares the variable as register, giving the compiler a hint that the variable should be placed in a hardware register for faster access. C compilers usually ignore this though and for good reasons: Often they do a better job without it anyway. In highly specific cases (a dispatch loop of a bytecode interpreter for example) it may provide benefits, though. ## global pragma The global pragma can be applied to a variable within a proc to instruct the compiler to store it in a global location and initialize it once at program startup. proc isHexNumber(s: string): bool = var pattern {.global.} = re"[0-9a-fA-F]+" result = s.match(pattern) When used within a generic proc, a separate unique global variable will be created for each instantiation of the proc. The order of initialization of the created global variables within a module is not defined, but all of them will be initialized after any top-level variables in their originating module and before any variable in a module that imports it. ## pragma pragma The pragma pragma can be used to declare user defined pragmas. This is useful because Nim's templates and macros do not affect pragmas. User defined pragmas are in a different module-wide scope than all other symbols. They cannot be imported from a module. Example: when appType == "lib": {.pragma: rtl, exportc, dynlib, cdecl.} else: {.pragma: rtl, importc, dynlib: "client.dll", cdecl.} proc p*(a, b: int): int {.rtl.} = result = a+b In the example a new pragma named rtl is introduced that either imports a symbol from a dynamic library or exports the symbol for dynamic library generation. ## Disabling certain messages Nim generates some warnings and hints ("line too long") that may annoy the user. A mechanism for disabling certain messages is provided: Each hint and warning message contains a symbol in brackets. This is the message's identifier that can be used to enable or disable it: {.hint[LineTooLong]: off.} # turn off the hint about too long lines This is often better than disabling all warnings at once. ## used pragma Nim produces a warning for symbols that are not exported and not used either. The used pragma can be attached to a symbol to suppress this warning. This is particularly useful when the symbol was generated by a macro: template implementArithOps(T) = proc echoAdd(a, b: T) {.used.} = echo a + b proc echoSub(a, b: T) {.used.} = echo a - b # no warning produced for the unused 'echoSub' implementArithOps(int) echoAdd 3, 5 ## experimental pragma The experimental pragma enables experimental language features. Depending on the concrete feature this means that the feature is either considered too unstable for an otherwise stable release or that the future of the feature is uncertain (it may be removed any time). Example: {.experimental: "parallel".} proc useParallel() = parallel: for i in 0..4: echo "echo in parallel" As a top level statement, the experimental pragma enables a feature for the rest of the module it's enabled in. This is problematic for macro and generic instantiations that cross a module scope. Currently these usages have to be put into a .push/pop environment: # client.nim proc useParallel*[T](unused: T) = # use a generic T here to show the problem. {.push experimental: "parallel".} parallel: for i in 0..4: echo "echo in parallel" {.pop.} import client useParallel(1) # Implementation Specific Pragmas This section describes additional pragmas that the current Nim implementation supports but which should not be seen as part of the language specification. ## Bitsize pragma The bitsize pragma is for object field members. It declares the field as a bitfield in C/C++. type mybitfield = object flag {.bitsize:1.}: cuint generates: struct mybitfield { unsigned int flag:1; }; ## Volatile pragma The volatile pragma is for variables only. It declares the variable as volatile, whatever that means in C/C++ (its semantics are not well defined in C/C++). Note: This pragma will not exist for the LLVM backend. ## NoDecl pragma The noDecl pragma can be applied to almost any symbol (variable, proc, type, etc.) and is sometimes useful for interoperability with C: It tells Nim that it should not generate a declaration for the symbol in the C code. For example: var EACCES {.importc, noDecl.}: cint # pretend EACCES was a variable, as # Nim does not know its value However, the header pragma is often the better alternative. Note: This will not work for the LLVM backend. ## Header pragma The header pragma is very similar to the noDecl pragma: It can be applied to almost any symbol and specifies that it should not be declared and instead the generated code should contain an #include: type PFile {.importc: "FILE*", header: "<stdio.h>".} = distinct pointer # import C's FILE* type; Nim will treat it as a new pointer type The header pragma always expects a string constant. The string contant contains the header file: As usual for C, a system header file is enclosed in angle brackets: <>. If no angle brackets are given, Nim encloses the header file in "" in the generated C code. Note: This will not work for the LLVM backend. ## IncompleteStruct pragma The incompleteStruct pragma tells the compiler to not use the underlying C struct in a sizeof expression: type DIR* {.importc: "DIR", header: "<dirent.h>", pure, incompleteStruct.} = object ## Compile pragma The compile pragma can be used to compile and link a C/C++ source file with the project: {.compile: "myfile.cpp".} Note: Nim computes a SHA1 checksum and only recompiles the file if it has changed. You can use the -f command line option to force recompilation of the file. ## Link pragma The link pragma can be used to link an additional file with the project: {.link: "myfile.o".} ## PassC pragma The passC pragma can be used to pass additional parameters to the C compiler like you would using the commandline switch --passC: {.passC: "-Wall -Werror".} Note that you can use gorge from the system module to embed parameters from an external command that will be executed during semantic analysis: {.passC: gorge("pkg-config --cflags sdl").} ## PassL pragma The passL pragma can be used to pass additional parameters to the linker like you would using the commandline switch --passL: {.passL: "-lSDLmain -lSDL".} Note that you can use gorge from the system module to embed parameters from an external command that will be executed during semantic analysis: {.passL: gorge("pkg-config --libs sdl").} ## Emit pragma The emit pragma can be used to directly affect the output of the compiler's code generator. So it makes your code unportable to other code generators/backends. Its usage is highly discouraged! However, it can be extremely useful for interfacing with C++ or Objective C code. Example: {.emit: """ static int cvariable = 420; """.} {.push stackTrace:off.} proc embedsC() = var nimVar = 89 # access Nim symbols within an emit section outside of string literals: {.emit: ["""fprintf(stdout, "%d\n", cvariable + (int)""", nimVar, ");"].} {.pop.} embedsC() nimbase.h defines NIM_EXTERNC C macro that can be used for extern "C" code to work with both nim c and nim cpp, eg: proc foobar() {.importc:"$1".}
{.emit: """
#include <stdio.h>
NIM_EXTERNC
void fun(){}
""".}

For backwards compatibility, if the argument to the emit statement is a single string literal, Nim symbols can be referred to via backticks. This usage is however deprecated.

For a toplevel emit statement the section where in the generated C/C++ file the code should be emitted can be influenced via the prefixes /*TYPESECTION*/ or /*VARSECTION*/ or /*INCLUDESECTION*/:

{.emit: """/*TYPESECTION*/
struct Vector3 {
public:
Vector3(): x(5) {}
Vector3(float x_): x(x_) {}
float x;
};
""".}

type Vector3 {.importcpp: "Vector3", nodecl} = object
x: cfloat

proc constructVector3(a: cfloat): Vector3 {.importcpp: "Vector3(@)", nodecl}

## ImportCpp pragma

Note: c2nim can parse a large subset of C++ and knows about the importcpp pragma pattern language. It is not necessary to know all the details described here.

Similar to the importc pragma for C, the importcpp pragma can be used to import C++ methods or C++ symbols in general. The generated code then uses the C++ method calling syntax: obj->method(arg). In combination with the header and emit pragmas this allows sloppy interfacing with libraries written in C++:

# Horrible example of how to interface with a C++ engine ... ;-)

{.emit: """
using namespace irr;
using namespace core;
using namespace scene;
using namespace video;
using namespace io;
using namespace gui;
""".}

const
irr = "<irrlicht/irrlicht.h>"

type
importcpp: "IrrlichtDevice".} = object
IrrlichtDevice = ptr IrrlichtDeviceObj

proc createDevice(): IrrlichtDevice {.
proc run(device: IrrlichtDevice): bool {.
header: irr, importcpp: "#.run(@)".}

The compiler needs to be told to generate C++ (command cpp) for this to work. The conditional symbol cpp is defined when the compiler emits C++ code.

### Namespaces

The sloppy interfacing example uses .emit to produce using namespace declarations. It is usually much better to instead refer to the imported name via the namespace::identifier notation:

type
importcpp: "irr::IrrlichtDevice".} = object

### Importcpp for enums

When importcpp is applied to an enum type the numerical enum values are annotated with the C++ enum type, like in this example: ((TheCppEnum)(3)). (This turned out to be the simplest way to implement it.)

### Importcpp for procs

Note that the importcpp variant for procs uses a somewhat cryptic pattern language for maximum flexibility:

• A hash # symbol is replaced by the first or next argument.
• A dot following the hash #. indicates that the call should use C++'s dot or arrow notation.
• An at symbol @ is replaced by the remaining arguments, separated by commas.

For example:

proc cppMethod(this: CppObj, a, b, c: cint) {.importcpp: "#.CppMethod(@)".}
var x: ptr CppObj
cppMethod(x[], 1, 2, 3)

Produces:

x->CppMethod(1, 2, 3)

As a special rule to keep backwards compatibility with older versions of the importcpp pragma, if there is no special pattern character (any of # ' @) at all, C++'s dot or arrow notation is assumed, so the above example can also be written as:

proc cppMethod(this: CppObj, a, b, c: cint) {.importcpp: "CppMethod".}

Note that the pattern language naturally also covers C++'s operator overloading capabilities:

proc vectorAddition(a, b: Vec3): Vec3 {.importcpp: "# + #".}
proc dictLookup(a: Dict, k: Key): Value {.importcpp: "#[#]".}
• An apostrophe ' followed by an integer i in the range 0..9 is replaced by the i'th parameter type. The 0th position is the result type. This can be used to pass types to C++ function templates. Between the ' and the digit an asterisk can be used to get to the base type of the type. (So it "takes away a star" from the type; T* becomes T.) Two stars can be used to get to the element type of the element type etc.

For example:

type Input {.importcpp: "System::Input".} = object
proc getSubsystem*[T](): ptr T {.importcpp: "SystemManager::getSubsystem<'*0>()", nodecl.}

let x: ptr Input = getSubsystem[Input]()

Produces:

x = SystemManager::getSubsystem<System::Input>()
• #@ is a special case to support a cnew operation. It is required so that the call expression is inlined directly, without going through a temporary location. This is only required to circumvent a limitation of the current code generator.

For example C++'s new operator can be "imported" like this:

proc cnew*[T](x: T): ptr T {.importcpp: "(new '*0#@)", nodecl.}

# constructor of 'Foo':
proc constructFoo(a, b: cint): Foo {.importcpp: "Foo(@)".}

let x = cnew constructFoo(3, 4)

Produces:

x = new Foo(3, 4)

However, depending on the use case new Foo can also be wrapped like this instead:

proc newFoo(a, b: cint): ptr Foo {.importcpp: "new Foo(@)".}

let x = newFoo(3, 4)

### Wrapping constructors

Sometimes a C++ class has a private copy constructor and so code like Class c = Class(1,2); must not be generated but instead Class c(1,2);. For this purpose the Nim proc that wraps a C++ constructor needs to be annotated with the constructor pragma. This pragma also helps to generate faster C++ code since construction then doesn't invoke the copy constructor:

# a better constructor of 'Foo':
proc constructFoo(a, b: cint): Foo {.importcpp: "Foo(@)", constructor.}

### Wrapping destructors

Since Nim generates C++ directly, any destructor is called implicitly by the C++ compiler at the scope exits. This means that often one can get away with not wrapping the destructor at all! However when it needs to be invoked explicitly, it needs to be wrapped. The pattern language provides everything that is required:

proc destroyFoo(this: var Foo) {.importcpp: "#.~Foo()".}

### Importcpp for objects

Generic importcpp'ed objects are mapped to C++ templates. This means that you can import C++'s templates rather easily without the need for a pattern language for object types:

type
StdMap {.importcpp: "std::map", header: "<map>".} [K, V] = object
proc []=[K, V](this: var StdMap[K, V]; key: K; val: V) {.
importcpp: "#[#] = #", header: "<map>".}

var x: StdMap[cint, cdouble]
x[6] = 91.4

Produces:

std::map<int, double> x;
x[6] = 91.4;
• If more precise control is needed, the apostrophe ' can be used in the supplied pattern to denote the concrete type parameters of the generic type. See the usage of the apostrophe operator in proc patterns for more details.
type
VectorIterator {.importcpp: "std::vector<'0>::iterator".} [T] = object

var x: VectorIterator[cint]

Produces:

std::vector<int>::iterator x;

## ImportObjC pragma

Similar to the importc pragma for C, the importobjc pragma can be used to import Objective C methods. The generated code then uses the Objective C method calling syntax: [obj method param1: arg]. In addition with the header and emit pragmas this allows sloppy interfacing with libraries written in Objective C:

# horrible example of how to interface with GNUStep ...

{.passL: "-lobjc".}
{.emit: """
#include <objc/Object.h>
@interface Greeter:Object
{
}

- (void)greet:(long)x y:(long)dummy;
@end

#include <stdio.h>
@implementation Greeter

- (void)greet:(long)x y:(long)dummy
{
printf("Hello, World!\n");
}
@end

#include <stdlib.h>
""".}

type
Id {.importc: "id", header: "<objc/Object.h>", final.} = distinct int

proc newGreeter: Id {.importobjc: "Greeter new", nodecl.}
proc greet(self: Id, x, y: int) {.importobjc: "greet", nodecl.}
proc free(self: Id) {.importobjc: "free", nodecl.}

var g = newGreeter()
g.greet(12, 34)
g.free()

The compiler needs to be told to generate Objective C (command objc) for this to work. The conditional symbol objc is defined when the compiler emits Objective C code.

## CodegenDecl pragma

The codegenDecl pragma can be used to directly influence Nim's code generator. It receives a format string that determines how the variable or proc is declared in the generated code.

For variables $1 in the format string represents the type of the variable and$2 is the name of the variable.

The following Nim code:

var
a {.codegenDecl: "$# progmem$#".}: int

will generate this C code:

int progmem a

For procedures $1 is the return type of the procedure,$2 is the name of the procedure and $3 is the parameter list. The following nim code: proc myinterrupt() {.codegenDecl: "__interrupt$# $#$#".} =
echo "realistic interrupt handler"

will generate this code:

__interrupt void myinterrupt()

## InjectStmt pragma

The injectStmt pragma can be used to inject a statement before every other statement in the current module. It is only supposed to be used for debugging:

{.injectStmt: gcInvariants().}

# ... complex code here that produces crashes ...

## compile time define pragmas

The pragmas listed here can be used to optionally accept values from the -d/--define option at compile time.

The implementation currently provides the following possible options (various others may be added later).

intdefineReads in a build-time define as an integer
strdefineReads in a build-time define as a string
booldefineReads in a build-time define as a bool
const FooBar {.intdefine.}: int = 5
echo FooBar
nim c -d:FooBar=42 foobar.nim

In the above example, providing the -d flag causes the symbol FooBar to be overwritten at compile time, printing out 42. If the -d:FooBar=42 were to be omitted, the default value of 5 would be used. To see if a value was provided, defined(FooBar) can be used.

The syntax -d:flag is actually just a shortcut for -d:flag=true.

## Custom annotations

It is possible to define custom typed pragmas. Custom pragmas do not effect code generation directly, but their presence can be detected by macros. Custom pragmas are defined using templates annotated with pragma pragma:

template dbTable(name: string, table_space: string = "") {.pragma.}
template dbKey(name: string = "", primary_key: bool = false) {.pragma.}
template dbForeignKey(t: typedesc) {.pragma.}
template dbIgnore {.pragma.}

Consider stylized example of possible Object Relation Mapping (ORM) implementation:

const tblspace {.strdefine.} = "dev" # switch for dev, test and prod environments

type
User {.dbTable("users", tblspace).} = object
id {.dbKey(primary_key = true).}: int
name {.dbKey"full_name".}: string
is_cached {.dbIgnore.}: bool
age: int

UserProfile {.dbTable("profiles", tblspace).} = object
id {.dbKey(primary_key = true).}: int
user_id {.dbForeignKey: User.}: int
write_access: bool
admin_acess: bool

In this example custom pragmas are used to describe how Nim objects are mapped to the schema of the relational database. Custom pragmas can have zero or more arguments. In order to pass multiple arguments use one of template call syntaxes. All arguments are typed and follow standard overload resolution rules for templates. Therefore, it is possible to have default values for arguments, pass by name, varargs, etc.

Custom pragmas can be used in all locations where ordinary pragmas can be specified. It is possible to annotate procs, templates, type and variable definitions, statements, etc.

Macros module includes helpers which can be used to simplify custom pragma access hasCustomPragma, getCustomPragmaVal. Please consult macros module documentation for details. These macros are no magic, they don't do anything you cannot do yourself by walking AST object representation.

More examples with custom pragmas:

• Better serialization/deserialization control:
type MyObj = object
a {.dontSerialize.}: int
b {.defaultDeserialize: 5.}: int
c {.serializationKey: "_c".}: string
• Adopting type for gui inspector in a game engine:
type MyComponent = object
position {.editable, animatable.}: Vector3
alpha {.editRange: [0.0..1.0], animatable.}: float32

# Foreign function interface

Nim's FFI (foreign function interface) is extensive and only the parts that scale to other future backends (like the LLVM/JavaScript backends) are documented here.

## Importc pragma

The importc pragma provides a means to import a proc or a variable from C. The optional argument is a string containing the C identifier. If the argument is missing, the C name is the Nim identifier exactly as spelled:

proc printf(formatstr: cstring) {.header: "<stdio.h>", importc: "printf", varargs.}

Note that this pragma is somewhat of a misnomer: Other backends do provide the same feature under the same name. Also, if one is interfacing with C++ the ImportCpp pragma and interfacing with Objective-C the ImportObjC pragma can be used.

The string literal passed to importc can be a format string:

## Extern pragma

Like exportc or importc, the extern pragma affects name mangling. The string literal passed to extern can be a format string:

proc p(s: string) {.extern: "prefix$1".} = echo s In the example the external name of p is set to prefixp. Only$1 is available and a literal dollar sign must be written as .

## Bycopy pragma

The bycopy pragma can be applied to an object or tuple type and instructs the compiler to pass the type by value to procs:

type
Vector {.bycopy.} = object
x, y, z: float

## Byref pragma

The byref pragma can be applied to an object or tuple type and instructs the compiler to pass the type by reference (hidden pointer) to procs.

## Varargs pragma

The varargs pragma can be applied to procedures only (and procedure types). It tells Nim that the proc can take a variable number of parameters after the last specified parameter. Nim string values will be converted to C strings automatically:

proc printf(formatstr: cstring) {.nodecl, varargs.}

printf("hallo %s", "world") # "world" will be passed as C string

## Union pragma

The union pragma can be applied to any object type. It means all of the object's fields are overlaid in memory. This produces a union instead of a struct in the generated C/C++ code. The object declaration then must not use inheritance or any GC'ed memory but this is currently not checked.

Future directions: GC'ed memory should be allowed in unions and the GC should scan unions conservatively.

## Packed pragma

The packed pragma can be applied to any object type. It ensures that the fields of an object are packed back-to-back in memory. It is useful to store packets or messages from/to network or hardware drivers, and for interoperability with C. Combining packed pragma with inheritance is not defined, and it should not be used with GC'ed memory (ref's).

Future directions: Using GC'ed memory in packed pragma will result in a static error. Usage with inheritance should be defined and documented.

## Dynlib pragma for import

With the dynlib pragma a procedure or a variable can be imported from a dynamic library (.dll files for Windows, lib*.so files for UNIX). The non-optional argument has to be the name of the dynamic library:

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:

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:

import os

proc getDllName: string =
result = "mylib.dll"
if existsFile(result): return
result = "mylib2.dll"
if existsFile(result): return

proc myImport(s: cstring) {.cdecl, importc, dynlib: getDllName().}

Note: Patterns like libtcl(|8.5|8.4).so are only supported in constant strings, because they are precompiled.

Note: Passing variables to the dynlib pragma will fail at runtime because of order of initialization problems.

Note: A dynlib import can be overridden with the --dynlibOverride:name command line option. The Compiler User Guide contains further information.

## Dynlib pragma for export

With the dynlib pragma a procedure can also be exported to a dynamic library. The pragma then has no argument and has to be used in conjunction with the exportc pragma:

proc exportme(): int {.cdecl, exportc, dynlib.}

This is only useful if the program is compiled as a dynamic library via the --app:lib command line option. This pragma only has an effect for the code generation on the Windows target, so when this pragma is forgotten and the dynamic library is only tested on Mac and/or Linux, there won't be an error. On Windows this pragma adds __declspec(dllexport) to the function declaration.

To enable thread support the --threads:on command line switch needs to be used. The system module then contains several threading primitives. See the threads and channels modules for the low level thread API. There are also high level parallelism constructs available. See spawn for further details.

Nim's memory model for threads is quite different than that of other common programming languages (C, Pascal, Java): Each thread has its own (garbage collected) heap and sharing of memory is restricted to global variables. This helps to prevent race conditions. GC efficiency is improved quite a lot, because the GC never has to stop other threads and see what they reference.

A proc that is executed as a new thread of execution should be marked by the thread pragma for reasons of readability. The compiler checks for violations of the no heap sharing restriction: This restriction implies that it is invalid to construct a data structure that consists of memory allocated from different (thread local) heaps.

A thread proc is passed to createThread or spawn and invoked indirectly; so the thread pragma implies procvar.

## GC safety

We call a proc p GC safe when it doesn't access any global variable that contains GC'ed memory (string, seq, ref or a closure) either directly or indirectly through a call to a GC unsafe proc.

The gcsafe annotation can be used to mark a proc to be gcsafe, otherwise this property is inferred by the compiler. Note that noSideEffect implies gcsafe. The only way to create a thread is via spawn or createThread. The invoked proc must not use var parameters nor must any of its parameters contain a ref or closure type. This enforces the no heap sharing restriction.

Routines that are imported from C are always assumed to be gcsafe. To disable the GC-safety checking the --threadAnalysis:off command line switch can be used. This is a temporary workaround to ease the porting effort from old code to the new threading model.

To override the compiler's gcsafety analysis a {.gcsafe.} pragma block can be used:

var
someGlobal: string = "some string here"

{.gcsafe.}:
deepCopy(perThread, someGlobal)

Future directions:

• A shared GC'ed heap might be provided.

var checkpoints* {.threadvar.}: seq[string]`