# Nimrod Manual

Authors: | Andreas Rumpf, Zahary Karadjov
Version: | 0.9.2

> "Complexity" seems to be a lot like "energy": you can transfer it from the end user to one/some of the other players, but the total amount seems to remain pretty much constant for a given task. -- Ran

# About this document

Note: This document is a draft! Several of Nimrod's features need more precise wording. This manual will evolve into a proper specification some day.

This document describes the lexis, the syntax, and the semantics of Nimrod.

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

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

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

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

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

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

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

Other parts of Nimrod - like scoping rules or runtime semantics are only described in an informal manner for now.

# Definitions

A Nimrod program 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. Expressions whose values can be determined statically are called constant expressions; they are never l-values.

A static error is an error that the implementation detects before program execution. Unless explicitly classified, an error is a static error.

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. However, the implementation provides a means to disable these runtime checks. See the section pragmas for details.

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.

# Lexical Analysis

## Encoding

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

## Indentation

Nimrod's standard grammar describes an indentation sensitive 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 preceeding number of spaces; indentation is not a separate token. This trick allows parsing of Nimrod with only 1 token of lookahead.

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

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

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

    simpleStmt = ifStmt / ...

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

## Comments

Comments start anywhere outside a string or character literal with the hash character `#`. Comments consist of a concatenation of comment pieces. A comment piece starts with `#` and runs until the end of the line. The end of line characters belong to the piece. If the next line only consists of a comment piece which is aligned to the preceding one, it does not start a new comment:

    i = 0     # This is a single comment over multiple lines belonging to the
              # assignment statement. The scanner merges these two pieces.
    # This is a new comment belonging to the current block, but to no particular
    # statement.
    i = i + 1 # This a new comment that is NOT
    echo(i)   # continued here, because this comment refers to the echo statement

The alignment requirement does not hold if the preceding comment piece ends in a backslash (followed by optional whitespace):

    type
      TMyObject {.final, pure, acyclic.} = object  # comment continues: \
        # we have lots of space here to comment 'TMyObject'.
        # This line belongs to the comment as it's properly aligned.

Comments are tokens; they are only allowed at certain places in the input file as they belong to the syntax tree! This feature enables perfect source-to-source transformations (such as pretty-printing) and superior documentation generators. A nice side-effect is that the human reader of the code always knows exactly which code snippet the comment refers to.

## Identifiers & Keywords

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

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

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 atomic
    bind block break
    case cast const continue converter
    discard distinct div do
    elif else end enum except export
    finally for from
    generic
    if import in include interface is isnot iterator
    lambda let
    macro method mixin mod
    nil not notin
    object of or out
    proc ptr
    raise ref return
    shared shl shr static
    template try tuple type
    var
    when while with without
    xor
    yield

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

Nimrod is a style-insensitive language. This means that it is not case-sensitive and even underscores are ignored: type is a reserved word, and so is TYPE or T_Y_P_E. The idea behind this is that this allows programmers to use their own preferred spelling style and libraries written by different programmers cannot use incompatible conventions. A Nimrod-aware editor or IDE can show the identifiers as preferred. Another advantage is that it frees the programmer from remembering the exact spelling of an identifier.

## String literals

Terminal symbol in the grammar: `STR_LIT`.

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

Escape sequence | Meaning
--- | ---
`\n` | newline
`\r`, `\c` | carriage return
`\l` | line feed
`\f` | form feed
`\t` | tabulator
`\v` | vertical tabulator
`\\` | backslash
`\"` | quotation mark
`\'` | apostrophe
`\` '0'..'9'+ | character with decimal value d; all decimal digits directly following are used for the character
`\a` | alert
`\b` | backspace
`\e` | escape [ESC]
`\x` HH | character with hex value HH; exactly two hex digits are allowed

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

## Triple quoted string literals

Terminal symbol in the grammar: `TRIPLESTR_LIT`.

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

    """"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 Nimrod (for example regular expressions).

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

## Character literals

Character literals are enclosed in single quotes `''` and can contain the same escape sequences as strings - with one exception: `\n` is not allowed as it may be wider than one character (often it is the pair CR/LF for example). A character is not an Unicode character but a single byte. The reason for this is efficiency: for the overwhelming majority of use-cases, the resulting programs will still handle UTF-8 properly as UTF-8 was specially designed for this. Another reason is that Nimrod can thus support `array[char, int]` or `set[char]` efficiently as many algorithms rely on this feature.

## Numerical constants

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

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

    INT_LIT ::= HEX_LIT
              | DEC_LIT
              | OCT_LIT
              | BIN_LIT

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

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

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

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

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

The type suffixes are:

Type Suffix | Resulting type of literal
--- | ---
`'i8` | int8
`'i16` | int16
`'i32` | int32
`'i64` | int64
`'u` | uint
`'u8` | uint8
`'u16` | uint16
`'u32` | uint32
`'u64` | uint64
`'f32` | float32
`'f64` | float64

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

## Operators

In Nimrod one can define his own operators. An operator is any combination of the following characters:

    =     +     -     *     /     <     >
    @     $     ~     &     %     |
    !     ?     ^     .     :     \

These keywords are also operators: `and or not xor shl shr div mod in notin is isnot of`.

=, :, :: are not available as general operators; they are used for other notational purposes.

`*:` is as a special case the two tokens * and : (to support `var v*: T`).

## 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 Nimrod's standard syntax. How the parser handles the indentation is already described in the Lexical Analysis section.

Nimrod allows user-definable operators. Binary operators have 10 different levels of precedence.

## Relevant character

An operator symbol's relevant character is its first character unless the first character is `\` and its length is greater than 1 then it is the second character.

This rule allows to escape operator symbols with `\` and keeps the operator's precedence and associativity; this is useful for meta programming.

## Associativity

All binary operators are left-associative, except binary operators whose relevant char is `^`.

## Precedence

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

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

If the operator's relevant 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)`.

Otherwise precedence is determined by the relevant character.

Precedence level | Operators | Relevant character | Terminal symbol
--- | --- | --- | ---
9 (highest) |  | `$ ^` | OP9
8 | `* / div mod shl shr %` | `* % \ /` | OP8
7 | `+ -` | `+ ~ |` | OP7
6 | `&` | `&` | OP6
5 | `..` | `.` | OP5
4 | `== <= < >= > != in not_in is isnot not of` | `= < > !` | OP4
3 | `and` |  | OP3
2 | `or xor` |  | OP2
1 |  | `@ : ?` | OP1
0 (lowest) | assignment operator (like `+=`, `*=`) |  | OP0

The grammar's start symbol is `module`.

    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' | 'addr' | 'static' | '..'

    prefixOperator = operator

    optInd = COMMENT?
    optPar = (IND{>} | IND{=})?

    simpleExpr = assignExpr (OP0 optInd assignExpr)*
    assignExpr = orExpr (OP1 optInd orExpr)*
    orExpr = andExpr (OP2 optInd andExpr)*
    andExpr = cmpExpr (OP3 optInd cmpExpr)*
    cmpExpr = sliceExpr (OP4 optInd sliceExpr)*
    sliceExpr = ampExpr (OP5 optInd ampExpr)*
    ampExpr = plusExpr (OP6 optInd plusExpr)*
    plusExpr = mulExpr (OP7 optInd mulExpr)*
    mulExpr = dollarExpr (OP8 optInd dollarExpr)*
    dollarExpr = primary (OP9 optInd primary)*
    symbol = '`' (KEYW|IDENT|operator|'(' ')'|'[' ']'|'{' '}'|'='|literal)+ '`'
           | IDENT
    indexExpr = expr
    indexExprList = indexExpr ^+ comma
    exprColonEqExpr = expr (':'|'=' expr)?
    exprList = expr ^+ comma
    dotExpr = expr '.' optInd ('type' | 'addr' | symbol)
    qualifiedIdent = symbol ('.' optInd ('type' | 'addr' | symbol))?
    exprColonEqExprList = exprColonEqExpr (comma exprColonEqExpr)* (comma)?
    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 ^+ ';'
                     | simpleExpr ('=' expr (';' complexOrSimpleStmt ^+ ';' )? )?
                                | (':' expr)? (',' (exprColonEqExpr comma?)*)?  )?
            optPar ')'
    generalizedLit = GENERALIZED_STR_LIT | GENERALIZED_TRIPLESTR_LIT
    identOrLiteral = generalizedLit | symbol
                   | 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
                   | par | arrayConstr | setOrTableConstr
                   | castExpr
    tupleConstr = '(' optInd (exprColonEqExpr comma?)* optPar ')'
    arrayConstr = '[' optInd (exprColonEqExpr comma?)* optPar ']'
    primarySuffix = '(' (exprColonEqExpr comma?)* ')' doBlocks?
                  | doBlocks
                  | '.' optInd ('type' | 'addr' | symbol) generalizedLit?
                  | '[' optInd indexExprList optPar ']'
                  | '{' optInd indexExprList optPar '}'
    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
    identWithPragma = identVis 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)*)?
    paramList = '(' declColonEquals ^* (comma/semicolon) ')'
    paramListArrow = paramList? ('->' optInd typeDesc)?
    paramListColon = paramList? (':' optInd typeDesc)?
    doBlock = 'do' paramListArrow pragmas? colcom stmt
    doBlocks = doBlock ^* IND{=}
    procExpr = 'proc' paramListColon pragmas? ('=' COMMENT? stmt)?
    expr = (ifExpr
          | whenExpr
          | caseExpr
          | tryStmt)
          / simpleExpr
    typeKeyw = 'var' | 'ref' | 'ptr' | 'shared' | 'type' | 'tuple'
             | 'proc' | 'iterator' | 'distinct' | 'object' | 'enum'
    primary = typeKeyw typeDescK
            /  prefixOperator* identOrLiteral primarySuffix*
            / 'addr' primary
            / 'static' primary
            / 'bind' primary
    typeDesc = simpleExpr
    typeDefAux = simpleExpr
    exprStmt = simpleExpr
             (( '=' optInd expr )
             / ( expr ^+ comma
                 doBlocks
                  / ':' stmt? ( IND{=} 'of' exprList ':' stmt
                              | IND{=} 'elif' expr ':' stmt
                              | IND{=} 'except' exprList ':' stmt
                              | IND{=} 'else' ':' stmt )*
               ))?
    importStmt = 'import' optInd expr
                  ((comma expr)*
                  / 'except' optInd (expr ^+ comma))
    includeStmt = 'include' optInd expr ^+ comma
    fromStmt = 'from' expr '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)?
    exceptBlock = 'except' colcom stmt
    forStmt = 'for' (identWithPragma ^+ comma) 'in' expr colcom stmt
    blockStmt = 'block' symbol? colcom stmt
    staticStmt = 'static' colcom stmt
    asmStmt = 'asm' pragma? (STR_LIT | RSTR_LIT | TRIPLE_STR_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' / declColonEquals
    object = 'object' pragma? ('of' typeDesc)? COMMENT? objectPart
    distinct = 'distinct' optInd typeDesc
    typeDef = identWithPragma genericParamList? '=' optInd typeDefAux
                indAndComment?
    varTuple = '(' optInd identWithPragma ^+ comma optPar ')' '=' optInd expr
    variable = (varTuple / identColonEquals) 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 | finallyStmt | exceptStmt | forStmt
                        | blockStmt | staticStmt | asmStmt
                        | 'proc' routine
                        | 'method' routine
                        | 'iterator' routine
                        | 'macro' routine
                        | 'template' routine
                        | 'converter' routine
                        | 'type' section(typeDef)
                        | 'const' section(constant)
                        | ('let' | 'var') section(variable)
                        | bindStmt | mixinStmt)
                        / simpleStmt
    stmt = (IND{>} complexOrSimpleStmt^+(IND{=} / ';') DED)
         / simpleStmt

# Types

All expressions have a type which is known at compile time. Nimrod is statically typed. One can declare new types, which is in essence defining an identifier that can be used to denote this custom type.

These are the major type classes:

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

## Ordinal types

Ordinal types 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 no ordinal types.

## Pre-defined integer types

These integer types are pre-defined:

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

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

operation | meaning
--- | ---
`a +% b` | unsigned integer addition
`a -% b` | unsigned integer subtraction
`a *% b` | unsigned integer multiplication
`a /% b` | unsigned integer division
`a %% b` | unsigned integer modulo operation
`a <% b` | treat `a` and `b` as unsigned and compare
`a <=% b` | treat `a` and `b` as unsigned and compare
`ze(a)` | extends the bits of `a` with zeros until it has the width of the `int` type
`toU8(a)` | treats `a` as unsigned and converts it to an unsigned integer of 8 bits (but still the `int8` type)
`toU16(a)` | treats `a` as unsigned and converts it to an unsigned integer of 16 bits (but still the `int16` type)
`toU32(a)` | treats `a` as unsigned and converts it to an unsigned integer of 32 bits (but still the `int32` type)

Automatic type conversion 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 Nimrod only widening type conversion 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 type (the base type). To define a subrange type, one must specify it's limiting values: the lowest and highest value of the type:

    type
      TSubrange = range[0..5]

`TSubrange` is a subrange of an integer which can only hold the values 0 to 5. Assigning any other value to a variable of type `TSubrange` is a checked runtime error (or static error if it can be statically determined). Assignments from the base type to one of its subrange types (and vice versa) are allowed.

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

Nimrod requires interval arithmetic for subrange types over a set of built-in operators that involve constants: `x %% 3` is of type `range[0..2]`. The following built-in operators for integers are affected by this rule: `-`, `+`, `*`, `min`, `max`, `succ`, `pred`, `mod`, `div`, `%%`, `and` (bitwise `and`).

Bitwise `and` only produces a `range` if one of its operands is a constant x so that (x+1) is a number of two. (Bitwise `and` is then a `%%` operation.)

This means that the following code is accepted:

    case (x and 3) + 7
    of 7: echo "A"
    of 8: echo "B"
    of 9: echo "C"
    of 10: echo "D"
    # note: no ``else`` required as (x and 3) + 7 has the type: range[7..10]

## Pre-defined floating point types

The following floating point types are pre-defined:

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

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

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

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

The IEEE exceptions are either ignored at runtime or mapped to the Nimrod exceptions: EFloatInvalidOp, EFloatDivByZero, EFloatOverflow, EFloatUnderflow, and EFloatInexact. These exceptions inherit from the EFloatingPoint base class.

Nimrod provides the pragmas NaNChecks and InfChecks to control whether the IEEE exceptions are ignored or trap a Nimrod exception:

    {.NanChecks: on, InfChecks: on.}
    var a = 1.0
    var b = 0.0
    echo b / b # raises EFloatInvalidOp
    echo a / b # raises EFloatOverflow

In the current implementation `EFloatDivByZero` and `EFloatInexact` are never raised. `EFloatOverflow` is raised instead of `EFloatDivByZero`. There is also a floatChecks pragma that is a short-cut for the combination of `NaNChecks` and `InfChecks` pragmas. `floatChecks` are turned off as default.

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

## Boolean type

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

This condition holds:

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

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

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

## Enumeration types

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

    type
      TDirection = enum
        north, east, south, west

Now the following holds:

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

Thus, north < east < south < west. The comparison operators can be used with enumeration types.

For better interfacing to other programming languages, the fields of enum types can be assigned an explicit ordinal value. However, the ordinal values have to be in ascending order. A field whose ordinal value is not explicitly given is assigned the value of the previous field + 1.

An explicit ordered enum can have holes:

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

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

The compiler supports the built-in stringify operator `$` for enumerations. The stringify's result can be controlled by explicitly giving the string values to use:

    type
      TMyEnum = enum
        valueA = (0, "my value A"),
        valueB = "value B",
        valueC = 2,
        valueD = (3, "abc")

As can be seen from the example, it is possible to both specify a field's ordinal value and its string value by using a tuple. It is also possible to only specify one of them.

An enum can be marked with the `pure` pragma so that it's fields are not added to the current scope, so they always need to be accessed via `TMyEnum.value`:

    type
      TMyEnum {.pure.} = enum
        valueA, valueB, valueC, valueD

    echo valueA # error: Unknown identifier
    echo TMyEnum.valueA # works

## String type

All string literals are of the type string. A string in Nimrod is very similar to a sequence of characters. However, strings in Nimrod are both zero-terminated and have a length field. One can retrieve the length with the builtin `len` procedure; the length never counts the terminating zero. The assignment operator for strings always copies the string. The `&` operator concatenates strings.

Strings are compared by their lexicographical order. All comparison operators are available. Strings can be indexed like arrays (lower bound is 0). Unlike arrays, they can be used in case statements:

    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 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 Nimrod `string` is implicitly convertible to `cstring` for convenience. If a Nimrod 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.

## Structured types

A variable of a structured type can hold multiple values at the same time. Structured types can be nested to unlimited levels. Arrays, sequences, tuples, objects and sets belong to the structured types.

## Array and sequence types

Arrays are a homogeneous type, meaning that each element in the array has the same type. Arrays always have a fixed length which is specified at compile time (except for open arrays). They can be indexed by any ordinal type. A parameter `A` may be an open array, in which case it is indexed by integers from 0 to `len(A)-1`. An array expression may be constructed by the array constructor `[]`.

Sequences are similar to arrays but of dynamic length which may change during runtime (like strings). 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
      TIntArray = array[0..5, int] # an array that is indexed with 0..5
      TIntSeq = seq[int] # a sequence of integers
    var
      x: TIntArray
      y: TIntSeq
    x = [1, 2, 3, 4, 5, 6]  # [] is the array constructor
    y = @[1, 2, 3, 4, 5, 6] # the @ turns the array into a sequence

The lower bound of an array or sequence may be received by the built-in proc `low()`, the higher bound by `high()`. The length may be received by `len()`. `low()` for a sequence or an open array always returns 0, as this is the first valid index. One can append elements to a sequence with the `add()` proc or the `&` operator, and remove (and get) the last element of a sequence with the `pop()` proc.

The notation `x[i]` can be used to access the i-th element of `x`.

Arrays are always bounds checked (at compile-time or at runtime). These checks can be disabled via pragmas or invoking the compiler with the `--boundChecks:off` command line switch.

## Open arrays

Often fixed size arrays turn out to be too inflexible; procedures should be able to deal with arrays of different sizes. The openarray type allows this; it can only be used for parameters. Openarrays are always indexed with an `int` starting at position 0. The `len`, `low` and `high` operations are available for open arrays too. Any array with a compatible base type can be passed to an openarray parameter, the index type does not matter. In addition to arrays sequences can also be passed to an open array parameter.

The openarray type cannot be nested: multidimensional openarrays are not supported because this is seldom needed and cannot be done efficiently.

## Varargs

A varargs parameter is an openarray parameter that additionally allows to pass a variable number of arguments to a procedure. The compiler converts the list of arguments to an array implicitly:

    proc myWriteln(f: TFile, a: varargs[string]) =
      for s in items(a):
        write(f, s)
      write(f, "\n")

    myWriteln(stdout, "abc", "def", "xyz")
    # is transformed to:
    myWriteln(stdout, ["abc", "def", "xyz"])

This transformation is only done if the varargs parameter is the last parameter in the procedure header. It is also possible to perform type conversions in this context:

    proc myWriteln(f: TFile, a: varargs[string, `$`]) =
      for s in items(a):
        write(f, s)
      write(f, "\n")

    myWriteln(stdout, 123, "abc", 4.0)
    # is transformed to:
    myWriteln(stdout, [$123, $"def", $4.0])

In this example `$` is applied to any argument that is passed to the parameter `a`. (Note that `$` applied to strings is a nop.)

## Tuples and object types

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

The assignment operator for tuples copies each component. The default assignment operator for objects copies each component. Overloading of the assignment operator for objects is not possible, but this will change in future versions of the compiler.

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

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

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

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

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

    type
      TPerson {.inheritable.} = object
        name*: string   # the * means that `name` is accessible from other modules
        age: int        # no * means that the field is hidden

      TStudent = object of TPerson # a student is a person
        id: int                    # with an id field

    var
      student: TStudent
      person: TPerson
    assert(student of TStudent) # is true

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

## Object construction

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

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

For a `ref object` type `system.new` is invoked implicitly.

## Object variants

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

An example:

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

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

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

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

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

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

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

In the example the `kind` field is called the discriminator: For safety its address cannot be taken and assignments to it are restricted: The new value must not lead to a change of the active object branch. For an object branch switch `system.reset` has to be used.

## Set type

The set type models the mathematical notion of a set. The set's basetype can only be an ordinal type. The reason is that sets are implemented as high performance bit vectors.

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

    {'a'..'z', '0'..'9'} # This constructs a set that contains the
                         # letters from 'a' to 'z' and the digits
                         # from '0' to '9'

These operations are supported by sets:

operation | meaning
--- | ---
`A + B` | union of two sets
`A * B` | intersection of two sets
`A - B` | difference of two sets (A without B's elements)
`A == B` | set equality
`A <= B` | subset relation (A is subset of B or equal to B)
`A < B` | strong subset relation (A is a real subset of B)
`e in A` | set membership (A contains element e)
`A -+- B` | symmetric set difference (= (A - B) + (B - A))
`card(A)` | the cardinality of A (number of elements in A)
`incl(A, elem)` | same as A = A + {elem}
`excl(A, elem)` | same as A = A - {elem}

## Reference and pointer types

References (similar to pointers 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).

Nimrod 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.

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
      PNode = ref TNode
      TNode = object
        le, ri: PNode
        data: int

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

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

    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
      TData = tuple[x, y: int, s: string]

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

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

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

    # free the memory:
    dealloc(d)

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

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

## Not nil annotation

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

    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)

    # but not this:
    var x: PObject
    p(x)

As shown in the example this is merely an annotation for documentation purposes; for now the compiler can only catch the most trivial type violations.

## Procedural type

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

Examples:

    type
      TCallback = proc (x: int) {.cdecl.}

    proc printItem(x: Int) = ...

    proc forEach(c: TCallback) =
      ...

    forEach(printItem)  # this will NOT work because calling conventions differ

    type
      TOnMouseMove = proc (x, y: int) {.closure.}

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

    proc setOnMouseMove(mouseMoveEvent: TOnMouseMove) = nil

    # 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`.

Nimrod supports these calling conventions:

nimcall
  is the default convention used for a Nimrod 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 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 Nimrod 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 Nimrod's default calling convention for procedures is `fastcall` to improve speed.

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

Assigning/passing a procedure to a procedural variable is only allowed if one of the following conditions hold:

1. The procedure that is accessed resists in the current module.
2. The procedure is marked with the `procvar` pragma (see procvar pragma).
3. The procedure has a calling convention that differs from `nimcall`.
4. The procedure is anonymous.

The rules' purpose is to prevent the case that extending a non-`procvar` procedure with default parameters breaks client code.

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.

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
      TDollar = distinct int
      TEuro = distinct int

    var
      d: TDollar
      e: TEuro

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

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

    proc `+` (x, y: TDollar): TDollar =
      result = TDollar(int(x) + int(y))

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

    proc `*` (x: TDollar, y: int): TDollar =
      result = TDollar(int(x) * y)

    proc `*` (x: int, y: TDollar): TDollar =
      result = TDollar(x * int(y))

    proc `div` ...

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

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

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

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

    template Additive(typ: typeDesc): stmt =
      proc `+` *(x, y: typ): typ {.borrow.}
      proc `-` *(x, y: typ): typ {.borrow.}

      # unary operators:
      proc `+` *(x: typ): typ {.borrow.}
      proc `-` *(x: typ): typ {.borrow.}

    template Multiplicative(typ, base: typeDesc): stmt =
      proc `*` *(x: typ, y: base): typ {.borrow.}
      proc `*` *(x: base, y: typ): typ {.borrow.}
      proc `div` *(x: typ, y: base): typ {.borrow.}
      proc `mod` *(x: typ, y: base): typ {.borrow.}

    template Comparable(typ: typeDesc): stmt =
      proc `<` * (x, y: typ): bool {.borrow.}
      proc `<=` * (x, y: typ): bool {.borrow.}
      proc `==` * (x, y: typ): bool {.borrow.}

    template DefineCurrency(typ, base: expr): stmt =
      type
        typ* = distinct base
      Additive(typ)
      Multiplicative(typ, base)
      Comparable(typ)

    DefineCurrency(TDollar, int)
    DefineCurrency(TEuro, int)

## Void type

The void type denotes the absense of any type. Parameters of type `void` are treated as non-existent, `void` as a return type means that the procedure does not return a value:

    proc nothing(x, y: void): void =
      echo "ha"

    nothing() # writes "ha" to stdout

The `void` type is particularly useful for generic code:

    proc callProc[T](p: proc (x: T), x: T) =
      when T is void:
        p()
      else:
        p(x)

    proc intProc(x: int) = nil
    proc emptyProc() = nil

    callProc[int](intProc, 12)
    callProc[void](emptyProc)

However, a `void` type cannot be inferred in generic code:

    callProc(emptyProc)
    # Error: type mismatch: got (proc ())
    # but expected one of:
    # callProc(p: proc (T), x: T)

The `void` type is only valid for parameters and return types; other symbols cannot have the type `void`.

# Type relations

The following section defines several relations on types that are needed to describe the type checking done by the compiler.

## Type equality

Nimrod uses structural type equivalence for most types. Only for objects, enumerations and distinct types name equivalence is used. The following algorithm (in pseudo-code) determines type equality:

    proc typeEqualsAux(a, b: PType,
                       s: var set[PType * PType]): bool =
      if (a,b) in s: return true
      incl(s, (a,b))
      if a.kind == b.kind:
        case a.kind
        of int, intXX, float, floatXX, char, string, cstring, pointer,
            bool, nil, void:
          # leaf type: kinds identical; nothing more to check
          result = true
        of ref, ptr, var, set, seq, openarray:
          result = typeEqualsAux(a.baseType, b.baseType, s)
        of range:
          result = typeEqualsAux(a.baseType, b.baseType, s) and
            (a.rangeA == b.rangeA) and (a.rangeB == b.rangeB)
        of array:
          result = typeEqualsAux(a.baseType, b.baseType, s) and
                   typeEqualsAux(a.indexType, b.indexType, s)
        of tuple:
          if a.tupleLen == b.tupleLen:
            for i in 0..a.tupleLen-1:
              if not typeEqualsAux(a[i], b[i], s): return false
            result = true
        of object, enum, distinct:
          result = a == b
        of proc:
          result = typeEqualsAux(a.parameterTuple, b.parameterTuple, s) and
                   typeEqualsAux(a.resultType, b.resultType, s) and
                   a.callingConvention == b.callingConvention

    proc typeEquals(a, b: PType): bool =
      var s: set[PType * PType] = {}
      result = typeEqualsAux(a, b, s)

Since types are graphs which can have cycles, the above algorithm needs an auxiliary set `s` to detect this case.

## Type equality modulo type distinction

The following algorithm (in pseudo-code) determines whether two types are equal with no respect to `distinct` types. For brevity the cycle check with an auxiliary set `s` is omitted:

    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:

    # XXX range types?
    proc isImplicitlyConvertible(a, b: PType): bool =
      case a.kind
      of int:     result = b in {int8, int16, int32, int64, uint, uint8, uint16,
                                 uint32, uint64, float, float32, float64}
      of int8:    result = b in {int16, int32, int64, int}
      of int16:   result = b in {int32, int64, int}
      of int32:   result = b in {int64, int}
      of uint:    result = b in {uint32, uint64}
      of uint8:   result = b in {uint16, uint32, uint64}
      of uint16:  result = b in {uint32, uint64}
      of uint32:  result = b in {uint64}
      of float:   result = b in {float32, float64}
      of float32: result = b in {float64, float}
      of float64: result = b in {float32, float}
      of seq:
        result = b == openArray and typeEquals(a.baseType, b.baseType)
      of array:
        result = b == openArray and typeEquals(a.baseType, b.baseType)
        if a.baseType == char and a.indexType.rangeA == 0:
          result = b = cstring
      of cstring, ptr:
        result = b == pointer
      of string:
        result = b == cstring

A type `a` is explicitly convertible to type `b` iff the following algorithm returns true:

    proc isIntegralType(t: PType): bool =
      result = isOrdinal(t) or t.kind in {float, float32, float64}

    proc isExplicitlyConvertible(a, b: PType): bool =
      if isImplicitlyConvertible(a, b): return true
      if typeEqualsOrDistinct(a, b): return true
      if isIntegralType(a) and isIntegralType(b): return true
      if isSubtype(a, b) or isSubtype(b, a): return true
      return false

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

To be written.

# Statements and expressions

Nimrod uses the common statement/expression paradigm: Statements do not produce a value in contrast to expressions. However, some expressions are statements.

Statements are separated into simple statements and complex statements. Simple statements are statements that cannot contain other statements like assignments, calls or the `return` statement; complex statements can contain other statements. To avoid the dangling else problem, complex statements always have to be intended. The details can be found in the grammar.

## Statement list expression

Statements can also occur in an expression context that looks like `(stmt1; stmt2; ...; ex)`. This is called an statement list expression or `(;)`. The type of `(stmt1; stmt2; ...; ex)` is the type of `ex`. All the other statements must be of type `void`. (One can use `discard` to produce a `void` type.) `(;)` does not introduce a new scope.

## Discard statement

Example:

    proc p(x, y: int): int =
      return x + y

    discard p(3, 4) # discard the return value of `p`

The discard statement evaluates its expression for side-effects and throws the expression's resulting value away.

Ignoring the return value of a procedure without using a discard statement is a static error.

The return value can be ignored implicitly if the called proc/iterator has been declared with the discardable pragma:

    proc p(x, y: int): int {.discardable.} =
      return x + y

    p(3, 4) # now valid

## Var statement

Var statements declare new local and global variables and initialize them. A comma separated list of variables can be used to specify variables of the same type:

    var
      a: int = 0
      x, y, z: int

If an initializer is given the type can be omitted: the variable is then of the same type as the initializing expression. Variables are always initialized with a default value if there is no initializing expression. The default value depends on the type and is always a zero in binary.

Type | default value
--- | ---
any integer type | 0
any float | 0.0
char | '\0'
bool | false
ref or pointer type | nil
procedural type | nil
sequence | nil (not `@[]`)
string | nil (not "")
tuple[x: A, y: B, ...] | (default(A), default(B), ...) (analogous for objects)
array[0..., T] | [default(T), ...]
range[T] | default(T); this may be out of the valid range
T = enum | cast[T](0); this may be an invalid value

The implicit initialization can be avoided for optimization reasons with the noinit pragma:

    var
      a {.noInit.}: array [0..1023, char]

If a proc is annotated with the `noinit` pragma this refers to its implicit `result` variable:

    proc returnUndefinedValue: int {.noinit.} = nil

## let statement

A Let statement declares new local and global single assignment variables and binds a value to them. The syntax is the of the `var` statement, except that the keyword `var` is replaced by the keyword `let`. Let variables are not l-values and can thus not be passed to `var` parameters nor can their address be taken. They cannot be assigned new values.

For let variables the same pragmas are available as for ordinary variables.

## Const section

Constants are symbols which are bound to a value. The constant's value cannot change. The compiler must be able to evaluate the expression in a constant declaration at compile time.

Nimrod contains a sophisticated compile-time evaluator, so procedures which have no side-effect can be used in constant expressions too:

    import strutils
    const
      constEval = contains("abc", 'b') # computed at compile time!

The rules for compile-time computability are:

1. Literals are compile-time computable.
2. Type conversions are compile-time computable.
3. Procedure calls of the form `p(X)` are compile-time computable if `p` is a proc without side-effects (see the noSideEffect pragma for details) and if `X` is a (possibly empty) list of compile-time computable arguments.

Constants cannot be of type `ptr`, `ref`, `var` or `object`, nor can they contain such a type.

## Static statement/expression

A static statement/expression can be used to enforce compile time evaluation explicitly. Enforced compile time evaluation can even evaluate code that has side effects:

    static:
      echo "echo at compile time"

It's a static error if the compiler cannot perform the evaluation at compile time.

The current implementation poses some restrictions for compile time evaluation: Code which contains `cast` or makes use of the foreign function interface cannot be evaluated at compile time. Later versions of Nimrod will support the FFI at compile time.

## If statement

Example:

    var name = readLine(stdin)

    if name == "Andreas":
      echo("What a nice name!")
    elif name == "":
      echo("Don't you have a name?")
    else:
      echo("Boring name...")

The if statement is a simple way to make a branch in the control flow: The expression after the keyword `if` is evaluated, if it is true the corresponding statements after the `:` are executed. Otherwise the expression after the `elif` is evaluated (if there is an `elif` branch), if it is true the corresponding statements after the `:` are executed. This goes on until the last `elif`. If all conditions fail, the `else` part is executed. If there is no `else` part, execution continues with the statement after the `if` statement.

The scoping for an `if` statement is slightly subtle to support an important use case. A new scope starts for the `if`/`elif` condition and ends after the corresponding then block:

    if {| (let m = input =~ re"(\w+)=\w+"; m.isMatch):
      echo "key ", m[0], " value ", m[1]  |}
    elif {| (let m = input =~ re""; m.isMatch):
      echo "new m in this scope" |}
    else:
      # 'm' not declared here

In the example the scopes have been enclosed in `{| |}`.

Note: These scoping rules will be active in 0.9.4.

## Case statement

Example:

    case readline(stdin)
    of "delete-everything", "restart-computer":
      echo("permission denied")
    of "go-for-a-walk":     echo("please yourself")
    else:                   echo("unknown command")

    # indentation of the branches is also allowed; and so is an optional colon
    # after the selecting expression:
    case readline(stdin):
      of "delete-everything", "restart-computer":
        echo("permission denied")
      of "go-for-a-walk":     echo("please yourself")
      else:                   echo("unknown command")

The case statement is similar to the if statement, but it represents a multi-branch selection. The expression after the keyword `case` is evaluated and if its value is in a slicelist the corresponding statements (after the `of` keyword) are executed. If the value is not in any given slicelist the `else` part is executed. If there is no `else` part and not all possible values that `expr` can hold occur in a `slicelist`, a static error occurs. This holds only for expressions of ordinal types. "All possible values" of `expr` are determined by `expr`'s type.

If the expression is not of an ordinal type, and no `else` part is given, control passes after the `case` statement.

To suppress the static error in the ordinal case an `else` part with a `nil` statement can be used.

As a special semantic extension, an expression in an `of` branch of a case statement may evaluate to a set constructor; the set is then expanded into a list of its elements:

    const
      SymChars: set[char] = {'a'..'z', 'A'..'Z', '\x80'..'\xFF'}

    proc classify(s: string) =
      case s[0]
      of SymChars, '_': echo "an identifier"
      of '0'..'9': echo "a number"
      else: echo "other"

    # is equivalent to:
    proc classify(s: string) =
      case s[0]
      of 'a'..'z', 'A'..'Z', '\x80'..'\xFF', '_': echo "an identifier"
      of '0'..'9': echo "a number"
      else: echo "other"

## When statement

Example:

    when sizeof(int) == 2:
      echo("running on a 16 bit system!")
    elif sizeof(int) == 4:
      echo("running on a 32 bit system!")
    elif sizeof(int) == 8:
      echo("running on a 64 bit system!")
    else:
      echo("cannot happen!")

The when statement is almost identical to the `if` statement with some exceptions:

- Each `expr` has to be a constant expression (of type `bool`).
- The statements do not open a new scope.
- The statements that belong to the expression that evaluated to true are translated by the compiler, the other statements are not checked for semantics! However, each `expr` is checked for semantics.

The `when` statement enables conditional compilation techniques. As a special syntactic extension, the `when` construct is also available within `object` definitions.

## Return statement

Example:

    return 40+2

The return statement ends the execution of the current procedure. It is only allowed in procedures. If there is an `expr`, this is syntactic sugar for:

    result = expr
    return result

`return` without an expression is a short notation for `return result` if the proc has a return type. The result variable is always the return value of the procedure. It is automatically declared by the compiler. As all variables, `result` is initialized to (binary) zero:

    proc returnZero(): int =
      # implicitly returns 0

## Yield statement

Example:

    yield (1, 2, 3)

The yield statement is used instead of the `return` statement in iterators. It is only valid in iterators. Execution is returned to the body of the for loop that called the iterator. Yield does not end the iteration process, but execution is passed back to the iterator if the next iteration starts. See the section about iterators (Iterators and the for statement) for further information.

## Block statement

Example:

    var found = false
    block myblock:
      for i in 0..3:
        for j in 0..3:
          if a[j][i] == 7:
            found = true
            break myblock # leave the block, in this case both for-loops
    echo(found)

The block statement is a means to group statements to a (named) block. Inside the block, the `break` statement is allowed to leave the block immediately. A `break` statement can contain a name of a surrounding block to specify which block is to leave.

## Break statement

Example:

    break

The break statement is used to leave a block immediately. If `symbol` is given, it is the name of the enclosing block that is to leave. If it is absent, the innermost block is left.

## While statement

Example:

    echo("Please tell me your password: \n")
    var pw = readLine(stdin)
    while pw != "12345":
      echo("Wrong password! Next try: \n")
      pw = readLine(stdin)

The while statement is executed until the `expr` evaluates to false. Endless loops are no error. `while` statements open an implicit block, so that they can be left with a `break` statement.

## Continue statement

A continue statement leads to the immediate next iteration of the surrounding loop construct. It is only allowed within a loop. A continue statement is syntactic sugar for a nested block:

    while expr1:
      stmt1
      continue
      stmt2

Is equivalent to:

    while expr1:
      block myBlockName:
        stmt1
        break myBlockName
        stmt2

## Assembler statement

The direct embedding of assembler code into Nimrod code is supported by the unsafe `asm` statement. Identifiers in the assembler code that refer to Nimrod identifiers shall be enclosed in a special character which can be specified in the statement's pragmas. The default special character is `'`'`:

    proc addInt(a, b: int): int {.noStackFrame.} =
      # a in eax, and b in edx
      asm """
          mov eax, `a`
          add eax, `b`
          jno theEnd
          call `raiseOverflow`
        theEnd:
      """

## If expression

An if expression is almost like an if statement, but it is an expression. Example:

    var y = if x > 8: 9 else: 10

An if expression always results in a value, so the `else` part is required. `Elif` parts are also allowed.

## When expression

Just like an if expression, but corresponding to the when statement.

## Case expression

The case expression is again very similar to the case statement:

    var favoriteFood = case animal
      of "dog": "bones"
      of "cat": "mice"
      elif animal.endsWith"whale": "plankton"
      else:
        echo "I'm not sure what to serve, but everybody loves ice cream"
        "ice cream"

As seen in the above example, the case expression can also introduce side effects. When multiple statements are given for a branch, Nimrod will use the last expression as the result value, much like in an expr template.

## Table constructor

A table constructor is syntactic sugar for an array constructor:

    {"key1": "value1", "key2", "key3": "value2"}

    # is the same as:
    [("key1", "value1"), ("key2", "value2"), ("key3", "value")]

The empty table can be written `{:}` (in contrast to the empty set which is `{}`) which is thus another way to write as the empty array constructor `[]`. This slightly unusal way of supporting tables has lots of advantages:

- The order of the (key,value)-pairs is preserved, thus it is easy to support ordered dicts with for example `{key: val}.newOrderedTable`.
- A table literal can be put into a `const` section and the compiler can easily put it into the executable's data section just like it can for arrays and the generated data section requires a minimal amount of memory.
- Every table implementation is treated equal syntactically.
- Apart from the minimal syntactic sugar the language core does not need to know about tables.

## Type conversions

Syntactically a type conversion is like a procedure call, but a type name replaces the procedure name. A type conversion is always safe in the sense that a failure to convert a type to another results in an exception (if it cannot be determined statically).

## Type casts

Example:

    cast[int](x)

Type casts are a crude mechanism to interpret the bit pattern of an expression as if it would be of another type. Type casts are only needed for low-level programming and are inherently unsafe.

## The addr operator

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

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

# Procedures

What most programming languages call methods or functions are called procedures in Nimrod (which is the correct terminology). A procedure declaration defines an identifier and associates it with a block of code. A procedure may call itself recursively. A parameter may be given a default value that is used if the caller does not provide a value for this parameter.

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 tries to find the proc that 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'

A procedure cannot modify its parameters (unless the parameters have the type var).

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.
      return intToStr(x)

Operators with one parameter are prefix operators, operators with two parameters are infix operators. (However, the parser distinguishes these from the operator's position within an expression.) There is no way to declare postfix operators: all postfix operators are built-in and handled by the grammar explicitly.

Any operator can be called like an ordinary proc with the 'opr' notation. (Thus an operator can have more than two parameters):

    proc `*+` (a, b, c: int): int =
      # Multiply and add
      return a * b + c

    assert `*+`(3, 4, 6) == `*`(a, `+`(b, c))

## Closures

Procedures can appear at the top level in a module as well as inside other scopes, in which case they are called nested procs. A nested proc can access local variables from its enclosing scope and if it does so it becomes a closure. Any captured variables are stored in a hidden additional argument to the closure (its environment) and they are accessed by reference by both the closure and its enclosing scope (i.e. any modifications made to them are visible in both places). The closure environment may be allocated on the heap or on the stack if the compiler determines that this would be safe.

## Anonymous Procs

Procs can also be treated as expressions, in which case it's allowed to omit the proc's name.

    var cities = @["Frankfurt", "Tokyo", "New York"]

    cities.sort(proc (x,y: string): int =
        cmp(x.len, y.len))

Procs as expressions can appear both as nested procs and inside top level executable code.

## Do notation

As a special more convenient notation, proc expressions involved in procedure calls can use the `do` keyword:

    sort(cities) do (x,y: string) -> int:
      cmp(x.len, y.len)

`do` is written after the parentheses enclosing the regular proc params. The proc expression represented by the do block is appended to them.

More than one `do` block can appear in a single call:

    proc performWithUndo(task: proc(), undo: proc()) = ...

    performWithUndo do:
      # multiple-line block of code
      # to perform the task
    do:
      # code to undo it

For compatibility with `stmt` templates and macros, the `do` keyword can be omitted if the supplied proc doesn't have any parameters and return value. The compatibility works in the other direction too as the `do` syntax can be used with macros and templates expecting `stmt` blocks.

## Nonoverloadable builtins

The following builtin procs cannot be overloaded for reasons of implementation simplicity (they require specialized semantic checking):

    defined, definedInScope, compiles, low, high, sizeOf,
    is, of, echo, shallowCopy, getAst

Thus they act more like keywords than like ordinary identifiers; unlike a keyword however, a redefinition may shadow the definition in the `system` module.

## 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] =
      return (a div b, a mod b)

    var t = divmod(8, 5)
    assert t.res == 1
    assert t.remainder = 3

One can use tuple unpacking to access the tuple's fields:

    var (x, y) = divmod(8, 5) # tuple unpacking
    assert x == 1
    assert y == 3

## Var return type

A proc, converter or iterator may return a `var` type which means that the returned value is an l-value and can be modified by the caller:

    var g = 0

    proc WriteAccessToG(): var int =
      result = g

    WriteAccessToG() = 6
    assert g == 6

It is a compile time error if the implicitly introduced pointer could be used to access a location beyond its lifetime:

    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.

## Overloading of the subscript operator

The `[]` subscript operator for arrays/openarrays/sequences can be overloaded. Overloading support is only possible if the first parameter has no type that already supports the built-in `[]` notation. Currently the compiler does not check this restriction.

# Multi-methods

Procedures always use static dispatch. Multi-methods use dynamic dispatch.

    type
      TExpr = object ## abstract base class for an expression
      TLiteral = object of TExpr
        x: int
      TPlusExpr = object of TExpr
        a, b: ref TExpr

    method eval(e: ref TExpr): int =
      # override this base method
      quit "to override!"

    method eval(e: ref TLiteral): int = return e.x

    method eval(e: ref TPlusExpr): int =
      # watch out: relies on dynamic binding
      return eval(e.a) + eval(e.b)

    proc newLit(x: int): ref TLiteral =
      new(result)
      result.x = x

    proc newPlus(a, b: ref TExpr): ref TPlusExpr =
      new(result)
      result.a = a
      result.b = b

    echo eval(newPlus(newPlus(newLit(1), newLit(2)), newLit(4)))

In the example the constructors `newLit` and `newPlus` are procs because they should use static binding, but `eval` is a method because it requires dynamic binding.

In a multi-method all parameters that have an object type are used for the dispatching:

    type
      TThing = object
      TUnit = object of TThing
        x: int

    method collide(a, b: TThing) {.inline.} =
      quit "to override!"

    method collide(a: TThing, b: TUnit) {.inline.} =
      echo "1"

    method collide(a: TUnit, b: TThing) {.inline.} =
      echo "2"

    var
      a, b: TUnit
    collide(a, b) # output: 2

Invocation of a multi-method cannot be ambiguous: collide 2 is preferred over collide 1 because the resolution works from left to right. In the example `TUnit, TThing` is preferred over `TThing, TUnit`.

Performance note: Nimrod does not produce a virtual method table, but generates dispatch trees. This avoids the expensive indirect branch for method calls and enables inlining. However, other optimizations like compile time evaluation or dead code elimination do not work with methods.

# Iterators and the for statement

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 overloadings of `items`/`pairs` are taken into account.

## First class iterators

There are 2 kinds of iterators in Nimrod: 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 increasee in code size. Inline iterators are second class citizens; one cannot pass them around like first class procs.

In contrast to that, a closure iterator can be passed around:

    iterator count0(): int {.closure.} =
      yield 0

    iterator count2(): int {.closure.} =
      var x = 1
      yield x
      inc x
      yield x

    proc invoke(iter: iterator(): int {.closure.}) =
      for x in iter(): echo x

    invoke(count0)
    invoke(count2)

Closure iterators have other restrictions than inline iterators:

1. `yield` in a closure iterator can not occur in a `try` statement.
2. For now, a closure iterator cannot be evaluated at compile time.
3. `return` is allowed in a closure iterator (but rarely useful).
4. Since closure iterators can be used as a collaborative tasking system, `void` is a valid return type for them.
5. Both inline and closure iterators cannot be recursive.

Iterators that are neither marked `{.closure.}` nor `{.inline.}` explicitly default to being inline, but that this may change in future versions of the implementation.

The `iterator` type is always of the calling convention `closure` implicitly; the following example shows how to use iterators to implement a collaborative tasking system:

    # simple tasking:
    type
      TTask = iterator (ticker: int)

    iterator a1(ticker: int) {.closure.} =
      echo "a1: A"
      yield
      echo "a1: B"
      yield
      echo "a1: C"
      yield
      echo "a1: D"

    iterator a2(ticker: int) {.closure.} =
      echo "a2: A"
      yield
      echo "a2: B"
      yield
      echo "a2: C"

    proc runTasks(t: varargs[TTask]) =
      var ticker = 0
      while true:
        let x = t[ticker mod t.len]
        if finished(x): break
        x(ticker)
        inc ticker

    runTasks(a1, a2)

The builtin `system.finished` can be used to determine if an iterator has finished its operation; no exception is raised on an attempt to invoke an iterator that has already finished its work.

# Type sections

Example:

    type # example demonstrating mutually recursive types
      PNode = ref TNode # a traced pointer to a TNode
      TNode = object
        le, ri: PNode   # left and right subtrees
        sym: ref TSym   # leaves contain a reference to a TSym

      TSym = object     # a symbol
        name: string    # the symbol's name
        line: int       # the line the symbol was declared in
        code: PNode     # the symbol's abstract syntax tree

A type section begins with the `type` keyword. It contains multiple type definitions. A type definition binds a type to a name. Type definitions can be recursive or even mutually recursive. Mutually recursive types are only possible within a single `type` section. Nominal types like `objects` or `enums` can only be defined in a `type` section.

# Exception handling

## Try statement

Example:

    # read the first two lines of a text file that should contain numbers
    # and tries to add them
    var
      f: TFile
    if open(f, "numbers.txt"):
      try:
        var a = readLine(f)
        var b = readLine(f)
        echo("sum: " & $(parseInt(a) + parseInt(b)))
      except EOverflow:
        echo("overflow!")
      except EInvalidValue:
        echo("could not convert string to integer")
      except EIO:
        echo("IO error!")
      except:
        echo("Unknown exception!")
      finally:
        close(f)

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

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).

## Except and finally statements

except and finally can also be used as a stand-alone statements. Any statements following them in the current block will be considered to be in an implicit try block:

    var f = open("numbers.txt")
    finally: close(f)
    ...

The `except` statement has a limitation in this form: you can't specify the type of the exception, you have to catch everything. Also, if you want to use both `finally` and `except` you need to reverse the usual sequence of the statements. Example:

    proc test() =
      raise newException(E_base, "Hey ho")

    proc tester() =
      finally: echo "3. Finally block"
      except: echo "2. Except block"
      echo "1. Pre exception"
      test()
      echo "4. Post exception"
    # --> 1, 2, 3 is printed, 4 is never reached

## Raise statement

Example:

    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 ENoExceptionToReraise exception is raised if there is no exception to re-raise. It follows that the `raise` statement always raises an exception (unless a raise hook has been provided).

## OnRaise builtin

`system.onRaise` can be used to override the behaviour of `raise` for a single `try` statement. onRaise has to be called within the `try` statement that should be affected.

This allows for a Lisp-like condition system:

    var myFile = open("broken.txt", fmWrite)
    try:
      onRaise do (e: ref E_Base)-> bool:
        if e of EIO:
          stdout.writeln "ok, writing to stdout instead"
        else:
          # do raise other exceptions:
          result = true
      myFile.writeln "writing to broken file"
    finally:
      myFile.close()

`OnRaise` can only filter raised exceptions, it cannot transform one exception into another. (Nor should `onRaise` raise an exception though this is currently not enforced.) This restriction keeps the exception tracking analysis sound.

# Effect system

## Exception tracking

Nimrod 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: [EIO, EOS].} =
      if what: raise newException(EIO, "IO")
      else: raise newException(EOS, "OS")

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

    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
      TCallback = proc (s: string) {.raises: [EIO].}
    var
      c: TCallback

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

    c = p # type error

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

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

Rules 1-2 ensure the following works:

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

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

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

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

## Tag tracking

The exception tracking is part of Nimrod'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
    proc readLine(): string {.tags: [IO].}

    proc no_IO_please() {.tags: [].} =
      # the compiler prevents this:
      let x = readLine()

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

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

## Read/Write tracking

Note: Read/write tracking is not yet implemented!

The inference for read/write tracking is analogous to the inference for exception tracking.

## Effects pragma

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

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

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

# Generics

Example:

    type
      TBinaryTree[T] = object      # TBinaryTree is a generic type with
                                   # with generic param ``T``
        le, ri: ref TBinaryTree[T] # left and right subtrees; may be nil
        data: T                    # the data stored in a node
      PBinaryTree[T] = ref TBinaryTree[T] # a shorthand for notational convenience

    proc newNode[T](data: T): PBinaryTree[T] = # constructor for a node
      new(result)
      result.data = data

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

    iterator inorder[T](root: PBinaryTree[T]): T =
      # inorder traversal of a binary tree
      # recursive iterators are not yet implemented, so this does not work in
      # the current compiler!
      if root.le != nil: yield inorder(root.le)
      yield root.data
      if root.ri != nil: yield inorder(root.ri)

    var
      root: PBinaryTree[string] # instantiate a PBinaryTree with the type string
    add(root, newNode("hallo")) # instantiates generic procs ``newNode`` and
    add(root, newNode("world")) # ``add``
    for str in inorder(root):
      writeln(stdout, str)

Generics are Nimrod'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.

## Is operator

The is operator checks for type equivalence at compile time. It is therefore very useful for type specialization within generic code:

    type
      TTable[TKey, TValue] = object
        keys: seq[TKey]
        values: seq[TValue]
        when not (TKey is string): # nil value for strings used for optimization
          deletedKeys: seq[bool]

## Type operator

The type (in many other languages called typeof) operator can be used to get the type of an expression:

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

If `type` is used to determine the result type of a proc/iterator/converter call `c(X)` (where `X` stands for a possibly empty list of arguments), the interpretation where `c` is an iterator is preferred over the other interpretations:

    import strutils

    # strutils contains both a ``split`` proc and iterator, but since an
    # an iterator is the preferred interpretation, `y` has the type ``string``:
    var y: type("a b c".split)

## Type Classes

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

type class | matches
--- | ---
`object` | any object type
`tuple` | any tuple type
`enum` | any enumeration
`proc` | any proc type
`ref` | any `ref` type
`ptr` | any `ptr` type
`var` | any `var` type
`distinct` | any distinct type
`array` | any array type
`set` | any set type
`seq` | any seq type
`auto` | any type

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

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

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

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

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

Nimrod 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) = nil

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

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

    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`.
      for a, b in fields(x, y):
        if a != b: return false
      return true

Alternatively, the `distinct` type modifier can be applied to the type class to allow each param matching the type class to bind to a different type.

If a proc param doesn't have a type specified, Nimrod will use the `distinct auto` type class (also known as `any`):

    # allow any combination of param types
    proc concat(a, b): string = $a & $b

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

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

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

If anonymous type classes are used, the `type` operator can be used to discover the instantiated type of each param.

## User defined type classes

To be written.

## Return Type Inference

If a type class is used as the return type of a proc and it won't be bound to a concrete type by some of the proc params, Nimrod will infer the return type from the proc body. This is usually used with the `auto` type class:

    proc makePair(a, b): auto = (first: a, second: b)

The return type will be treated as additional generic param and can be explicitly specified at call sites as any other generic param.

Future versions of nimrod may also support overloading based on the return type of the overloads. In such settings, the expected result type at call sites may also influence the inferred return type.

## Symbol lookup in generics

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

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

    type
      TIndex = distinct int

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

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

    echo a == b # works!

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

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

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

# Templates

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

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

Example:

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

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

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

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

The "types" of templates can be the symbols `expr` (stands for expression), `stmt` (stands for statement) or `typedesc` (stands for type description). These are no real types, they just help the compiler parsing. Real types can be used too; this implies that expressions are expected.

## Ordinary vs immediate templates

There are two different kinds of templates: immediate templates and ordinary templates. Ordinary templates take part in overloading resolution. As such their arguments need to be type checked before the template is invoked. So ordinary templates cannot receive undeclared identifiers:

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

    declareInt(x) # error: unknown identifier: 'x'

An `immediate` template does not participate in overload resolution and so its arguments are not checked for semantics before invocation. So they can receive undeclared identifiers:

    template declareInt(x: expr) {.immediate.} =
      var x: int

    declareInt(x) # valid

## Scoping in templates

The template body does not open a new scope. To open a new scope a `block` statement can be used:

    template declareInScope(x: expr, t: typeDesc): stmt {.immediate.} =
      var x: t

    template declareInNewScope(x: expr, t: typeDesc): stmt {.immediate.} =
      # open a new scope:
      block:
        var x: t

    declareInScope(a, int)
    a = 42  # works, `a` is known here

    declareInNewScope(b, int)
    b = 42  # does not work, `b` is unknown

## Passing a code block to a template

If there is a `stmt` parameter it should be the last in the template declaration, because statements are passed to a template via a special `:` syntax:

    template withFile(f, fn, mode: expr, actions: stmt): stmt {.immediate.} =
      block:
        var f: TFile
        if open(f, fn, mode):
          try:
            actions
          finally:
            close(f)
        else:
          quit("cannot open: " & fn)

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

In the example the two `writeln` statements are bound to the `actions` parameter.

Note: The symbol binding rules for templates might change!

Symbol binding within templates happens after template instantiation:

    # Module A
    var
      lastId = 0

    template genId*: expr =
      inc(lastId)
      lastId

    # Module B
    import A

    echo genId() # Error: undeclared identifier: 'lastId'

## Bind statement

Exporting a template is a often a leaky abstraction as it can depend on symbols that are not visible from a client module. However, to compensate for this case, a bind statement can be used: It declares all identifiers that should be bound early (i.e. when the template is parsed):

    # Module A
    var
      lastId = 0

    template genId*: expr =
      bind lastId
      inc(lastId)
      lastId

    # Module B
    import A

    echo genId() # Works

A `bind` statement can also be used in generics for the same purpose.

## Identifier construction

In templates identifiers can be constructed with the backticks notation:

    template typedef(name: expr, typ: typeDesc) {.immediate.} =
      type
        `T name`* {.inject.} = typ
        `P name`* {.inject.} = ref `T name`

    typedef(myint, int)
    var x: PMyInt

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

## Lookup rules for template parameters

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

    # module 'm'

    type
      TLev = enum
        levA, levB

    var abclev = levB

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

    tstLev(levA)
    # produces: 'levA levA'

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

    # module 'm'

    type
      TLev = enum
        levA, levB

    var abclev = levB

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

    tstLev(levA)
    # produces: 'levA levB'

## Hygiene in templates

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

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

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

Whether a symbol that is declared in a template is exposed to the instantiation scope is controlled by the inject 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: expr, actions: stmt): stmt {.immediate.} =
      block:
        var f: TFile  # since 'f' is a template param, it's injected implicitly
        ...

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

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

    {.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.

# Macros

A macro is a special kind of low level template. Macros can be used to implement domain specific languages. Like templates, macros come in the 2 flavors immediate and ordinary.

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

To write macros, one needs to know how the Nimrod concrete syntax is converted to an abstract syntax tree.

There are two ways to invoke a macro:

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

## Expression Macros

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

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

    macro debug(n: varargs[expr]): stmt =
      # `n` is a Nimrod AST that contains the whole macro invocation
      # this macro returns a list of statements:
      result = newNimNode(nnkStmtList, n)
      # iterate over any argument that is passed to this macro:
      for i in 0..n.len-1:
        # add a call to the statement list that writes the expression;
        # `toStrLit` converts an AST to its string representation:
        add(result, newCall("write", newIdentNode("stdout"), toStrLit(n[i])))
        # add a call to the statement list that writes ": "
        add(result, newCall("write", newIdentNode("stdout"), newStrLitNode(": ")))
        # add a call to the statement list that writes the expressions value:
        add(result, newCall("writeln", newIdentNode("stdout"), n[i]))

    var
      a: array [0..10, int]
      x = "some string"
    a[0] = 42
    a[1] = 45

    debug(a[0], a[1], x)

The macro call expands to:

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

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

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

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

## BindSym

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

    import macros

    macro debug(n: varargs[expr]): stmt =
      result = newNimNode(nnkStmtList, n)
      for i in 0..n.len-1:
        # we can bind symbols in scope via 'bindSym':
        add(result, newCall(bindSym"write", bindSym"stdout", toStrLit(n[i])))
        add(result, newCall(bindSym"write", bindSym"stdout", newStrLitNode(": ")))
        add(result, newCall(bindSym"writeln", bindSym"stdout", n[i]))

    var
      a: array [0..10, int]
      x = "some string"
    a[0] = 42
    a[1] = 45

    debug(a[0], a[1], x)

The macro call expands to:

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

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

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

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

## Statement Macros

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

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

    import macros

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

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

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: stmt) = nil

    proc p() {.m.} = nil

This is a simple syntactic transformation into:

    template m(s: stmt) = nil

    m:
      proc p() = nil

# Special Types

## typedesc

typedesc is a special type allowing one to treat types as compile-time values (i.e. if types are compile-time values and all values have a type, then typedesc must be their type).

When used as a regular proc param, typedesc acts as a type class. The proc will be instantiated for each unique type parameter and one can refer to the instantiation type using the param name:

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

    var n = TNode.new
    var tree = new(TBinaryTree[int])

When used with macros and .compileTime. procs on the other hand, the compiler does not need to instantiate the code multiple times, because types then can be manipulated using the unified internal symbol representation. In such context typedesc acts as any other type. One can create variables, store typedesc values inside containers and so on. For example, here is how one can create a type-safe wrapper for the unsafe printf function from C:

    macro safePrintF(formatString: string{lit}, args: vararg[expr]): expr =
      var i = 0
      for c in formatChars(formatString):
        var expectedType = case c
          of 'c': char
          of 'd', 'i', 'x', 'X': int
          of 'f', 'e', 'E', 'g', 'G': float
          of 's': string
          of 'p': pointer
          else: EOutOfRange

        var actualType = args[i].getType
        inc i

        if expectedType == EOutOfRange:
          error c & " is not a valid format character"
        elif expectedType != actualType:
          error "type mismatch for argument ", i, ". expected type: ",
                expectedType.name, ", actual type: ", actualType.name

      # keep the original callsite, but use cprintf instead
      result = callsite()
      result[0] = newIdentNode(!"cprintf")

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

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

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

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

# Term rewriting macros

Term rewriting macros are macros or templates that have not only a name but also a pattern that is searched for after the semantic checking phase of the compiler: This means they provide an easy way to enhance the compilation pipeline with user defined optimizations:

    template optMul{`*`(a, 2)}(a: int): int = a+a

    let x = 3
    echo x * 2

The compiler now rewrites `x * 2` as `x + x`. The code inside the curlies is the pattern to match against. The operators `*`, `**`, `|`, `~` have a special meaning in patterns if they are written in infix notation, so to match verbatim against `*` the ordinary function call syntax needs to be used.

Unfortunately optimizations are hard to get right and even the tiny example is wrong:

    template optMul{`*`(a, 2)}(a: int): int = a+a

    proc f(): int =
      echo "side effect!"
      result = 55

    echo f() * 2

We cannot duplicate 'a' if it denotes an expression that has a side effect! Fortunately Nimrod supports side effect analysis:

    template optMul{`*`(a, 2)}(a: int{noSideEffect}): int = a+a

    proc f(): int =
      echo "side effect!"
      result = 55

    echo f() * 2 # not optimized ;-)

So what about `2 * a`? We should tell the compiler `*` is commutative. We cannot really do that however as the following code only swaps arguments blindly:

    template mulIsCommutative{`*`(a, b)}(a, b: int): int = b*a

What optimizers really need to do is a canonicalization:

    template canonMul{`*`(a, b)}(a: int{lit}, b: int): int = b*a

The `int{lit}` parameter pattern matches against an expression of type `int`, but only if it's a literal.

## Parameter constraints

The parameter constraint expression can use the operators `|` (or), `&` (and) and `~` (not) and the following predicates:

Predicate | Meaning
--- | ---
`atom` | The matching node has no children.
`lit` | The matching node is a literal like "abc", 12.
`sym` | The matching node must be a symbol (a bound identifier).
`ident` | The matching node must be an identifier (an unbound identifier).
`call` | The matching AST must be a call/apply expression.
`lvalue` | The matching AST must be an lvalue.
`sideeffect` | The matching AST must have a side effect.
`nosideeffect` | The matching AST must have no side effect.
`param` | A symbol which is a parameter.
`genericparam` | A symbol which is a generic parameter.
`module` | A symbol which is a module.
`type` | A symbol which is a type.
`var` | A symbol which is a variable.
`let` | A symbol which is a `let` variable.
`const` | A symbol which is a constant.
`result` | The special `result` variable.
`proc` | A symbol which is a proc.
`method` | A symbol which is a method.
`iterator` | A symbol which is an iterator.
`converter` | A symbol which is a converter.
`macro` | A symbol which is a macro.
`template` | A symbol which is a template.
`field` | A symbol which is a field in a tuple or an object.
`enumfield` | A symbol which is a field in an enumeration.
`forvar` | A for loop variable.
`label` | A label (used in `block` statements).
`nk*` | The matching AST must have the specified kind. (Example: `nkIfStmt` denotes an `if` statement.)
`alias` | States that the marked parameter needs to alias with some other parameter.
`noalias` | States that every other parameter must not alias with the marked parameter.

The `alias` and `noalias` predicates refer not only to the matching AST, but also to every other bound parameter; syntactially they need to occur after the ordinary AST predicates:

    template ex{a = b + c}(a: int{noalias}, b, c: int) =
      # this transformation is only valid if 'b' and 'c' do not alias 'a':
      a = b
      inc a, b

## Pattern operators

The operators `*`, `**`, `|`, `~` have a special meaning in patterns if they are written in infix notation.

### The `|` operator

The `|` operator if used as infix operator creates an ordered choice:

    template t{0|1}(): expr = 3
    let a = 1
    # outputs 3:
    echo a

The matching is performed after the compiler performed some optimizations like constant folding, so the following does not work:

    template t{0|1}(): expr = 3
    # outputs 1:
    echo 1

The reason is that the compiler already transformed the 1 into "1" for the `echo` statement. However, a term rewriting macro should not change the semantics anyway. In fact they can be deactived with the `--patterns:off` command line option or temporarily with the `patterns` pragma.

### The `{}` operator

A pattern expression can be bound to a pattern parameter via the `expr{param}` notation:

    template t{(0|1|2){x}}(x: expr): expr = x+1
    let a = 1
    # outputs 2:
    echo a

### The `~` operator

The `~` operator is the not operator in patterns:

    template t{x = (~x){y} and (~x){z}}(x, y, z: bool): stmt =
      x = y
      if x: x = z

    var
      a = false
      b = true
      c = false
    a = b and c
    echo a

### The `*` operator

The `*` operator can flatten a nested binary expression like `a & b & c` to `&(a, b, c)`:

    var
      calls = 0

    proc `&&`(s: varargs[string]): string =
      result = s[0]
      for i in 1..len(s)-1: result.add s[i]
      inc calls

    template optConc{ `&&` * a }(a: string): expr = &&a

    let space = " "
    echo "my" && (space & "awe" && "some " ) && "concat"

    # check that it's been optimized properly:
    doAssert calls == 1

The second operator of * must be a parameter; it is used to gather all the arguments. The expression `"my" && (space & "awe" && "some " ) && "concat"` is passed to `optConc` in `a` as a special list (of kind `nkArgList`) which is flattened into a call expression; thus the invocation of `optConc` produces:

    `&&`("my", space & "awe", "some ", "concat")

### The `**` operator

The `**` is much like the `*` operator, except that it gathers not only all the arguments, but also the matched operators in reverse polish notation:

    import macros

    type
      TMatrix = object
        dummy: int

    proc `*`(a, b: TMatrix): TMatrix = nil
    proc `+`(a, b: TMatrix): TMatrix = nil
    proc `-`(a, b: TMatrix): TMatrix = nil
    proc `$`(a: TMatrix): string = result = $a.dummy
    proc mat21(): TMatrix =
      result.dummy = 21

    macro optM{ (`+`|`-`|`*`) ** a }(a: TMatrix): expr =
      echo treeRepr(a)
      result = newCall(bindSym"mat21")

    var x, y, z: TMatrix

    echo x + y * z - x

This passes the expression `x + y * z - x` to the `optM` macro as an `nnkArgList` node containing:

    Arglist
      Sym "x"
      Sym "y"
      Sym "z"
      Sym "*"
      Sym "+"
      Sym "x"
      Sym "-"

(Which is the reverse polish notation of `x + y * z - x`.)

## Parameters

Parameters in a pattern are type checked in the matching process. If a parameter is of the type `varargs` it is treated specially and it can match 0 or more arguments in the AST to be matched against:

    template optWrite{
      write(f, x)
      ((write|writeln){w})(f, y)
    }(x, y: varargs[expr], f: TFile, w: expr) =
      w(f, x, y)

## Example: Partial evaluation

The following example shows how some simple partial evaluation can be implemented with term rewriting:

    proc p(x, y: int; cond: bool): int =
      result = if cond: x + y else: x - y

    template optP1{p(x, y, true)}(x, y: expr): expr = x + y
    template optP2{p(x, y, false)}(x, y: expr): expr = x - y

## Example: hoisting

The following example how some form of hoisting can be implemented:

    import pegs

    template optPeg{peg(pattern)}(pattern: string{lit}): TPeg =
      var gl {.global, gensym.} = peg(pattern)
      gl

    for i in 0 .. 3:
      echo match("(a b c)", peg"'(' @ ')'")
      echo match("W_HI_Le", peg"\y 'while'")

The `optPeg` template optimizes the case of a peg constructor with a string literal, so that the pattern will only be parsed once at program startup and stored in a global `gl` which is then re-used. This optimization is called hoisting because it is comparable to classical loop hoisting.

# AST based overloading

Parameter constraints can also be used for ordinary routine parameters; these constraints affect ordinary overloading resolution then:

    proc optLit(a: string{lit|`const`}) =
      echo "string literal"
    proc optLit(a: string) =
      echo "no string literal"

    const
      constant = "abc"

    var
      variable = "xyz"

    optLit("literal")
    optLit(constant)
    optLit(variable)

However, the constraints `alias` and `noalias` are not available in ordinary routines.

## Move optimization

The `call` constraint is particularly useful to implement a move optimization for types that have copying semantics:

    proc `[]=`*(t: var TTable, key: string, val: string) =
      ## puts a (key, value)-pair into `t`. The semantics of string require
      ## a copy here:
      let idx = findInsertionPosition(key)
      t[idx] = key
      t[idx] = val

    proc `[]=`*(t: var TTable, key: string{call}, val: string{call}) =
      ## puts a (key, value)-pair into `t`. Optimized version that knows that
      ## the strings are unique and thus don't need to be copied:
      let idx = findInsertionPosition(key)
      shallowCopy t[idx], key
      shallowCopy t[idx], val

    var t: TTable
    # overloading resolution ensures that the optimized []= is called here:
    t["abc"] = "xyz"

# Modules

Nimrod 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.

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
      return x + 1

### Import statement

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

    import strutils except `%`

    # doesn't work then:
    echo "$1" % "abc"

### From import statement

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

    from strutils import `%`

    echo "$1" % "abc"
    # always possible: full qualification:
    echo strutils.replace("abc", "a", "z")

It's also possible to use `from module import nil` if one wants to import the module but wants to enforce fully qualified access to every symbol in `module`.

### Export statement

An export statement can be used for symbol fowarding so that client modules don't need to import a module's dependencies:

    # module B
    type TMyObject* = object

    # module A
    import B
    export B.TMyObject

    proc `$`*(x: TMyObject): string = "my object"

    # module C
    import A

    # B.TMyObject has been imported implicitly here:
    var x: TMyObject
    echo($x)

## Scope rules

Identifiers are valid from the point of their declaration until the end of the block in which the declaration occurred. The range where the identifier is known is the scope of the identifier. The exact scope of an identifier depends on the way it was declared.

### Block scope

The scope of a variable declared in the declaration part of a block is valid from the point of declaration until the end of the block. If a block contains a second block, in which the identifier is redeclared, then inside this block, the second declaration will be valid. Upon leaving the inner block, the first declaration is valid again. An identifier cannot be redefined in the same block, except if valid for procedure or iterator overloading purposes.

### Tuple or object scope

The field identifiers inside a tuple or object definition are valid in the following places:

- To the end of the tuple/object definition.
- Field designators of a variable of the given tuple/object type.
- In all descendant types of the object type.

### Module scope

All identifiers of a module are valid from the point of declaration until the end of the module. Identifiers from indirectly dependent modules are not available. The system module is automatically imported in every other module.

If a module imports an identifier by two different modules, each occurrence of the identifier has to be qualified, unless it is an overloaded procedure or iterator in which case the overloading resolution takes place:

    # 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

# Compiler Messages

The Nimrod 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 Nimrod's method to give the compiler additional information / commands without introducing a massive number of new keywords. Pragmas are processed on the fly during semantic checking. Pragmas are enclosed in the special `{.` and `.}` curly brackets. Pragmas are also often used as a first implementation to play with a language feature before a nicer syntax to access the feature becomes available.

## noSideEffect pragma

The noSideEffect 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`.

Future directions: `func` may become a keyword and syntactic sugar for a proc with no side effects:

    func `+` (x, y: int): int

## destructor pragma

The destructor pragma is used to mark a proc to act as a type destructor. The proc must have a single parameter with a concrete type (the name of a generic type is allowed too).

Destructors will be automatically invoked when a local stack variable goes out of scope.

If a record type features a field with destructable type and the user have not provided explicit implementation, Nimrod will automatically generate a destructor for the record type. Nimrod will automatically insert calls to any base class destructors in both user-defined and generated destructors.

A destructor is attached to the type it destructs; expressions of this type can then only be used in destructible contexts and as parameters:

    type
      TMyObj = object
        x, y: int
        p: pointer

    proc destruct(o: var TMyObj) {.destructor.} =
      if o.p != nil: dealloc o.p

    proc open: TMyObj =
      result = TMyObj(x: 1, y: 2, p: alloc(3))

    proc work(o: TMyObj) =
      echo o.x
      # No destructor invoked here for 'o' as 'o' is a parameter.

    proc main() =
      # destructor automatically invoked at the end of the scope:
      var x = open()
      # valid: pass 'x' to some other proc:
      work(x)

      # Error: usage of a type with a destructor in a non destructible context
      echo open()

A destructible context is currently only the following:

1. The `expr` in `var x = expr`.
2. The `expr` in `let x = expr`.
3. The `expr` in `return expr`.
4. The `expr` in `result = expr` where `result` is the special symbol introduced by the compiler.

These rules ensure that the construction is tied to a variable and can easily be destructed at its scope exit. Later versions of the language will improve the support of destructors.

## procvar pragma

The procvar pragma is used to mark a proc that it can be passed to a procedural variable.

## compileTime pragma

The compileTime pragma is used to mark a proc to be used at compile time only. No code will be generated for it. Compile time procs are useful as helpers for macros.

## noReturn pragma

The noreturn pragma is used to mark a proc that never returns.

## Acyclic pragma

The acyclic pragma can be used for object types to mark them as acyclic even though they seem to be cyclic. This is an optimization for the garbage collector to not consider objects of this type as part of a cycle:

    type
      PNode = ref TNode
      TNode {.acyclic, final.} = object
        left, right: PNode
        data: string

In the example a tree structure is declared with the `TNode` type. Note that the type definition is recursive and the GC has to assume that objects of this type may form a cyclic graph. The `acyclic` pragma passes the information that this cannot happen to the GC. If the programmer uses the `acyclic` pragma for data types that are in reality cyclic, the GC may leak memory, but nothing worse happens.

Future directions: The `acyclic` pragma may become a property of a `ref` type:

    type
      PNode = acyclic ref TNode
      TNode = object
        left, right: PNode
        data: string

## Final pragma

The final pragma can be used for an object type to specify that it cannot be inherited from.

## shallow pragma

The shallow pragma affects the semantics of a type: The compiler is allowed to make a shallow copy. This can cause serious semantic issues and break memory safety! However, it can speed up assignments considerably, because the semantics of Nimrod require deep copying of sequences and strings. This can be expensive, especially if sequences are used to build a tree structure:

    type
      TNodeKind = enum nkLeaf, nkInner
      TNode {.final, shallow.} = object
        case kind: TNodeKind
        of nkLeaf:
          strVal: string
        of nkInner:
          children: seq[TNode]

## Pure pragma

An object type can be marked with the pure pragma so that its type field which is used for runtime type identification is omitted. This is necessary for binary compatibility with other compiled languages.

## NoStackFrame pragma

A proc can be marked with the noStackFrame 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. This is useful for procs that only consist of an assembler statement.

## error pragma

The error pragma is used to make the compiler output an error message with the given content. Compilation does not necessarily abort after an error though.

The `error` pragma can also be used to annotate a symbol (like an iterator or proc). The usage of the symbol then triggers a compile-time error. This is especially useful to rule out that some operation is valid due to overloading and type conversions:

    ## 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.

## 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: expr, msg = "") =
      if not cond:
        # change run-time line information of the 'raise' statement:
        {.line: InstantiationInfo().}:
          raise newException(EAssertionFailed, msg)

If the `line` pragma is used with a parameter, the parameter needs be a `tuple[filename: string, line: int]`. If it is used without a parameter, `system.InstantiationInfo()` is used.

## linearScanEnd pragma

The linearScanEnd pragma can be used to tell the compiler how to compile a Nimrod 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.

## unroll pragma

The unroll pragma can be used to tell the compiler that it should unroll a for or while loop for runtime 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

See Ordinary vs immediate templates.

## compilation option pragmas

The listed pragmas here can be used to override the code generation options for a section of code.

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

pragma | allowed values | description
--- | --- | ---
checks | on|off | Turns the code generation for all runtime checks on or off.
boundChecks | on|off | Turns the code generation for array bound checks on or off.
overflowChecks | on|off | Turns the code generation for over- or underflow checks on or off.
nilChecks | on|off | Turns the code generation for nil pointer checks on or off.
assertions | on|off | Turns the code generation for assertions on or off.
warnings | on|off | Turns the warning messages of the compiler on or off.
hints | on|off | Turns the hint messages of the compiler on or off.
optimization | none|speed|size | Optimize the code for speed or size, or disable optimization.
patterns | on|off | Turns the term rewriting templates/macros on or off.
callconv | cdecl|... | Specifies the default calling convention for all procedures (and procedure types) that follow.

Example:

    {.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 an bytecode interpreter for example) it may provide benefits, though.

## global pragma

The global pragma can be applied to a variable within a proc to instruct the compiler to store it in a global location and initialize it once at program startup.

    proc isHexNumber(s: string): bool =
      var pattern {.global.} = re"[0-9a-fA-F]+"
      result = s.match(pattern)

When used within a generic proc, a separate unique global variable will be created for each instantiation of the proc. The order of initialization of the created global variables within a module is not defined, but all of them will be initialized after any top-level variables in their originating module and before any variable in a module that imports it.

## DeadCodeElim pragma

The deadCodeElim pragma only applies to whole modules: It tells the compiler to activate (or deactivate) dead code elimination for the module the pragma appears in.

The `--deadCodeElim:on` command line switch has the same effect as marking every module with `{.deadCodeElim:on}`. However, for some modules such as the GTK wrapper it makes sense to always turn on dead code elimination - no matter if it is globally active or not.

Example:

    {.deadCodeElim: on.}

## NoForward pragma

The noforward pragma can be used to turn on and off a special compilation mode that to large extent eliminates the need for forward declarations. In this mode, the proc definitions may appear out of order and the compiler will postpone their semantic analysis and compilation until it actually needs to generate code using the definitions. In this regard, this mode is similar to the modus operandi of dynamic scripting languages, where the function calls are not resolved until the code is executed. Here is the detailed algorithm taken by the compiler:

1. When a callable symbol is first encountered, the compiler will only note the symbol callable name and it will add it to the appropriate overload set in the current scope. At this step, it won't try to resolve any of the type expressions used in the signature of the symbol (so they can refer to other not yet defined symbols).

2. When a top level call is encountered (usually at the very end of the module), the compiler will try to determine the actual types of all of the symbols in the matching overload set. This is a potentially recursive process as the signatures of the symbols may include other call expressions, whoose types will be resolved at this point too.

3. Finally, after the best overload is picked, the compiler will start compiling the body of the respective symbol. This in turn will lead the compiler to discover more call expresions that need to be resolved and steps 2 and 3 will be repeated as necessary.

Please note that if a callable symbol is never used in this scenario, its body will never be compiled. This is the default behavior leading to best compilation times, but if exhaustive compilation of all definitions is required, using `nimrod check` provides this option as well.

Example:

    {.noforward: on.}

    proc foo(x: int) =
      bar x

    proc bar(x: int) =
      echo x

    foo(10)

## Pragma pragma

The pragma pragma can be used to declare user defined pragmas. This is useful because Nimrod's templates and macros do not affect pragmas. User defined pragmas are in a different module-wide scope than all other symbols. They cannot be imported from a module.

Example:

    when appType == "lib":
      {.pragma: rtl, exportc, dynlib, cdecl.}
    else:
      {.pragma: rtl, importc, dynlib: "client.dll", cdecl.}

    proc p*(a, b: int): int {.rtl.} =
      return a+b

In the example a new pragma named `rtl` is introduced that either imports a symbol from a dynamic library or exports the symbol for dynamic library generation.

## Disabling certain messages

Nimrod generates some warnings and hints ("line too long") that may annoy the user. A mechanism for disabling certain messages is provided: Each hint and warning message contains a symbol in brackets. This is the message's identifier that can be used to enable or disable it:

    {.warning[LineTooLong]: off.} # turn off warning about too long lines

This is often better than disabling all warnings at once.

# Foreign function interface

Nimrod's FFI (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 Nimrod identifier exactly as spelled:

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

Note that this pragma is somewhat of a misnomer: Other backends will provide the same feature under the same name.

## Exportc pragma

The exportc pragma provides a means to export a type, a variable, or a procedure to C. The optional argument is a string containing the C identifier. If the argument is missing, the C name is the Nimrod identifier exactly as spelled:

    proc callme(formatstr: cstring) {.exportc: "callMe", varargs.}

Note that this pragma is somewhat of a misnomer: Other backends will provide the same feature under the same name.

## 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
      TVector {.bycopy, pure.} = object
        x, y, z: float

## Byref pragma

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

## Varargs pragma

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

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

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

## 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
      quit("could not load dynamic library")

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

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

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

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

## Dynlib pragma for export

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

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

This is only useful if the program is compiled as a dynamic library via the `--app:lib` command line option.

# Threads

Even though Nimrod's thread support and semantics are preliminary, they should be quite usable already. 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 thread API.

Nimrod's memory model for threads is quite different than that of other common programming languages (C, Pascal, Java): Each thread has its own (garbage collected) heap and sharing of memory is restricted to global variables. This helps to prevent race conditions. GC efficiency is improved quite a lot, because the GC never has to stop other threads and see what they reference. Memory allocation requires no lock at all! This design easily scales to massive multicore processors that will become the norm in the future.

## Thread pragma

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

Since the semantic checking of threads requires whole program analysis, it is quite expensive and can be turned off with `--threadanalysis:off` to improve compile times.

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

## Threadvar pragma

A global variable can be marked with the threadvar pragma; it is a thread-local variable then:

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

Due to implementation restrictions thread local variables cannot be initialized within the `var` section. (Every thread local variable needs to be replicated at thread creation.)

## Actor model

Caution: This section is already outdated! XXX

Nimrod supports the actor model of concurrency natively:

    type
      TMsgKind = enum
        mLine, mEof
      TMsg = object
        case k: TMsgKind
        of mEof: nil
        of mLine: data: string

    var
      thr: TThread[TMsg]
      printedLines = 0
      m: TMsg

    proc print() {.thread.} =
      while true:
        var x = recv[TMsg]()
        if x.k == mEof: break
        echo x.data
        discard atomicInc(printedLines)

    createThread(thr, print)

    var input = open("readme.txt")
    while not endOfFile(input):
      m.data = input.readLine()
      thr.send(m)
    close(input)
    m.k = mEof
    thr.send(m)
    joinThread(thr)

    echo printedLines

In the actor model threads communicate only over sending messages (send and recv built-ins), not by sharing memory. Every thread has an inbox that keeps incoming messages until the thread requests a new message via the `recv` operation. The inbox is an unlimited FIFO queue.

In the above example the `print` thread also communicates with its parent thread over the `printedLines` global variable. In general, it is highly advisable to only read from globals, but not to write to them. In fact a write to a global that contains GC'ed memory is always wrong, because it violates the no heap sharing restriction:

    var
      global: string
      t: TThread[string]

    proc horrible() {.thread.} =
      global = "string in thread local heap!"

    createThread(t, horrible)
    joinThread(t)

For the above code the compiler produces "Warning: write to foreign heap". This warning might become an error message in future versions of the compiler.

Creating a thread is an expensive operation, because a new stack and heap needs to be created for the thread. It is therefore highly advisable that a thread handles a large amount of work. Nimrod prefers coarse grained over fine grained concurrency.

## Threads and exceptions

The interaction between threads and exceptions is simple: A handled exception in one thread cannot affect any other thread. However, an unhandled exception in one thread terminates the whole process!

# Taint mode

The Nimrod compiler and most parts of the standard library support a taint mode. Input strings are declared with the TaintedString string type declared in the `system` module.

If the taint mode is turned on (via the `--taintMode:on` command line option) it is a distinct string type which helps to detect input validation errors:

    echo "your name: "
    var name: TaintedString = stdin.readline
    # it is safe here to output the name without any input validation, so
    # we simply convert `name` to string to make the compiler happy:
    echo "hi, ", name.string

If the taint mode is turned off, `TaintedString` is simply an alias for `string`.

Generated: 2013-05-21 09:55:04 UTC