## Introduction

This is a reference manual for the Go programming language. For more information and other documents, see golang.org.

Go is a general-purpose language designed with systems programming in mind. It is strongly typed and garbage-collected and has explicit support for concurrent programming. Programs are constructed from packages, whose properties allow efficient management of dependencies. The existing implementations use a traditional compile/link model to generate executable binaries.

The grammar is compact and regular, allowing for easy analysis by automatic tools such as integrated development environments.

## Notation

The syntax is specified using Extended Backus-Naur Form (EBNF):

    Production  = production_name "=" [ Expression ] "." .
    Expression  = Alternative { "|" Alternative } .
    Alternative = Term { Term } .
    Term        = production_name | token [ "…" token ] | Group | Option | Repetition .
    Group       = "(" Expression ")" .
    Option      = "[" Expression "]" .
    Repetition  = "{" Expression "}" .

Productions are expressions constructed from terms and the following operators, in increasing precedence:

    |   alternation
    ()  grouping
    []  option (0 or 1 times)
    {}  repetition (0 to n times)

Lower-case production names are used to identify lexical tokens. Non-terminals are in CamelCase. Lexical tokens are enclosed in double quotes `""` or back quotes ````.

The form `a … b` represents the set of characters from `a` through `b` as alternatives. The horizontal ellipsis `…` is also used elsewhere in the spec to informally denote various enumerations or code snippets that are not further specified. The character `…` (as opposed to the three characters `...`) is not a token of the Go language.

## Source code representation

Source code is Unicode text encoded in UTF-8. The text is not canonicalized, so a single accented code point is distinct from the same character constructed from combining an accent and a letter; those are treated as two code points. For simplicity, this document will use the unqualified term character to refer to a Unicode code point in the source text.

Each code point is distinct; for instance, upper and lower case letters are different characters.

Implementation restriction: For compatibility with other tools, a compiler may disallow the NUL character (U+0000) in the source text.

Implementation restriction: For compatibility with other tools, a compiler may ignore a UTF-8-encoded byte order mark (U+FEFF) if it is the first Unicode code point in the source text. A byte order mark may be disallowed anywhere else in the source.

### Characters

The following terms are used to denote specific Unicode character classes:

    newline        = /* the Unicode code point U+000A */ .
    unicode_char   = /* an arbitrary Unicode code point except newline */ .
    unicode_letter = /* a Unicode code point classified as "Letter" */ .
    unicode_digit  = /* a Unicode code point classified as "Decimal Digit" */ .

In The Unicode Standard 6.3, Section 4.5 "General Category" defines a set of character categories. Go treats those characters in category Lu, Ll, Lt, Lm, or Lo as Unicode letters, and those in category Nd as Unicode digits.

### Letters and digits

The underscore character `_` (U+005F) is considered a letter.

    letter        = unicode_letter | "_" .
    decimal_digit = "0" … "9" .
    octal_digit   = "0" … "7" .
    hex_digit     = "0" … "9" | "A" … "F" | "a" … "f" .

## Lexical elements

### Comments

There are two forms of comments:

1. Line comments start with the character sequence `//` and stop at the end of the line. A line comment acts like a newline.
2. General comments start with the character sequence `/*` and continue through the character sequence `*/`. A general comment containing one or more newlines acts like a newline, otherwise it acts like a space.

Comments do not nest.

### Tokens

Tokens form the vocabulary of the Go language. There are four classes: identifiers, keywords, operators and delimiters, and literals. White space, formed from spaces (U+0020), horizontal tabs (U+0009), carriage returns (U+000D), and newlines (U+000A), is ignored except as it separates tokens that would otherwise combine into a single token. Also, a newline or end of file may trigger the insertion of a semicolon. While breaking the input into tokens, the next token is the longest sequence of characters that form a valid token.

### Semicolons

The formal grammar uses semicolons `";"` as terminators in a number of productions. Go programs may omit most of these semicolons using the following two rules:

1. When the input is broken into tokens, a semicolon is automatically inserted into the token stream at the end of a non-blank line if the line's final token is

   - an identifier
   - an integer, floating-point, imaginary, rune, or string literal
   - one of the keywords `break`, `continue`, `fallthrough`, or `return`
   - one of the operators and delimiters `++`, `--`, `)`, `]`, or `}`
2. To allow complex statements to occupy a single line, a semicolon may be omitted before a closing `")"` or `"}"`.

To reflect idiomatic use, code examples in this document elide semicolons using these rules.

### Identifiers

Identifiers name program entities such as variables and types. An identifier is a sequence of one or more letters and digits. The first character in an identifier must be a letter.

    identifier = letter { letter | unicode_digit } .

    a
    _x9
    ThisVariableIsExported
    αβ

Some identifiers are predeclared.

### Keywords

The following keywords are reserved and may not be used as identifiers.

    break        default      func         interface    select
    case         defer        go           map          struct
    chan         else         goto         package      switch
    const        fallthrough  if           range        type
    continue     for          import       return       var

### Operators and Delimiters

The following character sequences represent operators, delimiters, and other special tokens:

    +    &     +=    &=     &&    ==    !=    (    )
    -    |     -=    |=     ||    <     <=    [    ]
    *    ^     *=    ^=     <-    >     >=    {    }
    /    <<    /=    <<=    ++    =     :=    ,    ;
    %    >>    %=    >>=    --    !     ...   .    :
         &^          &^=

### Integer literals

An integer literal is a sequence of digits representing an integer constant. An optional prefix sets a non-decimal base: `0` for octal, `0x` or `0X` for hexadecimal. In hexadecimal literals, letters `a-f` and `A-F` represent values 10 through 15.

    int_lit     = decimal_lit | octal_lit | hex_lit .
    decimal_lit = ( "1" … "9" ) { decimal_digit } .
    octal_lit   = "0" { octal_digit } .
    hex_lit     = "0" ( "x" | "X" ) hex_digit { hex_digit } .

    42
    0600
    0xBadFace
    170141183460469231731687303715884105727

### Floating-point literals

A floating-point literal is a decimal representation of a floating-point constant. It has an integer part, a decimal point, a fractional part, and an exponent part. The integer and fractional part comprise decimal digits; the exponent part is an `e` or `E` followed by an optionally signed decimal exponent. One of the integer part or the fractional part may be elided; one of the decimal point or the exponent may be elided.

    float_lit = decimals "." [ decimals ] [ exponent ] |
                decimals exponent |
                "." decimals [ exponent ] .
    decimals  = decimal_digit { decimal_digit } .
    exponent  = ( "e" | "E" ) [ "+" | "-" ] decimals .

    0.
    72.40
    072.40  // == 72.40
    2.71828
    1.e+0
    6.67428e-11
    1E6
    .25
    .12345E+5

### Imaginary literals

An imaginary literal is a decimal representation of the imaginary part of a complex constant. It consists of a floating-point literal or decimal integer followed by the lower-case letter `i`.

    imaginary_lit = (decimals | float_lit) "i" .

    0i
    011i  // == 11i
    0.i
    2.71828i
    1.e+0i
    6.67428e-11i
    1E6i
    .25i
    .12345E+5i

### Rune literals

A rune literal represents a rune constant, an integer value identifying a Unicode code point. A rune literal is expressed as one or more characters enclosed in single quotes. Within the quotes, any character may appear except single quote and newline. A single quoted character represents the Unicode value of the character itself, while multi-character sequences beginning with a backslash encode values in various formats.

The simplest form represents the single character within the quotes; since Go source text is Unicode characters encoded in UTF-8, multiple UTF-8-encoded bytes may represent a single integer value. For instance, the literal `'a'` holds a single byte representing a literal `a`, Unicode U+0061, value `0x61`, while `'ä'` holds two bytes (`0xc3` `0xa4`) representing a literal `a`-dieresis, U+00E4, value `0xe4`.

Several backslash escapes allow arbitrary values to be encoded as ASCII text. There are four ways to represent the integer value as a numeric constant: `\x` followed by exactly two hexadecimal digits; `\u` followed by exactly four hexadecimal digits; `\U` followed by exactly eight hexadecimal digits, and a plain backslash `\` followed by exactly three octal digits. In each case the value of the literal is the value represented by the digits in the corresponding base.

Although these representations all result in an integer, they have different valid ranges. Octal escapes must represent a value between 0 and 255 inclusive. Hexadecimal escapes satisfy this condition by construction. The escapes `\u` and `\U` represent Unicode code points so within them some values are illegal, in particular those above `0x10FFFF` and surrogate halves.

After a backslash, certain single-character escapes represent special values:

    \a   U+0007 alert or bell
    \b   U+0008 backspace
    \f   U+000C form feed
    \n   U+000A line feed or newline
    \r   U+000D carriage return
    \t   U+0009 horizontal tab
    \v   U+000b vertical tab
    \\   U+005c backslash
    \'   U+0027 single quote  (valid escape only within rune literals)
    \"   U+0022 double quote  (valid escape only within string literals)

All other sequences starting with a backslash are illegal inside rune literals.

    rune_lit         = "'" ( unicode_value | byte_value ) "'" .
    unicode_value    = unicode_char | little_u_value | big_u_value | escaped_char .
    byte_value       = octal_byte_value | hex_byte_value .
    octal_byte_value = `\` octal_digit octal_digit octal_digit .
    hex_byte_value   = `\` "x" hex_digit hex_digit .
    little_u_value   = `\` "u" hex_digit hex_digit hex_digit hex_digit .
    big_u_value      = `\` "U" hex_digit hex_digit hex_digit hex_digit
                               hex_digit hex_digit hex_digit hex_digit .
    escaped_char     = `\` ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | `\` | "'" | `"` ) .

    'a'
    'ä'
    '本'
    '\t'
    '\000'
    '\007'
    '\377'
    '\x07'
    '\xff'
    '\u12e4'
    '\U00101234'
    'aa'         // illegal: too many characters
    '\xa'        // illegal: too few hexadecimal digits
    '\0'         // illegal: too few octal digits
    '\uDFFF'     // illegal: surrogate half
    '\U00110000' // illegal: invalid Unicode code point

### String literals

A string literal represents a string constant obtained from concatenating a sequence of characters. There are two forms: raw string literals and interpreted string literals.

Raw string literals are character sequences between back quotes ````. Within the quotes, any character is legal except back quote. The value of a raw string literal is the string composed of the uninterpreted (implicitly UTF-8-encoded) characters between the quotes; in particular, backslashes have no special meaning and the string may contain newlines. Carriage return characters ('\r') inside raw string literals are discarded from the raw string value.

Interpreted string literals are character sequences between double quotes `""`. The text between the quotes, which may not contain newlines, forms the value of the literal, with backslash escapes interpreted as they are in rune literals (except that `\'` is illegal and `\"` is legal), with the same restrictions. The three-digit octal (`\`nnn) and two-digit hexadecimal (`\x` nn) escapes represent individual bytes of the resulting string; all other escapes represent the (possibly multi-byte) UTF-8 encoding of individual characters. Thus inside a string literal `\377` and `\xFF` represent a single byte of value `0xFF`=255, while `ÿ`, `\u00FF`, `\U000000FF` and `\xc3\xbf` represent the two bytes `0xc3` `0xbf` of the UTF-8 encoding of character U+00FF.

    string_lit             = raw_string_lit | interpreted_string_lit .
    raw_string_lit         = "`" { unicode_char | newline } "`" .
    interpreted_string_lit = `"` { unicode_value | byte_value } `"` .

    `abc`  // same as "abc"
    `\n
    \n`    // same as "\\n\n\\n"
    "\n"
    ""
    "Hello, world!\n"
    "日本語"
    "\u65e5本\U00008a9e"
    "\xff\u00FF"
    "\uD800"       // illegal: surrogate half
    "\U00110000"   // illegal: invalid Unicode code point

These examples all represent the same string:

    "日本語"                                 // UTF-8 input text
    `日本語`                                 // UTF-8 input text as a raw literal
    "\u65e5\u672c\u8a9e"                    // the explicit Unicode code points
    "\U000065e5\U0000672c\U00008a9e"        // the explicit Unicode code points
    "\xe6\x97\xa5\xe6\x9c\xac\xe8\xaa\x9e"  // the explicit UTF-8 bytes

If the source code represents a character as two code points, such as a combining form involving an accent and a letter, the result will be an error if placed in a rune literal (it is not a single code point), and will appear as two code points if placed in a string literal.

## Constants

There are boolean constants, rune constants, integer constants, floating-point constants, complex constants, and string constants. Rune, integer, floating-point, and complex constants are collectively called numeric constants.

A constant value is represented by a rune, integer, floating-point, imaginary, or string literal, an identifier denoting a constant, a constant expression, a conversion with a result that is a constant, or the result value of some built-in functions such as `unsafe.Sizeof` applied to any value, `cap` or `len` applied to some expressions, `real` and `imag` applied to a complex constant and `complex` applied to numeric constants. The boolean truth values are represented by the predeclared constants `true` and `false`. The predeclared identifier iota denotes an integer constant.

In general, complex constants are a form of constant expression and are discussed in that section.

Numeric constants represent values of arbitrary precision and do not overflow.

Constants may be typed or untyped. Literal constants, `true`, `false`, `iota`, and certain constant expressions containing only untyped constant operands are untyped.

A constant may be given a type explicitly by a constant declaration or conversion, or implicitly when used in a variable declaration or an assignment or as an operand in an expression. It is an error if the constant value cannot be represented as a value of the respective type. For instance, `3.0` can be given any integer or any floating-point type, while `2147483648.0` (equal to `1<<31`) can be given the types `float32`, `float64`, or `uint32` but not `int32` or `string`.

An untyped constant has a default type which is the type to which the constant is implicitly converted in contexts where a typed value is required, for instance, in a short variable declaration such as `i := 0` where there is no explicit type. The default type of an untyped constant is `bool`, `rune`, `int`, `float64`, `complex128` or `string` respectively, depending on whether it is a boolean, rune, integer, floating-point, complex, or string constant.

There are no constants denoting the IEEE-754 infinity and not-a-number values, but the `math` package's Inf, NaN, IsInf, and IsNaN functions return and test for those values at run time.

Implementation restriction: Although numeric constants have arbitrary precision in the language, a compiler may implement them using an internal representation with limited precision. That said, every implementation must:

- Represent integer constants with at least 256 bits.
- Represent floating-point constants, including the parts of a complex constant, with a mantissa of at least 256 bits and a signed exponent of at least 32 bits.
- Give an error if unable to represent an integer constant precisely.
- Give an error if unable to represent a floating-point or complex constant due to overflow.
- Round to the nearest representable constant if unable to represent a floating-point or complex constant due to limits on precision.

These requirements apply both to literal constants and to the result of evaluating constant expressions.

## Variables

A variable is a storage location for holding a value. The set of permissible values is determined by the variable's type.

A variable declaration or, for function parameters and results, the signature of a function declaration or function literal reserves storage for a named variable. Calling the built-in function `new` or taking the address of a composite literal allocates storage for a variable at run time. Such an anonymous variable is referred to via a (possibly implicit) pointer indirection.

Structured variables of array, slice, and struct types have elements and fields that may be addressed individually. Each such element acts like a variable.

The static type (or just type) of a variable is the type given in its declaration, the type provided in the `new` call or composite literal, or the type of an element of a structured variable. Variables of interface type also have a distinct dynamic type, which is the concrete type of the value assigned to the variable at run time (unless the value is the predeclared identifier `nil`, which has no type). The dynamic type may vary during execution but values stored in interface variables are always assignable to the static type of the variable.

    var x interface{}  // x is nil and has static type interface{}
    var v *T           // v has value nil, static type *T
    x = 42             // x has value 42 and dynamic type int
    x = v              // x has value (*T)(nil) and dynamic type *T

A variable's value is retrieved by referring to the variable in an expression; it is the most recent value assigned to the variable. If a variable has not yet been assigned a value, its value is the zero value for its type.

## Types

A type determines the set of values and operations specific to values of that type. Types may be named or unnamed. Named types are specified by a (possibly qualified) type name; unnamed types are specified using a type literal, which composes a new type from existing types.

    Type      = TypeName | TypeLit | "(" Type ")" .
    TypeName  = identifier | QualifiedIdent .
    TypeLit   = ArrayType | StructType | PointerType | FunctionType | InterfaceType |
    	    SliceType | MapType | ChannelType .

Named instances of the boolean, numeric, and string types are predeclared. Composite types—array, struct, pointer, function, interface, slice, map, and channel types—may be constructed using type literals.

Each type `T` has an underlying type: If `T` is one of the predeclared boolean, numeric, or string types, or a type literal, the corresponding underlying type is `T` itself. Otherwise, `T`'s underlying type is the underlying type of the type to which `T` refers in its type declaration.

       type T1 string
       type T2 T1
       type T3 []T1
       type T4 T3

The underlying type of `string`, `T1`, and `T2` is `string`. The underlying type of `[]T1`, `T3`, and `T4` is `[]T1`.

### Method sets

A type may have a method set associated with it. The method set of an interface type is its interface. The method set of any other type `T` consists of all methods declared with receiver type `T`. The method set of the corresponding pointer type `*T` is the set of all methods declared with receiver `*T` or `T` (that is, it also contains the method set of `T`). Further rules apply to structs containing anonymous fields, as described in the section on struct types. Any other type has an empty method set. In a method set, each method must have a unique non-blank method name.

The method set of a type determines the interfaces that the type implements and the methods that can be called using a receiver of that type.

### Boolean types

A boolean type represents the set of Boolean truth values denoted by the predeclared constants `true` and `false`. The predeclared boolean type is `bool`.

### Numeric types

A numeric type represents sets of integer or floating-point values. The predeclared architecture-independent numeric types are:

    uint8       the set of all unsigned  8-bit integers (0 to 255)
    uint16      the set of all unsigned 16-bit integers (0 to 65535)
    uint32      the set of all unsigned 32-bit integers (0 to 4294967295)
    uint64      the set of all unsigned 64-bit integers (0 to 18446744073709551615)

    int8        the set of all signed  8-bit integers (-128 to 127)
    int16       the set of all signed 16-bit integers (-32768 to 32767)
    int32       the set of all signed 32-bit integers (-2147483648 to 2147483647)
    int64       the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807)

    float32     the set of all IEEE-754 32-bit floating-point numbers
    float64     the set of all IEEE-754 64-bit floating-point numbers

    complex64   the set of all complex numbers with float32 real and imaginary parts
    complex128  the set of all complex numbers with float64 real and imaginary parts

    byte        alias for uint8
    rune        alias for int32

The value of an n-bit integer is n bits wide and represented using two's complement arithmetic.

There is also a set of predeclared numeric types with implementation-specific sizes:

    uint     either 32 or 64 bits
    int      same size as uint
    uintptr  an unsigned integer large enough to store the uninterpreted bits of a pointer value

To avoid portability issues all numeric types are distinct except `byte`, which is an alias for `uint8`, and `rune`, which is an alias for `int32`. Conversions are required when different numeric types are mixed in an expression or assignment. For instance, `int32` and `int` are not the same type even though they may have the same size on a particular architecture.

### String types

A string type represents the set of string values. A string value is a (possibly empty) sequence of bytes. Strings are immutable: once created, it is impossible to change the contents of a string. The predeclared string type is `string`.

The length of a string `s` (its size in bytes) can be discovered using the built-in function `len`. The length is a compile-time constant if the string is a constant. A string's bytes can be accessed by integer indices 0 through `len(s)-1`. It is illegal to take the address of such an element; if `s[i]` is the `i`'th byte of a string, `&s[i]` is invalid.

### Array types

An array is a numbered sequence of elements of a single type, called the element type. The number of elements is called the length and is never negative.

    ArrayType   = "[" ArrayLength "]" ElementType .
    ArrayLength = Expression .
    ElementType = Type .

The length is part of the array's type; it must evaluate to a non-negative constant representable by a value of type `int`. The length of array `a` can be discovered using the built-in function `len`. The elements can be addressed by integer indices 0 through `len(a)-1`. Array types are always one-dimensional but may be composed to form multi-dimensional types.

    [32]byte
    [2*N] struct { x, y int32 }
    [1000]*float64
    [3][5]int
    [2][2][2]float64  // same as [2]([2]([2]float64))

### Slice types

A slice is a descriptor for a contiguous segment of an underlying array and provides access to a numbered sequence of elements from that array. A slice type denotes the set of all slices of arrays of its element type. The value of an uninitialized slice is `nil`.

    SliceType = "[" "]" ElementType .

Like arrays, slices are indexable and have a length. The length of a slice `s` can be discovered by the built-in function `len`; unlike with arrays it may change during execution. The elements can be addressed by integer indices 0 through `len(s)-1`. The slice index of a given element may be less than the index of the same element in the underlying array.

A slice, once initialized, is always associated with an underlying array that holds its elements. A slice therefore shares storage with its array and with other slices of the same array; by contrast, distinct arrays always represent distinct storage.

The array underlying a slice may extend past the end of the slice. The capacity is a measure of that extent: it is the sum of the length of the slice and the length of the array beyond the slice; a slice of length up to that capacity can be created by slicing a new one from the original slice. The capacity of a slice `a` can be discovered using the built-in function `cap(a)`.

A new, initialized slice value for a given element type `T` is made using the built-in function `make`, which takes a slice type and parameters specifying the length and optionally the capacity. A slice created with `make` always allocates a new, hidden array to which the returned slice value refers. That is, executing

    make([]T, length, capacity)

produces the same slice as allocating an array and slicing it, so these two expressions are equivalent:

    make([]int, 50, 100)
    new([100]int)[0:50]

Like arrays, slices are always one-dimensional but may be composed to construct higher-dimensional objects. With arrays of arrays, the inner arrays are, by construction, always the same length; however with slices of slices (or arrays of slices), the inner lengths may vary dynamically. Moreover, the inner slices must be initialized individually.

### Struct types

A struct is a sequence of named elements, called fields, each of which has a name and a type. Field names may be specified explicitly (IdentifierList) or implicitly (AnonymousField). Within a struct, non-blank field names must be unique.

    StructType     = "struct" "{" { FieldDecl ";" } "}" .
    FieldDecl      = (IdentifierList Type | AnonymousField) [ Tag ] .
    AnonymousField = [ "*" ] TypeName .
    Tag            = string_lit .

    // An empty struct.
    struct {}

    // A struct with 6 fields.
    struct {
    	x, y int
    	u float32
    	_ float32  // padding
    	A *[]int
    	F func()
    }

A field declared with a type but no explicit field name is an anonymous field, also called an embedded field or an embedding of the type in the struct. An embedded type must be specified as a type name `T` or as a pointer to a non-interface type name `*T`, and `T` itself may not be a pointer type. The unqualified type name acts as the field name.

    // A struct with four anonymous fields of type T1, *T2, P.T3 and *P.T4
    struct {
    	T1        // field name is T1
    	*T2       // field name is T2
    	P.T3      // field name is T3
    	*P.T4     // field name is T4
    	x, y int  // field names are x and y
    }

The following declaration is illegal because field names must be unique in a struct type:

    struct {
    	T     // conflicts with anonymous field *T and *P.T
    	*T    // conflicts with anonymous field T and *P.T
    	*P.T  // conflicts with anonymous field T and *T
    }

A field or method `f` of an anonymous field in a struct `x` is called promoted if `x.f` is a legal selector that denotes that field or method `f`.

Promoted fields act like ordinary fields of a struct except that they cannot be used as field names in composite literals of the struct.

Given a struct type `S` and a type named `T`, promoted methods are included in the method set of the struct as follows:

- If `S` contains an anonymous field `T`, the method sets of `S` and `*S` both include promoted methods with receiver `T`. The method set of `*S` also includes promoted methods with receiver `*T`.
- If `S` contains an anonymous field `*T`, the method sets of `S` and `*S` both include promoted methods with receiver `T` or `*T`.

A field declaration may be followed by an optional string literal tag, which becomes an attribute for all the fields in the corresponding field declaration. The tags are made visible through a reflection interface and take part in type identity for structs but are otherwise ignored.

    // A struct corresponding to the TimeStamp protocol buffer.
    // The tag strings define the protocol buffer field numbers.
    struct {
    	microsec  uint64 "field 1"
    	serverIP6 uint64 "field 2"
    	process   string "field 3"
    }

### Pointer types

A pointer type denotes the set of all pointers to variables of a given type, called the base type of the pointer. The value of an uninitialized pointer is `nil`.

    PointerType = "*" BaseType .
    BaseType    = Type .

    *Point
    *[4]int

### Function types

A function type denotes the set of all functions with the same parameter and result types. The value of an uninitialized variable of function type is `nil`.

    FunctionType   = "func" Signature .
    Signature      = Parameters [ Result ] .
    Result         = Parameters | Type .
    Parameters     = "(" [ ParameterList [ "," ] ] ")" .
    ParameterList  = ParameterDecl { "," ParameterDecl } .
    ParameterDecl  = [ IdentifierList ] [ "..." ] Type .

Within a list of parameters or results, the names (IdentifierList) must either all be present or all be absent. If present, each name stands for one item (parameter or result) of the specified type and all non-blank names in the signature must be unique. If absent, each type stands for one item of that type. Parameter and result lists are always parenthesized except that if there is exactly one unnamed result it may be written as an unparenthesized type.

The final parameter in a function signature may have a type prefixed with `...`. A function with such a parameter is called variadic and may be invoked with zero or more arguments for that parameter.

    func()
    func(x int) int
    func(a, _ int, z float32) bool
    func(a, b int, z float32) (bool)
    func(prefix string, values ...int)
    func(a, b int, z float64, opt ...interface{}) (success bool)
    func(int, int, float64) (float64, *[]int)
    func(n int) func(p *T)

### Interface types

An interface type specifies a method set called its interface. A variable of interface type can store a value of any type with a method set that is any superset of the interface. Such a type is said to implement the interface. The value of an uninitialized variable of interface type is `nil`.

    InterfaceType      = "interface" "{" { MethodSpec ";" } "}" .
    MethodSpec         = MethodName Signature | InterfaceTypeName .
    MethodName         = identifier .
    InterfaceTypeName  = TypeName .

As with all method sets, in an interface type, each method must have a unique non-blank name.

    // A simple File interface
    interface {
    	Read(b Buffer) bool
    	Write(b Buffer) bool
    	Close()
    }

More than one type may implement an interface. For instance, if two types `S1` and `S2` have the method set

    func (p T) Read(b Buffer) bool { return … }
    func (p T) Write(b Buffer) bool { return … }
    func (p T) Close() { … }

(where `T` stands for either `S1` or `S2`) then the `File` interface is implemented by both `S1` and `S2`, regardless of what other methods `S1` and `S2` may have or share.

A type implements any interface comprising any subset of its methods and may therefore implement several distinct interfaces. For instance, all types implement the empty interface:

    interface{}

Similarly, consider this interface specification, which appears within a type declaration to define an interface called `Locker`:

    type Locker interface {
    	Lock()
    	Unlock()
    }

If `S1` and `S2` also implement

    func (p T) Lock() { … }
    func (p T) Unlock() { … }

they implement the `Locker` interface as well as the `File` interface.

An interface `T` may use a (possibly qualified) interface type name `E` in place of a method specification. This is called embedding interface `E` in `T`; it adds all (exported and non-exported) methods of `E` to the interface `T`.

    type ReadWriter interface {
    	Read(b Buffer) bool
    	Write(b Buffer) bool
    }

    type File interface {
    	ReadWriter  // same as adding the methods of ReadWriter
    	Locker      // same as adding the methods of Locker
    	Close()
    }

    type LockedFile interface {
    	Locker
    	File        // illegal: Lock, Unlock not unique
    	Lock()      // illegal: Lock not unique
    }

An interface type `T` may not embed itself or any interface type that embeds `T`, recursively.

    // illegal: Bad cannot embed itself
    type Bad interface {
    	Bad
    }

    // illegal: Bad1 cannot embed itself using Bad2
    type Bad1 interface {
    	Bad2
    }
    type Bad2 interface {
    	Bad1
    }

### Map types

A map is an unordered group of elements of one type, called the element type, indexed by a set of unique keys of another type, called the key type. The value of an uninitialized map is `nil`.

    MapType     = "map" "[" KeyType "]" ElementType .
    KeyType     = Type .

The comparison operators `==` and `!=` must be fully defined for operands of the key type; thus the key type must not be a function, map, or slice. If the key type is an interface type, these comparison operators must be defined for the dynamic key values; failure will cause a run-time panic.

    map[string]int
    map[*T]struct{ x, y float64 }
    map[string]interface{}

The number of map elements is called its length. For a map `m`, it can be discovered using the built-in function `len` and may change during execution. Elements may be added during execution using assignments and retrieved with index expressions; they may be removed with the `delete` built-in function.

A new, empty map value is made using the built-in function `make`, which takes the map type and an optional capacity hint as arguments:

    make(map[string]int)
    make(map[string]int, 100)

The initial capacity does not bound its size: maps grow to accommodate the number of items stored in them, with the exception of `nil` maps. A `nil` map is equivalent to an empty map except that no elements may be added.

### Channel types

A channel provides a mechanism for concurrently executing functions to communicate by sending and receiving values of a specified element type. The value of an uninitialized channel is `nil`.

    ChannelType = ( "chan" | "chan" "<-" | "<-" "chan" ) ElementType .

The optional `<-` operator specifies the channel direction, send or receive. If no direction is given, the channel is bidirectional. A channel may be constrained only to send or only to receive by conversion or assignment.

    chan T          // can be used to send and receive values of type T
    chan<- float64  // can only be used to send float64s
    <-chan int      // can only be used to receive ints

The `<-` operator associates with the leftmost `chan` possible:

    chan<- chan int    // same as chan<- (chan int)
    chan<- <-chan int  // same as chan<- (<-chan int)
    <-chan <-chan int  // same as <-chan (<-chan int)
    chan (<-chan int)

A new, initialized channel value can be made using the built-in function `make`, which takes the channel type and an optional capacity as arguments:

    make(chan int, 100)

The capacity, in number of elements, sets the size of the buffer in the channel. If the capacity is zero or absent, the channel is unbuffered and communication succeeds only when both a sender and receiver are ready. Otherwise, the channel is buffered and communication succeeds without blocking if the buffer is not full (sends) or not empty (receives). A `nil` channel is never ready for communication.

A channel may be closed with the built-in function `close`. The multi-valued assignment form of the receive operator reports whether a received value was sent before the channel was closed.

A single channel may be used in send statements, receive operations, and calls to the built-in functions `cap` and `len` by any number of goroutines without further synchronization. Channels act as first-in-first-out queues. For example, if one goroutine sends values on a channel and a second goroutine receives them, the values are received in the order sent.

## Properties of types and values

### Type identity

Two types are either identical or different.

Two named types are identical if their type names originate in the same TypeSpec. A named and an unnamed type are always different. Two unnamed types are identical if the corresponding type literals are identical, that is, if they have the same literal structure and corresponding components have identical types. In detail:

- Two array types are identical if they have identical element types and the same array length.
- Two slice types are identical if they have identical element types.
- Two struct types are identical if they have the same sequence of fields, and if corresponding fields have the same names, and identical types, and identical tags. Two anonymous fields are considered to have the same name. Lower-case field names from different packages are always different.
- Two pointer types are identical if they have identical base types.
- Two function types are identical if they have the same number of parameters and result values, corresponding parameter and result types are identical, and either both functions are variadic or neither is. Parameter and result names are not required to match.
- Two interface types are identical if they have the same set of methods with the same names and identical function types. Lower-case method names from different packages are always different. The order of the methods is irrelevant.
- Two map types are identical if they have identical key and value types.
- Two channel types are identical if they have identical value types and the same direction.

Given the declarations

    type (
    	T0 []string
    	T1 []string
    	T2 struct{ a, b int }
    	T3 struct{ a, c int }
    	T4 func(int, float64) *T0
    	T5 func(x int, y float64) *[]string
    )

these types are identical:

    T0 and T0
    []int and []int
    struct{ a, b *T5 } and struct{ a, b *T5 }
    func(x int, y float64) *[]string and func(int, float64) (result *[]string)

`T0` and `T1` are different because they are named types with distinct declarations; `func(int, float64) *T0` and `func(x int, y float64) *[]string` are different because `T0` is different from `[]string`.

### Assignability

A value `x` is assignable to a variable of type `T` ("`x` is assignable to `T`") in any of these cases:

- `x`'s type is identical to `T`.
- `x`'s type `V` and `T` have identical underlying types and at least one of `V` or `T` is not a named type.
- `T` is an interface type and `x` implements `T`.
- `x` is a bidirectional channel value, `T` is a channel type, `x`'s type `V` and `T` have identical element types, and at least one of `V` or `T` is not a named type.
- `x` is the predeclared identifier `nil` and `T` is a pointer, function, slice, map, channel, or interface type.
- `x` is an untyped constant representable by a value of type `T`.

## Blocks

A block is a possibly empty sequence of declarations and statements within matching brace brackets.

    Block = "{" StatementList "}" .
    StatementList = { Statement ";" } .

In addition to explicit blocks in the source code, there are implicit blocks:

1. The universe block encompasses all Go source text.
2. Each package has a package block containing all Go source text for that package.
3. Each file has a file block containing all Go source text in that file.
4. Each "if", "for", and "switch" statement is considered to be in its own implicit block.
5. Each clause in a "switch" or "select" statement acts as an implicit block.

Blocks nest and influence scoping.

## Declarations and scope

A declaration binds a non-blank identifier to a constant, type, variable, function, label, or package. Every identifier in a program must be declared. No identifier may be declared twice in the same block, and no identifier may be declared in both the file and package block.

The blank identifier may be used like any other identifier in a declaration, but it does not introduce a binding and thus is not declared. In the package block, the identifier `init` may only be used for `init` function declarations, and like the blank identifier it does not introduce a new binding.

    Declaration   = ConstDecl | TypeDecl | VarDecl .
    TopLevelDecl  = Declaration | FunctionDecl | MethodDecl .

The scope of a declared identifier is the extent of source text in which the identifier denotes the specified constant, type, variable, function, label, or package.

Go is lexically scoped using blocks:

1. The scope of a predeclared identifier is the universe block.
2. The scope of an identifier denoting a constant, type, variable, or function (but not method) declared at top level (outside any function) is the package block.
3. The scope of the package name of an imported package is the file block of the file containing the import declaration.
4. The scope of an identifier denoting a method receiver, function parameter, or result variable is the function body.
5. The scope of a constant or variable identifier declared inside a function begins at the end of the ConstSpec or VarSpec (ShortVarDecl for short variable declarations) and ends at the end of the innermost containing block.
6. The scope of a type identifier declared inside a function begins at the identifier in the TypeSpec and ends at the end of the innermost containing block.

An identifier declared in a block may be redeclared in an inner block. While the identifier of the inner declaration is in scope, it denotes the entity declared by the inner declaration.

The package clause is not a declaration; the package name does not appear in any scope. Its purpose is to identify the files belonging to the same package and to specify the default package name for import declarations.

### Label scopes

Labels are declared by labeled statements and are used in the "break", "continue", and "goto" statements. It is illegal to define a label that is never used. In contrast to other identifiers, labels are not block scoped and do not conflict with identifiers that are not labels. The scope of a label is the body of the function in which it is declared and excludes the body of any nested function.

### Blank identifier

The blank identifier is represented by the underscore character `_`. It serves as an anonymous placeholder instead of a regular (non-blank) identifier and has special meaning in declarations, as an operand, and in assignments.

### Predeclared identifiers

The following identifiers are implicitly declared in the universe block:

    Types:
    	bool byte complex64 complex128 error float32 float64
    	int int8 int16 int32 int64 rune string
    	uint uint8 uint16 uint32 uint64 uintptr

    Constants:
    	true false iota

    Zero value:
    	nil

    Functions:
    	append cap close complex copy delete imag len
    	make new panic print println real recover

### Exported identifiers

An identifier may be exported to permit access to it from another package. An identifier is exported if both:

1. the first character of the identifier's name is a Unicode upper case letter (Unicode class "Lu"); and
2. the identifier is declared in the package block or it is a field name or method name.

All other identifiers are not exported.

### Uniqueness of identifiers

Given a set of identifiers, an identifier is called unique if it is different from every other in the set. Two identifiers are different if they are spelled differently, or if they appear in different packages and are not exported. Otherwise, they are the same.

### Constant declarations

A constant declaration binds a list of identifiers (the names of the constants) to the values of a list of constant expressions. The number of identifiers must be equal to the number of expressions, and the nth identifier on the left is bound to the value of the nth expression on the right.

    ConstDecl      = "const" ( ConstSpec | "(" { ConstSpec ";" } ")" ) .
    ConstSpec      = IdentifierList [ [ Type ] "=" ExpressionList ] .

    IdentifierList = identifier { "," identifier } .
    ExpressionList = Expression { "," Expression } .

If the type is present, all constants take the type specified, and the expressions must be assignable to that type. If the type is omitted, the constants take the individual types of the corresponding expressions. If the expression values are untyped constants, the declared constants remain untyped and the constant identifiers denote the constant values. For instance, if the expression is a floating-point literal, the constant identifier denotes a floating-point constant, even if the literal's fractional part is zero.

    const Pi float64 = 3.14159265358979323846
    const zero = 0.0         // untyped floating-point constant
    const (
    	size int64 = 1024
    	eof        = -1  // untyped integer constant
    )
    const a, b, c = 3, 4, "foo"  // a = 3, b = 4, c = "foo", untyped integer and string constants
    const u, v float32 = 0, 3    // u = 0.0, v = 3.0

Within a parenthesized `const` declaration list the expression list may be omitted from any but the first declaration. Such an empty list is equivalent to the textual substitution of the first preceding non-empty expression list and its type if any. Omitting the list of expressions is therefore equivalent to repeating the previous list. The number of identifiers must be equal to the number of expressions in the previous list. Together with the `iota` constant generator this mechanism permits light-weight declaration of sequential values:

    const (
    	Sunday = iota
    	Monday
    	Tuesday
    	Wednesday
    	Thursday
    	Friday
    	Partyday
    	numberOfDays  // this constant is not exported
    )

### Iota

Within a constant declaration, the predeclared identifier `iota` represents successive untyped integer constants. It is reset to 0 whenever the reserved word `const` appears in the source and increments after each ConstSpec. It can be used to construct a set of related constants:

    const (  // iota is reset to 0
    	c0 = iota  // c0 == 0
    	c1 = iota  // c1 == 1
    	c2 = iota  // c2 == 2
    )

    const (
    	a = 1 << iota  // a == 1 (iota has been reset)
    	b = 1 << iota  // b == 2
    	c = 1 << iota  // c == 4
    )

    const (
    	u         = iota * 42  // u == 0     (untyped integer constant)
    	v float64 = iota * 42  // v == 42.0  (float64 constant)
    	w         = iota * 42  // w == 84    (untyped integer constant)
    )

    const x = iota  // x == 0 (iota has been reset)
    const y = iota  // y == 0 (iota has been reset)

Within an ExpressionList, the value of each `iota` is the same because it is only incremented after each ConstSpec:

    const (
    	bit0, mask0 = 1 << iota, 1<<iota - 1  // bit0 == 1, mask0 == 0
    	bit1, mask1                           // bit1 == 2, mask1 == 1
    	_, _                                  // skips iota == 2
    	bit3, mask3                           // bit3 == 8, mask3 == 7
    )

This last example exploits the implicit repetition of the last non-empty expression list.

### Type declarations

A type declaration binds an identifier, the type name, to a new type that has the same underlying type as an existing type, and operations defined for the existing type are also defined for the new type. The new type is different from the existing type.

    TypeDecl     = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
    TypeSpec     = identifier Type .

    type IntArray [16]int

    type (
    	Point struct{ x, y float64 }
    	Polar Point
    )

    type TreeNode struct {
    	left, right *TreeNode
    	value *Comparable
    }

    type Block interface {
    	BlockSize() int
    	Encrypt(src, dst []byte)
    	Decrypt(src, dst []byte)
    }

The declared type does not inherit any methods bound to the existing type, but the method set of an interface type or of elements of a composite type remains unchanged:

    // A Mutex is a data type with two methods, Lock and Unlock.
    type Mutex struct         { /* Mutex fields */ }
    func (m *Mutex) Lock()    { /* Lock implementation */ }
    func (m *Mutex) Unlock()  { /* Unlock implementation */ }

    // NewMutex has the same composition as Mutex but its method set is empty.
    type NewMutex Mutex

    // The method set of the base type of PtrMutex remains unchanged,
    // but the method set of PtrMutex is empty.
    type PtrMutex *Mutex

    // The method set of *PrintableMutex contains the methods
    // Lock and Unlock bound to its anonymous field Mutex.
    type PrintableMutex struct {
    	Mutex
    }

    // MyBlock is an interface type that has the same method set as Block.
    type MyBlock Block

A type declaration may be used to define a different boolean, numeric, or string type and attach methods to it:

    type TimeZone int

    const (
    	EST TimeZone = -(5 + iota)
    	CST
    	MST
    	PST
    )

    func (tz TimeZone) String() string {
    	return fmt.Sprintf("GMT+%dh", tz)
    }

### Variable declarations

A variable declaration creates one or more variables, binds corresponding identifiers to them, and gives each a type and an initial value.

    VarDecl     = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .
    VarSpec     = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .

    var i int
    var U, V, W float64
    var k = 0
    var x, y float32 = -1, -2
    var (
    	i       int
    	u, v, s = 2.0, 3.0, "bar"
    )
    var re, im = complexSqrt(-1)
    var _, found = entries[name]  // map lookup; only interested in "found"

If a list of expressions is given, the variables are initialized with the expressions following the rules for assignments. Otherwise, each variable is initialized to its zero value.

If a type is present, each variable is given that type. Otherwise, each variable is given the type of the corresponding initialization value in the assignment. If that value is an untyped constant, it is first converted to its default type; if it is an untyped boolean value, it is first converted to type `bool`. The predeclared value `nil` cannot be used to initialize a variable with no explicit type.

    var d = math.Sin(0.5)  // d is float64
    var i = 42             // i is int
    var t, ok = x.(T)      // t is T, ok is bool
    var n = nil            // illegal

Implementation restriction: A compiler may make it illegal to declare a variable inside a function body if the variable is never used.

### Short variable declarations

A short variable declaration uses the syntax:

    ShortVarDecl = IdentifierList ":=" ExpressionList .

It is shorthand for a regular variable declaration with initializer expressions but no types:

    "var" IdentifierList = ExpressionList .

    i, j := 0, 10
    f := func() int { return 7 }
    ch := make(chan int)
    r, w := os.Pipe(fd)  // os.Pipe() returns two values
    _, y, _ := coord(p)  // coord() returns three values; only interested in y coordinate

Unlike regular variable declarations, a short variable declaration may redeclare variables provided they were originally declared earlier in the same block with the same type, and at least one of the non-blank variables is new. As a consequence, redeclaration can only appear in a multi-variable short declaration. Redeclaration does not introduce a new variable; it just assigns a new value to the original.

    field1, offset := nextField(str, 0)
    field2, offset := nextField(str, offset)  // redeclares offset
    a, a := 1, 2                              // illegal: double declaration of a or no new variable if a was declared elsewhere

Short variable declarations may appear only inside functions. In some contexts such as the initializers for "if", "for", or "switch" statements, they can be used to declare local temporary variables.

### Function declarations

A function declaration binds an identifier, the function name, to a function.

    FunctionDecl = "func" FunctionName ( Function | Signature ) .
    FunctionName = identifier .
    Function     = Signature FunctionBody .
    FunctionBody = Block .

If the function's signature declares result parameters, the function body's statement list must end in a terminating statement.

    func findMarker(c <-chan int) int {
    	for i := range c {
    		if x := <-c; isMarker(x) {
    			return x
    		}
    	}
    	// invalid: missing return statement.
    }

A function declaration may omit the body. Such a declaration provides the signature for a function implemented outside Go, such as an assembly routine.

    func min(x int, y int) int {
    	if x < y {
    		return x
    	}
    	return y
    }

    func flushICache(begin, end uintptr)  // implemented externally

### Method declarations

A method is a function with a receiver. A method declaration binds an identifier, the method name, to a method, and associates the method with the receiver's base type.

    MethodDecl   = "func" Receiver MethodName ( Function | Signature ) .
    Receiver     = Parameters .

The receiver is specified via an extra parameter section preceeding the method name. That parameter section must declare a single parameter, the receiver. Its type must be of the form `T` or `*T` (possibly using parentheses) where `T` is a type name. The type denoted by `T` is called the receiver base type; it must not be a pointer or interface type and it must be declared in the same package as the method. The method is said to be bound to the base type and the method name is visible only within selectors for that type.

A non-blank receiver identifier must be unique in the method signature. If the receiver's value is not referenced inside the body of the method, its identifier may be omitted in the declaration. The same applies in general to parameters of functions and methods.

For a base type, the non-blank names of methods bound to it must be unique. If the base type is a struct type, the non-blank method and field names must be distinct.

Given type `Point`, the declarations

    func (p *Point) Length() float64 {
    	return math.Sqrt(p.x * p.x + p.y * p.y)
    }

    func (p *Point) Scale(factor float64) {
    	p.x *= factor
    	p.y *= factor
    }

bind the methods `Length` and `Scale`, with receiver type `*Point`, to the base type `Point`.

The type of a method is the type of a function with the receiver as first argument. For instance, the method `Scale` has type

    func(p *Point, factor float64)

However, a function declared this way is not a method.

## Expressions

An expression specifies the computation of a value by applying operators and functions to operands.

### Operands

Operands denote the elementary values in an expression. An operand may be a literal, a (possibly qualified) non-blank identifier denoting a constant, variable, or function, a method expression yielding a function, or a parenthesized expression.

The blank identifier may appear as an operand only on the left-hand side of an assignment.

    Operand     = Literal | OperandName | MethodExpr | "(" Expression ")" .
    Literal     = BasicLit | CompositeLit | FunctionLit .
    BasicLit    = int_lit | float_lit | imaginary_lit | rune_lit | string_lit .
    OperandName = identifier | QualifiedIdent.

### Qualified identifiers

A qualified identifier is an identifier qualified with a package name prefix. Both the package name and the identifier must not be blank.

    QualifiedIdent = PackageName "." identifier .

A qualified identifier accesses an identifier in a different package, which must be imported. The identifier must be exported and declared in the package block of that package.

    math.Sin	// denotes the Sin function in package math

### Composite literals

Composite literals construct values for structs, arrays, slices, and maps and create a new value each time they are evaluated. They consist of the type of the value followed by a brace-bound list of composite elements. An element may be a single expression or a key-value pair.

    CompositeLit  = LiteralType LiteralValue .
    LiteralType   = StructType | ArrayType | "[" "..." "]" ElementType |
                    SliceType | MapType | TypeName .
    LiteralValue  = "{" [ ElementList [ "," ] ] "}" .
    ElementList   = Element { "," Element } .
    Element       = [ Key ":" ] Value .
    Key           = FieldName | Expression | LiteralValue .
    FieldName     = identifier .
    Value         = Expression | LiteralValue .

The LiteralType must be a struct, array, slice, or map type (the grammar enforces this constraint except when the type is given as a TypeName). The types of the expressions must be assignable to the respective field, element, and key types of the LiteralType; there is no additional conversion. The key is interpreted as a field name for struct literals, an index for array and slice literals, and a key for map literals. For map literals, all elements must have a key. It is an error to specify multiple elements with the same field name or constant key value.

For struct literals the following rules apply:

- A key must be a field name declared in the LiteralType.
- An element list that does not contain any keys must list an element for each struct field in the order in which the fields are declared.
- If any element has a key, every element must have a key.
- An element list that contains keys does not need to have an element for each struct field. Omitted fields get the zero value for that field.
- A literal may omit the element list; such a literal evaluates to the zero value for its type.
- It is an error to specify an element for a non-exported field of a struct belonging to a different package.

Given the declarations

    type Point3D struct { x, y, z float64 }
    type Line struct { p, q Point3D }

one may write

    origin := Point3D{}                            // zero value for Point3D
    line := Line{origin, Point3D{y: -4, z: 12.3}}  // zero value for line.q.x

For array and slice literals the following rules apply:

- Each element has an associated integer index marking its position in the array.
- An element with a key uses the key as its index; the key must be a constant integer expression.
- An element without a key uses the previous element's index plus one. If the first element has no key, its index is zero.

Taking the address of a composite literal generates a pointer to a unique variable initialized with the literal's value.

    var pointer *Point3D = &Point3D{y: 1000}

The length of an array literal is the length specified in the LiteralType. If fewer elements than the length are provided in the literal, the missing elements are set to the zero value for the array element type. It is an error to provide elements with index values outside the index range of the array. The notation `...` specifies an array length equal to the maximum element index plus one.

    buffer := [10]string{}             // len(buffer) == 10
    intSet := [6]int{1, 2, 3, 5}       // len(intSet) == 6
    days := [...]string{"Sat", "Sun"}  // len(days) == 2

A slice literal describes the entire underlying array literal. Thus, the length and capacity of a slice literal are the maximum element index plus one. A slice literal has the form

    []T{x1, x2, … xn}

and is shorthand for a slice operation applied to an array:

    tmp := [n]T{x1, x2, … xn}
    tmp[0 : n]

Within a composite literal of array, slice, or map type `T`, elements or map keys that are themselves composite literals may elide the respective literal type if it is identical to the element or key type of `T`. Similarly, elements or keys that are addresses of composite literals may elide the `&T` when the element or key type is `*T`.

    [...]Point{{1.5, -3.5}, {0, 0}}     // same as [...]Point{Point{1.5, -3.5}, Point{0, 0}}
    [][]int{{1, 2, 3}, {4, 5}}          // same as [][]int{[]int{1, 2, 3}, []int{4, 5}}
    [][]Point{{{0, 1}, {1, 2}}}         // same as [][]Point{[]Point{Point{0, 1}, Point{1, 2}}}
    map[string]Point{"orig": {0, 0}}    // same as map[string]Point{"orig": Point{0, 0}}

    [...]*Point{{1.5, -3.5}, {0, 0}}    // same as [...]*Point{&Point{1.5, -3.5}, &Point{0, 0}}

    map[Point]string{{0, 0}: "orig"}    // same as map[Point]string{Point{0, 0}: "orig"}

A parsing ambiguity arises when a composite literal using the TypeName form of the LiteralType appears as an operand between the keyword and the opening brace of the block of an "if", "for", or "switch" statement, and the composite literal is not enclosed in parentheses, square brackets, or curly braces. In this rare case, the opening brace of the literal is erroneously parsed as the one introducing the block of statements. To resolve the ambiguity, the composite literal must appear within parentheses.

    if x == (T{a,b,c}[i]) { … }
    if (x == T{a,b,c}[i]) { … }

Examples of valid array, slice, and map literals:

    // list of prime numbers
    primes := []int{2, 3, 5, 7, 9, 2147483647}

    // vowels[ch] is true if ch is a vowel
    vowels := [128]bool{'a': true, 'e': true, 'i': true, 'o': true, 'u': true, 'y': true}

    // the array [10]float32{-1, 0, 0, 0, -0.1, -0.1, 0, 0, 0, -1}
    filter := [10]float32{-1, 4: -0.1, -0.1, 9: -1}

    // frequencies in Hz for equal-tempered scale (A4 = 440Hz)
    noteFrequency := map[string]float32{
    	"C0": 16.35, "D0": 18.35, "E0": 20.60, "F0": 21.83,
    	"G0": 24.50, "A0": 27.50, "B0": 30.87,
    }

### Function literals

A function literal represents an anonymous function.

    FunctionLit = "func" Function .

    func(a, b int, z float64) bool { return a*b < int(z) }

A function literal can be assigned to a variable or invoked directly.

    f := func(x, y int) int { return x + y }
    func(ch chan int) { ch <- ACK }(replyChan)

Function literals are closures: they may refer to variables defined in a surrounding function. Those variables are then shared between the surrounding function and the function literal, and they survive as long as they are accessible.

### Primary expressions

Primary expressions are the operands for unary and binary expressions.

    PrimaryExpr =
    	Operand |
    	Conversion |
    	PrimaryExpr Selector |
    	PrimaryExpr Index |
    	PrimaryExpr Slice |
    	PrimaryExpr TypeAssertion |
    	PrimaryExpr Arguments .

    Selector       = "." identifier .
    Index          = "[" Expression "]" .
    Slice          = "[" ( [ Expression ] ":" [ Expression ] ) |
                         ( [ Expression ] ":" Expression ":" Expression )
                     "]" .
    TypeAssertion  = "." "(" Type ")" .
    Arguments      = "(" [ ( ExpressionList | Type [ "," ExpressionList ] ) [ "..." ] [ "," ] ] ")" .

    x
    2
    (s + ".txt")
    f(3.1415, true)
    Point{1, 2}
    m["foo"]
    s[i : j + 1]
    obj.color
    f.p[i].x()

### Selectors

For a primary expression `x` that is not a package name, the selector expression

    x.f

denotes the field or method `f` of the value `x` (or sometimes `*x`; see below). The identifier `f` is called the (field or method) selector; it must not be the blank identifier. The type of the selector expression is the type of `f`. If `x` is a package name, see the section on qualified identifiers.

A selector `f` may denote a field or method `f` of a type `T`, or it may refer to a field or method `f` of a nested anonymous field of `T`. The number of anonymous fields traversed to reach `f` is called its depth in `T`. The depth of a field or method `f` declared in `T` is zero. The depth of a field or method `f` declared in an anonymous field `A` in `T` is the depth of `f` in `A` plus one.

The following rules apply to selectors:

1. For a value `x` of type `T` or `*T` where `T` is not a pointer or interface type, `x.f` denotes the field or method at the shallowest depth in `T` where there is such an `f`. If there is not exactly one `f` with shallowest depth, the selector expression is illegal.
2. For a value `x` of type `I` where `I` is an interface type, `x.f` denotes the actual method with name `f` of the dynamic value of `x`. If there is no method with name `f` in the method set of `I`, the selector expression is illegal.
3. As an exception, if the type of `x` is a named pointer type and `(*x).f` is a valid selector expression denoting a field (but not a method), `x.f` is shorthand for `(*x).f`.
4. In all other cases, `x.f` is illegal.
5. If `x` is of pointer type and has the value `nil` and `x.f` denotes a struct field, assigning to or evaluating `x.f` causes a run-time panic.
6. If `x` is of interface type and has the value `nil`, calling or evaluating the method `x.f` causes a run-time panic.

For example, given the declarations:

    type T0 struct {
    	x int
    }

    func (*T0) M0()

    type T1 struct {
    	y int
    }

    func (T1) M1()

    type T2 struct {
    	z int
    	T1
    	*T0
    }

    func (*T2) M2()

    type Q *T2

    var t T2     // with t.T0 != nil
    var p *T2    // with p != nil and (*p).T0 != nil
    var q Q = p

one may write:

    t.z          // t.z
    t.y          // t.T1.y
    t.x          // (*t.T0).x

    p.z          // (*p).z
    p.y          // (*p).T1.y
    p.x          // (*(*p).T0).x

    q.x          // (*(*q).T0).x        (*q).x is a valid field selector

    p.M2()       // p.M2()              M2 expects *T2 receiver
    p.M1()       // ((*p).T1).M1()      M1 expects T1 receiver
    p.M0()       // ((&(*p).T0)).M0()   M0 expects *T0 receiver, see section on Calls

but the following is invalid:

    q.M0()       // (*q).M0 is valid but not a field selector

### Method expressions

If `M` is in the method set of type `T`, `T.M` is a function that is callable as a regular function with the same arguments as `M` prefixed by an additional argument that is the receiver of the method.

    MethodExpr    = ReceiverType "." MethodName .
    ReceiverType  = TypeName | "(" "*" TypeName ")" | "(" ReceiverType ")" .

Consider a struct type `T` with two methods, `Mv`, whose receiver is of type `T`, and `Mp`, whose receiver is of type `*T`.

    type T struct {
    	a int
    }
    func (tv  T) Mv(a int) int         { return 0 }  // value receiver
    func (tp *T) Mp(f float32) float32 { return 1 }  // pointer receiver

    var t T

The expression

    T.Mv

yields a function equivalent to `Mv` but with an explicit receiver as its first argument; it has signature

    func(tv T, a int) int

That function may be called normally with an explicit receiver, so these five invocations are equivalent:

    t.Mv(7)
    T.Mv(t, 7)
    (T).Mv(t, 7)
    f1 := T.Mv; f1(t, 7)
    f2 := (T).Mv; f2(t, 7)

Similarly, the expression

    (*T).Mp

yields a function value representing `Mp` with signature

    func(tp *T, f float32) float32

For a method with a value receiver, one can derive a function with an explicit pointer receiver, so

    (*T).Mv

yields a function value representing `Mv` with signature

    func(tv *T, a int) int

Such a function indirects through the receiver to create a value to pass as the receiver to the underlying method; the method does not overwrite the value whose address is passed in the function call.

The final case, a value-receiver function for a pointer-receiver method, is illegal because pointer-receiver methods are not in the method set of the value type.

Function values derived from methods are called with function call syntax; the receiver is provided as the first argument to the call. That is, given `f := T.Mv`, `f` is invoked as `f(t, 7)` not `t.f(7)`. To construct a function that binds the receiver, use a function literal or method value.

It is legal to derive a function value from a method of an interface type. The resulting function takes an explicit receiver of that interface type.

### Method values

If the expression `x` has static type `T` and `M` is in the method set of type `T`, `x.M` is called a method value. The method value `x.M` is a function value that is callable with the same arguments as a method call of `x.M`. The expression `x` is evaluated and saved during the evaluation of the method value; the saved copy is then used as the receiver in any calls, which may be executed later.

The type `T` may be an interface or non-interface type.

As in the discussion of method expressions above, consider a struct type `T` with two methods, `Mv`, whose receiver is of type `T`, and `Mp`, whose receiver is of type `*T`.

    type T struct {
    	a int
    }
    func (tv  T) Mv(a int) int         { return 0 }  // value receiver
    func (tp *T) Mp(f float32) float32 { return 1 }  // pointer receiver

    var t T
    var pt *T
    func makeT() T

The expression

    t.Mv

yields a function value of type

    func(int) int

These two invocations are equivalent:

    t.Mv(7)
    f := t.Mv; f(7)

Similarly, the expression

    pt.Mp

yields a function value of type

    func(float32) float32

As with selectors, a reference to a non-interface method with a value receiver using a pointer will automatically dereference that pointer: `pt.Mv` is equivalent to `(*pt).Mv`.

As with method calls, a reference to a non-interface method with a pointer receiver using an addressable value will automatically take the address of that value: `t.Mp` is equivalent to `(&t).Mp`.

    f := t.Mv; f(7)   // like t.Mv(7)
    f := pt.Mp; f(7)  // like pt.Mp(7)
    f := pt.Mv; f(7)  // like (*pt).Mv(7)
    f := t.Mp; f(7)   // like (&t).Mp(7)
    f := makeT().Mp   // invalid: result of makeT() is not addressable

Although the examples above use non-interface types, it is also legal to create a method value from a value of interface type.

    var i interface { M(int) } = myVal
    f := i.M; f(7)  // like i.M(7)

### Index expressions

A primary expression of the form

    a[x]

denotes the element of the array, pointer to array, slice, string or map `a` indexed by `x`. The value `x` is called the index or map key, respectively. The following rules apply:

If `a` is not a map:

- the index `x` must be of integer type or untyped; it is in range if `0 <= x < len(a)`, otherwise it is out of range
- a constant index must be non-negative and representable by a value of type `int`

For `a` of array type `A`:

- a constant index must be in range
- if `x` is out of range at run time, a run-time panic occurs
- `a[x]` is the array element at index `x` and the type of `a[x]` is the element type of `A`

For `a` of pointer to array type:

- `a[x]` is shorthand for `(*a)[x]`

For `a` of slice type `S`:

- if `x` is out of range at run time, a run-time panic occurs
- `a[x]` is the slice element at index `x` and the type of `a[x]` is the element type of `S`

For `a` of string type:

- a constant index must be in range if the string `a` is also constant
- if `x` is out of range at run time, a run-time panic occurs
- `a[x]` is the non-constant byte value at index `x` and the type of `a[x]` is `byte`
- `a[x]` may not be assigned to

For `a` of map type `M`:

- `x`'s type must be assignable to the key type of `M`
- if the map contains an entry with key `x`, `a[x]` is the map value with key `x` and the type of `a[x]` is the value type of `M`
- if the map is `nil` or does not contain such an entry, `a[x]` is the zero value for the value type of `M`

Otherwise `a[x]` is illegal.

An index expression on a map `a` of type `map[K]V` used in an assignment or initialization of the special form

    v, ok = a[x]
    v, ok := a[x]
    var v, ok = a[x]

yields an additional untyped boolean value. The value of `ok` is `true` if the key `x` is present in the map, and `false` otherwise.

Assigning to an element of a `nil` map causes a run-time panic.

### Slice expressions

Slice expressions construct a substring or slice from a string, array, pointer to array, or slice. There are two variants: a simple form that specifies a low and high bound, and a full form that also specifies a bound on the capacity.

#### Simple slice expressions

For a string, array, pointer to array, or slice `a`, the primary expression

    a[low : high]

constructs a substring or slice. The indices `low` and `high` select which elements of operand `a` appear in the result. The result has indices starting at 0 and length equal to `high` - `low`. After slicing the array `a`

    a := [5]int{1, 2, 3, 4, 5}
    s := a[1:4]

the slice `s` has type `[]int`, length 3, capacity 4, and elements

    s[0] == 2
    s[1] == 3
    s[2] == 4

For convenience, any of the indices may be omitted. A missing `low` index defaults to zero; a missing `high` index defaults to the length of the sliced operand:

    a[2:]  // same as a[2 : len(a)]
    a[:3]  // same as a[0 : 3]
    a[:]   // same as a[0 : len(a)]

If `a` is a pointer to an array, `a[low : high]` is shorthand for `(*a)[low : high]`.

For arrays or strings, the indices are in range if `0` <= `low` <= `high` <= `len(a)`, otherwise they are out of range. For slices, the upper index bound is the slice capacity `cap(a)` rather than the length. A constant index must be non-negative and representable by a value of type `int`; for arrays or constant strings, constant indices must also be in range. If both indices are constant, they must satisfy `low <= high`. If the indices are out of range at run time, a run-time panic occurs.

Except for untyped strings, if the sliced operand is a string or slice, the result of the slice operation is a non-constant value of the same type as the operand. For untyped string operands the result is a non-constant value of type `string`. If the sliced operand is an array, it must be addressable and the result of the slice operation is a slice with the same element type as the array.

If the sliced operand of a valid slice expression is a `nil` slice, the result is a `nil` slice. Otherwise, the result shares its underlying array with the operand.

#### Full slice expressions

For an array, pointer to array, or slice `a` (but not a string), the primary expression

    a[low : high : max]

constructs a slice of the same type, and with the same length and elements as the simple slice expression `a[low : high]`. Additionally, it controls the resulting slice's capacity by setting it to `max - low`. Only the first index may be omitted; it defaults to 0. After slicing the array `a`

    a := [5]int{1, 2, 3, 4, 5}
    t := a[1:3:5]

the slice `t` has type `[]int`, length 2, capacity 4, and elements

    t[0] == 2
    t[1] == 3

As for simple slice expressions, if `a` is a pointer to an array, `a[low : high : max]` is shorthand for `(*a)[low : high : max]`. If the sliced operand is an array, it must be addressable.

The indices are in range if `0 <= low <= high <= max <= cap(a)`, otherwise they are out of range. A constant index must be non-negative and representable by a value of type `int`; for arrays, constant indices must also be in range. If multiple indices are constant, the constants that are present must be in range relative to each other. If the indices are out of range at run time, a run-time panic occurs.

### Type assertions

For an expression `x` of interface type and a type `T`, the primary expression

    x.(T)

asserts that `x` is not `nil` and that the value stored in `x` is of type `T`. The notation `x.(T)` is called a type assertion.

More precisely, if `T` is not an interface type, `x.(T)` asserts that the dynamic type of `x` is identical to the type `T`. In this case, `T` must implement the (interface) type of `x`; otherwise the type assertion is invalid since it is not possible for `x` to store a value of type `T`. If `T` is an interface type, `x.(T)` asserts that the dynamic type of `x` implements the interface `T`.

If the type assertion holds, the value of the expression is the value stored in `x` and its type is `T`. If the type assertion is false, a run-time panic occurs. In other words, even though the dynamic type of `x` is known only at run time, the type of `x.(T)` is known to be `T` in a correct program.

    var x interface{} = 7  // x has dynamic type int and value 7
    i := x.(int)           // i has type int and value 7

    type I interface { m() }
    var y I
    s := y.(string)        // illegal: string does not implement I (missing method m)
    r := y.(io.Reader)     // r has type io.Reader and y must implement both I and io.Reader

A type assertion used in an assignment or initialization of the special form

    v, ok = x.(T)
    v, ok := x.(T)
    var v, ok = x.(T)

yields an additional untyped boolean value. The value of `ok` is `true` if the assertion holds. Otherwise it is `false` and the value of `v` is the zero value for type `T`. No run-time panic occurs in this case.

### Calls

Given an expression `f` of function type `F`,

    f(a1, a2, … an)

calls `f` with arguments `a1, a2, … an`. Except for one special case, arguments must be single-valued expressions assignable to the parameter types of `F` and are evaluated before the function is called. The type of the expression is the result type of `F`. A method invocation is similar but the method itself is specified as a selector upon a value of the receiver type for the method.

    math.Atan2(x, y)  // function call
    var pt *Point
    pt.Scale(3.5)     // method call with receiver pt

In a function call, the function value and arguments are evaluated in the usual order. After they are evaluated, the parameters of the call are passed by value to the function and the called function begins execution. The return parameters of the function are passed by value back to the calling function when the function returns.

Calling a `nil` function value causes a run-time panic.

As a special case, if the return values of a function or method `g` are equal in number and individually assignable to the parameters of another function or method `f`, then the call `f(g(parameters_of_g))` will invoke `f` after binding the return values of `g` to the parameters of `f` in order. The call of `f` must contain no parameters other than the call of `g`, and `g` must have at least one return value. If `f` has a final `...` parameter, it is assigned the return values of `g` that remain after assignment of regular parameters.

    func Split(s string, pos int) (string, string) {
    	return s[0:pos], s[pos:]
    }

    func Join(s, t string) string {
    	return s + t
    }

    if Join(Split(value, len(value)/2)) != value {
    	log.Panic("test fails")
    }

A method call `x.m()` is valid if the method set of (the type of) `x` contains `m` and the argument list can be assigned to the parameter list of `m`. If `x` is addressable and `&x`'s method set contains `m`, `x.m()` is shorthand for `(&x).m()`:

    var p Point
    p.Scale(3.5)

There is no distinct method type and there are no method literals.

### Passing arguments to `...` parameters

If `f` is variadic with a final parameter `p` of type `...T`, then within `f` the type of `p` is equivalent to type `[]T`. If `f` is invoked with no actual arguments for `p`, the value passed to `p` is `nil`. Otherwise, the value passed is a new slice of type `[]T` with a new underlying array whose successive elements are the actual arguments, which all must be assignable to `T`. The length and capacity of the slice is therefore the number of arguments bound to `p` and may differ for each call site.

Given the function and calls

    func Greeting(prefix string, who ...string)
    Greeting("nobody")
    Greeting("hello:", "Joe", "Anna", "Eileen")

within `Greeting`, `who` will have the value `nil` in the first call, and `[]string{"Joe", "Anna", "Eileen"}` in the second.

If the final argument is assignable to a slice type `[]T`, it may be passed unchanged as the value for a `...T` parameter if the argument is followed by `...`. In this case no new slice is created.

Given the slice `s` and call

    s := []string{"James", "Jasmine"}
    Greeting("goodbye:", s...)

within `Greeting`, `who` will have the same value as `s` with the same underlying array.

### Operators

Operators combine operands into expressions.

    Expression = UnaryExpr | Expression binary_op Expression .
    UnaryExpr  = PrimaryExpr | unary_op UnaryExpr .

    binary_op  = "||" | "&&" | rel_op | add_op | mul_op .
    rel_op     = "==" | "!=" | "<" | "<=" | ">" | ">=" .
    add_op     = "+" | "-" | "|" | "^" .
    mul_op     = "*" | "/" | "%" | "<<" | ">>" | "&" | "&^" .

    unary_op   = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .

Comparisons are discussed elsewhere. For other binary operators, the operand types must be identical unless the operation involves shifts or untyped constants. For operations involving constants only, see the section on constant expressions.

Except for shift operations, if one operand is an untyped constant and the other operand is not, the constant is converted to the type of the other operand.

The right operand in a shift expression must have unsigned integer type or be an untyped constant that can be converted to unsigned integer type. If the left operand of a non-constant shift expression is an untyped constant, the type of the constant is what it would be if the shift expression were replaced by its left operand alone.

    var s uint = 33
    var i = 1<<s           // 1 has type int
    var j int32 = 1<<s     // 1 has type int32; j == 0
    var k = uint64(1<<s)   // 1 has type uint64; k == 1<<33
    var m int = 1.0<<s     // 1.0 has type int
    var n = 1.0<<s != i    // 1.0 has type int; n == false if ints are 32bits in size
    var o = 1<<s == 2<<s   // 1 and 2 have type int; o == true if ints are 32bits in size
    var p = 1<<s == 1<<33  // illegal if ints are 32bits in size: 1 has type int, but 1<<33 overflows int
    var u = 1.0<<s         // illegal: 1.0 has type float64, cannot shift
    var u1 = 1.0<<s != 0   // illegal: 1.0 has type float64, cannot shift
    var u2 = 1<<s != 1.0   // illegal: 1 has type float64, cannot shift
    var v float32 = 1<<s   // illegal: 1 has type float32, cannot shift
    var w int64 = 1.0<<33  // 1.0<<33 is a constant shift expression

### Operator precedence

Unary operators have the highest precedence. As the `++` and `--` operators form statements, not expressions, they fall outside the operator hierarchy. As a consequence, statement `*p++` is the same as `(*p)++`.

There are five precedence levels for binary operators. Multiplication operators bind strongest, followed by addition operators, comparison operators, `&&` (logical AND), and finally `||` (logical OR):

    Precedence    Operator
        5             *  /  %  <<  >>  &  &^
        4             +  -  |  ^
        3             ==  !=  <  <=  >  >=
        2             &&
        1             ||

Binary operators of the same precedence associate from left to right. For instance, `x / y * z` is the same as `(x / y) * z`.

    +x
    23 + 3*x[i]
    x <= f()
    ^a >> b
    f() || g()
    x == y+1 && <-chanPtr > 0

### Arithmetic operators

Arithmetic operators apply to numeric values and yield a result of the same type as the first operand. The four standard arithmetic operators (`+`, `-`, `*`, `/`) apply to integer, floating-point, and complex types; `+` also applies to strings. All other arithmetic operators apply to integers only.

    +    sum                    integers, floats, complex values, strings
    -    difference             integers, floats, complex values
    *    product                integers, floats, complex values
    /    quotient               integers, floats, complex values
    %    remainder              integers

    &    bitwise AND            integers
    |    bitwise OR             integers
    ^    bitwise XOR            integers
    &^   bit clear (AND NOT)    integers

    <<   left shift             integer << unsigned integer
    >>   right shift            integer >> unsigned integer

Strings can be concatenated using the `+` operator or the `+=` assignment operator:

    s := "hi" + string(c)
    s += " and good bye"

String addition creates a new string by concatenating the operands.

For two integer values `x` and `y`, the integer quotient `q = x / y` and remainder `r = x % y` satisfy the following relationships:

    x = q*y + r  and  |r| < |y|

with `x / y` truncated towards zero ("truncated division").

     x     y     x / y     x % y
     5     3       1         2
    -5     3      -1        -2
     5    -3      -1         2
    -5    -3       1        -2

As an exception to this rule, if the dividend `x` is the most negative value for the int type of `x`, the quotient `q = x / -1` is equal to `x` (and `r = 0`).

    			 x, q
    int8                     -128
    int16                  -32768
    int32             -2147483648
    int64    -9223372036854775808

If the divisor is a constant, it must not be zero. If the divisor is zero at run time, a run-time panic occurs. If the dividend is non-negative and the divisor is a constant power of 2, the division may be replaced by a right shift, and computing the remainder may be replaced by a bitwise AND operation:

     x     x / 4     x % 4     x >> 2     x & 3
     11      2         3         2          3
    -11     -2        -3        -3          1

The shift operators shift the left operand by the shift count specified by the right operand. They implement arithmetic shifts if the left operand is a signed integer and logical shifts if it is an unsigned integer. There is no upper limit on the shift count. Shifts behave as if the left operand is shifted `n` times by 1 for a shift count of `n`. As a result, `x << 1` is the same as `x*2` and `x >> 1` is the same as `x/2` but truncated towards negative infinity.

For integer operands, the unary operators `+`, `-`, and `^` are defined as follows:

    +x                          is 0 + x
    -x    negation              is 0 - x
    ^x    bitwise complement    is m ^ x  with m = "all bits set to 1" for unsigned x
                                          and  m = -1 for signed x

For floating-point and complex numbers, `+x` is the same as `x`, while `-x` is the negation of `x`. The result of a floating-point or complex division by zero is not specified beyond the IEEE-754 standard; whether a run-time panic occurs is implementation-specific.

### Integer overflow

For unsigned integer values, the operations `+`, `-`, `*`, and `<<` are computed modulo 2n, where n is the bit width of the unsigned integer's type. Loosely speaking, these unsigned integer operations discard high bits upon overflow, and programs may rely on ``wrap around''.

For signed integers, the operations `+`, `-`, `*`, and `<<` may legally overflow and the resulting value exists and is deterministically defined by the signed integer representation, the operation, and its operands. No exception is raised as a result of overflow. A compiler may not optimize code under the assumption that overflow does not occur. For instance, it may not assume that `x < x + 1` is always true.

### Comparison operators

Comparison operators compare two operands and yield an untyped boolean value.

    ==    equal
    !=    not equal
    <     less
    <=    less or equal
    >     greater
    >=    greater or equal

In any comparison, the first operand must be assignable to the type of the second operand, or vice versa.

The equality operators `==` and `!=` apply to operands that are comparable. The ordering operators `<`, `<=`, `>`, and `>=` apply to operands that are ordered. These terms and the result of the comparisons are defined as follows:

- Boolean values are comparable. Two boolean values are equal if they are either both `true` or both `false`.
- Integer values are comparable and ordered, in the usual way.
- Floating point values are comparable and ordered, as defined by the IEEE-754 standard.
- Complex values are comparable. Two complex values `u` and `v` are equal if both `real(u) == real(v)` and `imag(u) == imag(v)`.
- String values are comparable and ordered, lexically byte-wise.
- Pointer values are comparable. Two pointer values are equal if they point to the same variable or if both have value `nil`. Pointers to distinct zero-size variables may or may not be equal.
- Channel values are comparable. Two channel values are equal if they were created by the same call to `make` or if both have value `nil`.
- Interface values are comparable. Two interface values are equal if they have identical dynamic types and equal dynamic values or if both have value `nil`.
- A value `x` of non-interface type `X` and a value `t` of interface type `T` are comparable when values of type `X` are comparable and `X` implements `T`. They are equal if `t`'s dynamic type is identical to `X` and `t`'s dynamic value is equal to `x`.
- Struct values are comparable if all their fields are comparable. Two struct values are equal if their corresponding non-blank fields are equal.
- Array values are comparable if values of the array element type are comparable. Two array values are equal if their corresponding elements are equal.

A comparison of two interface values with identical dynamic types causes a run-time panic if values of that type are not comparable. This behavior applies not only to direct interface value comparisons but also when comparing arrays of interface values or structs with interface-valued fields.

Slice, map, and function values are not comparable. However, as a special case, a slice, map, or function value may be compared to the predeclared identifier `nil`. Comparison of pointer, channel, and interface values to `nil` is also allowed and follows from the general rules above.

    const c = 3 < 4            // c is the untyped bool constant true

    type MyBool bool
    var x, y int
    var (
    	// The result of a comparison is an untyped bool.
    	// The usual assignment rules apply.
    	b3        = x == y // b3 has type bool
    	b4 bool   = x == y // b4 has type bool
    	b5 MyBool = x == y // b5 has type MyBool
    )

### Logical operators

Logical operators apply to boolean values and yield a result of the same type as the operands. The right operand is evaluated conditionally.

    &&    conditional AND    p && q  is  "if p then q else false"
    ||    conditional OR     p || q  is  "if p then true else q"
    !     NOT                !p      is  "not p"

### Address operators

For an operand `x` of type `T`, the address operation `&x` generates a pointer of type `*T` to `x`. The operand must be addressable, that is, either a variable, pointer indirection, or slice indexing operation; or a field selector of an addressable struct operand; or an array indexing operation of an addressable array. As an exception to the addressability requirement, `x` may also be a (possibly parenthesized) composite literal. If the evaluation of `x` would cause a run-time panic, then the evaluation of `&x` does too.

For an operand `x` of pointer type `*T`, the pointer indirection `*x` denotes the variable of type `T` pointed to by `x`. If `x` is `nil`, an attempt to evaluate `*x` will cause a run-time panic.

    &x
    &a[f(2)]
    &Point{2, 3}
    *p
    *pf(x)

    var x *int = nil
    *x   // causes a run-time panic
    &*x  // causes a run-time panic

### Receive operator

For an operand `ch` of channel type, the value of the receive operation `<-ch` is the value received from the channel `ch`. The channel direction must permit receive operations, and the type of the receive operation is the element type of the channel. The expression blocks until a value is available. Receiving from a `nil` channel blocks forever. A receive operation on a closed channel can always proceed immediately, yielding the element type's zero value after any previously sent values have been received.

    v1 := <-ch
    v2 = <-ch
    f(<-ch)
    <-strobe  // wait until clock pulse and discard received value

A receive expression used in an assignment or initialization of the special form

    x, ok = <-ch
    x, ok := <-ch
    var x, ok = <-ch

yields an additional untyped boolean result reporting whether the communication succeeded. The value of `ok` is `true` if the value received was delivered by a successful send operation to the channel, or `false` if it is a zero value generated because the channel is closed and empty.

### Conversions

Conversions are expressions of the form `T(x)` where `T` is a type and `x` is an expression that can be converted to type `T`.

    Conversion = Type "(" Expression [ "," ] ")" .

If the type starts with the operator `*` or `<-`, or if the type starts with the keyword `func` and has no result list, it must be parenthesized when necessary to avoid ambiguity:

    *Point(p)        // same as *(Point(p))
    (*Point)(p)      // p is converted to *Point
    <-chan int(c)    // same as <-(chan int(c))
    (<-chan int)(c)  // c is converted to <-chan int
    func()(x)        // function signature func() x
    (func())(x)      // x is converted to func()
    (func() int)(x)  // x is converted to func() int
    func() int(x)    // x is converted to func() int (unambiguous)

A constant value `x` can be converted to type `T` in any of these cases:

- `x` is representable by a value of type `T`.
- `x` is a floating-point constant, `T` is a floating-point type, and `x` is representable by a value of type `T` after rounding using IEEE 754 round-to-even rules. The constant `T(x)` is the rounded value.
- `x` is an integer constant and `T` is a string type. The same rule as for non-constant `x` applies in this case.

Converting a constant yields a typed constant as result.

    uint(iota)               // iota value of type uint
    float32(2.718281828)     // 2.718281828 of type float32
    complex128(1)            // 1.0 + 0.0i of type complex128
    float32(0.49999999)      // 0.5 of type float32
    string('x')              // "x" of type string
    string(0x266c)           // "♬" of type string
    MyString("foo" + "bar")  // "foobar" of type MyString
    string([]byte{'a'})      // not a constant: []byte{'a'} is not a constant
    (*int)(nil)              // not a constant: nil is not a constant, *int is not a boolean, numeric, or string type
    int(1.2)                 // illegal: 1.2 cannot be represented as an int
    string(65.0)             // illegal: 65.0 is not an integer constant

A non-constant value `x` can be converted to type `T` in any of these cases:

- `x` is assignable to `T`.
- `x`'s type and `T` have identical underlying types.
- `x`'s type and `T` are unnamed pointer types and their pointer base types have identical underlying types.
- `x`'s type and `T` are both integer or floating point types.
- `x`'s type and `T` are both complex types.
- `x` is an integer or a slice of bytes or runes and `T` is a string type.
- `x` is a string and `T` is a slice of bytes or runes.

Specific rules apply to (non-constant) conversions between numeric types or to and from a string type. These conversions may change the representation of `x` and incur a run-time cost. All other conversions only change the type but not the representation of `x`.

There is no linguistic mechanism to convert between pointers and integers. The package `unsafe` implements this functionality under restricted circumstances.

#### Conversions between numeric types

For the conversion of non-constant numeric values, the following rules apply:

1. When converting between integer types, if the value is a signed integer, it is sign extended to implicit infinite precision; otherwise it is zero extended. It is then truncated to fit in the result type's size. For example, if `v := uint16(0x10F0)`, then `uint32(int8(v)) == 0xFFFFFFF0`. The conversion always yields a valid value; there is no indication of overflow.
2. When converting a floating-point number to an integer, the fraction is discarded (truncation towards zero).
3. When converting an integer or floating-point number to a floating-point type, or a complex number to another complex type, the result value is rounded to the precision specified by the destination type. For instance, the value of a variable `x` of type `float32` may be stored using additional precision beyond that of an IEEE-754 32-bit number, but float32(x) represents the result of rounding `x`'s value to 32-bit precision. Similarly, `x + 0.1` may use more than 32 bits of precision, but `float32(x + 0.1)` does not.

In all non-constant conversions involving floating-point or complex values, if the result type cannot represent the value the conversion succeeds but the result value is implementation-dependent.

#### Conversions to and from a string type

1. Converting a signed or unsigned integer value to a string type yields a string containing the UTF-8 representation of the integer. Values outside the range of valid Unicode code points are converted to `"\uFFFD"`.

       string('a')       // "a"
       string(-1)        // "\ufffd" == "\xef\xbf\xbd"
       string(0xf8)      // "\u00f8" == "ø" == "\xc3\xb8"
       type MyString string
       MyString(0x65e5)  // "\u65e5" == "日" == "\xe6\x97\xa5"

2. Converting a slice of bytes to a string type yields a string whose successive bytes are the elements of the slice.

       string([]byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'})   // "hellø"
       string([]byte{})                                     // ""
       string([]byte(nil))                                  // ""

       type MyBytes []byte
       string(MyBytes{'h', 'e', 'l', 'l', '\xc3', '\xb8'})  // "hellø"

3. Converting a slice of runes to a string type yields a string that is the concatenation of the individual rune values converted to strings.

       string([]rune{0x767d, 0x9d6c, 0x7fd4})   // "\u767d\u9d6c\u7fd4" == "白鵬翔"
       string([]rune{})                         // ""
       string([]rune(nil))                      // ""

       type MyRunes []rune
       string(MyRunes{0x767d, 0x9d6c, 0x7fd4})  // "\u767d\u9d6c\u7fd4" == "白鵬翔"

4. Converting a value of a string type to a slice of bytes type yields a slice whose successive elements are the bytes of the string.

       []byte("hellø")   // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}
       []byte("")        // []byte{}

       MyBytes("hellø")  // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}

5. Converting a value of a string type to a slice of runes type yields a slice containing the individual Unicode code points of the string.

       []rune(MyString("白鵬翔"))  // []rune{0x767d, 0x9d6c, 0x7fd4}
       []rune("")                 // []rune{}

       MyRunes("白鵬翔")           // []rune{0x767d, 0x9d6c, 0x7fd4}

### Constant expressions

Constant expressions may contain only constant operands and are evaluated at compile time.

Untyped boolean, numeric, and string constants may be used as operands wherever it is legal to use an operand of boolean, numeric, or string type, respectively. Except for shift operations, if the operands of a binary operation are different kinds of untyped constants, the operation and, for non-boolean operations, the result use the kind that appears later in this list: integer, rune, floating-point, complex. For example, an untyped integer constant divided by an untyped complex constant yields an untyped complex constant.

A constant comparison always yields an untyped boolean constant. If the left operand of a constant shift expression is an untyped constant, the result is an integer constant; otherwise it is a constant of the same type as the left operand, which must be of integer type. Applying all other operators to untyped constants results in an untyped constant of the same kind (that is, a boolean, integer, floating-point, complex, or string constant).

    const a = 2 + 3.0          // a == 5.0   (untyped floating-point constant)
    const b = 15 / 4           // b == 3     (untyped integer constant)
    const c = 15 / 4.0         // c == 3.75  (untyped floating-point constant)
    const Θ float64 = 3/2      // Θ == 1.0   (type float64, 3/2 is integer division)
    const Π float64 = 3/2.     // Π == 1.5   (type float64, 3/2. is float division)
    const d = 1 << 3.0         // d == 8     (untyped integer constant)
    const e = 1.0 << 3         // e == 8     (untyped integer constant)
    const f = int32(1) << 33   // illegal    (constant 8589934592 overflows int32)
    const g = float64(2) >> 1  // illegal    (float64(2) is a typed floating-point constant)
    const h = "foo" > "bar"    // h == true  (untyped boolean constant)
    const j = true             // j == true  (untyped boolean constant)
    const k = 'w' + 1          // k == 'x'   (untyped rune constant)
    const l = "hi"             // l == "hi"  (untyped string constant)
    const m = string(k)        // m == "x"   (type string)
    const Σ = 1 - 0.707i       //            (untyped complex constant)
    const Δ = Σ + 2.0e-4       //            (untyped complex constant)
    const Φ = iota*1i - 1/1i   //            (untyped complex constant)

Applying the built-in function `complex` to untyped integer, rune, or floating-point constants yields an untyped complex constant.

    const ic = complex(0, c)   // ic == 3.75i  (untyped complex constant)
    const iΘ = complex(0, Θ)   // iΘ == 1i     (type complex128)

Constant expressions are always evaluated exactly; intermediate values and the constants themselves may require precision significantly larger than supported by any predeclared type in the language. The following are legal declarations:

    const Huge = 1 << 100         // Huge == 1267650600228229401496703205376  (untyped integer constant)
    const Four int8 = Huge >> 98  // Four == 4                                (type int8)

The divisor of a constant division or remainder operation must not be zero:

    3.14 / 0.0   // illegal: division by zero

The values of typed constants must always be accurately representable as values of the constant type. The following constant expressions are illegal:

    uint(-1)     // -1 cannot be represented as a uint
    int(3.14)    // 3.14 cannot be represented as an int
    int64(Huge)  // 1267650600228229401496703205376 cannot be represented as an int64
    Four * 300   // operand 300 cannot be represented as an int8 (type of Four)
    Four * 100   // product 400 cannot be represented as an int8 (type of Four)

The mask used by the unary bitwise complement operator `^` matches the rule for non-constants: the mask is all 1s for unsigned constants and -1 for signed and untyped constants.

    ^1         // untyped integer constant, equal to -2
    uint8(^1)  // illegal: same as uint8(-2), -2 cannot be represented as a uint8
    ^uint8(1)  // typed uint8 constant, same as 0xFF ^ uint8(1) = uint8(0xFE)
    int8(^1)   // same as int8(-2)
    ^int8(1)   // same as -1 ^ int8(1) = -2

Implementation restriction: A compiler may use rounding while computing untyped floating-point or complex constant expressions; see the implementation restriction in the section on constants. This rounding may cause a floating-point constant expression to be invalid in an integer context, even if it would be integral when calculated using infinite precision, and vice versa.

### Order of evaluation

At package level, initialization dependencies determine the evaluation order of individual initialization expressions in variable declarations. Otherwise, when evaluating the operands of an expression, assignment, or return statement, all function calls, method calls, and communication operations are evaluated in lexical left-to-right order.

For example, in the (function-local) assignment

    y[f()], ok = g(h(), i()+x[j()], <-c), k()

the function calls and communication happen in the order `f()`, `h()`, `i()`, `j()`, `<-c`, `g()`, and `k()`. However, the order of those events compared to the evaluation and indexing of `x` and the evaluation of `y` is not specified.

    a := 1
    f := func() int { a++; return a }
    x := []int{a, f()}            // x may be [1, 2] or [2, 2]: evaluation order between a and f() is not specified
    m := map[int]int{a: 1, a: 2}  // m may be {2: 1} or {2: 2}: evaluation order between the two map assignments is not specified
    n := map[int]int{a: f()}      // n may be {2: 3} or {3: 3}: evaluation order between the key and the value is not specified

At package level, initialization dependencies override the left-to-right rule for individual initialization expressions, but not for operands within each expression:

    var a, b, c = f() + v(), g(), sqr(u()) + v()

    func f() int        { return c }
    func g() int        { return a }
    func sqr(x int) int { return x*x }

    // functions u and v are independent of all other variables and functions

The function calls happen in the order `u()`, `sqr()`, `v()`, `f()`, `v()`, and `g()`.

Floating-point operations within a single expression are evaluated according to the associativity of the operators. Explicit parentheses affect the evaluation by overriding the default associativity. In the expression `x + (y + z)` the addition `y + z` is performed before adding `x`.

## Statements

Statements control execution.

    Statement =
    	Declaration | LabeledStmt | SimpleStmt |
    	GoStmt | ReturnStmt | BreakStmt | ContinueStmt | GotoStmt |
    	FallthroughStmt | Block | IfStmt | SwitchStmt | SelectStmt | ForStmt |
    	DeferStmt .

    SimpleStmt = EmptyStmt | ExpressionStmt | SendStmt | IncDecStmt | Assignment | ShortVarDecl .

### Terminating statements

A terminating statement is one of the following:

1. A "return" or "goto" statement.
2. A call to the built-in function `panic`.
3. A block in which the statement list ends in a terminating statement.
4. An "if" statement in which:
   - the "else" branch is present, and
   - both branches are terminating statements.
5. A "for" statement in which:
   - there are no "break" statements referring to the "for" statement, and
   - the loop condition is absent.
6. A "switch" statement in which:
   - there are no "break" statements referring to the "switch" statement,
   - there is a default case, and
   - the statement lists in each case, including the default, end in a terminating statement, or a possibly labeled "fallthrough" statement.
7. A "select" statement in which:
   - there are no "break" statements referring to the "select" statement, and
   - the statement lists in each case, including the default if present, end in a terminating statement.
8. A labeled statement labeling a terminating statement.

All other statements are not terminating.

A statement list ends in a terminating statement if the list is not empty and its final statement is terminating.

### Empty statements

The empty statement does nothing.

    EmptyStmt = .

### Labeled statements

A labeled statement may be the target of a `goto`, `break` or `continue` statement.

    LabeledStmt = Label ":" Statement .
    Label       = identifier .

    Error: log.Panic("error encountered")

### Expression statements

With the exception of specific built-in functions, function and method calls and receive operations can appear in statement context. Such statements may be parenthesized.

    ExpressionStmt = Expression .

The following built-in functions are not permitted in statement context:

    append cap complex imag len make new real
    unsafe.Alignof unsafe.Offsetof unsafe.Sizeof

    h(x+y)
    f.Close()
    <-ch
    (<-ch)
    len("foo")  // illegal if len is the built-in function

### Send statements

A send statement sends a value on a channel. The channel expression must be of channel type, the channel direction must permit send operations, and the type of the value to be sent must be assignable to the channel's element type.

    SendStmt = Channel "<-" Expression .
    Channel  = Expression .

Both the channel and the value expression are evaluated before communication begins. Communication blocks until the send can proceed. A send on an unbuffered channel can proceed if a receiver is ready. A send on a buffered channel can proceed if there is room in the buffer. A send on a closed channel proceeds by causing a run-time panic. A send on a `nil` channel blocks forever.

    ch <- 3  // send value 3 to channel ch

### IncDec statements

The "++" and "--" statements increment or decrement their operands by the untyped constant `1`. As with an assignment, the operand must be addressable or a map index expression.

    IncDecStmt = Expression ( "++" | "--" ) .

The following assignment statements are semantically equivalent:

    IncDec statement    Assignment
    x++                 x += 1
    x--                 x -= 1

### Assignments

    Assignment = ExpressionList assign_op ExpressionList .

    assign_op = [ add_op | mul_op ] "=" .

Each left-hand side operand must be addressable, a map index expression, or (for `=` assignments only) the blank identifier. Operands may be parenthesized.

    x = 1
    *p = f()
    a[i] = 23
    (k) = <-ch  // same as: k = <-ch

An assignment operation `x` op`=` `y` where op is a binary arithmetic operation is equivalent to `x` `=` `x` op `(y)` but evaluates `x` only once. The op`=` construct is a single token. In assignment operations, both the left- and right-hand expression lists must contain exactly one single-valued expression, and the left-hand expression must not be the blank identifier.

    a[i] <<= 2
    i &^= 1<<n

A tuple assignment assigns the individual elements of a multi-valued operation to a list of variables. There are two forms. In the first, the right hand operand is a single multi-valued expression such as a function call, a channel or map operation, or a type assertion. The number of operands on the left hand side must match the number of values. For instance, if `f` is a function returning two values,

    x, y = f()

assigns the first value to `x` and the second to `y`. In the second form, the number of operands on the left must equal the number of expressions on the right, each of which must be single-valued, and the nth expression on the right is assigned to the nth operand on the left:

    one, two, three = '一', '二', '三'

The blank identifier provides a way to ignore right-hand side values in an assignment:

    _ = x       // evaluate x but ignore it
    x, _ = f()  // evaluate f() but ignore second result value

The assignment proceeds in two phases. First, the operands of index expressions and pointer indirections (including implicit pointer indirections in selectors) on the left and the expressions on the right are all evaluated in the usual order. Second, the assignments are carried out in left-to-right order.

    a, b = b, a  // exchange a and b

    x := []int{1, 2, 3}
    i := 0
    i, x[i] = 1, 2  // set i = 1, x[0] = 2

    i = 0
    x[i], i = 2, 1  // set x[0] = 2, i = 1

    x[0], x[0] = 1, 2  // set x[0] = 1, then x[0] = 2 (so x[0] == 2 at end)

    x[1], x[3] = 4, 5  // set x[1] = 4, then panic setting x[3] = 5.

    type Point struct { x, y int }
    var p *Point
    x[2], p.x = 6, 7  // set x[2] = 6, then panic setting p.x = 7

    i = 2
    x = []int{3, 5, 7}
    for i, x[i] = range x {  // set i, x[2] = 0, x[0]
    	break
    }
    // after this loop, i == 0 and x == []int{3, 5, 3}

In assignments, each value must be assignable to the type of the operand to which it is assigned, with the following special cases:

1. Any typed value may be assigned to the blank identifier.
2. If an untyped constant is assigned to a variable of interface type or the blank identifier, the constant is first converted to its default type.
3. If an untyped boolean value is assigned to a variable of interface type or the blank identifier, it is first converted to type `bool`.

### If statements

"If" statements specify the conditional execution of two branches according to the value of a boolean expression. If the expression evaluates to true, the "if" branch is executed, otherwise, if present, the "else" branch is executed.

    IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" ( IfStmt | Block ) ] .

    if x > max {
    	x = max
    }

The expression may be preceded by a simple statement, which executes before the expression is evaluated.

    if x := f(); x < y {
    	return x
    } else if x > z {
    	return z
    } else {
    	return y
    }

### Switch statements

"Switch" statements provide multi-way execution. An expression or type specifier is compared to the "cases" inside the "switch" to determine which branch to execute.

    SwitchStmt = ExprSwitchStmt | TypeSwitchStmt .

There are two forms: expression switches and type switches. In an expression switch, the cases contain expressions that are compared against the value of the switch expression. In a type switch, the cases contain types that are compared against the type of a specially annotated switch expression.

#### Expression switches

In an expression switch, the switch expression is evaluated and the case expressions, which need not be constants, are evaluated left-to-right and top-to-bottom; the first one that equals the switch expression triggers execution of the statements of the associated case; the other cases are skipped. If no case matches and there is a "default" case, its statements are executed. There can be at most one default case and it may appear anywhere in the "switch" statement. A missing switch expression is equivalent to the boolean value `true`.

    ExprSwitchStmt = "switch" [ SimpleStmt ";" ] [ Expression ] "{" { ExprCaseClause } "}" .
    ExprCaseClause = ExprSwitchCase ":" StatementList .
    ExprSwitchCase = "case" ExpressionList | "default" .

In a case or default clause, the last non-empty statement may be a (possibly labeled) "fallthrough" statement to indicate that control should flow from the end of this clause to the first statement of the next clause. Otherwise control flows to the end of the "switch" statement. A "fallthrough" statement may appear as the last statement of all but the last clause of an expression switch.

The expression may be preceded by a simple statement, which executes before the expression is evaluated.

    switch tag {
    default: s3()
    case 0, 1, 2, 3: s1()
    case 4, 5, 6, 7: s2()
    }

    switch x := f(); {  // missing switch expression means "true"
    case x < 0: return -x
    default: return x
    }

    switch {
    case x < y: f1()
    case x < z: f2()
    case x == 4: f3()
    }

#### Type switches

A type switch compares types rather than values. It is otherwise similar to an expression switch. It is marked by a special switch expression that has the form of a type assertion using the reserved word `type` rather than an actual type:

    switch x.(type) {
    // cases
    }

Cases then match actual types `T` against the dynamic type of the expression `x`. As with type assertions, `x` must be of interface type, and each non-interface type `T` listed in a case must implement the type of `x`.

    TypeSwitchStmt  = "switch" [ SimpleStmt ";" ] TypeSwitchGuard "{" { TypeCaseClause } "}" .
    TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" .
    TypeCaseClause  = TypeSwitchCase ":" StatementList .
    TypeSwitchCase  = "case" TypeList | "default" .
    TypeList        = Type { "," Type } .

The TypeSwitchGuard may include a short variable declaration. When that form is used, the variable is declared at the beginning of the implicit block in each clause. In clauses with a case listing exactly one type, the variable has that type; otherwise, the variable has the type of the expression in the TypeSwitchGuard.

The type in a case may be `nil`; that case is used when the expression in the TypeSwitchGuard is a `nil` interface value.

Given an expression `x` of type `interface{}`, the following type switch:

    switch i := x.(type) {
    case nil:
    	printString("x is nil")                // type of i is type of x (interface{})
    case int:
    	printInt(i)                            // type of i is int
    case float64:
    	printFloat64(i)                        // type of i is float64
    case func(int) float64:
    	printFunction(i)                       // type of i is func(int) float64
    case bool, string:
    	printString("type is bool or string")  // type of i is type of x (interface{})
    default:
    	printString("don't know the type")     // type of i is type of x (interface{})
    }

could be rewritten:

    v := x  // x is evaluated exactly once
    if v == nil {
    	i := v                                 // type of i is type of x (interface{})
    	printString("x is nil")
    } else if i, isInt := v.(int); isInt {
    	printInt(i)                            // type of i is int
    } else if i, isFloat64 := v.(float64); isFloat64 {
    	printFloat64(i)                        // type of i is float64
    } else if i, isFunc := v.(func(int) float64); isFunc {
    	printFunction(i)                       // type of i is func(int) float64
    } else {
    	_, isBool := v.(bool)
    	_, isString := v.(string)
    	if isBool || isString {
    		i := v                         // type of i is type of x (interface{})
    		printString("type is bool or string")
    	} else {
    		i := v                         // type of i is type of x (interface{})
    		printString("don't know the type")
    	}
    }

The type switch guard may be preceded by a simple statement, which executes before the guard is evaluated.

The "fallthrough" statement is not permitted in a type switch.

### For statements

A "for" statement specifies repeated execution of a block. The iteration is controlled by a condition, a "for" clause, or a "range" clause.

    ForStmt = "for" [ Condition | ForClause | RangeClause ] Block .
    Condition = Expression .

In its simplest form, a "for" statement specifies the repeated execution of a block as long as a boolean condition evaluates to true. The condition is evaluated before each iteration. If the condition is absent, it is equivalent to the boolean value `true`.

    for a < b {
    	a *= 2
    }

A "for" statement with a ForClause is also controlled by its condition, but additionally it may specify an init and a post statement, such as an assignment, an increment or decrement statement. The init statement may be a short variable declaration, but the post statement must not. Variables declared by the init statement are re-used in each iteration.

    ForClause = [ InitStmt ] ";" [ Condition ] ";" [ PostStmt ] .
    InitStmt = SimpleStmt .
    PostStmt = SimpleStmt .

    for i := 0; i < 10; i++ {
    	f(i)
    }

If non-empty, the init statement is executed once before evaluating the condition for the first iteration; the post statement is executed after each execution of the block (and only if the block was executed). Any element of the ForClause may be empty but the semicolons are required unless there is only a condition. If the condition is absent, it is equivalent to the boolean value `true`.

    for cond { S() }    is the same as    for ; cond ; { S() }
    for      { S() }    is the same as    for true     { S() }

A "for" statement with a "range" clause iterates through all entries of an array, slice, string or map, or values received on a channel. For each entry it assigns iteration values to corresponding iteration variables if present and then executes the block.

    RangeClause = [ ExpressionList "=" | IdentifierList ":=" ] "range" Expression .

The expression on the right in the "range" clause is called the range expression, which may be an array, pointer to an array, slice, string, map, or channel permitting receive operations. As with an assignment, if present the operands on the left must be addressable or map index expressions; they denote the iteration variables. If the range expression is a channel, at most one iteration variable is permitted, otherwise there may be up to two. If the last iteration variable is the blank identifier, the range clause is equivalent to the same clause without that identifier.

The range expression is evaluated once before beginning the loop, with one exception: if the range expression is an array or a pointer to an array and at most one iteration variable is present, only the range expression's length is evaluated; if that length is constant, by definition the range expression itself will not be evaluated.

Function calls on the left are evaluated once per iteration. For each iteration, iteration values are produced as follows if the respective iteration variables are present:

    Range expression                          1st value          2nd value

    array or slice  a  [n]E, *[n]E, or []E    index    i  int    a[i]       E
    string          s  string type            index    i  int    see below  rune
    map             m  map[K]V                key      k  K      m[k]       V
    channel         c  chan E, <-chan E       element  e  E

1. For an array, pointer to array, or slice value `a`, the index iteration values are produced in increasing order, starting at element index 0. If at most one iteration variable is present, the range loop produces iteration values from 0 up to `len(a)-1` and does not index into the array or slice itself. For a `nil` slice, the number of iterations is 0.
2. For a string value, the "range" clause iterates over the Unicode code points in the string starting at byte index 0. On successive iterations, the index value will be the index of the first byte of successive UTF-8-encoded code points in the string, and the second value, of type `rune`, will be the value of the corresponding code point. If the iteration encounters an invalid UTF-8 sequence, the second value will be `0xFFFD`, the Unicode replacement character, and the next iteration will advance a single byte in the string.
3. The iteration order over maps is not specified and is not guaranteed to be the same from one iteration to the next. If map entries that have not yet been reached are removed during iteration, the corresponding iteration values will not be produced. If map entries are created during iteration, that entry may be produced during the iteration or may be skipped. The choice may vary for each entry created and from one iteration to the next. If the map is `nil`, the number of iterations is 0.
4. For channels, the iteration values produced are the successive values sent on the channel until the channel is closed. If the channel is `nil`, the range expression blocks forever.

The iteration values are assigned to the respective iteration variables as in an assignment statement.

The iteration variables may be declared by the "range" clause using a form of short variable declaration (`:=`). In this case their types are set to the types of the respective iteration values and their scope is the block of the "for" statement; they are re-used in each iteration. If the iteration variables are declared outside the "for" statement, after execution their values will be those of the last iteration.

    var testdata *struct {
    	a *[7]int
    }
    for i, _ := range testdata.a {
    	// testdata.a is never evaluated; len(testdata.a) is constant
    	// i ranges from 0 to 6
    	f(i)
    }

    var a [10]string
    for i, s := range a {
    	// type of i is int
    	// type of s is string
    	// s == a[i]
    	g(i, s)
    }

    var key string
    var val interface {}  // value type of m is assignable to val
    m := map[string]int{"mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6}
    for key, val = range m {
    	h(key, val)
    }
    // key == last map key encountered in iteration
    // val == map[key]

    var ch chan Work = producer()
    for w := range ch {
    	doWork(w)
    }

    // empty a channel
    for range ch {}

### Go statements

A "go" statement starts the execution of a function call as an independent concurrent thread of control, or goroutine, within the same address space.

    GoStmt = "go" Expression .

The expression must be a function or method call; it cannot be parenthesized. Calls of built-in functions are restricted as for expression statements.

The function value and parameters are evaluated as usual in the calling goroutine, but unlike with a regular call, program execution does not wait for the invoked function to complete. Instead, the function begins executing independently in a new goroutine. When the function terminates, its goroutine also terminates. If the function has any return values, they are discarded when the function completes.

    go Server()
    go func(ch chan<- bool) { for { sleep(10); ch <- true; }} (c)

### Select statements

A "select" statement chooses which of a set of possible send or receive operations will proceed. It looks similar to a "switch" statement but with the cases all referring to communication operations.

    SelectStmt = "select" "{" { CommClause } "}" .
    CommClause = CommCase ":" StatementList .
    CommCase   = "case" ( SendStmt | RecvStmt ) | "default" .
    RecvStmt   = [ ExpressionList "=" | IdentifierList ":=" ] RecvExpr .
    RecvExpr   = Expression .

A case with a RecvStmt may assign the result of a RecvExpr to one or two variables, which may be declared using a short variable declaration. The RecvExpr must be a (possibly parenthesized) receive operation. There can be at most one default case and it may appear anywhere in the list of cases.

Execution of a "select" statement proceeds in several steps:

1. For all the cases in the statement, the channel operands of receive operations and the channel and right-hand-side expressions of send statements are evaluated exactly once, in source order, upon entering the "select" statement. The result is a set of channels to receive from or send to, and the corresponding values to send. Any side effects in that evaluation will occur irrespective of which (if any) communication operation is selected to proceed. Expressions on the left-hand side of a RecvStmt with a short variable declaration or assignment are not yet evaluated.
2. If one or more of the communications can proceed, a single one that can proceed is chosen via a uniform pseudo-random selection. Otherwise, if there is a default case, that case is chosen. If there is no default case, the "select" statement blocks until at least one of the communications can proceed.
3. Unless the selected case is the default case, the respective communication operation is executed.
4. If the selected case is a RecvStmt with a short variable declaration or an assignment, the left-hand side expressions are evaluated and the received value (or values) are assigned.
5. The statement list of the selected case is executed.

Since communication on `nil` channels can never proceed, a select with only `nil` channels and no default case blocks forever.

    var a []int
    var c, c1, c2, c3, c4 chan int
    var i1, i2 int
    select {
    case i1 = <-c1:
    	print("received ", i1, " from c1\n")
    case c2 <- i2:
    	print("sent ", i2, " to c2\n")
    case i3, ok := (<-c3):  // same as: i3, ok := <-c3
    	if ok {
    		print("received ", i3, " from c3\n")
    	} else {
    		print("c3 is closed\n")
    	}
    case a[f()] = <-c4:
    	// same as:
    	// case t := <-c4
    	//	a[f()] = t
    default:
    	print("no communication\n")
    }

    for {  // send random sequence of bits to c
    	select {
    	case c <- 0:  // note: no statement, no fallthrough, no folding of cases
    	case c <- 1:
    	}
    }

    select {}  // block forever

### Return statements

A "return" statement in a function `F` terminates the execution of `F`, and optionally provides one or more result values. Any functions deferred by `F` are executed before `F` returns to its caller.

    ReturnStmt = "return" [ ExpressionList ] .

In a function without a result type, a "return" statement must not specify any result values.

    func noResult() {
    	return
    }

There are three ways to return values from a function with a result type:

1. The return value or values may be explicitly listed in the "return" statement. Each expression must be single-valued and assignable to the corresponding element of the function's result type.

       func simpleF() int {
       	return 2
       }

       func complexF1() (re float64, im float64) {
       	return -7.0, -4.0
       }

2. The expression list in the "return" statement may be a single call to a multi-valued function. The effect is as if each value returned from that function were assigned to a temporary variable with the type of the respective value, followed by a "return" statement listing these variables, at which point the rules of the previous case apply.

       func complexF2() (re float64, im float64) {
       	return complexF1()
       }

3. The expression list may be empty if the function's result type specifies names for its result parameters. The result parameters act as ordinary local variables and the function may assign values to them as necessary. The "return" statement returns the values of these variables.

       func complexF3() (re float64, im float64) {
       	re = 7.0
       	im = 4.0
       	return
       }

       func (devnull) Write(p []byte) (n int, _ error) {
       	n = len(p)
       	return
       }

Regardless of how they are declared, all the result values are initialized to the zero values for their type upon entry to the function. A "return" statement that specifies results sets the result parameters before any deferred functions are executed.

Implementation restriction: A compiler may disallow an empty expression list in a "return" statement if a different entity (constant, type, or variable) with the same name as a result parameter is in scope at the place of the return.

    func f(n int) (res int, err error) {
    	if _, err := f(n-1); err != nil {
    		return  // invalid return statement: err is shadowed
    	}
    	return
    }

### Break statements

A "break" statement terminates execution of the innermost "for", "switch", or "select" statement within the same function.

    BreakStmt = "break" [ Label ] .

If there is a label, it must be that of an enclosing "for", "switch", or "select" statement, and that is the one whose execution terminates.

    OuterLoop:
    	for i = 0; i < n; i++ {
    		for j = 0; j < m; j++ {
    			switch a[i][j] {
    			case nil:
    				state = Error
    				break OuterLoop
    			case item:
    				state = Found
    				break OuterLoop
    			}
    		}
    	}

### Continue statements

A "continue" statement begins the next iteration of the innermost "for" loop at its post statement. The "for" loop must be within the same function.

    ContinueStmt = "continue" [ Label ] .

If there is a label, it must be that of an enclosing "for" statement, and that is the one whose execution advances.

    RowLoop:
    	for y, row := range rows {
    		for x, data := range row {
    			if data == endOfRow {
    				continue RowLoop
    			}
    			row[x] = data + bias(x, y)
    		}
    	}

### Goto statements

A "goto" statement transfers control to the statement with the corresponding label within the same function.

    GotoStmt = "goto" Label .

    goto Error

Executing the "goto" statement must not cause any variables to come into scope that were not already in scope at the point of the goto. For instance, this example:

    	goto L  // BAD
    	v := 3
    L:

is erroneous because the jump to label `L` skips the creation of `v`.

A "goto" statement outside a block cannot jump to a label inside that block. For instance, this example:

    if n%2 == 1 {
    	goto L1
    }
    for n > 0 {
    	f()
    	n--
    L1:
    	f()
    	n--
    }

is erroneous because the label `L1` is inside the "for" statement's block but the `goto` is not.

### Fallthrough statements

A "fallthrough" statement transfers control to the first statement of the next case clause in a expression "switch" statement. It may be used only as the final non-empty statement in such a clause.

    FallthroughStmt = "fallthrough" .

### Defer statements

A "defer" statement invokes a function whose execution is deferred to the moment the surrounding function returns, either because the surrounding function executed a return statement, reached the end of its function body, or because the corresponding goroutine is panicking.

    DeferStmt = "defer" Expression .

The expression must be a function or method call; it cannot be parenthesized. Calls of built-in functions are restricted as for expression statements.

Each time a "defer" statement executes, the function value and parameters to the call are evaluated as usual and saved anew but the actual function is not invoked. Instead, deferred functions are invoked immediately before the surrounding function returns, in the reverse order they were deferred. If a deferred function value evaluates to `nil`, execution panics when the function is invoked, not when the "defer" statement is executed.

For instance, if the deferred function is a function literal and the surrounding function has named result parameters that are in scope within the literal, the deferred function may access and modify the result parameters before they are returned. If the deferred function has any return values, they are discarded when the function completes. (See also the section on handling panics.)

    lock(l)
    defer unlock(l)  // unlocking happens before surrounding function returns

    // prints 3 2 1 0 before surrounding function returns
    for i := 0; i <= 3; i++ {
    	defer fmt.Print(i)
    }

    // f returns 1
    func f() (result int) {
    	defer func() {
    		result++
    	}()
    	return 0
    }

## Built-in functions

Built-in functions are predeclared. They are called like any other function but some of them accept a type instead of an expression as the first argument.

The built-in functions do not have standard Go types, so they can only appear in call expressions; they cannot be used as function values.

### Close

For a channel `c`, the built-in function `close(c)` records that no more values will be sent on the channel. It is an error if `c` is a receive-only channel. Sending to or closing a closed channel causes a run-time panic. Closing the nil channel also causes a run-time panic. After calling `close`, and after any previously sent values have been received, receive operations will return the zero value for the channel's type without blocking. The multi-valued receive operation returns a received value along with an indication of whether the channel is closed.

### Length and capacity

The built-in functions `len` and `cap` take arguments of various types and return a result of type `int`. The implementation guarantees that the result always fits into an `int`.

    Call      Argument type    Result

    len(s)    string type      string length in bytes
              [n]T, *[n]T      array length (== n)
              []T              slice length
              map[K]T          map length (number of defined keys)
              chan T           number of elements queued in channel buffer

    cap(s)    [n]T, *[n]T      array length (== n)
              []T              slice capacity
              chan T           channel buffer capacity

The capacity of a slice is the number of elements for which there is space allocated in the underlying array. At any time the following relationship holds:

    0 <= len(s) <= cap(s)

The length of a `nil` slice, map or channel is 0. The capacity of a `nil` slice or channel is 0.

The expression `len(s)` is constant if `s` is a string constant. The expressions `len(s)` and `cap(s)` are constants if the type of `s` is an array or pointer to an array and the expression `s` does not contain channel receives or (non-constant) function calls; in this case `s` is not evaluated. Otherwise, invocations of `len` and `cap` are not constant and `s` is evaluated.

    const (
    	c1 = imag(2i)                    // imag(2i) = 2.0 is a constant
    	c2 = len([10]float64{2})         // [10]float64{2} contains no function calls
    	c3 = len([10]float64{c1})        // [10]float64{c1} contains no function calls
    	c4 = len([10]float64{imag(2i)})  // imag(2i) is a constant and no function call is issued
    	c5 = len([10]float64{imag(z)})   // invalid: imag(z) is a (non-constant) function call
    )
    var z complex128

### Allocation

The built-in function `new` takes a type `T`, allocates storage for a variable of that type at run time, and returns a value of type `*T` pointing to it. The variable is initialized as described in the section on initial values.

    new(T)

For instance

    type S struct { a int; b float64 }
    new(S)

allocates storage for a variable of type `S`, initializes it (`a=0`, `b=0.0`), and returns a value of type `*S` containing the address of the location.

### Making slices, maps and channels

The built-in function `make` takes a type `T`, which must be a slice, map or channel type, optionally followed by a type-specific list of expressions. It returns a value of type `T` (not `*T`). The memory is initialized as described in the section on initial values.

    Call             Type T     Result

    make(T, n)       slice      slice of type T with length n and capacity n
    make(T, n, m)    slice      slice of type T with length n and capacity m

    make(T)          map        map of type T
    make(T, n)       map        map of type T with initial space for n elements

    make(T)          channel    unbuffered channel of type T
    make(T, n)       channel    buffered channel of type T, buffer size n

The size arguments `n` and `m` must be of integer type or untyped. A constant size argument must be non-negative and representable by a value of type `int`. If both `n` and `m` are provided and are constant, then `n` must be no larger than `m`. If `n` is negative or larger than `m` at run time, a run-time panic occurs.

    s := make([]int, 10, 100)       // slice with len(s) == 10, cap(s) == 100
    s := make([]int, 1e3)           // slice with len(s) == cap(s) == 1000
    s := make([]int, 1<<63)         // illegal: len(s) is not representable by a value of type int
    s := make([]int, 10, 0)         // illegal: len(s) > cap(s)
    c := make(chan int, 10)         // channel with a buffer size of 10
    m := make(map[string]int, 100)  // map with initial space for 100 elements

### Appending to and copying slices

The built-in functions `append` and `copy` assist in common slice operations. For both functions, the result is independent of whether the memory referenced by the arguments overlaps.

The variadic function `append` appends zero or more values `x` to `s` of type `S`, which must be a slice type, and returns the resulting slice, also of type `S`. The values `x` are passed to a parameter of type `...T` where `T` is the element type of `S` and the respective parameter passing rules apply. As a special case, `append` also accepts a first argument assignable to type `[]byte` with a second argument of string type followed by `...`. This form appends the bytes of the string.

    append(s S, x ...T) S  // T is the element type of S

If the capacity of `s` is not large enough to fit the additional values, `append` allocates a new, sufficiently large underlying array that fits both the existing slice elements and the additional values. Otherwise, `append` re-uses the underlying array.

    s0 := []int{0, 0}
    s1 := append(s0, 2)                // append a single element     s1 == []int{0, 0, 2}
    s2 := append(s1, 3, 5, 7)          // append multiple elements    s2 == []int{0, 0, 2, 3, 5, 7}
    s3 := append(s2, s0...)            // append a slice              s3 == []int{0, 0, 2, 3, 5, 7, 0, 0}
    s4 := append(s3[3:6], s3[2:]...)   // append overlapping slice    s4 == []int{3, 5, 7, 2, 3, 5, 7, 0, 0}

    var t []interface{}
    t = append(t, 42, 3.1415, "foo")   //                             t == []interface{}{42, 3.1415, "foo"}

    var b []byte
    b = append(b, "bar"...)            // append string contents      b == []byte{'b', 'a', 'r' }

The function `copy` copies slice elements from a source `src` to a destination `dst` and returns the number of elements copied. Both arguments must have identical element type `T` and must be assignable to a slice of type `[]T`. The number of elements copied is the minimum of `len(src)` and `len(dst)`. As a special case, `copy` also accepts a destination argument assignable to type `[]byte` with a source argument of a string type. This form copies the bytes from the string into the byte slice.

    copy(dst, src []T) int
    copy(dst []byte, src string) int

Examples:

    var a = [...]int{0, 1, 2, 3, 4, 5, 6, 7}
    var s = make([]int, 6)
    var b = make([]byte, 5)
    n1 := copy(s, a[0:])            // n1 == 6, s == []int{0, 1, 2, 3, 4, 5}
    n2 := copy(s, s[2:])            // n2 == 4, s == []int{2, 3, 4, 5, 4, 5}
    n3 := copy(b, "Hello, World!")  // n3 == 5, b == []byte("Hello")

### Deletion of map elements

The built-in function `delete` removes the element with key `k` from a map `m`. The type of `k` must be assignable to the key type of `m`.

    delete(m, k)  // remove element m[k] from map m

If the map `m` is `nil` or the element `m[k]` does not exist, `delete` is a no-op.

### Manipulating complex numbers

Three functions assemble and disassemble complex numbers. The built-in function `complex` constructs a complex value from a floating-point real and imaginary part, while `real` and `imag` extract the real and imaginary parts of a complex value.

    complex(realPart, imaginaryPart floatT) complexT
    real(complexT) floatT
    imag(complexT) floatT

The type of the arguments and return value correspond. For `complex`, the two arguments must be of the same floating-point type and the return type is the complex type with the corresponding floating-point constituents: `complex64` for `float32`, `complex128` for `float64`. The `real` and `imag` functions together form the inverse, so for a complex value `z`, `z` `==` `complex(real(z),` `imag(z))`.

If the operands of these functions are all constants, the return value is a constant.

    var a = complex(2, -2)             // complex128
    var b = complex(1.0, -1.4)         // complex128
    x := float32(math.Cos(math.Pi/2))  // float32
    var c64 = complex(5, -x)           // complex64
    var im = imag(b)                   // float64
    var rl = real(c64)                 // float32

### Handling panics

Two built-in functions, `panic` and `recover`, assist in reporting and handling run-time panics and program-defined error conditions.

    func panic(interface{})
    func recover() interface{}

While executing a function `F`, an explicit call to `panic` or a run-time panic terminates the execution of `F`. Any functions deferred by `F` are then executed as usual. Next, any deferred functions run by `F's` caller are run, and so on up to any deferred by the top-level function in the executing goroutine. At that point, the program is terminated and the error condition is reported, including the value of the argument to `panic`. This termination sequence is called panicking.

    panic(42)
    panic("unreachable")
    panic(Error("cannot parse"))

The `recover` function allows a program to manage behavior of a panicking goroutine. Suppose a function `G` defers a function `D` that calls `recover` and a panic occurs in a function on the same goroutine in which `G` is executing. When the running of deferred functions reaches `D`, the return value of `D`'s call to `recover` will be the value passed to the call of `panic`. If `D` returns normally, without starting a new `panic`, the panicking sequence stops. In that case, the state of functions called between `G` and the call to `panic` is discarded, and normal execution resumes. Any functions deferred by `G` before `D` are then run and `G`'s execution terminates by returning to its caller.

The return value of `recover` is `nil` if any of the following conditions holds:

- `panic`'s argument was `nil`;
- the goroutine is not panicking;
- `recover` was not called directly by a deferred function.

The `protect` function in the example below invokes the function argument `g` and protects callers from run-time panics raised by `g`.

    func protect(g func()) {
    	defer func() {
    		log.Println("done")  // Println executes normally even if there is a panic
    		if x := recover(); x != nil {
    			log.Printf("run time panic: %v", x)
    		}
    	}()
    	log.Println("start")
    	g()
    }

### Bootstrapping

Current implementations provide several built-in functions useful during bootstrapping. These functions are documented for completeness but are not guaranteed to stay in the language. They do not return a result.

    Function   Behavior

    print      prints all arguments; formatting of arguments is implementation-specific
    println    like print but prints spaces between arguments and a newline at the end

## Packages

Go programs are constructed by linking together packages. A package in turn is constructed from one or more source files that together declare constants, types, variables and functions belonging to the package and which are accessible in all files of the same package. Those elements may be exported and used in another package.

### Source file organization

Each source file consists of a package clause defining the package to which it belongs, followed by a possibly empty set of import declarations that declare packages whose contents it wishes to use, followed by a possibly empty set of declarations of functions, types, variables, and constants.

    SourceFile       = PackageClause ";" { ImportDecl ";" } { TopLevelDecl ";" } .

### Package clause

A package clause begins each source file and defines the package to which the file belongs.

    PackageClause  = "package" PackageName .
    PackageName    = identifier .

The PackageName must not be the blank identifier.

    package math

A set of files sharing the same PackageName form the implementation of a package. An implementation may require that all source files for a package inhabit the same directory.

### Import declarations

An import declaration states that the source file containing the declaration depends on functionality of the imported package (§Program initialization and execution) and enables access to exported identifiers of that package. The import names an identifier (PackageName) to be used for access and an ImportPath that specifies the package to be imported.

    ImportDecl       = "import" ( ImportSpec | "(" { ImportSpec ";" } ")" ) .
    ImportSpec       = [ "." | PackageName ] ImportPath .
    ImportPath       = string_lit .

The PackageName is used in qualified identifiers to access exported identifiers of the package within the importing source file. It is declared in the file block. If the PackageName is omitted, it defaults to the identifier specified in the package clause of the imported package. If an explicit period (`.`) appears instead of a name, all the package's exported identifiers declared in that package's package block will be declared in the importing source file's file block and must be accessed without a qualifier.

The interpretation of the ImportPath is implementation-dependent but it is typically a substring of the full file name of the compiled package and may be relative to a repository of installed packages.

Implementation restriction: A compiler may restrict ImportPaths to non-empty strings using only characters belonging to Unicode's L, M, N, P, and S general categories (the Graphic characters without spaces) and may also exclude the characters `!"#$%&'()*,:;<=>?[\]^`{|}` and the Unicode replacement character U+FFFD.

Assume we have compiled a package containing the package clause `package math`, which exports function `Sin`, and installed the compiled package in the file identified by `"lib/math"`. This table illustrates how `Sin` is accessed in files that import the package after the various types of import declaration.

    Import declaration          Local name of Sin

    import   "lib/math"         math.Sin
    import m "lib/math"         m.Sin
    import . "lib/math"         Sin

An import declaration declares a dependency relation between the importing and imported package. It is illegal for a package to import itself, directly or indirectly, or to directly import a package without referring to any of its exported identifiers. To import a package solely for its side-effects (initialization), use the blank identifier as explicit package name:

    import _ "lib/math"

### An example package

Here is a complete Go package that implements a concurrent prime sieve.

    package main

    import "fmt"

    // Send the sequence 2, 3, 4, … to channel 'ch'.
    func generate(ch chan<- int) {
    	for i := 2; ; i++ {
    		ch <- i  // Send 'i' to channel 'ch'.
    	}
    }

    // Copy the values from channel 'src' to channel 'dst',
    // removing those divisible by 'prime'.
    func filter(src <-chan int, dst chan<- int, prime int) {
    	for i := range src {  // Loop over values received from 'src'.
    		if i%prime != 0 {
    			dst <- i  // Send 'i' to channel 'dst'.
    		}
    	}
    }

    // The prime sieve: Daisy-chain filter processes together.
    func sieve() {
    	ch := make(chan int)  // Create a new channel.
    	go generate(ch)       // Start generate() as a subprocess.
    	for {
    		prime := <-ch
    		fmt.Print(prime, "\n")
    		ch1 := make(chan int)
    		go filter(ch, ch1, prime)
    		ch = ch1
    	}
    }

    func main() {
    	sieve()
    }

## Program initialization and execution

### The zero value

When storage is allocated for a variable, either through a declaration or a call of `new`, or when a new value is created, either through a composite literal or a call of `make`, and no explicit initialization is provided, the variable or value is given a default value. Each element of such a variable or value is set to the zero value for its type: `false` for booleans, `0` for integers, `0.0` for floats, `""` for strings, and `nil` for pointers, functions, interfaces, slices, channels, and maps. This initialization is done recursively, so for instance each element of an array of structs will have its fields zeroed if no value is specified.

These two simple declarations are equivalent:

    var i int
    var i int = 0

After

    type T struct { i int; f float64; next *T }
    t := new(T)

the following holds:

    t.i == 0
    t.f == 0.0
    t.next == nil

The same would also be true after

    var t T

### Package initialization

Within a package, package-level variables are initialized in declaration order but after any of the variables they depend on.

More precisely, a package-level variable is considered ready for initialization if it is not yet initialized and either has no initialization expression or its initialization expression has no dependencies on uninitialized variables. Initialization proceeds by repeatedly initializing the next package-level variable that is earliest in declaration order and ready for initialization, until there are no variables ready for initialization.

If any variables are still uninitialized when this process ends, those variables are part of one or more initialization cycles, and the program is not valid.

The declaration order of variables declared in multiple files is determined by the order in which the files are presented to the compiler: Variables declared in the first file are declared before any of the variables declared in the second file, and so on.

Dependency analysis does not rely on the actual values of the variables, only on lexical references to them in the source, analyzed transitively. For instance, if a variable `x`'s initialization expression refers to a function whose body refers to variable `y` then `x` depends on `y`. Specifically:

- A reference to a variable or function is an identifier denoting that variable or function.
- A reference to a method `m` is a method value or method expression of the form `t.m`, where the (static) type of `t` is not an interface type, and the method `m` is in the method set of `t`. It is immaterial whether the resulting function value `t.m` is invoked.
- A variable, function, or method `x` depends on a variable `y` if `x`'s initialization expression or body (for functions and methods) contains a reference to `y` or to a function or method that depends on `y`.

Dependency analysis is performed per package; only references referring to variables, functions, and methods declared in the current package are considered.

For example, given the declarations

    var (
    	a = c + b
    	b = f()
    	c = f()
    	d = 3
    )

    func f() int {
    	d++
    	return d
    }

the initialization order is `d`, `b`, `c`, `a`.

Variables may also be initialized using functions named `init` declared in the package block, with no arguments and no result parameters.

    func init() { … }

Multiple such functions may be defined, even within a single source file. The `init` identifier is not declared and thus `init` functions cannot be referred to from anywhere in a program.

A package with no imports is initialized by assigning initial values to all its package-level variables followed by calling all `init` functions in the order they appear in the source, possibly in multiple files, as presented to the compiler. If a package has imports, the imported packages are initialized before initializing the package itself. If multiple packages import a package, the imported package will be initialized only once. The importing of packages, by construction, guarantees that there can be no cyclic initialization dependencies.

Package initialization—variable initialization and the invocation of `init` functions—happens in a single goroutine, sequentially, one package at a time. An `init` function may launch other goroutines, which can run concurrently with the initialization code. However, initialization always sequences the `init` functions: it will not invoke the next one until the previous one has returned.

To ensure reproducible initialization behavior, build systems are encouraged to present multiple files belonging to the same package in lexical file name order to a compiler.

### Program execution

A complete program is created by linking a single, unimported package called the main package with all the packages it imports, transitively. The main package must have package name `main` and declare a function `main` that takes no arguments and returns no value.

    func main() { … }

Program execution begins by initializing the main package and then invoking the function `main`. When that function invocation returns, the program exits. It does not wait for other (non-`main`) goroutines to complete.

## Errors

The predeclared type `error` is defined as

    type error interface {
    	Error() string
    }

It is the conventional interface for representing an error condition, with the nil value representing no error. For instance, a function to read data from a file might be defined:

    func Read(f *File, b []byte) (n int, err error)

## Run-time panics

Execution errors such as attempting to index an array out of bounds trigger a run-time panic equivalent to a call of the built-in function `panic` with a value of the implementation-defined interface type `runtime.Error`. That type satisfies the predeclared interface type `error`. The exact error values that represent distinct run-time error conditions are unspecified.

    package runtime

    type Error interface {
    	error
    	// and perhaps other methods
    }

## System considerations

### Package `unsafe`

The built-in package `unsafe`, known to the compiler, provides facilities for low-level programming including operations that violate the type system. A package using `unsafe` must be vetted manually for type safety and may not be portable. The package provides the following interface:

    package unsafe

    type ArbitraryType int  // shorthand for an arbitrary Go type; it is not a real type
    type Pointer *ArbitraryType

    func Alignof(variable ArbitraryType) uintptr
    func Offsetof(selector ArbitraryType) uintptr
    func Sizeof(variable ArbitraryType) uintptr

A `Pointer` is a pointer type but a `Pointer` value may not be dereferenced. Any pointer or value of underlying type `uintptr` can be converted to a `Pointer` type and vice versa. The effect of converting between `Pointer` and `uintptr` is implementation-defined.

    var f float64
    bits = *(*uint64)(unsafe.Pointer(&f))

    type ptr unsafe.Pointer
    bits = *(*uint64)(ptr(&f))

    var p ptr = nil

The functions `Alignof` and `Sizeof` take an expression `x` of any type and return the alignment or size, respectively, of a hypothetical variable `v` as if `v` was declared via `var v = x`.

The function `Offsetof` takes a (possibly parenthesized) selector `s.f`, denoting a field `f` of the struct denoted by `s` or `*s`, and returns the field offset in bytes relative to the struct's address. If `f` is an embedded field, it must be reachable without pointer indirections through fields of the struct. For a struct `s` with field `f`:

    uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f) == uintptr(unsafe.Pointer(&s.f))

Computer architectures may require memory addresses to be aligned; that is, for addresses of a variable to be a multiple of a factor, the variable's type's alignment. The function `Alignof` takes an expression denoting a variable of any type and returns the alignment of the (type of the) variable in bytes. For a variable `x`:

    uintptr(unsafe.Pointer(&x)) % unsafe.Alignof(x) == 0

Calls to `Alignof`, `Offsetof`, and `Sizeof` are compile-time constant expressions of type `uintptr`.

### Size and alignment guarantees

For the numeric types, the following sizes are guaranteed:

    type                                 size in bytes

    byte, uint8, int8                     1
    uint16, int16                         2
    uint32, int32, float32                4
    uint64, int64, float64, complex64     8
    complex128                           16

The following minimal alignment properties are guaranteed:

1. For a variable `x` of any type: `unsafe.Alignof(x)` is at least 1.
2. For a variable `x` of struct type: `unsafe.Alignof(x)` is the largest of all the values `unsafe.Alignof(x.f)` for each field `f` of `x`, but at least 1.
3. For a variable `x` of array type: `unsafe.Alignof(x)` is the same as `unsafe.Alignof(x[0])`, but at least 1.

A struct or array type has size zero if it contains no fields (or elements, respectively) that have a size greater than zero. Two distinct zero-size variables may have the same address in memory.