doc/go_spec.html

<!--
Open issues:
[ ] Semantics of type declaration:
	- creating a new type (status quo), or only a new type name?
	- declaration "type T S" strips methods of S. why/why not?
	- no mechanism to declare a local type name: type T P.T


Todo's:
[ ] document illegality of package-external tuple assignments to structs
	w/ private fields: P.T(1, 2) illegal since same as P.T(a: 1, b: 2) for
	a T struct { a b int }.
[ ] should probably write something about evaluation order of statements even
	though obvious
[ ] string conversion: string([]int{}) vs string(int) conversion. Former is
	"inverse" of string range iteration.
[ ] do we need explicit channel conversion (to change channel direction)?


Wish list:
[ ] enum symbols that are not mixable with ints or some other mechanism
	(requirement that basic type aliases need conversion for compatibility)
[ ] Helper syntax for composite types: allow names/keys/indices for
	structs/maps/arrays
[ ] built-in assert() ("conditional panic") (gri)
-->


<h2>Introduction</h2>

<p>
This is a reference manual for the Go programming language. For
more information and other documents, see <a
href="/">the Go home page</a>.
</p>

<p>
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
<i>packages</i>, whose properties allow efficient management of
dependencies. The existing implementations use a traditional
compile/link model to generate executable binaries.
</p>

<p>
The grammar is compact and regular, allowing for easy analysis by
automatic tools such as integrated development environments.
</p>
<hr/>
<h2>Notation</h2>
<p>
The syntax is specified using Extended Backus-Naur Form (EBNF):
</p>

<pre class="grammar">
Production  = production_name "=" Expression .
Expression  = Alternative { "|" Alternative } .
Alternative = Term { Term } .
Term        = production_name | token [ "..." token ] | Group | Option | Repetition .
Group       = "(" Expression ")" .
Option      = "[" Expression "]" .
Repetition  = "{" Expression "}" .
</pre>

<p>
Productions are expressions constructed from terms and the following
operators, in increasing precedence:
</p>
<pre class="grammar">
|   alternation
()  grouping
[]  option (0 or 1 times)
{}  repetition (0 to n times)
</pre>

<p>
Lower-case production names are used to identify lexical tokens.
Non-terminals are in CamelCase. Lexical symbols are enclosed in
double quotes <code>""</code> (the double quote symbol is written as
<code>'"'</code>).
</p>

<p>
The form <code>"a ... b"</code> represents the set of characters from
<code>a</code> through <code>b</code> as alternatives.
</p>

<hr/>

<h2>Source code representation</h2>

<p>
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 term <i>character</i> to refer to a Unicode code point.
</p>
<p>
Each code point is distinct; for instance, upper and lower case letters
are different characters.
</p>

<h3>Characters</h3>

<p>
The following terms are used to denote specific Unicode character classes:
</p>
<ul>
	<li>unicode_char      an arbitrary Unicode code point</li>
	<li>unicode_letter    a Unicode code point classified as "Letter"</li>
	<li>capital_letter    a Unicode code point classified as "Letter, uppercase"</li>
	<li>unicode_digit     a Unicode code point classified as "Digit"</li>
</ul>

(The Unicode Standard, Section 4.5 General Category - Normative.)

<h3>Letters and digits</h3>

<p>
The underscore character <code>_</code> (U+005F) is considered a letter.
</>
<pre class="grammar">
letter        = unicode_letter | "_" .
decimal_digit = "0" ... "9" .
octal_digit   = "0" ... "7" .
hex_digit     = "0" ... "9" | "A" ... "F" | "a" ... "f" .
</pre>
<hr/>

<h2>Lexical elements</h2>

<h3>Comments</h3>

<p>
There are two forms of comments.  The first starts at the character
sequence <code>//</code> and continues through the next newline.  The
second starts at the character sequence <code>/*</code> and continues
through the character sequence <code>*/</code>.  Comments do not nest.
</p>

<h3>Tokens</h3>

<p>
Tokens form the vocabulary of the Go language.
There are four classes: identifiers, keywords, operators
and delimiters, and literals.  <i>White space</i>, formed from
blanks, tabs, and newlines, is ignored except as it separates tokens
that would otherwise combine into a single token.  Comments
behave as white space.  While breaking the input into tokens,
the next token is the longest sequence of characters that form a
valid token.
</p>

<h3>Identifiers</h3>

<p>
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.
</p>
<pre class="grammar">
identifier    = letter { letter | unicode_digit } .
</pre>
<pre>
a
_x9
ThisVariableIsExported
αβ
</pre>
Some identifiers are predeclared (§Predeclared identifiers).

<h3>Keywords</h3>

<p>
The following keywords are reserved and may not be used as identifiers.
</p>
<pre class="grammar">
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
</pre>

<h3>Operators and Delimiters</h3>

<p>
The following character sequences represent operators, delimiters, and other special tokens:
</p>
<pre class="grammar">
+    &amp;     +=    &amp;=     &amp;&amp;    ==    !=    (    )
-    |     -=    |=     ||    &lt;     &lt;=    [    ]
*    ^     *=    ^=     &lt;-    &gt;     &gt;=    {    }
/    <<    /=    <<=    ++    =     :=    ,    ;
%    >>    %=    >>=    --    !     ...   .    :
     &amp;^          &amp;^=
</pre>

<h3>Integer literals</h3>

<p>
An integer literal is a sequence of one or more digits in the
corresponding base, which may be 8, 10, or 16.  An optional prefix
sets a non-decimal base: <code>0</code> for octal, <code>0x</code> or
<code>0X</code> for hexadecimal.  In hexadecimal literals, letters
<code>a-f</code> and <code>A-F</code> represent values 10 through 15.
</p>
<pre class="grammar">
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 } .
</pre>

<pre>
42
0600
0xBadFace
170141183460469231731687303715884105727
</pre>

<h3>Floating-point literals</h3>
<p>
A floating-point literal is a decimal representation of a floating-point
number.  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 <code>e</code> or <code>E</code>
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.
</p>
<pre class="grammar">
float_lit    = decimals "." [ decimals ] [ exponent ] |
               decimals exponent |
               "." decimals [ exponent ] .
decimals = decimal_digit { decimal_digit } .
exponent = ( "e" | "E" ) [ "+" | "-" ] decimals .
</pre>

<pre>
0.
2.71828
1.e+0
6.67428e-11
1E6
.25
.12345E+5
</pre>

<h3>Ideal numbers</h3>

<p>
Integer literals represent values of arbitrary precision, or <i>ideal
integers</i>.  Similarly, floating-point literals represent values
of arbitrary precision, or <i>ideal floats</i>.  These <i>ideal
numbers</i> have no size or type and cannot overflow.  However,
when (used in an expression) assigned to a variable or typed constant,
the destination must be able to represent the assigned value.
</p>
<p>
Implementation restriction: A compiler may implement ideal numbers
by choosing an internal representation with at least twice the precision
of any machine type.
</p>

<h3>Character literals</h3>

<p>
A character literal represents an integer value, typically a
Unicode code point, 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 itself,
while multi-character sequences beginning with a backslash encode
values in various formats.
</p>
<p>
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 <code>'a'</code> holds a single byte representing
a literal <code>a</code>, Unicode U+0061, value <code>0x61</code>, while
<code>'ä'</code> holds two bytes (<code>0xc3</code> <code>0xa4</code>) representing
a literal <code>a</code>-dieresis, U+00E4, value <code>0xe4</code>.
</p>
<p>
Several backslash escapes allow arbitrary values to be represented
as ASCII text.  There are four ways to represent the integer value
as a numeric constant: <code>\x</code> followed by exactly two hexadecimal
digits; <code>\u</code> followed by exactly four hexadecimal digits;
<code>\U</code> followed by exactly eight hexadecimal digits, and a
plain backslash <code>\</code> 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.
</p>
<p>
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 `Unicode' escapes <code>\u</code> and <code>\U</code>
represent Unicode code points so within them some values are illegal,
in particular those above <code>0x10FFFF</code> and surrogate halves.
</p>
<p>
After a backslash, certain single-character escapes represent special values:
</p>
<pre class="grammar">
\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 character literals)
\"   U+0022 double quote  (valid escape only within string literals)
</pre>
<p>
All other sequences are illegal inside character literals.
</p>
<pre class="grammar">
char_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" | "\" | "'" | """ ) .
</pre>
<pre>
'a'
'ä'
'本'
'\t'
'\000'
'\007'
'\377'
'\x07'
'\xff'
'\u12e4'
'\U00101234'
</pre>

<p>
The value of a character literal is an ideal integer, just as with
integer literals.
</p>

<h3>String literals</h3>

<p>
String literals represent constant values of type <code>string</code>.
There are two forms: raw string literals and interpreted string
literals.
</p>
<p>
Raw string literals are character sequences between back quotes
<code>``</code>.  Within the quotes, any character is legal except
newline and back quote. The value of a raw string literal is the
string composed of the uninterpreted bytes between the quotes;
in particular, backslashes have no special meaning.
</p>
<p>
Interpreted string literals are character sequences between double
quotes <code>&quot;&quot;</code>. The text between the quotes forms the
value of the literal, with backslash escapes interpreted as they
are in character literals (except that <code>\'</code> is illegal and
<code>\"</code> is legal).  The three-digit octal (<code>\000</code>)
and two-digit hexadecimal (<code>\x00</code>) escapes represent individual
<i>bytes</i> of the resulting string; all other escapes represent
the (possibly multi-byte) UTF-8 encoding of individual <i>characters</i>.
Thus inside a string literal <code>\377</code> and <code>\xFF</code> represent
a single byte of value <code>0xFF</code>=255, while <code>ÿ</code>,
<code>\u00FF</code>, <code>\U000000FF</code> and <code>\xc3\xbf</code> represent
the two bytes <code>0xc3 0xbf</code> of the UTF-8 encoding of character
U+00FF.
</p>

<pre class="grammar">
string_lit             = raw_string_lit | interpreted_string_lit .
raw_string_lit         = "`" { unicode_char } "`" .
interpreted_string_lit = """ { unicode_value | byte_value } """ .
</pre>

<pre>
`abc`
`\n`
"hello, world\n"
"\n"
""
"Hello, world!\n"
"日本語"
"\u65e5本\U00008a9e"
"\xff\u00FF"
</pre>

<p>
These examples all represent the same string:
</p>

<pre>
"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
</pre>

<p>
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 character literal (it is not a single code
point), and will appear as two code points if placed in a string
literal.
</p>
<hr/>

<h2>Types</h2>

<p>
A type determines the set of values and operations specific to values of that type.
A type may be specified by a (possibly qualified (§Qualified identifiers))
type name (§Type declarations) or a <i>type literal</i>,
which composes a new type in terms of previously declared types.
</p>

<pre class="grammar">
Type      = TypeName | TypeLit | "(" Type ")" .
TypeName  = QualifiedIdent.
TypeLit   = ArrayType | StructType | PointerType | FunctionType | InterfaceType |
	    SliceType | MapType | ChannelType .
</pre>

<p>
<i>Basic types</i> such as <code>int</code> are predeclared (§Predeclared identifiers).
Other types may be constructed from these, recursively,
including arrays, structs, pointers, functions, interfaces, slices, maps, and
channels.
</p>

<p>
At any point in the source code, a type may be <i>complete</i> or
<i>incomplete</i>.  An incomplete type is one whose size is not
yet known, such as a struct whose fields are not yet fully
defined or a forward declared type (§Forward declarations).
Most types are always complete; for instance, a pointer
type is always complete even if it points to an incomplete type
because the size of the pointer itself is always known.
(TODO: Need to figure out how forward declarations of
interface fit in here.)
</p>
<p>
The <i>interface</i> of a type is the set of methods bound to it
(§Method declarations); for pointer types, it is the interface
of the pointer base type (§Pointer types). All types have an interface;
if they have no methods, it is the <i>empty interface</i>.
</p>
<p>
The <i>static type</i> (or just <i>type</i>) of a variable is the
type defined by its declaration.  Variables of interface type
(§Interface types) also have a distinct <i>dynamic type</i>, which
is the actual type of the value stored in the variable at run-time.
The dynamic type may vary during execution but is always compatible
with the static type of the interface variable.  For non-interfaces
types, the dynamic type is always the static type.
</p>

<h3>Basic types</h3>

<p>
Basic types include traditional numeric types, booleans, and strings. All are predeclared.
</p>

<h3>Numeric types</h3>

<p>
The architecture-independent numeric types are:
</p>

<pre class="grammar">
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 valid IEEE-754 32-bit floating point numbers
float64  the set of all valid IEEE-754 64-bit floating point numbers

byte     familiar alias for uint8
</pre>

<p>
Integer types are represented in the usual binary format; the value of
an n-bit integer is n bits wide. A negative signed integer is represented
as the two's complement of its absolute value.
</p>

<p>
There is also a set of architecture-independent basic numeric types
whose size depends on the architecture:
</p>

<pre class="grammar">
uint     at least 32 bits, at most the size of the largest uint type
int      at least 32 bits, at most the size of the largest int type
float    at least 32 bits, at most the size of the largest float type
uintptr  smallest uint type large enough to store the uninterpreted
		 bits of a pointer value
</pre>

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


<h3>Booleans</h3>

The type <code>bool</code> comprises the Boolean truth values
represented by the predeclared constants <code>true</code>
and <code>false</code>.


<h3>Strings</h3>

<p>
The <code>string</code> type represents the set of textual string values.
Strings behave like arrays of bytes but are immutable: once created,
it is impossible to change the contents of a string.

<p>
The elements of strings have type <code>byte</code> and may be
accessed using the usual indexing operations (§Indexes).  It is
illegal to take the address of such an element, that is, even if
<code>s[i]</code> is the <code>i</code><sup>th</sup> byte of a
string, <code>&amp;s[i]</code> is invalid.  The length of a string
can be computed by the function <code>len(s1)</code>.
</p>

<p>
A sequence of string literals is concatenated into a single string.
</p>
<pre class="grammar">
StringLit   = string_lit { string_lit } .
</pre>

<pre>
"Alea iacta est."
"Alea " /* The die */ `iacta est` /* is cast */ "."
</pre>

<h3>Array types</h3>

<p>
An array is a numbered sequence of elements of a single
type, called the element type, which must be complete
(§Types). The number of elements is called the length and is never
negative.
</p>

<pre class="grammar">
ArrayType   = "[" ArrayLength "]" ElementType .
ArrayLength = Expression .
ElementType = CompleteType .
</pre>

<p>
The length is part of the array's type and must must be a constant
expression (§Constant expressions) that evaluates to a non-negative
integer value.  The length of array <code>a</code> can be discovered
using the built-in function <code>len(a)</code>, which is a
compile-time constant.  The elements can be indexed by integer
indices 0 through the <code>len(a)-1</code> (§Indexes).
</p>

<pre>
[32]byte
[2*N] struct { x, y int32 }
[1000]*float64
</pre>

<h3>Slice types</h3>

<p>
A slice is a reference to a contiguous segment of an array and
contains a numbered sequence of elements from that array.  A slice
type denotes the set of all slices of arrays of its element type.
A slice value may be <code>nil</code>.
</p>

<pre class="grammar">
SliceType = "[" "]" ElementType .
</pre>

<p>
Like arrays, slices are indexable and have a length.  The length of a
slice <code>s</code> can be discovered by the built-in function
<code>len(s)</code>; unlike with arrays it may change during
execution.  The elements can be addressed by integer indices 0
through <code>len(s)-1</code> (§Indexes).  The slice index of a
given element may be less than the index of the same element in the
underlying array.
</p>
<p>
A slice, once initialized, is always associated with an underlying
array that holds its elements.  A slice therfore shares storage
with its array and with other slices of the same array; by contrast,
distinct arrays always represent distinct storage.
</p>
<p>
The array underlying a slice may extend past the end of the slice.
The <i>capacity</i> 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 (§Slices).
The capacity of a slice <code>a</code> can be discovered using the
built-in function
</p>

<pre>
cap(s)
</pre>

<p>
and the relationship between <code>len()</code> and <code>cap()</code> is:
</p>

<pre>
0 <= len(a) <= cap(a)
</pre>

<p>
The value of an uninitialized slice is <code>nil</code>.
The length and capacity of a <code>nil</code> slice
are 0. A new, initialized slice value for a given element type <code>T</code> is
made using the built-in function <code>make</code>, which takes a slice type
and parameters specifying the length and optionally the capacity:
</p>

<pre>
make([]T, length)
make([]T, length, capacity)
</pre>

<p>
The <code>make()</code> call allocates a new, hidden array to which the returned
slice value refers. That is, calling <code>make</code>
</p>

<pre>
make([]T, length, capacity)
</pre>

<p>
produces the same slice as allocating an array and slicing it, so these two examples
result in the same slice:
</p>

<pre>
make([]int, 50, 100)
new([100]int)[0:50]
</pre>


<h3>Struct types</h3>

<p>
A struct is a sequence of named
elements, called fields, with various types. A struct type declares
an identifier and type for each field. Within a struct, field identifiers
must be unique and  field types must be complete (§Types).
</p>

<pre class="grammar">
StructType = "struct" "{" [ FieldDeclList ] "}" .
FieldDeclList = FieldDecl { ";" FieldDecl } [ ";" ] .
FieldDecl = (IdentifierList CompleteType | [ "*" ] TypeName) [ Tag ] .
Tag = StringLit .
</pre>

<pre>
// An empty struct.
struct {}

// A struct with 5 fields.
struct {
	x, y int;
	u float;
	A *[]int;
	F func();
}
</pre>

<p>
A field declared with a type but no field identifier is an <i>anonymous field</i>.
Such a field type must be specified as
a type name <code>T</code> or as a pointer to a type name <code>*T</code>,
and <code>T</code> itself, may not be
a pointer or interface type. The unqualified type name acts as the field identifier.
</p>

<pre>
// A struct with four anonymous fields of type T1, *T2, P.T3 and *P.T4
struct {
	T1;        // the field name is T1
	*T2;       // the field name is T2
	P.T3;      // the field name is T3
	*P.T4;     // the field name is T4
	x, y int;
}
</pre>

<p>
The unqualified type name of an anonymous field must not conflict with the
field identifier (or unqualified type name for an anonymous field) of any
other field within the struct. The following declaration is illegal:
</p>

<pre>
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
}
</pre>

<p>
Fields and methods (§Method declarations) of an anonymous field are
promoted to be ordinary fields and methods of the struct (§Selectors).
</p>
<p>
A field declaration may be followed by an optional string literal <i>tag</i>, which
becomes an attribute for all the identifiers in the corresponding
field declaration. The tags are made
visible through a reflection library (TODO: reference?)
but are otherwise ignored.
</p>

<pre>
// A struct corresponding to the EventIdMessage protocol buffer.
// The tag strings define the protocol buffer field numbers.
struct {
	time_usec uint64 "field 1";
	server_ip uint32 "field 2";
	process_id uint32 "field 3";
}
</pre>

<h3>Pointer types</h3>

<p>
A pointer type denotes the set of all pointers to variables of a given
type, called the <i>base type</i> of the pointer.
A pointer value may be <code>nil</code>.
</p>

<pre class="grammar">
PointerType = "*" BaseType .
BaseType = Type .
</pre>

<pre>
*int
*map[string] *chan int
</pre>

<h3>Function types</h3>

<p>
A function type denotes the set of all functions with the same parameter
and result types.
A function value may be <code>nil</code>.
</p>

<pre class="grammar">
FunctionType   = "func" Signature .
Signature      = Parameters [ Result ] .
Result         = Parameters | CompleteType .
Parameters     = "(" [ ParameterList ] ")" .
ParameterList  = ParameterDecl { "," ParameterDecl } .
ParameterDecl  = [ IdentifierList ] ( CompleteType | "..." ) .
</pre>

<p>
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; 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 that is not a function type it may writen as an unparenthesized type.
The types of parameters and results must be complete.
(TODO: is completeness necessary?)
</p>
<p>
For the last parameter only, instead of a type one may write
<code>...</code> to indicate that the function may be invoked with
zero or more additional arguments of any
type. If parameters of such a function are named, the final identifier
list must be a single name, that of the <code>...</code> parameter.
</p>

<pre>
func ()
func (x int)
func () int
func (string, float, ...)
func (a, b int, z float) bool
func (a, b int, z float) (bool)
func (a, b int, z float, opt ...) (success bool)
func (int, int, float) (float, *[]int)
func (n int) (func (p* T))
</pre>


<h3>Interface types</h3>

<p>
An interface type specifies an unordered set of methods. A variable
of interface type can store, dynamically, any value that implements
at least that set of methods.
An interface value may be <code>nil</code>.
</p>

<pre class="grammar">
InterfaceType      = "interface" "{" [ MethodSpecList ] "}" .
MethodSpecList     = MethodSpec { ";" MethodSpec } [ ";" ] .
MethodSpec         = IdentifierList Signature | InterfaceTypeName .
InterfaceTypeName  = TypeName .
</pre>

<pre>
// A simple File interface
interface {
	Read, Write	(b Buffer) bool;
	Close		();
}
</pre>

<p>
Any type (including interface types) whose interface includes,
possibly as a subset, the complete set of methods of an interface <code>I</code>
is said to implement interface <code>I</code>.
For instance, if two types <code>S1</code> and <code>S2</code>
have the methods
</p>

<pre>
func (p T) Read(b Buffer) bool { return ... }
func (p T) Write(b Buffer) bool { return ... }
func (p T) Close() { ... }
</pre>

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

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

<pre>
interface { }
</pre>

<p>
Similarly, consider this interface specification,
which appears within a type declaration (§Type declarations)
to define an interface called <code>Lock</code>:
</p>

<pre>
type Lock interface {
	Lock, Unlock	();
}
</pre>

<p>
If <code>S1</code> and <code>S2</code> also implement
</p>

<pre>
func (p T) Lock() { ... }
func (p T) Unlock() { ... }
</pre>

<p>
they implement the <code>Lock</code> interface as well
as the <code>File</code> interface.
</p>
<p>
An interface may contain an interface type name <code>T</code>
in place of a method specification.
In this notation, <code>T</code> must denote a different, complete interface type
and the effect is equivalent to enumerating the methods of <code>T</code> explicitly
in the interface.
</p>

<pre>
type ReadWrite interface {
	Read, Write	(b Buffer) bool;
}

type File interface {
	ReadWrite;  // same as enumerating the methods in ReadWrite
	Lock;       // same as enumerating the methods in Lock
	Close();
}
</pre>

<h3>Map types</h3>

<p>
A map is an unordered group of elements of one type, called the
value type, indexed by a set of unique <i>keys</i> of another type,
called the key type.  Both key and value types must be complete.
(§Types).
(TODO: is completeness necessary here?)
A map value may be <code>nil</code>.

</p>

<pre class="grammar">
MapType     = "map" "[" KeyType "]" ValueType .
KeyType     = CompleteType .
ValueType   = CompleteType .
</pre>

<p>
The comparison operators <code>==</code> and <code>!=</code>
(§Comparison operators) must be fully defined for operands of the
key type; thus the key type must be a basic, pointer, interface,
map, or channel type. 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 error.

</p>

<pre>
map [string] int
map [*T] struct { x, y float }
map [string] interface {}
</pre>

<p>
The number of elements is called the length and is never negative.
The length of a map <code>m</code> can be discovered using the
built-in function <code>len(m)</code> and may change during execution.
The value of an uninitialized map is <code>nil</code>.
</p>
<p>
Upon creation, a map is empty.  Values may be added and removed
during execution using special forms of assignment (§Assignments).
A new, empty map value is made using the built-in
function <code>make</code>, which takes the map type and an optional
capacity hint as arguments:
</p>

<pre>
make(map[string] int)
make(map[string] int, 100)
</pre>

<p>
The initial capacity does not bound its size:
maps grow to accommodate the number of items
stored in them.
</p>

<h3>Channel types</h3>

<p>
A channel provides a mechanism for two concurrently executing functions
to synchronize execution and communicate by passing a value of a
specified element type. The element type must be complete (§Types).
(TODO: is completeness necessary here?)
A channel value may be <code>nil</code>.
</p>

<pre class="grammar">
ChannelType   = Channel | SendChannel | RecvChannel .
Channel       = "chan" ValueType .
SendChannel   = "chan" "&lt;-" ValueType .
RecvChannel   = "&lt;-" "chan" ValueType .
</pre>

<p>
Upon creation, a channel can be used both to send and to receive values.
By conversion or assignment, a channel may be constrained only to send or
to receive. This constraint is called a channel's <i>direction</i>; either
<i>send</i>, <i>receive</i>, or <i>bi-directional</i> (unconstrained).
</p>

<pre>
chan T         // can be used to send and receive values of type T
chan &lt;- float  // can only be used to send floats
&lt;-chan int     // can only be used to receive ints
</pre>

<p>
The value of an uninitialized channel is <code>nil</code>. A new, initialized channel
value is made using the built-in function <code>make</code>,
which takes the channel type and an optional capacity as arguments:
</p>


<pre>
make(chan int, 100)
</pre>

<p>
The capacity, in number of elements, sets the size of the buffer in the channel. If the
capacity is greater than zero, the channel is asynchronous and, provided the
buffer is not full, sends can succeed without blocking. If the capacity is zero
or absent, the communication succeeds only when both a sender and receiver are ready.
</p>

<p>
For a channel <code>c</code>, the predefined function <code>close(c)</code>
marks the channel as unable to accept more
values through a send operation.  After any previously
sent values have been received, receives will return
the zero value for the channel's type.  After at least one such zero value has been
received, <code>closed(c)</code> returns true.
</p>

<h2>General properties of types and values</h2>

<p>
Types may be <i>different</i>, <i>structurally equal</i> (or just <i>equal</i>),
or <i>identical</i>.
Go is <i>type safe</i>: different types cannot be mixed
in binary operations and values cannot be assigned to variables of different
types. Values can be assigned to variables of equal type.
</p>

<h3>Type equality and identity </h3>

<p>
Two type names denote equal types if the types in the corresponding declarations
are equal (§Declarations and Scope).
Two type literals specify equal types if they have the same
literal structure and corresponding components have equal types.
In detail:
</p>

<ul>
	<li>Two pointer types are equal if they have equal base types.</li>

	<li>Two array types are equal if they have equal element types and
	  the same array length.</li>

	<li>Two struct types are equal if they have the same sequence of fields,
	    with the same names and equal types. Two anonymous fields are
	    considered to have the same name.</li>

	<li>Two function types are equal if they have the same number of parameters
	  and result values and if corresponding parameter and result types are
	  the same. All "..." parameters have equal type.
	  Parameter and result names are not required to match.</li>

	<li>Two slice types are equal if they have equal element types.</li>

	<li>Two channel types are equal if they have equal value types and
	  the same direction.</li>

	<li>Two map types are equal if they have equal key and value types.</li>

	<li>Two interface types are equal if they have the same set of methods
	  with the same names and equal function types. The order
	  of the methods is irrelevant.</li>
</ul>

<p>
Type identity is more stringent than type equality.
It requires for type names
that they originate in the same type declaration, while for equality it requires
only that they originate in equal type declarations.
Also, the names of parameters and results must match for function types.
In all other respects, the definition of type identity is the
same as for type equality listed above but with ``identical''
substitued for ``equal''.
</p>
<p>
By definition, identical types are also equal types.
Two types are different if they are not equal.
</p>

<p>
Given the declarations
</p>

<pre>
type (
	T0 []string;
	T1 []string
	T2 struct { a, b int };
	T3 struct { a, c int };
	T4 func (int, float) *T0
	T5 func (x int, y float) *[]string
)
</pre>

<p>
these types are equal:
</p>

<pre>
T0 and T0
T0 and T1
T0 and []string
T4 and T5
T3 and struct { a int; c int }
</pre>

<p>
<code>T2</code> and <code>T3</code> are not equal because
they have different field names.
</p>

<p>
These types are identical:
</p>

<pre>
T0 and T0
[]int and []int
struct { a, b *T5 } and struct { a, b *T5 }
</pre>

<p>
<code>T0</code> and <code>T1</code> are equal but not
identical because they have distinct declarations.
</p>

<h3>Assignment compatibility</h3>

<p>
Values of any type may always be assigned to variables
of equal static type. Some types and values have conditions under which they may
be assigned to different types:
</p>
<ul>
<li>
The predeclared constant <code>nil</code> can be assigned to any
pointer, function, slice, map, channel, or interface variable.
<li>
A pointer to an array can be assigned to a slice variable with equal element type.
The slice variable then refers to the original array; the data is not copied.
</li>
<li>
A value can be assigned to an interface variable if the static
type of the value implements the interface.
</li>
<li>
A value of bidirectional channel type can be assigned to any channel
variable of equal channel value type.
</li>
</ul>

<h3>Comparison compatibility</h3>

<p>
Values of any type may be compared to other values of equal static
type.  Values of numeric and string type may be compared using the
full range of comparison operators as described in §Comparison operators;
booleans may be compared only for equality or inequality.
</p>

<p>
Values of composite type may be
compared for equality or inequality using the <code>==</code> and
<code>!=</code> operators, with the following provisos:
</p>
<ul>
<li>
Arrays and structs may not be compared to anything.
</li>
<li>
A slice value may only be compared explicitly against <code>nil</code>.
A slice value is equal to <code>nil</code> if it has been assigned the explicit
value <code>nil</code> or if it is a variable (or array element,
field, etc.) that has not been modified since it was created
uninitialized.
</li>
<li>
Similarly, an interface value is equal to <code>nil</code> if it has
been assigned the explicit value <code>nil</code> or if it is a
variable (or array element, field, etc.) that has not been modified
since it was created uninitialized.
</li>
<li>
For types that can be compared to <code>nil</code>,
two values of the same type are equal if they both equal <code>nil</code>,
unequal if one equals <code>nil</code> and one does not.
</li>
<li>
Pointer values are equal if they point to the same location.
</li>
<li>
Function values are equal if they refer to the same function.
</li>
<li>
Channel and map values are equal if they were created by the same call to <code>make</code>
(§Making slices, maps, and channels).
</li>
<li>
Interface values may be compared if they have the same static type.
They will be equal only if they have the same dynamic type and the underlying values are equal.
</li>
</ul>
<hr/>


<h2>Declarations and Scope</h2>

<p>
A declaration binds an identifier to a language entity such as
a variable or function and specifies properties such as its type.
Every identifier in a program must be declared.
</p>

<pre class="grammar">
Declaration = ConstDecl | TypeDecl | VarDecl | FunctionDecl | MethodDecl .
</pre>

<p>
The <i>scope</i> of an identifier is the extent of source text within which the
identifier denotes the bound entity. No identifier may be declared twice in a
single scope, but inner blocks can declare a new entity with the same
identifier, in which case the scope created by the outer declaration excludes
that created by the inner.
</p>
<p>
There are levels of scoping in effect before each source file is compiled.
In order from outermost to innermost:
</p>
<ol>
	<li>The <i>universe</i> scope contains all predeclared identifiers.</li>
	<li>An implicit scope contains only the package name.</li>
	<li>The <i>package-level</i> scope surrounds all declarations at the
	    top level of the file, that is, outside the body of any
	    function or method.  That scope is shared across all
	    source files within the package (§Packages), allowing
	    package-level identifiers to be shared between source
	    files.</li>
</ol>

<p>
The scope of an identifier depends on the entity declared:
</p>

<ol>
	<li> The scope of predeclared identifiers is the universe scope.</li>

	<li> The scope of an identifier denoting a type, function or package
	     extends from the point of the identifier in the declaration
	     to the end of the innermost surrounding block.</li>

	<li> The scope of a constant or variable extends textually from
	     the end of its declaration to the end of the innermost
	     surrounding block. If the variable is declared in the
	     <i>init</i> statement of an <code>if</code>,  <code>for</code>,
	     or  <code>switch </code> statement, the
	     innermost surrounding block is the block associated
	     with that statement.</li>

	<li> The scope of a parameter or result is the body of the
	     corresponding function.</li>

	<li> The scope of a field or method is selectors for the
	     corresponding type containing the field or method (§Selectors).</li>

	<li> The scope of a label is a special scope emcompassing
	     the body of the innermost surrounding function, excluding
	     nested functions.  Labels do not conflict with non-label identifiers.</li>
</ol>

<h3>Predeclared identifiers</h3>

<p>
The following identifiers are implicitly declared in the outermost scope:
</p>
<pre class="grammar">
Basic types:
	bool byte float32 float64 int8 int16 int32 int64
	string uint8 uint16 uint32 uint64

Architecture-specific convenience types:
	float int uint uintptr

Constants:
	true false iota nil

Functions:
	cap len make new panic panicln print println

Packages:
	sys (TODO: does sys endure?)
</pre>

<h3>Exported identifiers</h3>

<p>
By default, identifiers are visible only within the package in which they are declared.
Some identifiers are <i>exported</i> and can be referenced using
<i>qualified identifiers</i> in other packages (§Qualified identifiers).
If an identifier satisfies these two conditions:
</p>
<ol>
<li>the first character of the identifier's name is a Unicode upper case letter;
<li>the identifier is declared at the package level or is a field or method of a type
declared at the top level;
</ol>
<p>
it will be exported automatically.
</p>

<h3>Const declarations</h3>

<p>
A constant declaration binds a list of identifiers (the names of
the constants) to the values of a list of constant expressions
(§Constant expressions).  The number of identifiers must be equal
to the number of expressions, and the n<sup>th</sup> identifier on
the left is bound to value of the n<sup>th</sup> expression on the
right.
</p>

<pre class="grammar">
ConstDecl      = "const" ( ConstSpec | "(" [ ConstSpecList ] ")" ) .
ConstSpecList  = ConstSpec { ";" ConstSpec } [ ";" ] .
ConstSpec      = IdentifierList [ [ CompleteType ] "=" ExpressionList ] .

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

CompleteType = Type .
</pre>

<p>
If the type (CompleteType) is omitted, the constants take the
individual types of the corresponding expressions, which may be
<i>ideal integer</i> or <i>ideal float</i> (§Ideal number).  If the type
is present, all constants take the type specified, and the types
of all the expressions must be assignment-compatible
with that type.
</p>

<pre>
const Pi float64 = 3.14159265358979323846
const E = 2.718281828
const (
	size int64 = 1024;
	eof = -1;
)
const a, b, c = 3, 4, "foo"  // a = 3, b = 4, c = "foo"
const u, v float = 0, 3      // u = 0.0, v = 3.0
</pre>

<p>
Within a parenthesized <code>const</code> 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 <code>iota</code> constant generator
(§Iota) this mechanism permits light-weight declaration of sequential values:
</p>

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


<h3>Iota</h3>

<p>
Within a constant declaration, the predeclared pseudo-constant
<code>iota</code> represents successive integers. It is reset to 0
whenever the reserved word <code>const</code> appears in the source
and increments with each semicolon. It can be used to construct a
set of related constants:
</p>

<pre>
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     (ideal integer)
	v float = iota * 42;  // v == 42.0  (float)
	w       = iota * 42;  // w == 84    (ideal integer)
)

const x = iota;  // x == 0 (iota has been reset)
const y = iota;  // y == 0 (iota has been reset)
</pre>

<p>
Within an ExpressionList, the value of each <code>iota</code> is the same because
it is only incremented at a semicolon:
</p>

<pre>
const (
	bit0, mask0 = 1 << iota, 1 << iota - 1;  // bit0 == 1, mask0 == 0
	bit1, mask1;                             // bit1 == 2, mask1 == 1
	bit2, mask2;                             // bit2 == 4, mask2 == 3
)
</pre>

<p>
This last example exploits the implicit repetition of the
last non-empty expression list.
</p>


<h3>Type declarations</h3>

<p>
A type declaration binds an identifier, the <i>type name</i>,
to a new type.  <font color=red>TODO: what exactly is a "new type"?</font>
</p>

<pre class="grammar">
TypeDecl     = "type" ( TypeSpec | "(" [ TypeSpecList ] ")" ) .
TypeSpecList = TypeSpec { ";" TypeSpec } [ ";" ] .
TypeSpec     = identifier ( Type | "struct" | "interface" ) .
</pre>

<pre>
type IntArray [16] int

type (
	Point struct { x, y float };
	Polar Point
)

type Comparable interface

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

type Comparable interface {
	cmp(Comparable) int
}
</pre>

<h3>Variable declarations</h3>

<p>
A variable declaration creates a variable, binds an identifier to it and
gives it a type and optionally an initial value.
The type must be complete (§Types).
</p>
<pre class="grammar">
VarDecl     = "var" ( VarSpec | "(" [ VarSpecList ] ")" ) .
VarSpecList = VarSpec { ";" VarSpec } [ ";" ] .
VarSpec     = IdentifierList ( CompleteType [ "=" ExpressionList ] | "=" ExpressionList ) .
</pre>

<pre>
var i int
var U, V, W float
var k = 0
var x, y float = -1.0, -2.0
var (
	i int;
	u, v, s = 2.0, 3.0, "bar"
)
</pre>

<p>
If there are expressions, their number must be equal
to the number of identifiers, and the n<sup>th</sup> variable
is initialized to the value of the n<sup>th</sup> expression.
Otherwise, each variable is initialized to the <i>zero</i>
of the type (§The zero value).
The expressions can be general expressions; they need not be constants.
</p>
<p>
Either the type or the expression list must be present.  If the
type is present, it sets the type of each variable and the expressions
(if any) must be assignment-compatible to that type.  If the type
is absent, the variables take the types of the corresponding
expressions.
</p>
<p>
If the type is absent and the corresponding expression is a constant
expression of ideal integer or ideal float type, the type of the
declared variable is <code>int</code> or <code>float</code>
respectively:
</p>

<pre>
var i = 0       // i has type int
var f = 3.1415  // f has type float
</pre>

<h3>Short variable declarations</h3>

A <i>short variable declaration</i> uses the syntax

<pre class="grammar">
SimpleVarDecl = IdentifierList ":=" ExpressionList .
</pre>

and is shorthand for the declaration syntax

<pre class="grammar">
"var" IdentifierList = ExpressionList .
</pre>

<pre>
i, j := 0, 10;
f := func() int { return 7; }
ch := make(chan int);
</pre>

<p>
Unlike regular variable declarations, short variable declarations
can be used, by analogy with tuple assignment (§Assignments), to
receive the individual elements of a multi-valued expression such
as a call to a multi-valued function.  In this form, the ExpressionList
must be a single such multi-valued expression, the number of
identifiers must equal the number of values, and the declared
variables will be assigned the corresponding values.
</p>

<pre>
r, w := os.Pipe(fd);  // os.Pipe() returns two values
</pre>

<p>
A short variable declaration may redeclare variables provided they
were originally declared in the same block with the same type, and at
least one of the 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.
</p>

<pre>
field1, offset := nextField(str, 0);
field2, offset := nextField(str, offset);  // redeclares offset
</pre>

<p>
Short variable declarations may appear only inside functions.
In some contexts such as the initializers for <code>if</code>,
<code>for</code>, or <code>switch</code> statements,
they can be used to declare local temporary variables (§Statements).
</p>

<h3>Function declarations</h3>

<p>
A function declaration binds an identifier to a function (§Function types).
</p>

<pre class="grammar">
FunctionDecl = "func" identifier Signature [ Block ] .
</pre>

<pre>
func min(x int, y int) int {
	if x &lt; y {
		return x;
	}
	return y;
}
</pre>

<p>
A function must be declared or forward-declared before it can be invoked (§Forward declarations).
Implementation restriction: Functions can only be declared at the package level.
</p>

<h3>Method declarations</h3>

<p>
A method declaration binds an identifier to a method,
which is a function with a <i>receiver</i>.
</p>
<pre class="grammar">
MethodDecl = "func" Receiver identifier Signature [ Block ] .
Receiver = "(" [ identifier ] [ "*" ] TypeName ")" .
</pre>

<p>
The receiver type must be a type name or a pointer to a type name,
and that name is called the <i>receiver base type</i> or just <i>base type</i>.
The base type must not be a pointer type and must be
declared in the same source file as the method.
The method is said to be <i>bound</i> to the base type
and is visible only within selectors for that type
(§Type declarations, §Selectors).
</p>

<p>
All methods bound to a base type must have the same receiver type,
either all pointers to the base type or all the base type itself.
Given type <code>Point</code>, the declarations
</p>

<pre>
func (p *Point) Length() float {
	return Math.sqrt(p.x * p.x + p.y * p.y);
}

func (p *Point) Scale(factor float) {
	p.x = p.x * factor;
	p.y = p.y * factor;
}
</pre>

<p>
bind the methods <code>Length</code> and <code>Scale</code>
to the base type <code>Point</code>.
</p>

<p>
If the
receiver's value is not referenced inside the the body of the method,
its identifier may be omitted in the declaration. The same applies in
general to parameters of functions and methods.
</p>

<p>
Methods can be declared
only after their base type is declared or forward-declared, and invoked
only after their own declaration or forward-declaration (§Forward declarations).
Implementation restriction: They can only be declared at package level.
</p>

<p>
The type of a method is the type of a function with the receiver as first
argument.  For instance, the method <code>Scale</code> has type
</p>

<pre>
(p *Point, factor float)
</pre>

<p>
However, a function declared this way is not a method.
</p>

<h3>Forward declarations</h3>

<p>
Mutually-recursive types require that one be
<i>forward declared</i> so that it may be named in the other.
A forward declaration of a type omits the block containing the fields
or methods of the type.
</p>

<pre>
type List struct  // forward declaration of List
type Item struct {
	value int;
	next *List;
}
type List struct {
	head, tail *Item
}
</pre>
<p>
A forward-declared type is incomplete (§Types)
until it is fully declared. The full declaration must follow
before the end of the block containing the forward declaration;
it cannot be contained in an inner block.
</p>
<p>
Functions and methods may similarly be forward-declared by omitting their body.
</p>
<pre>
func F(a int) int  // forward declaration of F
func G(a, b int) int {
	return F(a) + F(b)
}
func F(a int) int {
	if a <= 0 { return 0 }
	return G(a-1, b+1)
}
</pre>

<hr/>

<h2>Expressions</h2>

<p>
An expression specifies the computation of a value by applying
operators and functions to operands. An expression has a value and
a type.
</p>

<h3>Operands</h3>

Operands denote the elementary values in an expression.

<pre class="grammar">
Operand    = Literal | QualifiedIdent | "(" Expression ")" .
Literal    = BasicLit | CompositeLit | FunctionLit .
BasicLit   = int_lit | float_lit | char_lit | StringLit .
StringLit  = string_lit { string_lit } .
</pre>


<h3>Constants</h3>

<p>
A <i>constant</i> is a literal of a basic type
(including the predeclared constants <code>true</code>, <code>false</code>
and <code>nil</code>
and values denoted by <code>iota</code>)
or a constant expression (§Constant expressions).
Constants have values that are known at compile time.
</p>

<h3>Qualified identifiers</h3>

<p>
A qualified identifier is an identifier qualified by a package name prefix.
</p>

<pre class="grammar">
QualifiedIdent = [ ( LocalPackageName | PackageName ) "." ] identifier .
LocalPackageName = identifier .
PackageName = identifier .
</pre>

<p>
A qualified identifier accesses an identifier in
a separate package.  The identifier must be exported by that package, which
means that it must begin with a Unicode upper case letter (§Exported identifiers).
</p>
<p>
The LocalPackageName is that of the package in which the qualified identifier
appears and is only necessary to access names hidden by intervening declarations
of a package-level identifier.
</p>

<pre>
Math.Sin
mypackage.hiddenName
mypackage.Math.Sin  // if Math is declared in an intervening scope
</pre>

TODO: 6g does not implement LocalPackageName.  Is this new?
Is it needed?

<h3>Composite literals</h3>

<p>
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 expressions,
or a list of key-value pairs for map literals.
</p>

<pre class="grammar">
CompositeLit  = LiteralType "{" [ ( ExpressionList | KeyValueList ) [ "," ] ] "}" .
LiteralType   = StructType | ArrayType | "[" "..." "]" ElementType |
                SliceType | MapType | TypeName .
KeyValueList  = KeyValueExpr { "," KeyValueExpr } .
KeyValueExpr  = Expression ":" Expression .
</pre>

<p>
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 assignment compatible to
the respective field, element, and key types of the LiteralType;
there is no additional conversion.
</p>

<pre>
type Rat struct { num, den int }
type Num struct { r Rat; f float; s string }
</pre>

<p>
one may write
</p>

<pre>
pi := Num{Rat{22, 7}, 3.14159, "pi"}
</pre>

<p>
Taking the address of a composite literal (§Address operators)
generates a unique pointer to an instance of the literal's value.
</p>
<pre>
var pi_ptr *Rat = &amp;Rat{22, 7}
</pre>

<p>
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 more elements than specified in the type. The
notation <code>...</code> specifies an array length equal
to the number of elements in the literal.
</p>

<pre>
buffer := [10]string{};               // len(buffer) == 10
primes := [6]int{2, 3, 5, 7, 9, 11};  // len(primes) == 6
days := [...]string{"Sat", "Sun"};    // len(days) == 2
</pre>

<p>
A slice literal describes the entire underlying array literal.
Thus, the length and capacity of a slice literal is the number of elements
(of the array) provided in the literal. A slice literal has the form
</p>

<pre>
[]T{x1, x2, ... xn}
</pre>

<p>
and is a shortcut for a slice operation applied to an array literal:
</p>

<pre>
[n]T{x1, x2, ... xn}[0 : n]
</pre>

<p>
In map literals only, the list contains
key-value pairs separated by a colon:
</p>

<pre>
m := map[string]int{"good": 0, "bad": 1, "indifferent": 7};
</pre>

<p>
A parsing ambiguity arises when a composite literal using the
TypeName form of the LiteralType appears in the condition of an
"if", "for", or "switch" statement, because the braces surrounding
the expressions in the literal are confused with those introducing
a block of statements. To resolve the ambiguity in this rare case,
the composite literal must appear within
parentheses.
</p>

<pre>
if x == (T{a,b,c}[i]) { ... }
if (x == T{a,b,c}[i]) { ... }
</pre>

<h3>Function literals</h3>

<p>
A function literal represents an anonymous function.
It consists of a specification of the function type and a function body.
</p>

<pre class="grammar">
FunctionLit   = FunctionType Block .
Block         = "{" StatementList "}" .
</pre>

<pre>
func (a, b int, z float) bool { return a*b &lt; int(z) }
</pre>

<p>
A function literal can be assigned to a variable or invoked directly.
</p>

<pre>
f := func(x, y int) int { return x + y }
func(ch chan int) { ch &lt;- ACK } (reply_chan)
</pre>

<p>
Function literals are <i>closures</i>: 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.
</p>


<h3>Primary expressions</h3>

<pre class="grammar">
PrimaryExpr =
	Operand |
	PrimaryExpr Selector |
	PrimaryExpr Index |
	PrimaryExpr Slice |
	PrimaryExpr TypeAssertion |
	PrimaryExpr Call .

Selector       = "." identifier .
Index          = "[" Expression "]" .
Slice          = "[" Expression ":" Expression "]" .
TypeAssertion  = "." "(" Type ")" .
Call           = "(" [ ExpressionList ] ")" .
</pre>


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


<h3>Selectors</h3>

<p>
A primary expression of the form
</p>

<pre>
x.f
</pre>

<p>
denotes the field or method <code>f</code> of the value denoted by <code>x</code>
(or of <code>*x</code> if
<code>x</code> is of pointer type). The identifier <code>f</code>
is called the (field or method)
<i>selector</i>.
The type of the expression is the type of <code>f</code>.
</p>
<p>
A selector <code>f</code> may denote a field or method <code>f</code> of
a type <code>T</code>, or it may refer
to a field or method <code>f</code> of a nested anonymous field of
<code>T</code>.
The number of anonymous fields traversed
to reach <code>f</code> is called its <i>depth</i> in <code>T</code>.
The depth of a field or method <code>f</code>
declared in <code>T</code> is zero.
The depth of a field or method <code>f</code> declared in
an anonymous field <code>A</code> in <code>T</code> is the
depth of <code>f</code> in <code>A</code> plus one.
</p>
<p>
The following rules apply to selectors:
</p>
<ol>
<li>
For a value <code>x</code> of type <code>T</code> or <code>*T</code>
where <code>T</code> is not an interface type,
<code>x.f</code> denotes the field or method at the shallowest depth
in <code>T</code> where there
is such an <code>f</code>.
If there is not exactly one <code>f</code> with shallowest depth, the selector
expression is illegal.
</li>
<li>
For a variable <code>x</code> of type <code>I</code> or <code>*I</code>
where <code>I</code> is an interface type,
<code>x.f</code> denotes the actual method with name <code>f</code> of the value assigned
to <code>x</code> if there is such a method.
If no value or <code>nil</code> was assigned to <code>x</code>, <code>x.f</code> is illegal.
</li>
<li>
In all other cases, <code>x.f</code> is illegal.
</ol>
<p>
Selectors automatically dereference pointers as necessary.
If <code>x</code> is of pointer type, <code>x.y</code>
is shorthand for <code>(*x).y</code>; if <code>y</code>
is also of pointer type, <code>x.y.z</code> is shorthand
for <code>(*(*x).y).z</code>, and so on.
If <code>*x</code> is of pointer type, dereferencing
must be explicit;
only one level of automatic dereferencing is provided.
For an <code>x</code> of type <code>T</code> containing an
anonymous field declared as <code>*A</code>,
<code>x.f</code> is a shortcut for <code>(*x.A).f</code>.
</p>
<p>
For example, given the declarations:
</p>

<pre>
type T0 struct {
	x int;
}

func (recv *T0) M0()

type T1 struct {
	y int;
}

func (recv T1) M1()

type T2 struct {
	z int;
	T1;
	*T0;
}

func (recv *T2) M2()

var p *T2;  // with p != nil and p.T1 != nil
</pre>

<p>
one may write:
</p>

<pre>
p.z         // (*p).z
p.y         // ((*p).T1).y
p.x         // (*(*p).T0).x

p.M2        // (*p).M2
p.M1        // ((*p).T1).M1
p.M0        // ((*p).T0).M0
</pre>


<font color=red>
TODO: Specify what happens to receivers.
</font>


<h3>Indexes</h3>

<p>
A primary expression of the form
</p>

<pre>
a[x]
</pre>

<p>
denotes the array or map element of <code>a</code> indexed by <code>x</code>.
The value <code>x</code> is called the
<i>array index</i> or <i>map key</i>, respectively. The following
rules apply:
</p>
<p>
For <code>a</code> of type <code>A</code> or <code>*A</code>
where <code>A</code> is an array type (§Array types):
</p>
<ul>
	<li><code>x</code> must be an integer value and <code>0 &lt;= x &lt; len(a)</code>
	<li><code>a[x]</code> is the array element at index <code>x</code> and the type of
	  <code>a[x]</code> is the element type of <code>A</code>
</ul>
<p>
For <code>a</code> of type <code>M</code> or <code>*M</code>
where <code>M</code> is a map type (§Map types):
</p>
<ul>
	<li><code>x</code>'s type must be equal to the key type of <code>M</code>
	  and the map must contain an entry with key <code>x</code> (but see special forms below)
	<li><code>a[x]</code> is the map value with key <code>x</code>
	  and the type of <code>a[x]</code> is the value type of <code>M</code>
</ul>

<p>
Otherwise <code>a[x]</code> is illegal.  If the index or key is out of range evaluating
an otherwise legal index expression, a run-time exception occurs.
</p>

<p>
However, if an index expression on a map <code>a</code> of type <code>map[K] V</code>
is used in an assignment of one of the special forms
</p>

<pre>
r, ok = a[x]
r, ok := a[x]
</pre>

<p>
the result of the index expression is a pair of values with types
<code>(K, bool)</code>.
If the key is present in the map,
the expression returns the pair <code>(a[x], true)</code>;
otherwise it returns <code>(Z, false)</code> where <code>Z</code> is
the zero value for <code>V</code> (§The zero value).
No run-time exception occurs in this case.
The index expression in this construct thus acts like a function call
returning a value and a boolean indicating success.  (§Assignments)
</p>

<p>
Similarly, if an assignment to a map has the special form
</p>

<pre>
a[x] = r, ok
</pre>

<p>
and boolean <code>ok</code> has the value <code>false</code>,
the entry for key <code>x</code> is deleted from the map; if
<code>ok</code> is <code>true</code>, the construct acts like
a regular assignment to an element of the map.
</p>

<h3>Slices</h3>

<p>
Strings, arrays, and slices can be <i>sliced</i> to construct substrings or descriptors
of subarrays. The index expressions in the slice select which elements appear
in the result.  The result has indexes starting at 0 and length equal to the
difference in the index values in the slice.  After slicing the array <code>a</code>
</p>

<pre>
a := [4]int{1, 2, 3, 4};
s := a[1:3];
</pre>

<p>
the slice <code>s</code> has type <code>[]int</code>, length 2, capacity 3, and elements
</p>

<pre>
s[0] == 2
s[1] == 3
</pre>

<p>
The slice length must be non-negative.
For arrays or strings, the indexes
<code>lo</code> and <code>hi</code> must satisfy
0 &lt;= <code>lo</code> &lt;= <code>hi</code> &lt;= length;
for slices, the upper bound is the capacity rather than the length.
<p>
If the sliced operand is a string, the result of the slice operation is another, new
string (§String types). If the sliced operand is an array or slice, the result
of the slice operation is a slice (§Slice types).
</p>


<h3>Type assertions</h3>

<p>
For an expression <code>x</code> and a type <code>T</code>, the primary expression
</p>

<pre>
x.(T)
</pre>

<p>
asserts that the value stored in <code>x</code> is of type <code>T</code>.
The notation <code>x.(T)</code> is called a <i>type assertion</i>.
The type of <code>x</code> must be an interface type.
</p>
<p>
More precisely, if <code>T</code> is not an interface type, <code>x.(T)</code> asserts
that the dynamic type of <code>x</code> is identical to the type <code>T</code>
(§Type equality and identity).
If <code>T</code> is an interface type, <code>x.(T)</code> asserts that the dynamic type
of <code>T</code> implements the interface <code>T</code> (§Interface types).
<font color=red>TODO: gri wants an error if x is already of type T.</font>
</p>
<p>
If the type assertion holds, the value of the expression is the value
stored in <code>x</code> and its type is <code>T</code>. If the type assertion is false, a run-time
exception occurs. In other words, even though the dynamic type of <code>x</code>
is known only at run-time, the type of <code>x.(T)</code> is
known to be <code>T</code> in a correct program.
</p>
<p>
If a type assertion is used in an assignment of one of the special forms,
</p>

<pre>
v, ok = x.(T)
v, ok := x.(T)
</pre>

<p>
the result of the assertion is a pair of values with types <code>(T, bool)</code>.
If the assertion holds, the expression returns the pair <code>(x.(T), true)</code>;
otherwise, the expression returns <code>(Z, false)</code> where <code>Z</code>
is the zero value for type <code>T</code> (§The zero value).
No run-time exception occurs in this case.
The type assertion in this construct thus acts like a function call
returning a value and a boolean indicating success.  (§Assignments)
</p>


<h3>Calls</h3>

<p>
Given an expression <code>f</code> of function type
<code>F</code>,
</p>

<pre>
f(a1, a2, ... an)
</pre>

<p>
calls <code>f</code> with arguments <code>a1, a2, ... an</code>.
The arguments must be single-valued expressions
assignment compatible with the parameters of
<code>F</code> and are evaluated before the function is called.
The type of the expression is the result type
of <code>F</code>.
A method invocation is similar but the method itself
is specified as a selector upon a value of the receiver type for
the method.
</p>

<pre>
Atan2(x, y)    // function call
var pt *Point;
pt.Scale(3.5)  // method call with receiver pt
</pre>

<p>
If the receiver type of the method is declared as a pointer of type <code>*T</code>,
the actual receiver may be a value of type <code>T</code>;
in such cases method invocation implicitly takes the
receiver's address:
</p>

<pre>
var p Point;
p.Scale(3.5)
</pre>

<p>
There is no distinct method type and there are no method literals.
</p>

<h3>Passing arguments to <code>...</code> parameters</h3>

<p>
When a function <code>f</code> has a <code>...</code> parameter,
it is always the last formal parameter. Within calls to <code>f</code>,
the arguments before the <code>...</code> are treated normally.
After those, an arbitrary number (including zero) of trailing
arguments may appear in the call and are bound to the <code>...</code>
parameter.
</p>

<p>
Within <code>f</code>, the <code>...</code> parameter has static
type <code>interface{}</code> (the empty interface). For each call,
its dynamic type is a structure whose sequential fields are the
trailing arguments of the call.  That is, the actual arguments
provided for a <code>...</code> parameter are wrapped into a struct
that is passed to the function instead of the actual arguments.
Using the reflection library (TODO: reference), <code>f</code> may
unpack the elements of the dynamic type to recover the actual
arguments.
</p>

<p>
Given the function and call
</p>
<pre>
func Fprintf(f io.Write, format string, args ...)
Fprintf(os.Stdout, "%s %d", "hello", 23);
</pre>

<p>
Within <code>Fprintf</code>, the dynamic type of <code>args</code> for this
call will be, schematically,
<code> struct { string; int }</code>.
</p>


<p>
As a special case, if a function passes its own <code>...</code> parameter as the argument
for a <code>...</code> in a call to another function with a <code>...</code> parameter,
the parameter is not wrapped again but passed directly. In short, a formal <code>...</code>
parameter is passed unchanged as an actual <code>...</code> parameter.

<h3>Operators</h3>

<p>
Operators combine operands into expressions.
</p>

<pre class="grammar">
Expression = UnaryExpr | Expression binary_op UnaryExpr .
UnaryExpr  = PrimaryExpr | unary_op UnaryExpr .

binary_op  = log_op | com_op | rel_op | add_op | mul_op .
log_op     = "||" | "&amp;&amp;" .
com_op     = "&lt;-" .
rel_op     = "==" | "!=" | "&lt;" | "&lt;=" | ">" | ">=" .
add_op     = "+" | "-" | "|" | "^" .
mul_op     = "*" | "/" | "%" | "&lt;&lt;" | ">>" | "&amp;" | "&amp;^" .

unary_op   = "+" | "-" | "!" | "^" | "*" | "&amp;" | "&lt;-" .
</pre>

<p>
The operand types in binary operations must be equal, with the following exceptions:
</p>
<ul>
	<li>Except in shift expressions, if one operand has numeric type and the other operand is
	  an ideal number, the ideal number is converted to match the type of
	  the other operand (§Expressions).</li>

	<li>If both operands are ideal numbers, the conversion is to ideal floats
	  if one of the operands is an ideal float
	  (relevant for <code>/</code> and <code>%</code>).</li>

	<li>The right operand in a shift operation must be always be of unsigned integer type
	  or an ideal number that can be safely converted into an unsigned integer type
	  (§Arithmetic operators).</li>

	<li>The operands in channel sends differ in type: one is always a channel and the
	other is a variable or value of the channel's element type.</li>

	<li>When comparing two operands of channel type, the channel value types
	  must be equal but the channel direction is ignored.</li>
</ul>

<p>
Unary operators have the highest precedence. They are evaluated from
right to left. As the  <code>++</code> and <code>--</code> operators form
statements, not expressions, they fall
outside the unary operator hierarchy and apply
to the operand on the left.
As a consequence, statement <code>*p++</code> is the same as <code>(*p)++</code>.
<p>
There are six precedence levels for binary operators.
Multiplication operators bind strongest, followed by addition
operators, comparison operators, communication operators,
<code>&amp;&amp;</code> (logical and), and finally <code>||</code> (logical or):
</p>

<pre class="grammar">
Precedence    Operator
    6             *  /  %  &lt;&lt;  >>  &amp;  &amp;^
    5             +  -  |  ^
    4             ==  !=  &lt;  &lt;=  >  >=
    3             &lt;-
    2             &amp;&amp;
    1             ||
</pre>

<p>
Binary operators of the same precedence associate from left to right.
For instance, <code>x / y / z</code> is the same as <code>(x / y) / z</code>.
</p>
<p>
Examples:
</p>

<pre>
+x
23 + 3*x[i]
x &lt;= f()
^a >> b
f() || g()
x == y + 1 &amp;&amp; &lt;-chan_ptr > 0
</pre>


<h3>Arithmetic operators</h3>
<p>
Arithmetic operators apply to numeric types and yield a result of the same
type as the first operand. The four standard arithmetic operators (<code>+</code>,
<code>-</code>,  <code>*</code>, <code>/</code>) apply both to integer and
floating point types, while <code>+</code> applies also
to strings; all other arithmetic operators apply to integers only.
</p>

<pre class="grammar">
+    sum                    integers, floats, strings
-    difference             integers, floats
*    product                integers, floats
/    quotient               integers, floats
%    remainder              integers

&amp;    bitwise and            integers
|    bitwise or             integers
^    bitwise xor            integers
&amp;^   bit clear (and not)    integers

<<   left shift             integer << unsigned integer
>>   right shift            integer >> unsigned integer
</pre>

<p>
Strings can be concatenated using the <code>+</code> operator
or the <code>+=</code> assignment operator:
</p>

<pre>
s := "hi" + string(c);
s += " and good bye";
</pre>

<p>
String addition creates a new string by concatenating the operands.
</p>
<p>
For integer values, <code>/</code> and <code>%</code> satisfy the following relationship:
</p>

<pre>
(a / b) * b + a % b == a
</pre>

<p>
with <code>(a / b)</code> truncated towards zero.
Examples:
</p>

<pre>
 x     y     x / y     x % y
 5     3       1         2
-5     3      -1        -2
 5    -3      -1         2
-5    -3       1        -2
</pre>

<p>
If the dividend is positive and the divisor is a constant power of 2,
the division may be replaced by a left shift, and computing the remainder may
be replaced by a bitwise "and" operation:
</p>

<pre>
 x     x / 4     x % 4     x >> 2     x &amp; 3
 11      2         3         2          3
-11     -2        -3        -3          1
</pre>

<p>
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. The shift count must
be an unsigned integer. There is no upper limit on the shift count. Shifts behave
as if the left operand is shifted <code>n</code> times by 1 for a shift
count of <code>n</code>.
As a result, <code>x << 1</code> is the same as <code>x*2</code>
and <code>x >> 1</code> is the same as
<code>x/2</code> truncated towards negative infinity.
</p>

<p>
For integer operands, the unary operators
<code>+</code>, <code>-</code>, and <code>^</code> are defined as
follows:
</p>

<pre class="grammar">
+x                          is 0 + x
-x    negation              is 0 - x
^x    bitwise complement    is m ^ x  with m = "all bits set to 1"
</pre>

<p>
For floating point numbers,
<code>+x</code> is the same as <code>x</code>,
while <code>-x</code> is the negation of <code>x</code>.
</p>

<h3>Integer overflow</h3>

<p>
For unsigned integer values, the operations <code>+</code>,
<code>-</code>, <code>*</code>, and <code>&lt;&lt;</code> are
computed modulo 2<sup><i>n</i></sup>, where <i>n</i> is the bit width of
the unsigned integer's type
(§Numeric types). Loosely speaking, these unsigned integer operations
discard high bits upon overflow, and programs may rely on ``wrap around''.
</p>
<p>
For signed integers, the operations <code>+</code>,
<code>-</code>, <code>*</code>, and <code>&lt;&lt;</code> 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 <code>x &lt; x + 1</code> is always true.
</p>


<h3>Comparison operators</h3>

<p>
Comparison operators yield a boolean result. All comparison operators apply
to basic types except bools.
The operators <code>==</code> and <code>!=</code> apply, at least in some cases,
to all types except arrays and structs.
</p>

<pre class="grammar">
==    equal
!=    not equal
<     less
<=    less or equal
>     greater
>=    greater or equal
</pre>

<p>
Numeric basic types are compared in the usual way.
</p>
<p>
Strings are compared byte-wise (lexically).
</p>
<p>
Booleans are equal if they are either both "true" or both "false".
</p>
<p>
The rules for comparison of composite types are described in the
section on §Comparison compatibility.
</p>


<h3>Logical operators</h3>

<p>
Logical operators apply to boolean operands and yield a boolean result.
The right operand is evaluated conditionally.
</p>

<pre class="grammar">
&amp;&amp;    conditional and    p &amp;&amp; q  is  "if p then q else false"
||    conditional or     p || q  is  "if p then true else q"
!     not                !p      is  "not p"
</pre>


<h3>Address operators</h3>

<p>
The unary prefix address-of operator <code>&amp;</code> generates the address of its operand, which must be a variable,
pointer indirection, field selector, or array or slice indexing operation. It is illegal to take the address of a function
result variable.
Given an operand of pointer type, the unary prefix pointer indirection operator <code>*</code> retrieves the value pointed
to by the operand.
</p>

<pre>
&amp;x
&amp;a[f(2)]
*p
*pf(x)
</pre>

<p>
<font color=red>TODO: This text needs to be cleaned up and go elsewhere, there are no address
operators involved.
</font>
</p>
<p>
Methods are a form of function and a method ``value'' has a function type.
Consider the type T with method M:
</p>

<pre>
type T struct {
	a int;
}
func (tp *T) M(a int) int;
var t *T;
</pre>

<p>
To construct the value of method M, one writes
</p>

<pre>
t.M
</pre>

<p>
using the variable t (not the type T).
<font color=red>TODO: It makes perfect sense to be able to say T.M (in fact, it makes more
sense then t.M, since only the type T is needed to find the method M, i.e.,
its address). TBD.
</font>
</p>

<p>
The expression t.M is a function value with type
</p>

<pre>
func (t *T, a int) int
</pre>

<p>
and may be invoked only as a function, not as a method:
</p>

<pre>
var f func (t *T, a int) int;
f = t.M;
x := f(t, 7);
</pre>

<p>
Note that one does not write t.f(7); taking the value of a method demotes
it to a function.
</p>

<p>
In general, given type T with method M and variable t of type T,
the method invocation
</p>

<pre>
t.M(args)
</pre>

<p>
is equivalent to the function call
</p>

<pre>
(t.M)(t, args)
</pre>

<p>
<font color=red>
TODO: should probably describe the effect of (t.m) under §Expressions if t.m
denotes a method: Effect is as described above, converts into function.
</font>
</p>
<p>
If T is an interface type, the expression t.M does not determine which
underlying type's M is called until the point of the call itself. Thus given
T1 and T2, both implementing interface I with method M, the sequence
</p>

<pre>
var t1 *T1;
var t2 *T2;
var i I = t1;
m := i.M;
m(t2, 7);
</pre>

<p>
will invoke t2.M() even though m was constructed with an expression involving
t1. Effectively, the value of m is a function literal
</p>

<pre>
func (recv I, a int) {
	recv.M(a);
}
</pre>

<p>
that is automatically created.
</p>
<p>
<font color=red>
TODO: Document implementation restriction: It is illegal to take the address
of a result parameter (e.g.: func f() (x int, p *int) { return 2, &amp;x }).
(TBD: is it an implementation restriction or fact?)
</font>
</p>

<h3>Communication operators</h3>

<p>
The term <i>channel</i> means "variable of channel type" (§Channel types).
</p>
<p>
The send operation uses the binary operator "&lt;-", which operates on
a channel and a value (expression):
</p>

<pre>
ch <- 3
</pre>

<p>
The send operation sends the value on the channel.  Both the channel
and the expression are evaluated before communication begins.
Communication blocks until the send can proceed, at which point the
value is transmitted on the channel.  A send can proceed if the
channel is asynchronous and there is room in its buffer or the
channel is synchronous and a receiver is ready.
</p>
<p>
If the send operation appears in an expression context, the value
of the expression is a boolean and the operation is non-blocking.
The value of the boolean reports true if the communication succeeded,
false if it did not. (The channel and
the expression to be sent are evaluated regardless.)
These two examples are equivalent:
</p>

<pre>
ok := ch <- 3;
if ok { print("sent") } else { print("not sent") }

if ch <- 3 { print("sent") } else { print("not sent") }
</pre>

<p>
In other words, if the program tests the value of a send operation,
the send is non-blocking and the value of the expression is the
success of the operation.  If the program does not test the value,
the operation blocks until it succeeds.
</p>
<p>
The receive operation uses the prefix unary operator "&lt;-".
The value of the expression is the value received, whose type
is the element type of the channel.
</p>

<pre>
<-ch
</pre>

<p>
The expression blocks until a value is available, which then can
be assigned to a variable or used like any other expression.
If the receive expression does not save the value, the value is
discarded.
</p>

<pre>
v1 := <-ch
v2 = <-ch
f(<-ch)
<-strobe  // wait until clock pulse
</pre>

<p>
If a receive expression is used in a tuple assignment of the form
</p>

<pre>
x, ok = <-ch;  // or: x, ok := <-ch
</pre>

<p>
the receive operation becomes non-blocking.
If the operation can proceeed, the boolean variable
<code>ok</code> will be set to <code>true</code>
and the value stored in <code>x</code>; otherwise
<code>ok</code> is set
to <code>false</code> and <code>x</code> is set to the
zero value for its type (§The zero value).
</p>

<p>
<font color=red>TODO: Probably in a separate section, communication semantices
need to be presented regarding send, receive, select, and goroutines.</font>
</p>

<h3>Constant expressions</h3>

<p>
Constant expressions may contain only constants, <code>iota</code>,
numeric literals, string literals, and
some constant-valued built-in functions such as <code>unsafe.Sizeof</code>
and <code>len</code> applied to an array.
In practice, constant expressions are those that can be evaluated at compile time.
<p>
The type of a constant expression is determined by the type of its
elements.  If it contains only numeric literals, its type is <i>ideal
integer</i> or <i>ideal float</i> (§Ideal number).  Whether a literal
is an integer or float depends on the syntax of the literals (123 vs. 123.0).
The nature of the arithmetic
operations within the expression depends, elementwise, on the values;
for example, 3/2 is an integer division yielding 1, while 3./2. is
a floating point division yielding 1.5.  Thus
</p>

<pre>
const x = 3./2. + 3/2;
</pre>

<p>
yields a floating point constant of ideal float value 2.5 (1.5 +
1); its constituent expressions are evaluated using distinct rules
for division.
</p>

<p>
Intermediate values and the constants themselves
may require precision significantly larger than any concrete type
in the language.  The following are legal declarations:
</p>

<pre>
const Huge = 1 << 100;
const Four int8 = Huge >> 98;
</pre>

<p>
A constant expression may appear in any context, such as assignment
to a variable of any numeric type, as long as the value of the
expression can be represented accurately in that context.
It is erroneous to assign a value with a non-zero fractional part
to an integer, or if the assignment would overflow or underflow,
or in general if the value cannot be represented by the type of
the variable.
For
instance, <code>3</code> can be assigned to any integer variable but also to any
floating point variable, while <code>-1e12</code> can be assigned to a
<code>float32</code>, <code>float64</code>, or even <code>int64</code>
but not <code>uint64</code> or <code>string</code>.
</p>

<p>
If a typed constant expression evaluates to a value that is not
representable by that type, the compiler reports an error.
</p>

<pre>
uint8(-1)         // error, out of range
uint8(100) * 100  // error, out of range
</pre>

<p>
The size of the mask used by the unary bitwise complement
operator in a typed constant expression is equal to the size of the
expression's type.  In an ideal constant expression, the bitwise
complement operator inverts all the bits, producing a negative value.
</p>

<pre>
^1          // ideal constant, equal to -2
uint8(^1)   // error, same as uint8(-2), out of range
^uint8(1)   // typed uint8 constant, same as 0xFF ^ uint8(1) = uint8(0xFE)
int8(^1)    // same as int8(-2)
^int8(1)    // error, same as 0xFF ^ int8(1) = int8(0xFE), out of range
</pre>

<p>
TODO: perhaps ^ should be disallowed on non-uints instead of assuming twos complement.
Also it may be possible to make typed constants more like variables, at the cost of fewer
overflow etc. errors being caught.
</p>

<h3>Order of evaluation</h3>

<p>
When evaluating the elements of an assignment or expression,
all function calls, method calls and
communication operations are evaluated in lexical left-to-right
order.  Otherwise, the order of evaluation is unspecified.
</p>

<p>
For example, while evaluating the arguments for this call
to function <code>f</code>,
</p>
<pre>
f(g(), h() + x[i()], <-c)
</pre>
<p>
the call to <code>g()</code> happens before the call to <code>h()</code>,
which happens before the call to <code>i()</code>, all of
of which happen before receiving the value from the channel
<code>c</code>.
However, the order of those events compared to the evaluation of
<code>f</code>, the evaluation of <code>x</code>, and the indexing
of <code>x</code> by the return value of
<code>i()</code> is not specified.
</p>

<hr/>

<h2>Statements</h2>

<p>
Statements control execution.
</p>

<pre class="grammar">
Statement =
	Declaration | EmptyStmt | LabeledStmt |
	SimpleStmt | GoStmt | ReturnStmt | BreakStmt | ContinueStmt | GotoStmt |
	FallthroughStmt | Block | IfStmt | SwitchStmt | SelectStmt | ForStmt |
	DeferStmt .

SimpleStmt = ExpressionStmt | IncDecStmt | Assignment | SimpleVarDecl .

StatementList = Statement { Separator Statement } .
Separator     = [ ";" ]
</pre>

<p>
Elements of a list of statements are separated by semicolons,
which may be omitted only if the previous statement:
</p>
<ul>
	<li>ends with the closing parenthesis ")" of a list of declarations
	    (§Declarations and Scope); or</li>
	<li>ends with a closing brace "}" that is not part of an expression.
</ul>


<h3>Empty statements</h3>

<p>
The empty statement does nothing.
</p>

<pre class="grammar">
EmptyStmt = .
</pre>

<p>
A statement list can always in effect be terminated with a semicolon by
adding an empty statement.
</p>


<h3>Labeled statements</h3>

<p>
A labeled statement may be the target of a <code>goto</code>,
<code>break</code> or <code>continue</code> statement.
</p>

<pre class="grammar">
LabeledStmt = Label ":" Statement .
Label       = identifier .
</pre>

<pre>
Error: log.Fatal("error encountered")
</pre>


<h3>Expression statements</h3>

<p>
Function calls, method calls, and channel operations
can appear in statement context.
</p>


<pre class="grammar">
ExpressionStmt = Expression .
</pre>

<pre>
f(x+y)
<-ch
</pre>


<h3>IncDec statements</h3>

<p>
The "++" and "--" statements increment or decrement their operands
by the ideal numeric value 1.  As with an assignment, the operand
must be a variable, pointer indirection, field selector or index expression.
</p>

<pre class="grammar">
IncDecStmt = Expression ( "++" | "--" ) .
</pre>

<p>
The following assignment statements (§Assignments) are semantically
equivalent:
</p>

<pre class="grammar">
IncDec statement    Assignment
x++                 x += 1
x--                 x -= 1
</pre>

<h3>Assignments</h3>

<pre class="grammar">
Assignment = ExpressionList assign_op ExpressionList .

assign_op = [ add_op | mul_op ] "=" .
</pre>

<p>
Each left-hand side operand must be a variable, pointer indirection,
field selector, or index expression.
</p>

<pre>
x = 1
*p = f()
a[i] = 23
k = <-ch
i &amp;^= 1&lt;&lt;n
</pre>

<p>
An <i>assignment operation</i> <code>x</code> <i>op</i><code>=</code>
<code>y</code> where <i>op</i> is a binary arithmetic operation is equivalent
to <code>x</code> <code>=</code> <code>x</code> <i>op</i>
<code>y</code> but evalutates <code>x</code>
only once.  The <i>op</i><code>=</code> construct is a single token.
</p>

<pre>
a[i] <<= 2
</pre>

<p>
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 evaluation or channel or map operation (§Channel
operations, §Map operations) or a type assertion (§Type assertions).
The number of operands on the left
hand side must match the number of values.  For instance, If
<code>f</code> is a function returning two values,
</p>

<pre>
x, y = f()
</pre>

<p>
assigns the first value to <code>x</code> and the second to <code>y</code>.
</p>

<p>
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.
The expressions on the right are evaluated before assigning to
any of the operands on the left, but otherwise the evaluation
order is unspecified.
</p>

<pre>
a, b = b, a  // exchange a and b
</pre>

<p>
In assignments, the type of each value must be assignment compatible
(§Assignment compatibility) with the type of the
operand to which it is assigned.
</p>


<h3>If statements</h3>

<p>
"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.  A missing condition
is equivalent to <code>true</code>.
</p>

<pre class="grammar">
IfStmt    = "if" [ [ SimpleStmt ] ";" ] [ Expression ] Block [ "else" Statement ] .
</pre>

<pre>
if x > 0 {
	return true;
}
</pre>

<p>
An "if" statement may include a simple statement before the expression.
The scope of any variables declared by that statement
extends to the end of the "if" statement
and the variables are initialized once before the statement is entered.
</p>

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


<h3>Switch statements</h3>

<p>
"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.
</p>

<pre class="grammar">
SwitchStmt = ExprSwitchStmt | TypeSwitchStmt .
</pre>

<p>
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.
</p>

<h4>Expression switches</h4>

<p>
In an expression switch,
the switch expression is evaluated and
the case expressions, which need not be constants,
are evaluated 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 expression is equivalent to
the expression <code>true</code>.
</p>

<pre class="grammar">
ExprSwitchStmt = "switch" [ [ SimpleStmt ] ";" ] [ Expression ] "{" { ExprCaseClause } "}" .
ExprCaseClause = ExprSwitchCase ":" [ StatementList ] .
ExprSwitchCase = "case" ExpressionList | "default" .
</pre>

<p>
In a case or default clause,
the last statement only may be a "fallthrough" statement
(§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.
</p>
<p>
Each case clause acts as a block for scoping purposes
(§Declarations and scope rules).
</p>
<p>
A "switch" statement may include a simple statement before the
expression.
The scope of any variables declared by that statement
extends to the end of the "switch" statement
and the variables are initialized once before the statement is entered.
</p>

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

switch x := f(); {
case x &lt; 0: return -x
default: return x
}

switch {          // missing expression means "true"
case x < y: f1();
case x < z: f2();
case x == 4: f3();
}
</pre>

<h4>Type switches</h4>

<p>
A type switch compares types rather than values. It is otherwise similar
to an expression switch. It is introduced by special
notation in the form of a simple declaration whose right hand side
has the form of a type assertion (§Type assertions)
using the reserved word <code>type</code> rather than an actual type.
Cases then match literal types against the dynamic type of the expression
in the type assertion.
</p>

<pre class="grammar">
TypeSwitchStmt  = "switch" [ [ SimpleStmt ] ";" ] TypeSwitchGuard "{" { TypeCaseClause } "}" .
TypeSwitchGuard = identifier ":=" Expression "." "(" "type" ")" .
TypeCaseClause  = TypeSwitchCase ":" [ StatementList ] .
TypeSwitchCase  = "case" ( type | "nil" ) | "default" .
</pre>

<p>
If the interface value equals <code>nil</code>,
only an explict <code>nil</code> case or "default"
case will execute.
</p>

<p>
Given a function <code>f</code>
that returns a value of interface type,
the following type switch:
</p>

<pre>
switch i := f().(type) {
case nil:
	printString("f() returns nil");
case int:
	printInt(i);	// i is an int
case float:
	printFloat(i);	// i is a float
case func(int) float:
	printFunction(i);	// i is a function
default:
	printString("don't know the type");
}
</pre>

<p>
could be rewritten:
</p>

<pre>
v := f();
if v == nil {
	printString("f() returns nil");
} else if i, is_int := v.(int); is_int {
	printInt(i);	// i is an int
} else if i, is_float := v.(float); is_float {
	printFloat(i);	// i is a float
} else if i, is_func := v.(func(int) float); is_func {
	printFunction(i);	// i is a function
} else {
	printString("don't know the type");
}
</pre>

<p>
In a type switch, the guard is mandatory,
there can be only one type per "case", and
the "fallthrough" statement is not allowed.
</p>

<h3>For statements</h3>

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

<pre class="grammar">
ForStmt = "for" [ Condition | ForClause | RangeClause ] Block .
Condition = Expression .
</pre>

<p>
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 <code>true</code>.
</p>

<pre>
for a &lt; b {
	a *= 2
}
</pre>

<p>
A "for" statement with a "for" clause is also controlled by its condition, but
additionally it may specify an <i>init</i>
and a <i>post</i> statement, such as an assignment,
an increment or decrement statement. The init statement (but not the post
statement) may also be a short variable declaration; the scope of the variables
it declares ends at the end of the statement
(§Declarations and scope rules).
</p>

<pre class="grammar">
ForClause = [ InitStmt ] ";" [ Condition ] ";" [ PostStmt ] .
InitStmt = SimpleStmt .
PostStmt = SimpleStmt .
</pre>

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

<p>
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 "for" clause may be empty but the semicolons are
required unless there is only a condition.
If the condition is absent, it is equivalent to <code>true</code>.
</p>

<pre>
for ; cond ; { S() }    is the same as    for cond { S() }
for true { S() }        is the same as    for      { S() }
</pre>

<p>
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 first assigns the current index or key to an iteration
variable - or the current (index, element) or (key, value) pair to a pair
of iteration variables - and then executes the block.
</p>

<pre class="grammar">
RangeClause = ExpressionList ( "=" | ":=" ) "range" Expression .
</pre>

<p>
The type of the right-hand expression in the "range" clause must be an
array, slice, string or map, or a pointer to an array, slice, string or map;
or it may be a channel.
Except for channels,
the identifier list must contain one or two expressions
(as in assignments, these must be a
variable, pointer indirection, field selector, or index expression)
denoting the
iteration variables. On each iteration,
the first variable is set to the string, array or slice index or
map key, and the second variable, if present, is set to the corresponding
string or array element or map value.
The types of the array or slice index (always <code>int</code>)
and element, or of the map key and value respectively,
must be assignment compatible to the iteration variables.
</p>
<p>
For strings, the "range" clause iterates over the Unicode code points
in the string.  On successive iterations, the index variable will be the
index of successive UTF-8-encoded code points in the string, and
the second variable, of type <code>int</code>, will be the value of
the corresponding code point.  If the iteration encounters an invalid
UTF-8 sequence, the second variable will be <code>0xFFFD</code>,
the Unicode replacement character, and the next iteration will advance
a single byte in the string.
</p>
<p>
For channels, the identifier list must contain one identifier.
The iteration recieves values sent on the channel until the channel is closed;
it does not process the zero value sent before the channel is closed.
</p>
<p>
The iteration variables may be declared by the "range" clause (":="), in which
case their scope ends at the end of the "for" statement (§Declarations and
scope rules). In this case their types are set to
<code>int</code> and the array element type, or the map key and value types, respectively.
If the iteration variables are declared outside the "for" statement,
after execution their values will be those of the last iteration.
</p>

<pre>
var a [10]string;
m := map[string]int{"mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6};

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 assignment-compatible to val
for key, value = range m {
	h(key, value)
}
// key == last map key encountered in iteration
// val == map[key]
</pre>

<p>
If map entries that have not yet been processed are deleted during iteration,
they will not be processed. If map entries are inserted during iteration, the
behavior is implementation-dependent, but each entry will be processed at most once.
</p>

<h3>Go statements</h3>

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

<pre class="grammar">
GoStmt = "go" Expression .
</pre>

<p>
The expression must be a call, and
unlike with a regular call, program execution does not wait
for the invoked function to complete.
</p>

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


<h3>Select statements</h3>

<p>
A "select" statement chooses which of a set of possible communications
will proceed.  It looks similar to a "switch" statement but with the
cases all referring to communication operations.
</p>

<pre class="grammar">
SelectStmt = "select" "{" { CommClause } "}" .
CommClause = CommCase ":" StatementList .
CommCase = "case" ( SendExpr | RecvExpr) | "default" .
SendExpr =  Expression "&lt;-" Expression .
RecvExpr =  [ Expression ( "=" | ":=" ) ] "&lt;-" Expression .
</pre>

<p>
Each communication clause acts as a block for the purpose of scoping
(§Declarations and scope rules).
</p>
<p>
For all the send and receive expressions in the "select"
statement, the channel expression is evaluated.  Any expressions
that appear on the right hand side of send expressions are also
evaluated. If any of the resulting channels can proceed, one is
chosen and the corresponding communication and statements are
evaluated.  Otherwise, if there is a default case, that executes;
if not, the statement blocks until one of the communications can
complete.  The channels and send expressions are not re-evaluated.
A channel pointer may be <code>nil</code>,
which is equivalent to that case not
being present in the select statement
except, if a send, its expression is still evaluated.
</p>
<p>
Since all the channels and send expressions are evaluated, any side
effects in that evaluation will occur for all the communications
in the "select" statement.
</p>
<p>
If multiple cases can proceed, a uniform fair choice is made to decide
which single communication will execute.
<p>
The receive case may declare a new variable using a short variable declaration
(§Short variable declarations).
The scope of such variables continues to the end of the
respective case's statements.
</p>

<pre>
var c, c1, c2 chan int;
var i1, i2 int;
select {
case i1 = &lt;-c1:
	print("received ", i1, " from c1\n");
case c2 &lt;- i2:
	print("sent ", i2, " to c2\n");
default:
	print("no communication\n");
}

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

<font color=red>
TODO: Make semantics more precise.
</font>


<h3>Return statements</h3>

<p>
A "return" statement terminates execution of the containing function
and optionally provides a result value or values to the caller.
</p>

<pre class="grammar">
ReturnStmt = "return" [ ExpressionList ] .
</pre>

<pre>
func procedure() {
	return
}
</pre>

<p>
There are two ways to return values from a function with a result
type.  The first is to explicitly list the return value or values
in the "return" statement.
Normally, the expressions
must be single-valued and assignment-compatible to the elements of
the result type of the function.
</p>

<pre>
func simple_f() int {
	return 2
}

func complex_f1() (re float, im float) {
	return -7.0, -4.0
}
</pre>

<p>
However, if the expression list in the "return" statement is a single call
to a multi-valued function, the values returned from the called function
will be returned from this one.  The result types of the current function
and the called function must match.
</p>

<pre>
func complex_f2() (re float, im float) {
	return complex_f1()
}
</pre>

<p>
The second way to return values is to use the elements of the
result list of the function as variables.  When the function begins
execution, these variables are initialized to the zero values for
their type (§The zero value).  The function can assign them as
necessary; if the "return" provides no values, those of the variables
will be returned to the caller.
</p>

<pre>
func complex_f3() (re float, im float) {
	re = 7.0;
	im = 4.0;
	return;
}
</pre>

<p>
TODO: Define when return is required.
</p>

<h3>Break statements</h3>

<p>
A "break" statement terminates execution of the innermost
"for", "switch" or "select" statement.
</p>

<pre class="grammar">
BreakStmt = "break" [ Label ].
</pre>

<p>
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
(§For statements, §Switch statements, §Select statements).
</p>

<pre>
L: for i < n {
	switch i {
		case 5: break L
	}
}
</pre>

<h3>Continue statements</h3>

<p>
A "continue" statement begins the next iteration of the
innermost "for" loop at the post statement (§For statements).
</p>

<pre class="grammar">
ContinueStmt = "continue" [ Label ].
</pre>

<p>
The optional label is analogous to that of a "break" statement.
</p>

<h3>Goto statements</h3>

<p>
A "goto" statement transfers control to the statement with the corresponding label.
</p>

<pre class="grammar">
GotoStmt = "goto" Label .
</pre>

<pre>
goto Error
</pre>

<p>
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:
</p>

<pre>
goto L;  // BAD
v := 3;
L:
</pre>

<p>
is erroneous because the jump to label <code>L</code> skips
the creation of <code>v</code>.
(TODO: Eliminate in favor of used and not set errors?)
</p>

<h3>Fallthrough statements</h3>

<p>
A "fallthrough" statement transfers control to the first statement of the
next case clause in a expression "switch" statement (§Expression switches). It may
be used only as the final non-empty statement in a case or default clause in an
expression "switch" statement.
</p>

<pre class="grammar">
FallthroughStmt = "fallthrough" .
</pre>


<h3>Defer statements</h3>

<p>
A "defer" statement invokes a function whose execution is deferred to the moment
the surrounding function returns.
</p>

<pre class="grammar">
DeferStmt = "defer" Expression .
</pre>

<p>
The expression must be a function or method call.
Each time the "defer" statement
executes, the parameters to the function call are evaluated and saved anew but the
function is not invoked. Immediately before the innermost function surrounding
the "defer" statement returns, but after its return value (if any) is evaluated,
each deferred function is executed with its saved parameters. Deferred functions
are executed in LIFO order.
</p>

<pre>
lock(l);
defer unlock(l);  // unlocking happens before surrounding function returns

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

<hr/>

<h2>Predeclared functions</h2>
<ul>
	<li>cap
	<li>close
	<li>closed
	<li>len
	<li>make
	<li>new
	<li>panic
	<li>panicln
	<li>print
	<li>println
</ul>

<h3>Length and capacity</h3>

<pre class="grammar">
Call       Argument type       Result

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

cap(s)    []T, *[]T            capacity of s
          map[K]T, *map[K]T    capacity of s
          chan T               channel buffer capacity
</pre>

<p>
The type of the result is always <code>int</code> and the
implementation guarantees that
the result always fits into an <code>int</code>.
<p>
The capacity of a slice or map is the number of elements for which there is
space allocated in the underlying array (for a slice) or map. For a slice
<code>s</code>, at any time the following relationship holds:

<pre>
0 <= len(s) <= cap(s)
</pre>


<h3>Conversions</h3>

<p>
Conversions look like function calls of the form
</p>

<pre class="grammar">
T(value)
</pre>

<p>
where <code>T</code> is a type
and <code>value</code> is an expression
that can be converted to a value
of result type <code>T</code>.
<p>
The following conversion rules apply:
</p>
<ul>
<li>
1) Between equal types.  The conversion always succeeds.
</li>
<li>
2) Between integer types.  If the value is a signed quantity, it is
sign extended to implicit infinite precision; otherwise it is zero
extended.  It is then truncated to fit in the result type size.
For example, <code>uint32(int8(0xFF))</code> is <code>0xFFFFFFFF</code>.
The conversion always yields a valid value; there is no signal for overflow.
</li>
<li>
3) Between integer and floating point types, or between floating point
types.  To avoid overdefining the properties of the conversion, for
now it is defined as a ``best effort'' conversion.  The conversion
always succeeds but the value may be a NaN or other problematic
result. <font color=red>TODO: clarify?</font>
</li>
<li>
4) Strings permit two special conversions.
</li>
<li>
4a) Converting an integer value yields a string containing the UTF-8
representation of the integer.

<pre>
string(0x65e5)  // "\u65e5"
</pre>

</li>
<li>
4b) Converting a slice of bytes yields a string whose successive
bytes are those of the slice.

<pre>
string([]byte{'h', 'e', 'l', 'l', 'o'}) // "hello"
</pre>
</li>
</ul>

<p>
There is no linguistic mechanism to convert between pointers and integers.
The <code>unsafe</code> package
implements this functionality under
restricted circumstances (§Package <code>unsafe</code>).
</p>


<h3>Allocation</h3>

<p>
The built-in function <code>new</code> takes a type <code>T</code> and
returns a value of type <code>*T</code>.
The memory is initialized as described in the section on initial values
(§The zero value).
</p>

<pre>
new(T)
</pre>

<p>
For instance
</p>

<pre>
type S struct { a int; b float }
new(S)
</pre>

<p>
dynamically allocates memory for a variable of type <code>S</code>,
initializes it (<code>a=0</code>, <code>b=0.0</code>),
and returns a value of type <code>*S</code> containing the address
of the memory.
</p>

<h3>Making slices, maps and channels</h3>

<p>
Slices, maps and channels are reference types that do not require the
extra indirection of an allocation with <code>new</code>.
The built-in function <code>make</code> takes a type <code>T</code>,
which must be a slice, map or channel type,
optionally followed by a type-specific list of expressions.
It returns a value of type <code>T</code> (not <code>*T</code>).
The memory is initialized as described in the section on initial values
(§The zero value).
</p>

<pre>
make(T [, optional list of expressions])
</pre>

<p>
For instance
</p>

<pre>
make(map[string] int)
</pre>

<p>
creates a new map value and initializes it to an empty map.
</p>

<p>
The parameters affect sizes for allocating slices, maps, and
buffered channels:
</p>

<pre>
s := make([]int, 10, 100);        # slice with len(s) == 10, cap(s) == 100
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
</pre>

<hr/>

<h2>Packages</h2>

<p>
Go programs are constructed by linking together <i>packages</i>.
A package is in turn constructed from one or more source files that
together provide access to a set of types, constants, functions,
and variables.  Those elements may be <i>imported</i> and used in
another package.
</p>

<h3>Source file organization</h3>

<p>
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.  The source text following the
package clause acts as a block for scoping (§Declarations and scope
rules).
</p>

<pre class="grammar">
SourceFile       = PackageClause { ImportDecl [ ";" ] } { Declaration [ ";" ] } .
</pre>

<h3>Package clause</h3>

<p>
A package clause begins each source file and defines the package
to which the file belongs.
</p>

<pre class="grammar">
PackageClause    = "package" PackageName .
</pre>

<pre>
package math
</pre>

<p>
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.
</p>

<h3>Import</h3>

<p>
A source file gains access to exported identifiers (§Exported
identifiers) from another package through an import declaration.
In the general form, an import declaration provides an identifier
that code in the source file may use to access the imported package's
contents and a file name referring to the (compiled) implementation of
the package.  The file name may be relative to a repository of
installed packages.
</p>

<pre class="grammar">
ImportDecl       = "import" ( ImportSpec | "(" [ ImportSpecList ] ")" ) .
ImportSpecList   = ImportSpec { ";" ImportSpec } [ ";" ] .
ImportSpec       = [ "." | PackageName ] PackageFileName .
PackageFileName  = StringLit .
</pre>

<p>
After an import, in the usual case an exported name <i>N</i> from the imported
package <i>P</i> may be accessed by the qualified identifier
<i>P</i><code>.</code><i>N</i> (§Qualified identifiers).  The actual
name <i>P</i> depends on the form of the import declaration.  If
an explicit package name <code>p1</code> is provided, the qualified
identifer will have the form <code>p1.</code><i>N</i>.  If no name
is provided in the import declaration, <i>P</i> will be the package
name declared within the source files of the imported package.
Finally, if the import declaration uses an explicit period
(<code>.</code>) for the package name, <i>N</i> will appear
in the package-level scope of the current file and the qualified name is
unnecessary and erroneous.  In this form, it is an error if the import introduces
a name conflict.
</p>
<p>
In this table, assume we have compiled a package named
<code>math</code>, which exports function <code>Sin</code>, and
installed the compiled package in file
<code>"lib/math"</code>.
</p>

<pre class="grammar">
Import syntax               Local name of Sin

import M "lib/math"         M.Sin
import   "lib/math"         math.Sin
import . "lib/math"         Sin
</pre>

<h3>Multi-file packages</h3>

<p>
If a package is constructed from multiple source files, all names
at package-level scope, not just exported names, are visible to all the
files in the package. An import declaration is still necessary to
declare intention to use the names,
but the imported names do not need a qualified identifer to be
accessed.
</p>

<p>
The compilation of a multi-file package may require
that the files be compiled and installed in an order that satisfies
the resolution of names imported within the package.
</p>

<p>
If source file <code>math1.go</code> contains
</p>
<pre>
package math

const twoPi = 6.283185307179586

function Sin(x float) float { return ... }
</pre>

<p>
and file <code>"math2.go"</code> begins
</p>
<pre>
package math

import "lib/math"
</pre>

<p>
then, provided <code>"math1.go"</code> is compiled first and
installed in <code>"lib/math"</code>, <code>math2.go</code>
may refer directly to <code>Sin</code> and <code>twoPi</code>
without a qualified identifier.
</p>

<h3>An example package</h3>

<p>
Here is a complete Go package that implements a concurrent prime sieve.
</p>

<pre>
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 'in' to channel 'out',
// 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()
}
</pre>

<hr/>

<h2>Program initialization and execution</h2>

<h3>The zero value</h3>
<p>
When memory is allocated to store a value, either through a declaration
or <code>new()</code>, and no explicit initialization is provided, the memory is
given a default initialization.  Each element of such a value is
set to the zero value for its type: <code>false</code> for booleans,
<code>0</code> for integers, <code>0.0</code> for floats, <code>""</code>
for strings, and <code>nil</code> for pointers and interfaces.
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.
</p>
<p>
These two simple declarations are equivalent:
</p>

<pre>
var i int;
var i int = 0;
</pre>

<p>
After
</p>

<pre>
type T struct { i int; f float; next *T };
t := new(T);
</pre>

<p>
the following holds:
</p>

<pre>
t.i == 0
t.f == 0.0
t.next == nil
</pre>

<p>
The same would also be true after
</p>

<pre>
var t T
</pre>

<h3>Program execution</h3>
<p>
A package with no imports is initialized by assigning initial values to
all its package-level variables in declaration order and then calling any
package-level function with the name and signature of
</p>
<pre>
func init()
</pre>
<p>
defined in its source. Since a package may contain more
than one source file, there may be more than one
<code>init()</code> function in a package, but
only one per source file.
</p>
<p>
Initialization code may contain "go" statements, but the functions
they invoke do not begin execution until initialization of the entire
program is complete. Therefore, all initialization code is run in a single
goroutine.
</p>
<p>
An <code>init()</code> function cannot be referred to from anywhere
in a program. In particular, <code>init()</code> cannot be called explicitly,
nor can a pointer to <code>init</code> be assigned to a function variable.
</p>
<p>
If a package has imports, the imported packages are initialized
before initializing the package itself. If multiple packages import
a package <code>P</code>, <code>P</code> will be initialized only once.
</p>
<p>
The importing of packages, by construction, guarantees that there can
be no cyclic dependencies in initialization.
</p>
<p>
A complete program, possibly created by linking multiple packages,
must have one package called <code>main</code>, with a function
</p>

<pre>
func main() { ... }
</pre>

<p>
defined.
The function <code>main.main()</code> takes no arguments and returns no value.
</p>
<p>
Program execution begins by initializing the <code>main</code> package and then
invoking <code>main.main()</code>.
</p>
<p>
When <code>main.main()</code> returns, the program exits.
</p>
<p>
Implementation restriction: The compiler assumes package <code>main</code>
is created by a single source file and that it is not imported by any other package.
</p>

<hr/>

<h2>System considerations</h2>

<h3>Package <code>unsafe</code></h3>

<p>
The built-in package <code>unsafe</code>, known to the compiler,
provides facilities for low-level programming including operations
that violate the type system. A package using <code>unsafe</code>
must be vetted manually for type safety.  The package provides the
following interface:
</p>

<pre class="grammar">
package unsafe

const Maxalign int

type Pointer *any  // "any" is shorthand for any Go type; it is not a real type.

func Alignof(variable any) int
func Offsetof(selector any) int
func Sizeof(variable any) int
</pre>

<p>
Any pointer or value of type <code>uintptr</code> can be converted into
a <code>Pointer</code> and vice versa.
</p>
<p>
The function <code>Sizeof</code> takes an expression denoting a
variable of any (complete) type and returns the size of the variable in bytes.
</p>
<p>
The function <code>Offsetof</code> takes a selector (§Selectors) denoting a struct
field of any type and returns the field offset in bytes relative to the
struct's address. For a struct <code>s</code> with field <code>f</code>:
</p>

<pre>
uintptr(unsafe.Pointer(&amp;s)) + uintptr(unsafe.Offsetof(s.f)) == uintptr(unsafe.Pointer(&amp;s.f))
</pre>

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

<pre>
uintptr(unsafe.Pointer(&amp;x)) % uintptr(unsafe.Alignof(x)) == 0
</pre>

<p>
The maximum alignment is given by the constant <code>Maxalign</code>.
It usually corresponds to the value of <code>Sizeof(x)</code> for
a variable <code>x</code> of the largest numeric type (8 for a
<code>float64</code>), but may
be smaller on systems with weaker alignment restrictions.
</p>
<p>
Calls to <code>Alignof</code>, <code>Offsetof</code>, and
<code>Sizeof</code> are constant expressions of type <code>int</code>.
</p>


<h3>Size and alignment guarantees</h3>

For the numeric types (§Numeric types), the following sizes are guaranteed:

<pre class="grammar">
type                      size in bytes

byte, uint8, int8         1
uint16, int16             2
uint32, int32, float32    4
uint64, int64, float64    8
</pre>

<p>
The following minimal alignment properties are guaranteed:
</p>
<ol>
<li>For a variable <code>x</code> of any type: <code>1 <= unsafe.Alignof(x) <= unsafe.Maxalign</code>.

<li>For a variable <code>x</code> of numeric type: <code>unsafe.Alignof(x)</code> is the smaller
   of <code>unsafe.Sizeof(x)</code> and <code>unsafe.Maxalign</code>, but at least 1.

<li>For a variable <code>x</code> of struct type: <code>unsafe.Alignof(x)</code> is the largest of
   all the values <code>unsafe.Alignof(x.f)</code> for each field <code>f</code> of x, but at least 1.

<li>For a variable <code>x</code> of array type: <code>unsafe.Alignof(x)</code> is the same as
   <code>unsafe.Alignof(x[0])</code>, but at least 1.
</ol>

<hr/>

<h2><font color=red>Differences between this doc and implementation - TODO</font></h2>
<p>
<font color=red>
Implementation accepts only ASCII digits for digits; doc says Unicode.
<br/>
Implementation does not honor the restriction on goto statements and targets (no intervening declarations).
<br/>
cap() does not work on maps or chans.
<br/>
len() does not work on chans.
</font>
</p>

</div>
</body>
</html>