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go/doc/go_spec.html
Rob Pike df3183f528 finish types. 
expressions.  (they were in pretty good shape; mostly cosmetic and HTML edits)

R=gri
DELTA=655  (226 added, 97 deleted, 332 changed)
OCL=25459
CL=25481
2009-02-26 16:37:23 -08:00

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<!--
Biggest open issues:
[ ] General iterators
[ ] Conversions:
- current situation is messy
- 2 (3?) different notations for the same thing
- unclear when a type guard is needed
- unclear where conversions can be applied
- for type T int; can we say T(3.0) ?
- do we need channel conversion (channel direction)
[ ] Semantics of type declaration:
- creating a new type (status quo), or only a new type name?
- also: declaration type T S; strips methods of S. why/why not?
Decisions in need of integration into the doc:
[ ] pair assignment is required to get map, and receive ok.
[ ] len() returns an int, new(array_type, n) n must be an int
Todo's:
[ ] there is some funny-ness regarding ';' and empty statements and label decls
[ ] 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 }.
[ ] clarification on interface types, rules
[ ] clarify tuples
[ ] need to talk about precise int/floats clearly
[ ] iant suggests to use abstract/precise int for len(), cap() - good idea
(issue: what happens in len() + const - what is the type?)
[ ] cleanup convert() vs T() vs x.(T) - convert() should go away?
[ ] fix "else" part of if statement
[ ] cleanup: 6g allows: interface { f F } where F is a function type.
fine, but then we should also allow: func f F {}, where F is a function type.
Wish list:
[ ] enum facility (enum symbols that are not mixable with ints) or some other
mechanism to obtain type-safety which we don't have with int-only tags
[ ] Gri: built-in assert() - alternatively: allow entire expressions
as statements so we can write: some_condition || panic(); (along these lines)
[ ] Helper syntax for composite types: allow names/keys/indices for
structs/maps/arrays, remove need for type in elements of composites
Smaller issues:
[ ] need for type switch? (or use type guard with ok in tuple assignment?)
[ ] Is . import implemented / do we still need it?
[ ] Do we allow empty statements? If so, do we allow empty statements after a label?
and if so, does a label followed by an empty statement (a semicolon) still denote
a for loop that is following, and can break L be used inside it?
Closed:
[x] Russ: If we use x.(T) for all conversions, we could use T() for "construction"
and type literals - would resolve the parsing ambiguity of T{} in if's -
switching to () for literals, conversion discussion still open
[x] Russ: consider re-introducing "func" for function type. Make function literals
behave like slices, etc. Require no &'s to get a function value (solves issue
of func{} vs &func{} vs &func_name).
[x] onreturn/undo statement - now: defer statement
[x] comparison of non-basic types: what do we allow? what do we allow in interfaces
what about maps (require ==, copy and hash)
maybe: no maps with non-basic type keys, and no interface comparison unless
with nil[x]
[x] clarify slice rules
[x] what are the permissible ranges for the indices in slices? The spec
doesn't correspond to the implementation. The spec is wrong when it
comes to the first index i: it should allow (at least) the range 0 <= i <= len(a).
also: document different semantics for strings and arrays (strings cannot be grown).
[x] reopening & and func issue: Seems inconsistent as both &func(){} and func(){} are
permitted. Suggestion: func literals are pointers. We need to use & for all other
functions. This would be in consistency with the declaration of function pointer
variables and the use of '&' to convert methods into function pointers.
- covered by other entry
[x] composite types should uniformly create an instance instead of a pointer - fixed
[x] like to have assert() in the language, w/ option to disable code gen for it
- added to wish list
[x] convert should not be used for composite literals anymore,
in fact, convert() should go away - made a todo
[x] type switch or some form of type test needed - duplicate entry
[x] provide composite literal notation to address array indices: []int{ 0: x1, 1: x2, ... }
and struct field names (both seem easy to do). - under "Missing" list
[x] passing a "..." arg to another "..." parameter doesn't wrap the argument again
(so "..." args can be passed down easily) - this is documented
[x] consider syntactic notation for composite literals to make them parsable w/o type information
(require ()'s in control clauses) - use heuristics for now
[x] do we need anything on package vs file names? - current package scheme workable for now
[x] what is the meaning of typeof() - we don't have it
[x] old-style export decls (still needed, but ideally should go away)
[x] packages of multiple files - we have a working approach
[x] partial export of structs, methods
[x] new as it is now is weird - need to go back to previous semantics and introduce
literals for slices, maps, channels - done
[x] determine if really necessary to disallow array assignment - allow array assignment
[x] semantics of statements - we just need to fill in the language, the semantics is mostly clear
[x] range statement: to be defined more reasonably
[x] need to be specific on (unsigned) integer operations: one must be able
to rely on wrap-around on overflow
[x] global var decls: "var a, b, c int = 0, 0, 0" is ok, but "var a, b, c = 0, 0, 0" is not
(seems inconsistent with "var a = 0", and ":=" notation)
[x] const decls: "const a, b = 1, 2" is not allowed - why not? Should be symmetric to vars.
[x] new(arraytype, n1, n2): spec only talks about length, not capacity
(should only use new(arraytype, n) - this will allow later
extension to multi-dim arrays w/o breaking the language) - documented
[x] should we have a shorter list of alias types? (byte, int, uint, float) - done
[x] reflection support
[x] syntax for var args
[x] Do composite literals create a new literal each time (gri thinks yes) (Russ is putting in a change
to this effect, essentially)
[x] comparison operators: can we compare interfaces?
[x] can we add methods to types defined in another package? (probably not)
[x] optional semicolons: too complicated and unclear
[x] anonymous types are written using a type name, which can be a qualified identifier.
this might be a problem when referring to such a field using the type name.
[x] nil and interfaces - can we test for nil, what does it mean, etc.
[x] talk about underflow/overflow of 2's complement numbers (defined vs not defined).
[x] change wording on array composite literals: the types are always fixed arrays
for array composites
[x] meaning of nil
[x] remove "any"
[x] methods for all types
[x] should binary <- be at lowest precedence level? when is a send/receive non-blocking? (NO - 9/19/08)
[x] func literal like a composite type - should probably require the '&' to get address (NO)
[x] & needed to get a function pointer from a function? (NO - there is the "func" keyword - 9/19/08)
-->
<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>
<p>
Where possible, recursive productions are used to express evaluation order
and operator precedence syntactically.
</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;= { }
/ << /= <<= ++ = := , ;
% >> %= >>= -- ! ... . :
</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.
</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 arithmetic types, booleans, and strings. All are predeclared.
</p>
<h3>Arithmetic 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>
Except for <code>byte</code>, which is an alias for <code>uint8</code>,
to avoid portability issues all numeric types are distinct. 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>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 the unqualified type name T3
*P.T4; // the field name is the unqualified type name 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>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>Pointer types</h3>
<p>
A pointer type denotes the set of all pointers to variables of a given
type, called the ``base type'' 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
an arbitrary number (including zero) of 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>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>, and its length and capacity
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:
</p>
<pre>
make([capacity]T)[0 : length]
</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, an allocation hint, as arguments:
</p>
<pre>
make(map[string] int, 100);
</pre>
<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 send and receive values of type T
chan &lt;- float // can only be used to send floats
&lt;-chan int // can only 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>
<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. They 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
T2 and T3
T4 and T5
T3 and struct { a int; int }
</pre>
<p>
and 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>
Arrays can be assigned to slice variables with equal element type.
When assigning to a slice variable, the array is not copied but a
slice comprising the entire array is created.
</li>
<li>
A value can be assigned to an interface variable if the dynamic
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>
and 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 point to the same function.
</li>
<li>
Channel and map values are equal if they were created by the same call of <code>make</code>
(§Making slices, maps, and channels).
</li>
<li>
Interface values are comparison compatible if they have the same static type and
equal if they have the same dynamic type.
</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 the 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 unique 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 convert len make new panic panicln print println typeof (TODO: typeof??)
Packages:
sys unsafe (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
``ideal integer'' or ``ideal float'' (§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. 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 .
</pre>
<pre>
type IntArray [16] int
type (
Point struct { x, y float };
Polar Point
)
type TreeNode struct {
left, right *TreeNode;
value Point;
}
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 := new(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>
count, error := os.Close(fd); // os.Close() returns two values
</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.
</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>
An operand is called <i>constant</i> if it is a literal of a basic type
(including the predeclared constants <code>true</code> and <code>false</code>,
and values denoted by <code>iota</code>),
the predeclared constant <code>nil</code>, or a parenthesized
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>
<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 parenthesized list of expressions,
or a list of expression pairs for map literals.
</p>
<pre class="grammar">
CompositeLit = LiteralType "(" [ ( ExpressionList | ExprPairList ) [ "," ] ] ")" .
LiteralType = StructType | ArrayType | "[" "..." "]" ElementType |
SliceType | MapType | TypeName .
ExprPairList = ExprPair { "," ExprPair } .
ExprPair = Expression ":" Expression .
</pre>
<p>
The LiteralType must be a struct, array, slice, or map type.
<font color=red>TODO: then why doesn't the grammar say that?</font>
The types of the expressions must match the respective field, element, and
key types of the LiteralType; there is no automatic type conversion.
Given
</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>
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>
<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 = "func" Signature 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 TypeGuard |
PrimaryExpr Call .
Selector = "." identifier .
Index = "[" Expression "]" .
Slice = "[" Expression ":" Expression "]" .
TypeGuard = "." "(" 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> must be of the same type as 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, and elements
</p>
<pre>
s[0] == 2
s[1] == 3
</pre>
<p>
The slice length must be non-negative.
For arrays or strings,
the index values in the slice must be in bounds for the original
array or string;
for slices, the index values must be between 0 and the capacity of the slice.
<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 guards</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>.(T)</code> is called a <i>type guard</i>, and <code>x.(T)</code> is called
a <i>guarded expression</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, the type guard 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, the type guard asserts that the dynamic type
of <code>T</code> implements the interface <code>T</code> (§Interface types).
The type guard is said to succeed if the assertion holds.
<font color=red>TODO: gri wants an error if x is already of type T.</font>
</p>
<p>
If the type guard succeeds, the value of the guarded expression is the value
stored in <code>x</code> and its type is <code>T</code>. If the type guard fails, 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 the guarded expression <code>x.(T)</code> is
known to be <code>T</code> in a correct program.
</p>
<p>
If a guarded expression 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 guarded expression is a pair of values with types <code>(T, bool)</code>.
If the type guard succeeds, 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 guard in this construct thus acts like a function call
returning a value and a boolean indicating success. (§Assignments)
</p>
<h3>Calls</h3>
<p>
Given a function or a function variable <code>f</code> of function type
<code>F</code>, the expression
</p>
<pre>
f(a, b, c)
</pre>
<p>
calls the function with arguments <code>a, b, c</code>.
The arguments must be 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 binaryOp 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;" .
unary_op = "+" | "-" | "!" | "^" | "*" | "&amp;" | "&lt;-" .
</pre>
<p>
The operand types in binary operations must be equal, with the following exceptions:
</p>
<ul>
<li>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 operations 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;
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
<< 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>
<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
(§Arithmetic 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>
<!--TODO(r): This section is a mess. Skipping it for now.-->
<p>
<font color=red>TODO: Need to talk about unary "*", clean up section below.</font>
<p>
<font color=red>TODO: This text needs to be cleaned up and go elsewhere, there are no address
operators involved.
</font>
<p>
Methods are a form of function, and a method ``value'' has a function type.
Consider the type T with method M:
<pre>
type T struct {
a int;
}
func (tp *T) M(a int) int;
var t *T;
</pre>
To construct the value of method M, one writes
<pre>
t.M
</pre>
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>
The expression t.M is a function value with type
<pre>
func (t *T, a int) int
</pre>
and may be invoked only as a function, not as a method:
<pre>
var f func (t *T, a int) int;
f = t.M;
x := f(t, 7);
</pre>
Note that one does not write t.f(7); taking the value of a method demotes
it to a function.
In general, given type T with method M and variable t of type T,
the method invocation
<pre>
t.M(args)
</pre>
is equivalent to the function call
<pre>
(t.M)(t, args)
</pre>
<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>
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
<pre>
var t1 *T1;
var t2 *T2;
var i I = t1;
m := i.M;
m(t2, 7);
</pre>
will invoke t2.M() even though m was constructed with an expression involving
t1. Effectively, the value of m is a function literal
<pre>
func (recv I, a int) {
recv.M(a);
}
</pre>
that is automatically created.
<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>
<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 it is an
integer or float depends on whether the value can be represented
precisely as an integer (123 vs. 1.23). 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>
<hr/>
<h2>Statements</h2>
Statements control execution.
<pre class="grammar">
Statement =
Declaration | LabelDecl | EmptyStat |
SimpleStat | GoStat | ReturnStat | BreakStat | ContinueStat | GotoStat |
FallthroughStat | Block | IfStat | SwitchStat | SelectStat | ForStat |
DeferStat .
SimpleStat =
ExpressionStat | IncDecStat | Assignment | SimpleVarDecl .
</pre>
Statements in a statement list are separated by semicolons, which can be
omitted in some cases as expressed by the OptSemicolon production.
<pre class="grammar">
StatementList = Statement { OptSemicolon Statement } .
</pre>
<p>
A semicolon may be omitted immediately following:
</p>
<ul>
<li>a closing parenthesis ")" ending a list of declarations (§Declarations and Scope)
<li>a closing brace "}" ending a type declaration (§Type declarations)
<li>a closing brace "}" ending a block (including switch and select statements)
<li>a label declaration (§Label declarations)
</ul>
In all other cases a semicolon is required to separate two statements. Since there
is an empty statement, a statement list can always be ``terminated'' with a semicolon.
<h3>Empty statements</h3>
The empty statement does nothing.
<pre class="grammar">
EmptyStat = .
</pre>
<h3>Expression statements</h3>
<pre class="grammar">
ExpressionStat = Expression .
</pre>
<pre>
f(x+y)
</pre>
<font color=red>
TODO: specify restrictions. 6g only appears to allow calls here.
</font>
<h3>IncDec statements</h3>
The "++" and "--" statements increment or decrement their operands
by the (ideal) constant value 1.
<pre class="grammar">
IncDecStat = Expression ( "++" | "--" ) .
</pre>
The following assignment statements (§Assignments) are semantically
equivalent:
<pre class="grammar">
IncDec statement Assignment
x++ x += 1
x-- x -= 1
</pre>
Both operators apply to integer and floating point types only.
<p>
Note that increment and decrement are statements, not expressions.
For instance, "x++" cannot be used as an operand in an expression.
<h3>Assignments</h3>
<pre class="grammar">
Assignment = ExpressionList assign_op ExpressionList .
assign_op = [ add_op | mul_op ] "=" .
</pre>
The left-hand side must be an l-value such as a variable, pointer indirection,
or an array index.
<pre>
x = 1
*p = f()
a[i] = 23
k = <-ch
</pre>
As in C, arithmetic binary operators can be combined with assignments:
<pre>
j <<= 2
</pre>
A tuple assignment assigns the individual elements of a multi-valued operation,
such as function evaluation or some channel and map operations, into individual
variables. For instance, a tuple assignment such as
<pre>
v1, v2, v3 = e1, e2, e3
</pre>
assigns the expressions e1, e2, e3 to temporaries and then assigns the temporaries
to the variables v1, v2, v3. Thus
<pre>
a, b = b, a
</pre>
exchanges the values of a and b. The tuple assignment
<pre>
x, y = f()
</pre>
calls the function f, which must return two values, and assigns them to x and y.
As a special case, retrieving a value from a map, when written as a two-element
tuple assignment, assign a value and a boolean. If the value is present in the map,
the value is assigned and the second, boolean variable is set to true. Otherwise,
the variable is unchanged, and the boolean value is set to false.
<pre>
value, present = map_var[key]
</pre>
To delete a value from a map, use a tuple assignment with the map on the left
and a false boolean expression as the second expression on the right, such
as:
<pre>
map_var[key] = value, false
</pre>
In assignments, the type of the expression must match the type of the left-hand side.
<h3>If statements</h3>
If statements specify the conditional execution of two branches; the "if"
and the "else" branch. If Expression evaluates to true,
the "if" branch is executed. Otherwise the "else" branch is executed if present.
If Condition is omitted, it is equivalent to true.
<pre class="grammar">
IfStat = "if" [ [ SimpleStat ] ";" ] [ Expression ] Block [ "else" Statement ] .
</pre>
<pre>
if x > 0 {
return true;
}
</pre>
An "if" statement may include the declaration of a single temporary variable.
The scope of the declared variable extends to the end of the if statement, and
the variable is initialized once before the statement is entered.
<pre>
if x := f(); x < y {
return x;
} else if x > z {
return z;
} else {
return y;
}
</pre>
<!--
TODO: gri thinks that Statement needs to be changed as follows:
IfStat =
"if" [ [ SimpleStat ] ";" ] [ Expression ] Block
[ "else" ( IfStat | Block ) ] .
To facilitate the "if else if" code pattern, if the "else" branch is
simply another "if" statement, that "if" statement may be written
without the surrounding Block:
if x > 0 {
return 0;
} else if x > 10 {
return 1;
} else {
return 2;
}
-->
<h3>Switch statements</h3>
Switches provide multi-way execution.
<pre class="grammar">
SwitchStat = "switch" [ [ SimpleStat ] ";" ] [ Expression ] "{" { CaseClause } "}" .
CaseClause = SwitchCase ":" [ StatementList ] .
SwitchCase = "case" ExpressionList | "default" .
</pre>
There can be at most one default case in a switch statement. In a case clause,
the last statement only may be a fallthrough statement ($Fallthrough statement).
It indicates that the control should flow from the end of this case clause to
the first statement of the next clause.
<p>
Each case clause effectively acts as a block for scoping purposes
($Declarations and scope rules).
<p>
The expressions do not need to be constants. They will
be evaluated top to bottom until the first successful non-default case is reached.
If none matches and there is a default case, the statements of the default
case are executed.
<pre>
switch tag {
default: s3()
case 0, 1: s1()
case 2: s2()
}
</pre>
A switch statement may include the declaration of a single temporary variable.
The scope of the declared variable extends to the end of the switch statement, and
the variable is initialized once before the switch is entered.
<pre>
switch x := f(); true {
case x &lt; 0: return -x
default: return x
}
</pre>
Cases do not fall through unless explicitly marked with a "fallthrough" statement.
<pre>
switch a {
case 1:
b();
fallthrough
case 2:
c();
}
</pre>
If the expression is omitted, it is equivalent to "true".
<pre>
switch {
case x < y: f1();
case x < z: f2();
case x == 4: f3();
}
</pre>
<h3>For statements</h3>
A for statement specifies repeated execution of a block. The iteration is
controlled by a condition, a for clause, or a range clause.
<pre class="grammar">
ForStat = "for" [ Condition | ForClause | RangeClause ] Block .
Condition = Expression .
</pre>
In its simplest form, a for statement specifies the repeated execution of
a block as long as a condition evaluates to true. The condition is evaluated
before each iteration. The type of the condition expression must be boolean.
If the condition is absent, it is equivalent to "true".
<pre>
for a &lt; b {
a *= 2
}
</pre>
A for statement with a for clause is also controlled by its condition, but
additionally it may specify an init and post statement, such as an assignment,
an increment or decrement statement. The init statement may also be a (simple)
variable declaration; no variables can be declared in the post statement.
<pre class="grammar">
ForClause = [ InitStat ] ";" [ Condition ] ";" [ PostStat ] .
InitStat = SimpleStat .
PostStat = SimpleStat .
</pre>
For instance, one may declare an iteration variable in the init statement:
<pre>
for i := 0; i < 10; i++ {
f(i)
}
</pre>
If present, the init statement is executed once before commencing the iteration;
the post statement is executed after each execution of the statement block (and
only if the block was executed). The scope of any variable declared in the init
statement ends with the end of the for statement block ($Declarations and scope
rules, Rule 3).
<p>
The init and post statement as well as the condition may be omitted; however
if either the init or post statement are present, the separating semicolons
must be present. If the condition is absent, it is equivalent to "true".
The following statements are equivalent:
<pre>
for ; cond ; { S() } is the same as for cond { S() }
for true { S() } is the same as for { S() }
</pre>
Alternatively, a for statement may be controlled by a range clause. A
range clause specifies iteration through all entries of an array or map.
For each entry it first assigns the current index or key to an iteration
variable - or the current (index, element) or (key, value) pair to a pair
of iteration variables - and then executes the block. Iteration terminates
when all entries have been processed, or if the for statement is terminated
early, for instance by a break or return statement.
<pre class="grammar">
RangeClause = IdentifierList ( "=" | ":=" ) "range" Expression .
</pre>
The type of the right-hand expression in the range clause must be an array or
map, or a pointer to an array or map. If it is a pointer, it must not be nil.
The left-hand identifier list must contain one or two identifiers denoting the
iteration variables. The first variable is set to the current array index or
map key, and the second variable, if present, is set to the corresponding
array element or map value. The types of the array index (int) and element,
or of the map key and value respectively, must be assignment-compatible to
the iteration variables.
<p>
The iteration variables may be declared by the range clause (":="), in which
case their scope ends at the end of the for statement block ($Declarations and
scope rules, Rule 3). In this case their types are the array index and element,
or the map key and value types, respectively.
<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>
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. Likewise, if the range expression is a
pointer variable, the behavior of assigning to that variable is implementation-
dependent. Assigning to the iteration variables during iteration simply changes
the values of those variables for the current iteration; it does not affect any
subsequent iterations.
<h3>Go statements</h3>
A go statement starts the execution of a function as an independent
concurrent thread of control within the same address space. The expression
must be a function or method call.
<pre class="grammar">
GoStat = "go" Expression .
</pre>
Unlike with a regular function call, program execution does not wait
for the invoked function to complete.
<pre>
go Server()
go func(ch chan <- bool) { for { sleep(10); ch <- true; }} (c)
</pre>
<h3>Select statements</h3>
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.
<pre class="grammar">
SelectStat = "select" "{" { CommClause } "}" .
CommClause = CommCase ":" [ StatementList ] .
CommCase = "case" ( SendExpr | RecvExpr) | "default" .
SendExpr = Expression "&lt;-" Expression .
RecvExpr = [ Expression ( "=" | ":=" ) ] "&lt;-" Expression .
</pre>
Each communication clause acts as a block for the purpose of scoping
(§Declarations and scope rules).
<p>
For all the send and receive expressions in the select
statement, the channel expression is evaluated. Any values
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 nil, which is equivalent to that case not
being present in the select statement.
<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.
<p>
If the channel sends or receives an interface type, its
communication can proceed only if the type of the communication
clause matches that of the dynamic value to be exchanged.
<p>
If multiple cases can proceed, a uniform fair choice is made regarding
which single communication will execute.
<p>
The receive case may declare a new variable (via a ":=" assignment). The
scope of such variables begins immediately after the variable identifier
and ends at the end of the respective "select" case (that is, before the
next "case", "default", or closing brace).
<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:
}
}
var ca chan interface {};
var i int;
var f float;
select {
case i = &lt;-ca:
print("received int ", i, " from ca\n");
case f = &lt;-ca:
print("received float ", f, " from ca\n");
}
</pre>
<font color=red>
TODO: Make semantics more precise.
</font>
<h3>Return statements</h3>
A return statement terminates execution of the containing function
and optionally provides a result value or values to the caller.
<pre class="grammar">
ReturnStat = "return" [ ExpressionList ] .
</pre>
There are two ways to return values from a function. The first is to
explicitly list the return value or values in the return statement:
<pre>
func simple_f() int {
return 2;
}
</pre>
A function may return multiple values.
The syntax of the return clause in that case is the same as
that of a parameter list; in particular, names must be provided for
the elements of the return value.
<pre>
func complex_f1() (re float, im float) {
return -7.0, -4.0;
}
</pre>
A second method to return values
is to use those names within the function as variables
to be assigned explicitly; the return statement will then provide no
values:
<pre>
func complex_f2() (re float, im float) {
re = 7.0;
im = 4.0;
return;
}
</pre>
<h3>Break statements</h3>
Within a for, switch, or select statement, a break statement terminates
execution of the innermost such statement.
<pre class="grammar">
BreakStat = "break" [ identifier ].
</pre>
If there is an identifier, it must be a label marking an enclosing
for, switch, or select statement, and that is the one whose execution
terminates.
<pre>
L: for i < n {
switch i {
case 5: break L
}
}
</pre>
<h3>Continue statements</h3>
Within a for loop a continue statement begins the next iteration of the
loop at the post statement.
<pre class="grammar">
ContinueStat = "continue" [ identifier ].
</pre>
The optional identifier is analogous to that of a break statement.
<h3>Label declarations</h3>
A label declaration serves as the target of a goto, break or continue statement.
<pre class="grammar">
LabelDecl = identifier ":" .
</pre>
Example:
<pre>
Error:
</pre>
<h3>Goto statements</h3>
A goto statement transfers control to the corresponding label statement.
<pre class="grammar">
GotoStat = "goto" identifier .
</pre>
<pre>
goto Error
</pre>
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:
<pre>
goto L; // BAD
v := 3;
L:
</pre>
is erroneous because the jump to label L skips the creation of v.
<h3>Fallthrough statements</h3>
A fallthrough statement transfers control to the first statement of the
next case clause in a switch statement (§Switch statements). It may only
be used in a switch statement, and only as the last statement in a case
clause of the switch statement.
<pre class="grammar">
FallthroughStat = "fallthrough" .
</pre>
<h3>Defer statements</h3>
A defer statement invokes a function whose execution is deferred to the moment
when the surrounding function returns.
<pre class="grammar">
DeferStat = "defer" Expression .
</pre>
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.
<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>convert
<li>len
<li>make
<li>new
<li>panic
<li>panicln
<li>print
<li>println
<li>typeof
</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>
Conversions syntactically look like function calls of the form
<pre class="grammar">
T(value)
</pre>
where <code>T</code> is the type name of an arithmetic type or string (§Basic types),
and <code>value</code> is the value of an expression that can be converted to a value
of result type <code>T</code>.
<p>
The following conversion rules apply:
<p>
1) 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, uint32(int8(0xFF)) is 0xFFFFFFFF. The conversion always
yields a valid value; there is no signal for overflow.
<p>
2) 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>
<p>
3) Strings permit two special conversions.
<p>
3a) Converting an integer value yields a string containing the UTF-8
representation of the integer.
(TODO: this one could be done just as well by a library.)
</p>
<pre>
string(0x65e5) // "\u65e5"
</pre>
<p>
3b) Converting an array of <code>uint8s</code> yields a string whose successive
bytes are those of the array.
(Recall <code>byte</code> is a synonym for <code>uint8</code>.)
</p>
<pre>
string([]byte('h', 'e', 'l', 'l', 'o')) // "hello"
</pre>
<p>
There is no linguistic mechanism to convert between pointers
and integers. A library may be provided under restricted circumstances
to acccess this conversion in low-level code.
<font color=red>
TODO: Do we allow interface/ptr conversions in this form or do they
have to be written as type guards? (§Type guards)
</font>
</p>
<h3>Allocation</h3>
The built-in function "new" takes a type "T" and returns a value of type "*T".
The memory is initialized as described in the section on initial values
(§The zero value).
<pre>
new(T)
</pre>
For instance
<pre>
type S struct { a int; b float }
new(S)
</pre>
dynamically allocates memory for a variable of type S, initializes it
(a=0, b=0.0), and returns a value of type *S pointing to that variable.
<p>
<font color=red>
TODO Once this has become clearer, connect new() and make() (new() may be
explained by make() and vice versa).
</font>
<h3>Making slices, maps, and channels</h3>
The built-in function "make" takes a type "T", optionally followed by a
type-specific list of expressions. It returns a value of type "T". "T"
must be a slice, map, or channel type.
The memory is initialized as described in the section on initial values
(§The zero value).
<pre>
make(T [, optional list of expressions])
</pre>
For instance
<pre>
make(map[string] int)
</pre>
creates a new map value and initializes it to an empty map.
The only defined parameters affect sizes for allocating slices, maps, and
buffered channels:
<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>
<font color=red>
TODO Once this has become clearer, connect new() and make() (new() may be
explained by make() and vice versa).
</font>
<hr/>
<h2>Packages</h2>
A package is a package clause, optionally followed by import declarations,
followed by a series of declarations.
<pre class="grammar">
Package = PackageClause { ImportDecl [ ";" ] } { Declaration [ ";" ] } .
</pre>
The source text following the package clause acts like a block for scoping
purposes ($Declarations and scope rules).
<p>
Every source file identifies the package to which it belongs.
The file must begin with a package clause.
<pre class="grammar">
PackageClause = "package" PackageName .
package Math
</pre>
A package can gain access to exported identifiers from another package
through an import declaration:
<pre class="grammar">
ImportDecl = "import" ( ImportSpec | "(" [ ImportSpecList ] ")" ) .
ImportSpecList = ImportSpec { ";" ImportSpec } [ ";" ] .
ImportSpec = [ "." | PackageName ] PackageFileName .
PackageFileName = StringLit .
</pre>
An import statement makes the exported package-level identifiers of the named
package file accessible to this package.
<p>
In the following discussion, assume we have a package in the
file "/lib/math", called package "math", which exports the identifiers
"Sin" and "Cos" denoting the respective trigonometric functions.
<p>
In the general form, with an explicit package name, the import
statement declares that package name as an identifier whose
contents are the exported elements of the imported package.
For instance, after
<pre>
import M "/lib/math"
</pre>
the contents of the package /lib/math can be accessed by
"M.Sin", "M.Cos", etc.
<p>
In its simplest form, with no package name, the import statement
implicitly uses the imported package name itself as the local
package name. After
<pre>
import "/lib/math"
</pre>
the contents are accessible by "math.Sin", "math.Cos".
<p>
Finally, if instead of a package name the import statement uses
an explicit period, the contents of the imported package are added
to the current package. After
<pre>
import . "/lib/math"
</pre>
the contents are accessible by "Sin" and "Cos". In this instance, it is
an error if the import introduces name conflicts.
<p>
Here is a complete example Go package that implements a concurrent prime sieve:
<pre>
package main
// 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(in chan <- int, out *<-chan int, prime int) {
for {
i := <-in; // Receive value of new variable 'i' from 'in'.
if i % prime != 0 {
out <- i // Send 'i' to channel 'out'.
}
}
}
// 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;
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>
<h3>Program execution</h3>
<p>
A package with no imports is initialized by assigning initial values to
all its global variables in declaration order and then calling any init()
functions defined in its source. Since a package may contain more
than one source file, there may be more than one init() function, 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
thread of execution.
</p>
<p>
Furthermore, an "init()" function cannot be referred to from anywhere
in a program. In particular, "init()" cannot be called explicitly, nor
can a pointer to "init" 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 P, P 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 main, 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 main package and then
invoking <code>main.main()</code>.
</p>
<p>
When main.main() returns, the program exits.
</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 arithmetic 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 arithmetic types (§Arithmetic 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 arithmetic 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 allow p.x where p is the local package name.
<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.
<br/>
Conversions work for any type; doc says only arithmetic types and strings.
</font>
</p>
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