The Go Programming Language
----
-(April 17, 2008)
+(April 18, 2008)
This document is an informal specification/proposal for a new systems programming
language.
The syntax is specified using Extended Backus-Naur Form (EBNF).
In particular:
-- "" encloses lexical symbols (a backslash precedes a literal quote within a symbol)
-- | separates alternatives
+- | separates alternatives (least binding strength)
- () groups
- [] specifies an option (0 or 1 times)
- {} specifies repetition (0 to n times)
+Lexical symbols are enclosed in double quotes '''' (the
+double quote symbol is written as ''"'').
+
A production may be referenced from various places in this document
but is usually defined close to its first use. Productions and code
examples are indented.
identifier = letter { letter | dec_digit } .
- a
- _x
- ThisIsVariable9
+ a
+ _x
+ ThisIsVariable9
Types
predeclared type names. There are signed and unsigned integer
and floating point types:
- bool the truth values true and false
+ bool the truth values true and false
- uint8 the set of all unsigned 8-bit integers
- uint16 the set of all unsigned 16-bit integers
- uint32 the set of all unsigned 32-bit integers
- unit64 the set of all unsigned 64-bit integers
+ uint8 the set of all unsigned 8-bit integers
+ uint16 the set of all unsigned 16-bit integers
+ uint32 the set of all unsigned 32-bit integers
+ unit64 the set of all unsigned 64-bit integers
- byte alias for uint8
+ byte alias for uint8
- int8 the set of all signed 8-bit integers, in 2's complement
- int16 the set of all signed 16-bit integers, in 2's complement
- int32 the set of all signed 32-bit integers, in 2's complement
- int64 the set of all signed 64-bit integers, in 2's complement
+ int8 the set of all signed 8-bit integers, in 2's complement
+ int16 the set of all signed 16-bit integers, in 2's complement
+ int32 the set of all signed 32-bit integers, in 2's complement
+ int64 the set of all signed 64-bit integers, in 2's complement
- 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
- float80 the set of all valid IEEE-754 80-bit floating point numbers
+ 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
+ float80 the set of all valid IEEE-754 80-bit floating point numbers
Additionally, Go declares 4 basic types, uint, int, float, and double,
which are platform-specific. The bit width of these types corresponds to
int_lit = [ sign ] unsigned_int_lit .
unsigned_int_lit = decimal_int_lit | octal_int_lit | hex_int_lit .
decimal_int_lit = ( "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" )
- { dec_digit } .
+ { dec_digit } .
octal_int_lit = "0" { oct_digit } .
hex_int_lit = "0" ( "x" | "X" ) hex_digit { hex_digit } .
float_lit = [ sign ] ( fractional_lit | exponential_lit ) .
fractional_lit = { dec_digit } ( dec_digit "." | "." dec_digit )
{ dec_digit } [ exponent ] .
exponential_lit = dec_digit { dec_digit } exponent .
- exponent = ( "e" | "E" ) [ sign ] dec_digit { dec_digit }
+ exponent = ( "e" | "E" ) [ sign ] dec_digit { dec_digit } .
07
0xFF
contents of a string.
- No internal pointers: it is illegal to create a pointer to an inner
element of a string.
-- They can be indexed: given string s1, s1[i] is a byte value.
-- They can be concatenated: given strings s1 and s2, s1 + s2 is a value
- combining the elements of s1 and s2 in sequence.
-- Known length: the length of a string s1 can be obtained by the function/
- operator len(s1). The length of a string is the number of bytes within.
+- They can be indexed: given string "s1", "s1[i]" is a byte value.
+- They can be concatenated: given strings "s1" and "s2", "s1 + s2" is a value
+ combining the elements of "s1" and "s2" in sequence.
+- Known length: the length of a string "s1" can be obtained by the function/
+ operator "len(s1)". The length of a string is the number of bytes within.
Unlike in C, there is no terminal NUL byte.
- Creation 1: a string can be created from an integer value by a conversion;
the result is a string containing the UTF-8 encoding of that code point.
- string('x') yields "x"; string(0x1234) yields the equivalent of "\u1234"
+ "string('x')" yields "x"; "string(0x1234)" yields the equivalent of "\u1234"
- Creation 2: a string can by created from an array of integer values (maybe
just array of bytes) by a conversion
a [3]byte; a[0] = 'a'; a[1] = 'b'; a[2] = 'c'; string(a) == "abc";
Character and string literals
----
-Character and string literals are almost the same as in C, but with
-UTF-8 required. This section is precise but can be skipped on first
-reading.
+Character and string literals are almost the same as in C, with the
+following differences:
-Character and string literals are similar to C except:
- - Octal character escapes are always 3 digits (\077 not \77)
- - Hexadecimal character escapes are always 2 digits (\x07 not \x7)
- - Strings are UTF-8 and represent Unicode
+ - The encoding is UTF-8
- `` strings exist; they do not interpret backslashes
+ - Octal character escapes are always 3 digits ("\077" not "\77")
+ - Hexadecimal character escapes are always 2 digits ("\x07" not "\x7")
-The rules are:
-
- char_lit = "'" ( utf8_char_no_single_quote | "\" esc_seq ) "'" .
+This section is precise but can be skipped on first reading. The rules are:
- esc_seq =
- "a" | "b" | "f" | "n" | "r" | "t" | "v" | "\" | "'" | "\"" |
- oct_digit oct_digit oct_digit |
- "x" hex_digit hex_digit |
- "u" hex_digit hex_digit hex_digit hex_digit |
- "U" hex_digit hex_digit hex_digit hex_digit
- hex_digit hex_digit hex_digit hex_digit .
+ char_lit = "'" ( unicode_value | byte_value ) "'" .
+ unicode_value = utf8_char | little_u_value | big_u_value | escaped_char .
+ byte_value = octal_byte_value | hex_byte_value .
+ octal_byte_value = "\" oct_digit oct_digit oct_digit .
+ hex_byte_value = "\" "x" hex_digit hex_digit .
+ little_u_value = "\" "u" hex_digit hex_digit hex_digit hex_digit .
+ big_u_value = "\" "U" hex_digit hex_digit hex_digit hex_digit
+ hex_digit hex_digit hex_digit hex_digit .
+ escaped_char = "\" ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | "\" | "'" | """ ) .
A unicode_value takes one of four forms:
* The UTF-8 encoding of a Unicode code point. Since Go source
text is in UTF-8, this is the obvious translation from input
text into Unicode characters.
-* The usual list of C backslash escapes: \n \t etc.
-* A `little u' value, such as \u12AB. This represents the Unicode
+* The usual list of C backslash escapes: "\n", "\t", etc.
+* A `little u' value, such as "\u12AB". This represents the Unicode
code point with the corresponding hexadecimal value. It always
has exactly 4 hexadecimal digits.
-* A `big U' value, such as \U00101234. This represents the
+* A `big U' value, such as "\U00101234". This represents the
Unicode code point with the corresponding hexadecimal value.
It always has exactly 8 hexadecimal digits.
is that of the Unicode code point represented by the text between the
quotes.
- 'a'
- 'ä'
- '本'
- '\t'
- '\000'
- '\007'
- '\377'
- '\x07'
- '\xff'
- '\u12e4'
- '\U00101234'
+ 'a'
+ 'ä'
+ '本'
+ '\t'
+ '\000'
+ '\007'
+ '\377'
+ '\x07'
+ '\xff'
+ '\u12e4'
+ '\U00101234'
String literals come in two forms: double-quoted and back-quoted.
Double-quoted strings have the usual properties; back-quoted strings
do not interpret backslashes at all.
string_lit = raw_string_lit | interpreted_string_lit .
- raw_string_lit = "`" { utf8_char_no_back_quote } "`" .
- interpreted_string_lit = "\"" { utf8_char_no_double_quote | "\\" esc_seq } "\"" .
+ raw_string_lit = "`" { utf8_char } "`" .
+ interpreted_string_lit = """ { unicode_value | byte_value } """ .
A string literal has type 'string'. Its value is constructed by
taking the byte values formed by the successive elements of the
literal. For byte_values, these are the literal bytes; for
unicode_values, these are the bytes of the UTF-8 encoding of the
corresponding Unicode code points. Note that
- "\u00FF"
+ "\u00FF"
and
- "\xFF"
+ "\xFF"
are
different strings: the first contains the two-byte UTF-8 expansion of
the value 255, while the second contains a single byte of value 255.
These examples all represent the same string:
- "日本語" // UTF-8 input text
- `日本語` // UTF-8 input text as a raw literal
- "\u65e5\u672c\u8a9e" // The explicit Unicode code points
- "\U000065e5\U0000672c\U00008a9e" // The explicit Unicode code points
- "\xe6\x97\xa5\xe6\x9c\xac\xe8\xaa\x9e" // The explicit UTF-8 bytes
+ "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
The language does not canonicalize Unicode text or evaluate combining
forms. The text of source code is passed uninterpreted.
FieldDeclList = FieldDecl { ";" FieldDecl } .
FieldDecl = IdentifierList Type .
- // An empty struct.
- struct {}
+ // An empty struct.
+ struct {}
- // A struct with 5 fields.
- struct {
- x, y int;
- u float;
- a []int;
- f func();
- }
+ // A struct with 5 fields.
+ struct {
+ x, y int;
+ u float;
+ a []int;
+ f func();
+ }
Compound Literals
----
ChannelType = "chan" [ "<" | ">" ] ValueType .
- chan any // a generic channel
- chan int // a channel that can exchange only ints
- chan> float // a channel that can only be used to send floats
- chan< any // a channel that can receive (only) values of any type
+ chan any // a generic channel
+ chan int // a channel that can exchange only ints
+ chan> float // a channel that can only be used to send floats
+ chan< any // a channel that can receive (only) values of any type
Channel variables always have type pointer to channel.
It is an error to attempt to use a channel value and in
particular to dereference a channel pointer.
- var ch *chan int;
- ch = new(chan int); // new returns type *chan int
+ var ch *chan int;
+ ch = new(chan int); // new returns type *chan int
There are no channel literals.
ParameterSection = [ IdentifierList ] Type .
Result = Type | "(" ParameterList ")" .
- // Function types
- func ()
- func (a, b int, z float) bool
- func (a, b int, z float) (success bool)
- func (a, b int, z float) (success bool, result float)
+ // Function types
+ func ()
+ func (a, b int, z float) bool
+ func (a, b int, z float) (success bool)
+ func (a, b int, z float) (success bool, result float)
- // Method types
- func (p *T) . ()
- func (p *T) . (a, b int, z float) bool
- func (p *T) . (a, b int, z float) (success bool)
- func (p *T) . (a, b int, z float) (success bool, result float)
+ // Method types
+ func (p *T) . ()
+ func (p *T) . (a, b int, z float) bool
+ func (p *T) . (a, b int, z float) (success bool)
+ func (p *T) . (a, b int, z float) (success bool, result float)
A variable can hold only a pointer to a function, not a function value.
In particular, v := func() {} creates a variable of type *func(). To call the
For now, a function literal can reference only its parameters, global
variables, and variables declared within the function literal.
- // Function literal
- func (a, b int, z float) bool { return a*b < int(z); }
+ // Function literal
+ func (a, b int, z float) bool { return a*b < int(z); }
- // Method literal
- func (p *T) . (a, b int, z float) bool { return a*b < int(z) + p.x; }
+ // Method literal
+ func (p *T) . (a, b int, z float) bool { return a*b < int(z) + p.x; }
Unresolved issues: Are there method literals? How do you use them?
the declaration
- func (p *Point) distance(float scale) float {
+ func (p *Point) distance(scale float) float {
return scale * (p.x*p.x + p.y*p.y);
}
No other operations are defined (yet).
Note that type
- interface {}
+ interface {}
is a special case that can match any struct type, while type
- any
+ any
can match any type at all, including basic types, arrays, etc.
TODO: details about reflection
(in increasing precedence order):
Precedence Operator
- 1 ||
- 2 &&
- 3 == != < <= > >=
- 4 + - | ^
- 5 * / % << >> &
- 6 + - ! ^ < > * & (unary)
+ 1 ||
+ 2 &&
+ 3 == != < <= > >=
+ 4 + - | ^
+ 5 * / % << >> &
+ 6 + - ! ^ < > * & (unary)
For integer values, / and % satisfy the following relationship:
- (a / b) * b + a % b == a
+ (a / b) * b + a % b == a
and
- (a / b) is "truncated towards zero".
+ (a / b) is "truncated towards zero".
There are no implicit type conversions except for
constants and literals. In particular, unsigned and signed integer
complement).
There is no '->' operator. Given a pointer p to a struct, one writes
- p.f
+ p.f
to access field f of the struct. Similarly, given an array or map
pointer, one writes
- p[i]
+ p[i]
to access an element. Given a function pointer, one writes
- p()
+ p()
to call the function.
Other operators behave as in C.
If multiple cases can proceed, a uniform fair choice is made regarding
which single communication will execute.
- var c, c1, c2 *chan int;
- select {
- case i1 = <c1:
- printf("received %d from c1\n", i1);
- case >c2 = i2:
- printf("sent %d to c2\n", i2);
- default:
- printf("no communication\n");
- }
-
- for { // send random sequence of bits to c
- select {
- case >c = 0: // note: no statement, no fallthrough, no folding of cases
- case >c = 1:
- }
- }
+ var c, c1, c2 *chan int;
+ select {
+ case i1 = <c1:
+ printf("received %d from c1\n", i1);
+ case >c2 = i2:
+ printf("sent %d to c2\n", i2);
+ default:
+ printf("no communication\n");
+ }
- var ca *chan any;
- var i int;
- var f float;
+ for { // send random sequence of bits to c
select {
- case i = <ca:
- printf("received int %d from ca\n", i);
- case f = <ca:
- printf("received float %f from ca\n", f);
+ case >c = 0: // note: no statement, no fallthrough, no folding of cases
+ case >c = 1:
}
+ }
+
+ var ca *chan any;
+ var i int;
+ var f float;
+ select {
+ case i = <ca:
+ printf("received int %d from ca\n", i);
+ case f = <ca:
+ printf("received float %f from ca\n", f);
+ }
TODO: do we allow case i := <c: ?
TODO: need to precise about all the details but this is not the right doc for that
scope that were not already in scope at the point of the goto. For
instance, this example:
- goto L; // BAD
- v := 3;
- L:
+ goto L; // BAD
+ v := 3;
+ L:
is erroneous because the jump to label L skips the creation of v.
- TODO: type switch?
- TODO: words about slices
-- TODO: what is nil? do we type-test by a nil conversion or something else?
+- TODO: I (gri) would like to say that sizeof(int) == sizeof(pointer), always.