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Commit 68fb2d04 authored by Rob Pike's avatar Rob Pike
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use tabs for indentation consistently 

R=gri
OCL=14125
CL=14125
parent f97832e4
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doc/go_lang.txt
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......@@ -182,42 +182,42 @@ comprehensible composability of types.
Here is a complete example Go program that implements a concurrent prime sieve:
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 := new(chan int); // Create a new channel.
go Generate(ch); // Start Generate() as a subprocess.
for {
prime := <-ch;
printf("%d\n", prime);
ch1 := new(chan int);
go Filter(ch, ch1, prime);
ch = ch1
}
}
func main() {
Sieve()
}
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 := new(chan int); // Create a new channel.
go Generate(ch); // Start Generate() as a subprocess.
for {
prime := <-ch;
printf("%d\n", prime);
ch1 := new(chan int);
go Filter(ch, ch1, prime);
ch = ch1
}
}
func main() {
Sieve()
}
Notation
......@@ -246,11 +246,11 @@ productions are in CamelCase.
Common productions
----
IdentifierList = identifier { "," identifier } .
ExpressionList = Expression { "," Expression } .
IdentifierList = identifier { "," identifier } .
ExpressionList = Expression { "," Expression } .
QualifiedIdent = [ PackageName "." ] identifier .
PackageName = identifier .
QualifiedIdent = [ PackageName "." ] identifier .
PackageName = identifier .
Source code representation
......@@ -274,11 +274,11 @@ Characters
In the grammar we use the notation
utf8_char
utf8_char
to refer to an arbitrary Unicode code point encoded in UTF-8. We use
non_ascii
non_ascii
to refer to the subset of "utf8_char" code points with values >= 128.
......@@ -286,11 +286,12 @@ to refer to the subset of "utf8_char" code points with values >= 128.
Digits and Letters
----
oct_digit = { "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" } .
dec_digit = { "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" } .
hex_digit = { "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" | "a" |
"A" | "b" | "B" | "c" | "C" | "d" | "D" | "e" | "E" | "f" | "F" } .
letter = "A" | "a" | ... "Z" | "z" | "_" | non_ascii .
oct_digit = { "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" } .
dec_digit = { "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" } .
hex_digit =
{ "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" | "a" |
"A" | "b" | "B" | "c" | "C" | "d" | "D" | "e" | "E" | "f" | "F" } .
letter = "A" | "a" | ... "Z" | "z" | "_" | non_ascii .
All non-ASCII code points are considered letters; digits are always ASCII.
......@@ -301,25 +302,25 @@ Identifiers
An identifier is a name for a program entity such as a variable, a
type, a function, etc.
identifier = letter { letter | dec_digit } .
identifier = letter { letter | dec_digit } .
a
_x
ThisIsVariable9
αβ
a
_x
ThisIsVariable9
αβ
Reserved words
----
break fallthrough interface return
case false iota select
const for map struct
chan func new switch
continue go nil true
default goto package type
else if range var
export import
break fallthrough interface return
case false iota select
const for map struct
chan func new switch
continue go nil true
default goto package type
else if range var
export import
With the exception of structure fields and methods, reserved words may
not be declared as identifiers.
......@@ -343,19 +344,19 @@ strings, and a special polymorphic type.
The arithmetic types are:
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
uint64 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
uint64 the set of all unsigned 64-bit integers
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 several platform-specific type aliases:
ushort, short, uint, int, ulong, long, float, and double. The bit
......@@ -380,9 +381,9 @@ sized types to maximize portability.
Other basic types include:
bool the truth values true and false
string immutable strings of bytes
any polymorphic type
bool the truth values true and false
string immutable strings of bytes
any polymorphic type
Two reserved words, ``true'' and ``false'', represent the
corresponding boolean constant values.
......@@ -418,23 +419,21 @@ variable.
Floating point constants also represent an abstract, ideal floating
point value that is constrained only upon assignment.
sign = "+" | "-" .
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 } .
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 } .
07
0xFF
-44
+3.24e-7
sign = "+" | "-" .
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 } .
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 } .
07
0xFF
-44
+3.24e-7
The string type
......@@ -444,21 +443,28 @@ The string type represents the set of string values (strings).
Strings behave like arrays of bytes, with the following properties:
- They are immutable: after creation, it is not possible to change the
contents of a string.
contents of a string.
- No internal pointers: it is illegal to create a pointer to an inner
element of a string.
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.
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.
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"
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"
- 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";
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
......@@ -467,22 +473,23 @@ Character and string literals
Character and string literals are almost the same as in C, with the
following differences:
- 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 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")
This section is precise but can be skipped on first reading. The rules are:
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" | "\" | "'" | """ ) .
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:
......@@ -512,57 +519,57 @@ A character literal is a form of unsigned integer constant. Its value
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 } "`" .
interpreted_string_lit = """ { unicode_value | byte_value } """ .
string_lit = raw_string_lit | interpreted_string_lit .
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.
The same rules apply to raw string literals, except the contents are
uninterpreted UTF-8.
`abc`
`\n`
"hello, world\n"
"\n"
""
"Hello, world!\n"
"日本語"
"\u65e5本\U00008a9e"
"\xff\u00FF"
`abc`
`\n`
"hello, world\n"
"\n"
""
"Hello, world!\n"
"日本語"
"\u65e5本\U00008a9e"
"\xff\u00FF"
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.
......@@ -589,9 +596,10 @@ with the static type of the variable.
Types may be composed from other types by assembling arrays, maps,
channels, structures, and functions. They are called composite types.
Type = TypeName | ArrayType | ChannelType | InterfaceType |
FunctionType | MapType | StructType | PointerType .
TypeName = QualifiedIdent.
Type =
TypeName | ArrayType | ChannelType | InterfaceType |
FunctionType | MapType | StructType | PointerType .
TypeName = QualifiedIdent.
Array types
......@@ -612,19 +620,19 @@ Any array may be assigned to an open array variable with the
same element type. Typically, open arrays are used as
formal parameters for functions.
ArrayType = "[" [ ArrayLength ] "]" ElementType .
ArrayLength = Expression .
ElementType = Type .
ArrayType = "[" [ ArrayLength ] "]" ElementType .
ArrayLength = Expression .
ElementType = Type .
[] uint8
[2*n] int
[64] struct { x, y: int32; }
[1000][1000] float64
[] uint8
[2*n] int
[64] struct { x, y: int32; }
[1000][1000] float64
The length of an array can be discovered at run time (or compile time, if
its length is a constant) using the built-in special function len():
len(a)
len(a)
Map types
......@@ -639,13 +647,13 @@ during execution. The number of entries in a map is called its length.
A map whose value type is 'any' can store values of all types.
END]
MapType = "map" "[" KeyType "]" ValueType .
KeyType = Type .
ValueType = Type | "any" .
MapType = "map" "[" KeyType "]" ValueType .
KeyType = Type .
ValueType = Type | "any" .
map [string] int
map [struct { pid int; name string }] *chan Buffer
map [string] any
map [string] int
map [struct { pid int; name string }] *chan Buffer
map [string] any
Implementation restriction: Currently, only pointers to maps are supported.
......@@ -658,20 +666,20 @@ Struct types are similar to C structs.
Each field of a struct represents a variable within the data
structure.
StructType = "struct" "{" [ FieldDeclList [ ";" ] ] "}" .
FieldDeclList = FieldDecl { ";" FieldDecl } .
FieldDecl = IdentifierList Type .
StructType = "struct" "{" [ FieldDeclList [ ";" ] ] "}" .
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();
}
Composite Literals
......@@ -683,25 +691,25 @@ conversion from expression list to composite value.
Structure literals follow this form directly. Given
type Rat struct { num, den int };
type Num struct { r Rat, f float, s string };
type Rat struct { num, den int };
type Num struct { r Rat, f float, s string };
we can write
pi := Num(Rat(22,7), 3.14159, "pi")
pi := Num(Rat(22,7), 3.14159, "pi")
For array literals, if the size is present the constructed array has that many
elements; trailing elements are given the approprate zero value for that type.
If it is absent, the size of the array is the number of elements. It is an error
if a specified size is less than the number of elements in the expression list.
primes := [6]int(2, 3, 5, 7, 9, 11)
weekdays := []string("mon", "tue", "wed", "thu", "fri", "sat", "sun")
primes := [6]int(2, 3, 5, 7, 9, 11)
weekdays := []string("mon", "tue", "wed", "thu", "fri", "sat", "sun")
Map literals are similar except the elements of the expression list are
key-value pairs separated by a colon:
m := map[string]int("good":0, "bad":1, "indifferent":7)
m := map[string]int("good":0, "bad":1, "indifferent":7)
TODO: helper syntax for nested arrays etc? (avoids repeating types but
complicates the spec needlessly.)
......@@ -712,22 +720,22 @@ Pointer types
Pointer types are similar to those in C.
PointerType = "*" ElementType.
PointerType = "*" ElementType.
Pointer arithmetic of any kind is not permitted.
*int
*map[string] *chan
*int
*map[string] *chan
For pointer types (only), the pointer element type may be an
identifier referring to an incomplete (not yet fully defined) or undeclared
type. This allows the construction of recursive and mutually recursive types
such as:
type S struct { s *S }
type S struct { s *S }
type S1 struct { s2 *S2 }
type S2 struct { s1 *S1 }
type S1 struct { s2 *S2 }
type S2 struct { s1 *S1 }
If the element type is an undeclared identifier, the declaration implicitly
forward-declares an (incomplete) type with the respective name. By the end
......@@ -746,19 +754,19 @@ By conversion or assignment, it may be restricted only to send or
to receive; such a restricted channel
is called a 'send channel' or a 'receive channel'.
ChannelType = "chan" [ "<-" | "-<" ] ValueType .
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
Function types
----
......@@ -772,25 +780,25 @@ END]
Functions can return multiple values simultaneously.
FunctionType = "func" AnonymousSignature .
AnonymousSignature = [ Receiver "." ] Parameters [ Result ] .
Receiver = "(" identifier Type ")" .
Parameters = "(" [ ParameterList ] ")" .
ParameterList = ParameterSection { "," ParameterSection } .
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)
// 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)
FunctionType = "func" AnonymousSignature .
AnonymousSignature = [ Receiver "." ] Parameters [ Result ] .
Receiver = "(" identifier Type ")" .
Parameters = "(" [ ParameterList ] ")" .
ParameterList = ParameterSection { "," ParameterSection } .
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)
// 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
......@@ -806,19 +814,19 @@ Function Literals
Function literals represent anonymous functions.
FunctionLit = FunctionType Block .
Block = "{" [ StatementList [ ";" ] ] "}" .
FunctionLit = FunctionType Block .
Block = "{" [ StatementList [ ";" ] ] "}" .
A function literal can be invoked
or assigned to a variable of the corresponding function pointer type.
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?
......@@ -829,13 +837,13 @@ Methods
A method is a function bound to a particular type T, where T is the
type of the receiver. For instance, given type Point
type Point struct { x, y float }
type Point struct { x, y float }
the declaration
func (p *Point) distance(scale float) float {
return scale * (p.x*p.x + p.y*p.y);
}
func (p *Point) distance(scale float) float {
return scale * (p.x*p.x + p.y*p.y);
}
creates a method of type *Point. Note that methods may appear anywhere
after the declaration of the receiver type and may be forward-declared.
......@@ -844,11 +852,11 @@ When invoked, a method behaves like a function whose first argument
is the receiver, but at the call site the receiver is bound to the method
using the notation
receiver.method()
receiver.method()
For instance, given a *Point variable pt, one may call
pt.distance(3.5)
pt.distance(3.5)
Interface of a type
......@@ -863,24 +871,24 @@ Interface types
An interface type denotes a set of methods.
InterfaceType = "interface" "{" [ MethodDeclList [ ";" ] ] "}" .
MethodDeclList = MethodDecl { ";" MethodDecl } .
MethodDecl = identifier Parameters [ Result ] .
InterfaceType = "interface" "{" [ MethodDeclList [ ";" ] ] "}" .
MethodDeclList = MethodDecl { ";" MethodDecl } .
MethodDecl = identifier Parameters [ Result ] .
// A basic file interface.
type File interface {
Read(b Buffer) bool;
Write(b Buffer) bool;
Close();
}
// A basic file interface.
type File interface {
Read(b Buffer) bool;
Write(b Buffer) bool;
Close();
}
Any type whose interface has, possibly as a subset, the complete
set of methods of an interface I is said to implement interface I.
For instance, if two types S1 and S2 have the methods
func (p T) Read(b Buffer) bool { return ... }
func (p T) Write(b Buffer) bool { return ... }
func (p T) Close() { ... }
func (p T) Read(b Buffer) bool { return ... }
func (p T) Write(b Buffer) bool { return ... }
func (p T) Close() { ... }
(where T stands for either S1 or S2) then the File interface is
implemented by both S1 and S2, regardless of what other methods
......@@ -888,20 +896,20 @@ S1 and S2 may have or share.
All types implement the empty interface:
interface {}
interface {}
In general, a type implements an arbitrary number of interfaces.
For instance, if we have
type Lock interface {
lock();
unlock();
}
type Lock interface {
lock();
unlock();
}
and S1 and S2 also implement
func (p T) lock() { ... }
func (p T) unlock() { ... }
func (p T) lock() { ... }
func (p T) unlock() { ... }
they implement the Lock interface as well as the File interface.
......@@ -922,18 +930,18 @@ plain assignment or implicitly, such as through a function parameter
or channel operation. Given an "any" variable v storing an underlying
value of type T, one may:
- copy v's value to another variable of type "any"
- extract the stored value by an explicit conversion operation T(v)
- copy v's value to a variable of type T
- copy v's value to another variable of type "any"
- extract the stored value by an explicit conversion operation T(v)
- copy v's value to a variable of type T
Attempts to convert/extract to an incompatible type will yield nil.
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
......@@ -960,19 +968,19 @@ are ignored for the purpose of type equivalence.
For instance, the struct type
struct {
a int;
b int;
f *func (m *[32] float, x int, y int) bool
}
struct {
a int;
b int;
f *func (m *[32] float, x int, y int) bool
}
is equivalent to
struct {
a, b int;
f *F
}
struct {
a, b int;
f *F
}
where "F" is declared as "func (a *[30 + 2] float, b, c int) (ok bool)".
Finally, two interface types are equivalent if they both declare the same set of
......@@ -984,7 +992,7 @@ vice versa. Note that the declaration order of the methods is not relevant.
Literals
----
Literal = char_lit | string_lit | int_lit | float_lit | FunctionLit | "nil" .
Literal = char_lit | string_lit | int_lit | float_lit | FunctionLit | "nil" .
Declarations
......@@ -993,7 +1001,7 @@ Declarations
A declaration associates a name with a language entity such as a constant, type,
variable, or function.
Declaration = [ "export" ] ( ConstDecl | TypeDecl | VarDecl | FunctionDecl ) .
Declaration = [ "export" ] ( ConstDecl | TypeDecl | VarDecl | FunctionDecl ) .
Global declarations optionally may be marked for export with the reserved word
"export". Local declarations can never be exported.
......@@ -1004,8 +1012,8 @@ If the declaration defines a type, the type structure is exported as well. In
particular, if the declaration defines a new "struct" or "interface" type,
all structure fields and all structure and interface methods are exported also.
export const pi float = 3.14159265
export func Parse(source string);
export const pi float = 3.14159265
export func Parse(source string);
Note that at the moment the old-style export via ExportDecl is still supported.
......@@ -1015,7 +1023,7 @@ TODO: Eventually we need to be able to restrict visibility of fields and methods
TODO: specify range of visibility, scope rules.
[OLD
Declaration = ConstDecl | TypeDecl | VarDecl | FunctionDecl | ExportDecl .
Declaration = ConstDecl | TypeDecl | VarDecl | FunctionDecl | ExportDecl .
END]
......@@ -1024,16 +1032,16 @@ Const declarations
A constant declaration gives a name to the value of a constant expression.
ConstDecl = "const" ( ConstSpec | "(" ConstSpecList [ ";" ] ")" ).
ConstSpec = identifier [ Type ] [ "=" Expression ] .
ConstSpecList = ConstSpec { ";" ConstSpec }.
ConstDecl = "const" ( ConstSpec | "(" ConstSpecList [ ";" ] ")" ).
ConstSpec = identifier [ Type ] [ "=" Expression ] .
ConstSpecList = ConstSpec { ";" ConstSpec }.
const pi float = 3.14159265
const e = 2.718281828
const (
one int = 1;
two = 3
)
const pi float = 3.14159265
const e = 2.718281828
const (
one int = 1;
two = 3
)
The constant expression may be omitted, in which case the expression is
the last expression used after the reserved word "const". If no such expression
......@@ -1100,26 +1108,26 @@ Type declarations
A type declaration introduces a name as a shorthand for a type.
TypeDecl = "type" ( TypeSpec | "(" TypeSpecList [ ";" ] ")" ).
TypeSpec = identifier Type .
TypeSpecList = TypeSpec { ";" TypeSpec }.
TypeDecl = "type" ( TypeSpec | "(" TypeSpecList [ ";" ] ")" ).
TypeSpec = identifier Type .
TypeSpecList = TypeSpec { ";" TypeSpec }.
The name refers to an incomplete type until the type specification is complete.
Incomplete types can be referred to only by pointer types. Consequently, in a
type declaration a type may not refer to itself unless it does so with a pointer
type.
type IntArray [16] int
type IntArray [16] int
type (
Point struct { x, y float };
Polar Point
)
type (
Point struct { x, y float };
Polar Point
)
type TreeNode struct {
left, right *TreeNode;
value Point;
}
type TreeNode struct {
left, right *TreeNode;
value Point;
}
Variable declarations
......@@ -1129,18 +1137,18 @@ A variable declaration creates a variable and gives it a type and a name.
It may optionally give the variable an initial value; in some forms of
declaration the type of the initial value defines the type of the variable.
VarDecl = "var" ( VarSpec | "(" VarSpecList [ ";" ] ")" ) .
VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .
VarSpecList = VarSpec { ";" VarSpec } .
var i int
var u, v, w float
var k = 0
var x, y float = -1.0, -2.0
var (
i int;
u, v = 2.0, 3.0
)
VarDecl = "var" ( VarSpec | "(" VarSpecList [ ";" ] ")" ) .
VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .
VarSpecList = VarSpec { ";" VarSpec } .
var i int
var u, v, w float
var k = 0
var x, y float = -1.0, -2.0
var (
i int;
u, v = 2.0, 3.0
)
If the expression list is present, it must have the same number of elements
as there are variables in the variable specification.
......@@ -1154,21 +1162,21 @@ If the variable type is omitted, and the corresponding initialization expression
is a constant expression of abstract int or floating point type, the type
of the variable is "int" or "float" respectively:
var i = 0 // i has int type
var f = 3.1415 // f has float type
var i = 0 // i has int type
var f = 3.1415 // f has float type
The syntax
SimpleVarDecl = identifier ":=" Expression .
SimpleVarDecl = identifier ":=" Expression .
is shorthand for
var identifier = Expression.
var identifier = Expression.
i := 0
f := func() int { return 7; }
ch := new(chan int);
i := 0
f := func() int { return 7; }
ch := new(chan int);
Also, in some contexts such as "if", "for", or "switch" statements,
this construct can be used to declare local temporary variables.
......@@ -1183,40 +1191,40 @@ in the signature.
Implementation restriction: Functions and methods can only be declared
at the global level.
FunctionDecl = "func" NamedSignature ( ";" | Block ) .
NamedSignature = [ Receiver ] identifier Parameters [ Result ] .
FunctionDecl = "func" NamedSignature ( ";" | Block ) .
NamedSignature = [ Receiver ] identifier Parameters [ Result ] .
func min(x int, y int) int {
if x < y {
return x;
}
return y;
}
func min(x int, y int) int {
if x < y {
return x;
}
return y;
}
func foo(a, b int, z float) bool {
return a*b < int(z);
}
func foo(a, b int, z float) bool {
return a*b < int(z);
}
A method is a function that also declares a receiver.
func (p *T) foo(a, b int, z float) bool {
return a*b < int(z) + p.x;
}
func (p *T) foo(a, b int, z float) bool {
return a*b < int(z) + p.x;
}
func (p *Point) Length() float {
return Math.sqrt(p.x * p.x + p.y * p.y);
}
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;
}
func (p *Point) Scale(factor float) {
p.x = p.x * factor;
p.y = p.y * factor;
}
Functions and methods can be forward declared by omitting the body:
func foo(a, b int, z float) bool;
func (p *T) foo(a, b int, z float) bool;
func foo(a, b int, z float) bool;
func (p *T) foo(a, b int, z float) bool;
Initial values
......@@ -1232,19 +1240,19 @@ be set to 0 if no other value is specified.
These two simple declarations are equivalent:
var i int;
var i int = 0;
var i int;
var i int = 0;
After
type T struct { i int; f float; next *T };
t := new(T);
type T struct { i int; f float; next *T };
t := new(T);
the following holds:
t.i == 0
t.f == 0.0
t.next == nil
t.i == 0
t.f == 0.0
t.next == nil
[OLD
......@@ -1267,11 +1275,11 @@ source than the export directive itself, but it is an error to specify
an identifier not declared anywhere in the source file containing the
export directive.
ExportDecl = "export" ExportIdentifier { "," ExportIdentifier } .
ExportIdentifier = QualifiedIdent .
ExportDecl = "export" ExportIdentifier { "," ExportIdentifier } .
ExportIdentifier = QualifiedIdent .
export sin, cos
export math.abs
export sin, cos
export math.abs
TODO: complete this section
......@@ -1284,53 +1292,53 @@ Expressions
Expression syntax is based on that of C but with fewer precedence levels.
Expression = BinaryExpr | UnaryExpr | PrimaryExpr .
BinaryExpr = Expression binary_op Expression .
UnaryExpr = unary_op Expression .
PrimaryExpr =
identifier | Literal | "(" Expression ")" | "iota" |
Call | Conversion | Allocation | Index |
Expression "." identifier | Expression "." "(" Type ")" .
Call = Expression "(" [ ExpressionList ] ")" .
Conversion = "convert" "(" Type [ "," ExpressionList ] ")" |
ConversionType "(" [ ExpressionList ] ")" .
ConversionType = TypeName | ArrayType | MapType | StructType | InterfaceType .
Allocation = "new" "(" Type [ "," ExpressionList ] ")" .
Index = SimpleIndex | Slice .
SimpleIndex = Expression "[" Expression"]" .
Slice = Expression "[" Expression ":" Expression "]" .
binary_op = log_op | comm_op | rel_op | add_op | mul_op .
log_op = "||" | "&&" .
comm_op = "<-" | "-<" .
rel_op = "==" | "!=" | "<" | "<=" | ">" | ">=" .
add_op = "+" | "-" | "|" | "^" .
mul_op = "*" | "/" | "%" | "<<" | ">>" | "&" .
unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .
Expression = BinaryExpr | UnaryExpr | PrimaryExpr .
BinaryExpr = Expression binary_op Expression .
UnaryExpr = unary_op Expression .
PrimaryExpr =
identifier | Literal | "(" Expression ")" | "iota" |
Call | Conversion | Allocation | Index |
Expression "." identifier | Expression "." "(" Type ")" .
Call = Expression "(" [ ExpressionList ] ")" .
Conversion =
"convert" "(" Type [ "," ExpressionList ] ")" | ConversionType "(" [ ExpressionList ] ")" .
ConversionType = TypeName | ArrayType | MapType | StructType | InterfaceType .
Allocation = "new" "(" Type [ "," ExpressionList ] ")" .
Index = SimpleIndex | Slice .
SimpleIndex = Expression "[" Expression"]" .
Slice = Expression "[" Expression ":" Expression "]" .
binary_op = log_op | comm_op | rel_op | add_op | mul_op .
log_op = "||" | "&&" .
comm_op = "<-" | "-<" .
rel_op = "==" | "!=" | "<" | "<=" | ">" | ">=" .
add_op = "+" | "-" | "|" | "^" .
mul_op = "*" | "/" | "%" | "<<" | ">>" | "&" .
unary_op = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .
Field selection and type assertions ('.') bind tightest, followed by indexing ('[]')
and then calls and conversions. The remaining precedence levels are as follows
(in increasing precedence order):
Precedence Operator
1 ||
2 &&
3 <- -<
4 == != < <= > >=
5 + - | ^
6 * / % << >> &
7 + - ! ^ * <- (unary) & (unary)
Precedence Operator
1 ||
2 &&
3 <- -<
4 == != < <= > >=
5 + - | ^
6 * / % << >> &
7 + - ! ^ * <- (unary) & (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 the shift operators
"<<" and ">>", both operands of a binary operator must have the same type.
......@@ -1347,59 +1355,59 @@ Unary "^" corresponds to C "~" (bitwise complement). There is no "~" operator
in Go.
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.
The reserved word "iota" is discussed in a later section.
Examples of primary expressions
x
2
(s + ".txt")
f(3.1415, true)
Point(1, 2)
new([]int, 100)
m["foo"]
s[i : j + 1]
obj.color
Math.sin
f.p[i].x()
x
2
(s + ".txt")
f(3.1415, true)
Point(1, 2)
new([]int, 100)
m["foo"]
s[i : j + 1]
obj.color
Math.sin
f.p[i].x()
Examples of general expressions
+x
23 + 3*x[i]
x <= f()
^a >> b
f() || g()
x == y + 1 && <-chan_ptr > 0
+x
23 + 3*x[i]
x <= f()
^a >> b
f() || g()
x == y + 1 && <-chan_ptr > 0
The nil value
----
The reserved word
nil
nil
represents the ``zero'' value for a pointer type or interface type.
The only operations allowed for nil are to assign it to a pointer or
interface variable and to compare it for equality or inequality with a
pointer or interface value.
var p *int;
if p != nil {
print(p)
} else {
print("p points nowhere")
}
var p *int;
if p != nil {
print(p)
} else {
print("p points nowhere")
}
By default, pointers are initialized to nil.
......@@ -1426,8 +1434,8 @@ initial values.
For instance,
type S struct { a int; b float }
new(S)
type S struct { a int; b float }
new(S)
allocates storage for an S, initializes it (a=0, b=0.0), and returns a
value of type *S pointing to that storage.
......@@ -1435,10 +1443,10 @@ value of type *S pointing to that storage.
The only defined parameters affect sizes for allocating arrays,
buffered channels, and maps.
ap := new([]int, 10); # a pointer to an array of 10 ints
aap := new([][]int, 5, 10); # a pointer to an array of 5 arrays of 10 ints
c := new(chan int, 10); # a pointer to a channel with a buffer size of 10
m := new(map[string] int, 100); # a pointer to a map with space for 100 elements preallocated
ap := new([]int, 10); # a pointer to an array of 10 ints
aap := new([][]int, 5, 10); # a pointer to an array of 5 arrays of 10 ints
c := new(chan int, 10); # a pointer to a channel with a buffer size of 10
m := new(map[string] int, 100); # a pointer to a map with space for 100 elements preallocated
TODO: argument order for dimensions in multidimensional arrays
......@@ -1571,13 +1579,13 @@ 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
a := []int(1,2,3,4)
slice := a[1:3]
a := []int(1,2,3,4)
slice := a[1:3]
The array ``slice'' has length two and elements
slice[0] == 2
slice[1] == 3
slice[0] == 2
slice[1] == 3
The index values in the slice must be in bounds for the original
array (or string) and the slice length must be non-negative.
......@@ -1586,15 +1594,15 @@ Slices are new arrays (or strings) storing copies of the elements, so
changes to the elements of the slice do not affect the original.
In the example, a subsequent assignment to element 0,
slice[0] = 5
slice[0] = 5
would have no effect on ``a''.
Strings and arrays can also be concatenated using the ``+'' (or ``+='')
operator.
a += []int(5, 6, 7)
s := "hi" + string(c)
a += []int(5, 6, 7)
s := "hi" + string(c)
Like slices, addition creates a new array or string by copying the
elements.
......@@ -1609,35 +1617,35 @@ introduces a new declaration and increments as each identifier
is declared. For instance, 'iota' can be used to construct
a set of related constants:
const (
enum0 = iota; // sets enum0 to 0, etc.
enum1 = iota;
enum2 = iota
)
const (
a = 1 << iota; // sets a to 1 (iota has been reset)
b = 1 << iota; // sets b to 2
c = 1 << iota; // sets c to 4
)
const x = iota; // sets x to 0
const y = iota; // sets y to 0
const (
enum0 = iota; // sets enum0 to 0, etc.
enum1 = iota;
enum2 = iota
)
const (
a = 1 << iota; // sets a to 1 (iota has been reset)
b = 1 << iota; // sets b to 2
c = 1 << iota; // sets c to 4
)
const x = iota; // sets x to 0
const y = iota; // sets y to 0
Since the expression in constant declarations repeats implicitly
if omitted, the first two examples above can be abbreviated:
const (
enum0 = iota; // sets enum0 to 0, etc.
enum1;
enum2
)
const (
enum0 = iota; // sets enum0 to 0, etc.
enum1;
enum2
)
const (
a = 1 << iota; // sets a to 1 (iota has been reset)
b; // sets b to 2
c; // sets c to 4
)
const (
a = 1 << iota; // sets a to 1 (iota has been reset)
b; // sets b to 2
c; // sets c to 4
)
TODO: should iota work in var, type, func decls too?
......@@ -1648,15 +1656,15 @@ Statements
Statements control execution.
Statement =
Declaration |
SimpleStat | GoStat | ReturnStat | BreakStat | ContinueStat | GotoStat |
Block | IfStat | SwitchStat | SelectStat | ForStat | RangeStat |
SimpleStat =
ExpressionStat | IncDecStat | Assignment | SimpleVarDecl .
Statement =
Declaration |
SimpleStat | GoStat | ReturnStat | BreakStat | ContinueStat | GotoStat |
Block | IfStat | SwitchStat | SelectStat | ForStat | RangeStat |
SimpleStat =
ExpressionStat | IncDecStat | Assignment | SimpleVarDecl .
Statement lists
----
......@@ -1664,7 +1672,7 @@ Semicolons are used to separate individual statements of a statement list.
They are optional immediately before or after a closing curly brace "}",
immediately after "++" or "--", and immediately before a reserved word.
StatementList = Statement { [ ";" ] Statement } .
StatementList = Statement { [ ";" ] Statement } .
TODO: This still seems to be more complicated then necessary.
......@@ -1673,17 +1681,17 @@ TODO: This still seems to be more complicated then necessary.
Expression statements
----
ExpressionStat = Expression .
ExpressionStat = Expression .
f(x+y)
f(x+y)
IncDec statements
----
IncDecStat = Expression ( "++" | "--" ) .
IncDecStat = Expression ( "++" | "--" ) .
a[i]++
a[i]++
Note that ++ and -- are not operators for expressions.
......@@ -1691,54 +1699,54 @@ Note that ++ and -- are not operators for expressions.
Assignments
----
Assignment = SingleAssignment | TupleAssignment .
SingleAssignment = PrimaryExpr assign_op Expression .
TupleAssignment = PrimaryExprList assign_op ExpressionList .
PrimaryExprList = PrimaryExpr { "," PrimaryExpr } .
assign_op = [ add_op | mul_op ] "=" .
Assignment = SingleAssignment | TupleAssignment .
SingleAssignment = PrimaryExpr assign_op Expression .
TupleAssignment = PrimaryExprList assign_op ExpressionList .
PrimaryExprList = PrimaryExpr { "," PrimaryExpr } .
assign_op = [ add_op | mul_op ] "=" .
The left-hand side must be an l-value such as a variable, pointer indirection,
or an array index.
x = 1
*p = f()
a[i] = 23
k = <-ch
x = 1
*p = f()
a[i] = 23
k = <-ch
As in C, arithmetic binary operators can be combined with assignments:
j <<= 2
j <<= 2
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
v1, v2, v3 = e1, e2, e3
v1, v2, v3 = e1, e2, e3
assigns the expressions e1, e2, e3 to temporaries and then assigns the temporaries
to the variables v1, v2, v3. Thus
a, b = b, a
a, b = b, a
exchanges the values of a and b. The tuple assignment
x, y = f()
x, y = f()
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.
value, present = map_var[key]
value, present = map_var[key]
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:
map_var[key] = value, false
map_var[key] = value, false
In assignments, the type of the expression must match the type of the left-hand side.
Communication
......@@ -1751,18 +1759,18 @@ Here the term "channel" means "variable of type *chan".
A channel is created by allocating it:
ch := new(chan int)
ch := new(chan int)
An optional argument to new() specifies a buffer size for an
asynchronous channel; if absent or zero, the channel is synchronous:
sync_chan := new(chan int)
buffered_chan := new(chan int, 10)
sync_chan := new(chan int)
buffered_chan := new(chan int, 10)
The send operator is the binary operator "-<", which operates on
a channel and a value (expression):
ch -< 3
ch -< 3
In this form, the send operation is an (expression) statement that
blocks until the send can proceed, at which point the value is
......@@ -1773,10 +1781,10 @@ 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. These two examples are equivalent:
ok := ch -< 3;
if ok { print("sent") } else { print("not sent") }
ok := ch -< 3;
if ok { print("sent") } else { print("not sent") }
if ch -< 3 { print("sent") } else { print("not sent") }
if ch -< 3 { print("sent") } else { print("not sent") }
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
......@@ -1786,35 +1794,35 @@ the operation blocks until it succeeds.
The receive uses the binary operator "<-", analogous to send but
with the channel on the right:
v1 <- ch
v1 <- ch
As with send operations, in expression context this form may
be used as a boolean and makes the receive non-blocking:
ok := e <- ch;
if ok { print("received", e) } else { print("did not receive") }
ok := e <- ch;
if ok { print("received", e) } else { print("did not receive") }
The receive operator may also be used as a prefix unary operator
on a channel.
<- ch
<- ch
The expression blocks until a value is available, which then can
be assigned to a variable or used like any other expression:
v1 := <-ch
v2 = <-ch
f(<-ch)
v1 := <-ch
v2 = <-ch
f(<-ch)
If the receive expression does not save the value, the value is
discarded:
<- strobe // wait until clock pulse
<- strobe // wait until clock pulse
Finally, as a special case unique to receive, the forms
e, ok := <-ch
e, ok = <-ch
e, ok := <-ch
e, ok = <-ch
allow the operation to declare and/or assign the received value and
the boolean indicating success. These two forms are always
......@@ -1828,11 +1836,11 @@ concurrent thread of control within the same address space. Unlike
with a function, the next line of the program does not wait for the
function to complete.
GoStat = "go" Call .
GoStat = "go" Call .
go Server()
go func(ch chan-< bool) { for { sleep(10); ch -< true; }} (c)
go Server()
go func(ch chan-< bool) { for { sleep(10); ch -< true; }} (c)
Return statements
......@@ -1841,35 +1849,35 @@ Return statements
A return statement terminates execution of the containing function
and optionally provides a result value or values to the caller.
ReturnStat = "return" [ ExpressionList ] .
ReturnStat = "return" [ ExpressionList ] .
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:
func simple_f() int {
return 2;
}
func simple_f() int {
return 2;
}
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.
func complex_f1() (re float, im float) {
return -7.0, -4.0;
}
func complex_f1() (re float, im float) {
return -7.0, -4.0;
}
The 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:
func complex_f2() (re float, im float) {
re = 7.0;
im = 4.0;
return;
}
func complex_f2() (re float, im float) {
re = 7.0;
im = 4.0;
return;
}
If statements
----
......@@ -1879,31 +1887,31 @@ condition need not be parenthesized and the "then" statement
must be in brace brackets. The condition may be omitted, in which
case it is assumed to have the value "true".
IfStat = "if" [ [ Simplestat ] ";" ] [ Condition ] Block [ "else" Statement ] .
IfStat = "if" [ [ Simplestat ] ";" ] [ Condition ] Block [ "else" Statement ] .
if x > 0 {
return true;
}
if x > 0 {
return true;
}
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.
if x := f(); x < y {
return x;
} else if x > z {
return z;
} else {
return y;
}
if x := f(); x < y {
return x;
} else if x > z {
return z;
} else {
return y;
}
TODO: We should fix this and move to:
IfStat =
"if" [ [ Simplestat ] ";" ] [ Condition ] Block
{ "else" "if" Condition Block }
[ "else" Block ] .
IfStat =
"if" [ [ Simplestat ] ";" ] [ Condition ] Block
{ "else" "if" Condition Block }
[ "else" Block ] .
Switch statements
......@@ -1911,9 +1919,9 @@ Switch statements
Switches provide multi-way execution.
SwitchStat = "switch" [ [ Simplestat ] ";" ] [ Expression ] "{" { CaseClause } "}" .
CaseClause = Case [ StatementList [ ";" ] ] [ "fallthrough" [ ";" ] ] .
Case = ( "case" ExpressionList | "default" ) ":" .
SwitchStat = "switch" [ [ Simplestat ] ";" ] [ Expression ] "{" { CaseClause } "}" .
CaseClause = Case [ StatementList [ ";" ] ] [ "fallthrough" [ ";" ] ] .
Case = ( "case" ExpressionList | "default" ) ":" .
There can be at most one default case in a switch statement.
......@@ -1925,38 +1933,38 @@ be evaluated top to bottom until the first successful non-default case is reache
If none matches and there is a default case, the statements of the default
case are executed.
switch tag {
default: s3()
case 0, 1: s1()
case 2: s2()
}
switch tag {
default: s3()
case 0, 1: s1()
case 2: s2()
}
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.
switch x := f(); true {
case x < 0: return -x
default: return x
}
switch x := f(); true {
case x < 0: return -x
default: return x
}
Cases do not fall through unless explicitly marked with a "fallthrough" statement.
switch a {
case 1:
b();
fallthrough
case 2:
c();
}
switch a {
case 1:
b();
fallthrough
case 2:
c();
}
If the expression is omitted, it is equivalent to "true".
switch {
case x < y: f1();
case x < z: f2();
case x == 4: f3();
}
switch {
case x < y: f1();
case x < z: f2();
case x == 4: f3();
}
Select statements
......@@ -1966,13 +1974,13 @@ 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.
SelectStat = "select" "{" { CommClause } "}" .
CommClause = CommCase [ StatementList [ ";" ] ] .
CommCase = ( "default" | ( "case" ( SendCase | RecvCase) ) ) ":" .
SendCase = SendExpr .
RecvCase = RecvExpr .
SendExpr = Expression "-<" Expression .
RecvExpr = [ identifier ] "<-" Expression .
SelectStat = "select" "{" { CommClause } "}" .
CommClause = CommCase [ StatementList [ ";" ] ] .
CommCase = ( "default" | ( "case" ( SendCase | RecvCase) ) ) ":" .
SendCase = SendExpr .
RecvCase = RecvExpr .
SendExpr = Expression "-<" Expression .
RecvExpr = [ identifier ] "<-" Expression .
The select statement evaluates all the channel (pointers) involved.
If any of the channels can proceed, the corresponding communication
......@@ -1988,32 +1996,32 @@ clause matches that of the dynamic value to be exchanged.
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 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);
}
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 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
......@@ -2024,33 +2032,33 @@ For statements
For statements are a combination of the "for" and "while" loops of C.
ForStat = "for" [ Condition | ForClause ] Block .
ForClause = [ InitStat ] ";" [ Condition ] ";" [ PostStat ] .
InitStat = SimpleStat .
Condition = Expression .
PostStat = SimpleStat .
ForStat = "for" [ Condition | ForClause ] Block .
ForClause = [ InitStat ] ";" [ Condition ] ";" [ PostStat ] .
InitStat = SimpleStat .
Condition = Expression .
PostStat = SimpleStat .
A SimpleStat is a simple statement such as an assignment, a SimpleVarDecl,
or an increment or decrement statement. Therefore one may declare a loop
variable in the init statement.
for i := 0; i < 10; i++ {
printf("%d\n", i)
}
for i := 0; i < 10; i++ {
printf("%d\n", i)
}
A for statement with just a condition executes until the condition becomes
false. Thus it is the same as C's while statement.
for a < b {
a *= 2
}
for a < b {
a *= 2
}
If the condition is absent, it is equivalent to "true".
for {
f()
}
for {
f()
}
Range statements
......@@ -2059,8 +2067,8 @@ Range statements
Range statements are a special control structure for iterating over
the contents of arrays and maps.
RangeStat = "range" IdentifierList ":=" RangeExpression Block .
RangeExpression = Expression .
RangeStat = "range" IdentifierList ":=" RangeExpression Block .
RangeExpression = Expression .
A range expression must evaluate to an array, map or string. The identifier list must contain
either one or two identifiers. If the range expression is a map, a single identifier is declared
......@@ -2068,20 +2076,20 @@ to range over the keys of the map; two identifiers range over the keys and corre
values. For arrays and strings, the behavior is analogous for integer indices (the keys) and
array elements (the values).
a := []int(1, 2, 3);
m := [string]map int("fo",2, "foo",3, "fooo",4)
a := []int(1, 2, 3);
m := [string]map int("fo",2, "foo",3, "fooo",4)
range i := a {
f(a[i]);
}
range i := a {
f(a[i]);
}
range v, i := a {
f(v);
}
range v, i := a {
f(v);
}
range k, v := m {
assert(len(k) == v);
}
range k, v := m {
assert(len(k) == v);
}
TODO: is this right?
......@@ -2092,17 +2100,17 @@ Break statements
Within a for or switch statement, a break statement terminates execution of
the innermost for or switch statement.
BreakStat = "break" [ identifier ].
BreakStat = "break" [ identifier ].
If there is an identifier, it must be the label name of an enclosing
for or switch
statement, and that is the one whose execution terminates.
L: for i < n {
switch i {
case 5: break L
}
}
L: for i < n {
switch i {
case 5: break L
}
}
Continue statements
......@@ -2111,7 +2119,7 @@ Continue statements
Within a for loop a continue statement begins the next iteration of the
loop at the post statement.
ContinueStat = "continue" [ identifier ].
ContinueStat = "continue" [ identifier ].
The optional identifier is analogous to that of a break statement.
......@@ -2121,9 +2129,9 @@ Label declaration
A label declaration serves as the target of a goto, break or continue statement.
LabelDecl = identifier ":" .
LabelDecl = identifier ":" .
Error:
Error:
Goto statements
......@@ -2131,17 +2139,17 @@ Goto statements
A goto statement transfers control to the corresponding label statement.
GotoStat = "goto" identifier .
GotoStat = "goto" identifier .
goto Error
goto Error
Executing the goto statement must not cause any variables to come into
scope that were not already in scope at the point of the goto. For
instance, this example:
goto L; // BAD
v := 3;
L:
goto L; // BAD
v := 3;
L:
is erroneous because the jump to label L skips the creation of v.
......@@ -2151,9 +2159,9 @@ Packages
Every source file identifies the package to which it belongs.
The file must begin with a package clause.
PackageClause = "package" PackageName .
PackageClause = "package" PackageName .
package Math
package Math
Import declarations
......@@ -2162,9 +2170,9 @@ Import declarations
A program can gain access to exported items from another package
through an import declaration:
ImportDecl = "import" ( ImportSpec | "(" ImportSpecList [ ";" ] ")" ) .
ImportSpec = [ "." | PackageName ] PackageFileName .
ImportSpecList = ImportSpec { ";" ImportSpec } .
ImportDecl = "import" ( ImportSpec | "(" ImportSpecList [ ";" ] ")" ) .
ImportSpec = [ "." | PackageName ] PackageFileName .
ImportSpecList = ImportSpec { ";" ImportSpec } .
An import statement makes the exported contents of the named
package file accessible in this package.
......@@ -2178,7 +2186,7 @@ statement declares that package name as an identifier whose
contents are the exported elements of the imported package.
For instance, after
import M "/lib/math"
import M "/lib/math"
the contents of the package /lib/math can be accessed by
M.cos, M.sin, etc.
......@@ -2187,7 +2195,7 @@ In its simplest form, with no package name, the import statement
implicitly uses the imported package name itself as the local
package name. After
import "/lib/math"
import "/lib/math"
the contents are accessible by Math.sin, Math.cos.
......@@ -2195,7 +2203,7 @@ 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
import . "/lib/math"
import . "/lib/math"
the contents are accessible by sin and cos. In this instance, it is
an error if the import introduces name conflicts.
......@@ -2207,7 +2215,7 @@ Program
A program is a package clause, optionally followed by import declarations,
followed by a series of declarations.
Program = PackageClause { ImportDecl [ ";" ] } { Declaration [ ";" ] } .
Program = PackageClause { ImportDecl [ ";" ] } { Declaration [ ";" ] } .
Initialization and program execution
......@@ -2228,7 +2236,7 @@ be no cyclic dependencies in initialization.
A complete program, possibly created by linking multiple packages,
must have one package called main, with a function
func main() { ... }
func main() { ... }
defined. The function main.main() takes no arguments and returns no
value.
......
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