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- explained function and method pointers

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- removed need for method types and literals
(gri & r)

R=r
DELTA=178  (101 added, 51 deleted, 26 changed)
OCL=14402
CL=14405
This commit is contained in:
Robert Griesemer 2008-08-21 16:55:38 -07:00
parent c2c49bec77
commit 2dabb6525a
1 changed files with 121 additions and 71 deletions
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190
doc/go_lang.txt
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@ -4,7 +4,7 @@ The Go Programming Language (DRAFT)
Robert Griesemer, Rob Pike, Ken Thompson
----
(August 20, 2008)
(August 21, 2008)
This document is a semi-formal specification/proposal for a new
systems programming language. The document is under active
@ -791,16 +791,10 @@ Function types
A function type denotes the set of all functions with the same signature.
A method is a function with a receiver declaration.
[OLD
, which is of type pointer to struct.
END]
Functions can return multiple values simultaneously.
FunctionType = "func" AnonymousSignature .
AnonymousSignature = [ Receiver "." ] Parameters [ Result ] .
Receiver = "(" identifier Type ")" .
FunctionType = "func" Signature .
Signature = Parameters [ Result ] .
Parameters = "(" [ ParameterList ] ")" .
ParameterList = ParameterSection { "," ParameterSection } .
ParameterSection = IdentifierList Type .
@ -812,12 +806,6 @@ Functions can return multiple values simultaneously.
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
function referenced by v, one writes v(). It is illegal to dereference a
@ -843,45 +831,13 @@ variables, and variables declared within the function literal.
// 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; }
Unresolved issues: Are there method literals? How do you use them?
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 }
the declaration
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.
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()
For instance, given a *Point variable pt, one may call
pt.distance(3.5)
Interface of a type
----
The interface of a type is defined to be the unordered set of methods
associated with that type.
associated with that type. Methods are defined in a later section;
they are functions bound to a type.
Interface types
@ -891,7 +847,7 @@ An interface type denotes a set of methods.
InterfaceType = "interface" "{" [ MethodDeclList [ ";" ] ] "}" .
MethodDeclList = MethodDecl { ";" MethodDecl } .
MethodDecl = identifier Parameters [ Result ] .
MethodDecl = identifier Signature .
// A basic file interface.
type File interface {
@ -1021,7 +977,7 @@ and "true", are predeclared. A declaration associates an identifier
with a language entity (package, constant, type, variable, function, method,
or label) and may specify properties of that entity such as its type.
Declaration = [ "export" ] ( ConstDecl | TypeDecl | VarDecl | FunctionDecl ) .
Declaration = [ "export" ] ( ConstDecl | TypeDecl | VarDecl | FunctionDecl | MethodDecl ) .
The ``scope'' of a language entity named 'x' extends textually from the point
immediately after the identifier 'x' in the declaration to the end of the
@ -1219,18 +1175,12 @@ Also, in some contexts such as "if", "for", or "switch" statements,
this construct can be used to declare local temporary variables.
Function and method declarations
Function declarations
----
Functions and methods have a special declaration syntax, slightly
different from the type syntax because an identifier must be present
in the signature.
A function declaration declares an identifier of type function.
Implementation restriction: Functions and methods can only be declared
at the global level.
FunctionDecl = "func" NamedSignature ( ";" | Block ) .
NamedSignature = [ Receiver ] identifier Parameters [ Result ] .
FunctionDecl = "func" identifier Signature ( ";" | Block ) .
func min(x int, y int) int {
if x < y {
@ -1239,16 +1189,24 @@ at the global level.
return y;
}
func foo(a, b int, z float) bool {
return a*b < int(z);
}
A function declaration without a body serves as a forward declaration:
func MakeNode(left, right *Node) *Node;
A method is a function that also declares a receiver.
Implementation restriction: Functions can only be declared at the global level.
func (p *T) foo(a, b int, z float) bool {
return a*b < int(z) + p.x;
}
Method declarations
----
A method declaration declares a function with a receiver.
MethodDecl = "func" Receiver identifier Signature ( ";" | Block ) .
Receiver = "(" identifier Type ")" .
A method is bound to the type of its receiver.
For instance, given type Point, the declarations
func (p *Point) Length() float {
return Math.sqrt(p.x * p.x + p.y * p.y);
@ -1259,10 +1217,31 @@ A method is a function that also declares a receiver.
p.y = p.y * factor;
}
Functions and methods can be forward declared by omitting the body:
create methods for type *Point. Note that methods may appear anywhere
after the declaration of the receiver type and may be forward-declared.
func foo(a, b int, z float) bool;
func (p *T) foo(a, b int, z float) bool;
Method invocation
----
A method is invoked using the notation
receiver.method()
where receiver is a value of the receive type of the method.
For instance, given a *Point variable pt, one may call
pt.Scale(3.5)
The type of a method is the type of a function with the receiver as first
argument. For instance, the method "Scale" has type
func(p *Point, factor float)
However, a function declared this way is not a method.
There is no distinct method type and there are no method literals.
Initial values
@ -1418,6 +1397,7 @@ Examples of primary expressions
obj.color
Math.sin
f.p[i].x()
&point.distance
Examples of general expressions
@ -1462,6 +1442,76 @@ TODO: if interfaces were explicitly pointers, this gets simpler.
END]
Function and method pointers
----
Given a function f, declared as
func f(a int) int;
taking the address of f with the expression
&f
creates a pointer to the function that may be stored in a value of type pointer
to function:
var fp *func(a int) int = &f;
The function pointer may be invoked with the usual syntax; no explicit
indirection is required:
fp(7)
Methods are a form of function, and the address of a method has the type
pointer to function. Consider the type T with method M:
type T struct {
a int;
}
func (tp *T) M(a int) int;
var t *T;
To construct the address of method M, we write
&t.M
using the variable t (not the type T). The expression is a pointer to a
function, with type
*func(t *T, a int) int
and may be invoked only as a function, not a method:
var f *func(t *T, a int) int;
f = &t.M;
x := f(t, 7);
Note that one does not write t.f(7); taking the address 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
t.M(args)
is equivalent to the function call
(&t.M)(t, args)
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 interface M, the sequence
var t1 *T1;
var t2 *T2;
var i I = t1;
m := &i.M;
m(t2);
will invoke t2.M() even though m was constructed with an expression involving
t1.
Allocation
----