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ES4 draft: Function

Lars Hansen Mon, 10 Mar 2008 14:58:07 -0700

First draft of the spec for the Function class.  Please comment.

--lars

Title: The class "Function"

The class `Function`



NAME:                       "The class 'Function'"
FILE:                       spec/library/Function.html
CATEGORY:                   Pre-defined classes
SOURCES:                    REFERENCES [1], [2], [3], [5]
SPEC AUTHOR:                Lars
DRAFT STATUS:               DRAFT 1 - 2008-03-10
REVIEWED AGAINST ES3:       YES
REVIEWED AGAINST ERRATA:    YES
REVIEWED AGAINST BASE DOC:  YES
REVIEWED AGAINST PROPOSALS: YES
REVIEWED AGAINST CODE:      YES
REVIEWED AGAINST TICKETS:   YES
IMPLEMENTATION STATUS:      ES4RI
TEST CASE STATUS:           ?


OPEN ISSUES

  * ES3 restrictions on argArray having to be an Array or an arguments
    object in Function.prototype.apply have been removed here.


NOTES

  * The annotation '/*: function*/' below looks like that because the
    reference implementation does not yet accept plain 'function' as a
    type annotation to mean 'anything callable' (ticket #153).

  * The use of 'Private' instead of 'private' is a workaround for a
    bug in the reference implementation (#368).


REFERENCES

[1] ECMAScript 3rd Edition specification section 15.3
[2] http://wiki.ecmascript.org/doku.php?id=proposals:static_generics
[2] http://bugs.ecmascript.org/ticket/172
[4] http://bugs.ecmascript.org/ticket/173
[5] http://bugs.ecmascript.org/ticket/174




 The class Function is a dynamic, non-final, direct subclass of
Object (see class Object).

 All objects defined by function definitions or expressions in
ECMAScript are instances of the class Function.

 Not all objects that can be called as functions are instances of
subclasses of the Function class, however.  Any object that has a
meta::invoke method can be called as a function.

NOTE    Host functions may also not be instances of Function or its
subclasses, but must to some extent behave as if they are (see Host objects).  However, host functions need not provide
meta::invoke methods to be callable.  The type function,
defined elsewhere, matches invokable host functions even if they do
not have a meta::invoke method.


Synopsis

 The class Function provides the following interface:

dynamic class Function extends Object
{
    public function Function(...args) …
    static meta function invoke(...args) …

    static public function apply(fn /*: function*/, thisArg: Object=null, argArray: Object=null) …
    static public function bind(method /*: function*/, thisObj: Object=null, ...args) …
    static public function call(fn /*: function*/, thisObj: Object=null, ...args) …

    static public const length = 1

    meta final function invoke( … ) …

    override intrinsic function toString() : string …

    intrinsic function apply(thisArg: Object=null, argArray: Object=null) …
    intrinsic function bind(thisObj: Object=null, ...args) …
    intrinsic function call(thisObj: Object=null, ...args) …

    public const length = …
    public var   prototype = …
}


The Function prototype object provides these direct properties:

    meta::invoke: function () …
    length:       0
    toString:     function () …
    apply:        function(thisArg, argArray) …
    bind:         function(thisArg, ...args) …
    call:         function(thisArg, ...args) …



Methods on the Function class object

 new Function (p1, p2, … , pn, body) 

Description 
 When the Function constructor is called with some arguments
as part of a new _expression_, it creates a new Function
instance whose parameter list is given by the concatenation of the
p_i arguments separated by "," and whose executable code
is given by the body argument.

 There may be no p_i arguments, and body is optional too,
defaulting to the empty string.

 If the first character of the comma-separated concatenation of the
p_i is a left parenthesis then the list of parameters
must be parseable as a FormalParameterList_opt enclosed
in parentheses and optionally followed by a colon and a return type.

 Otherwise, the list of parameters must be parsable as a
FormalParameterList_opt.

 If the list of parameters is not parseable as outlined in the
previous two paragraphs, or if the body is not parsable as a
FunctionBody, then a SyntaxError exception is thrown (see the
grammar in section ECMAScript grammar).

 Regardless of the form of the parameter list, it may include type
annotations, default parameter values, and rest arguments.

Returns 
 The Function constructor returns a new Function instance.

Implementation
public function Function(...args)
    helper::createFunction(args);

helper function createFunction(args) {
    let parameters = "";
    let body = "";
    if (args.length > 0) {
        body = args[args.length-1];
        args.length = args.length-1;
        parameters = args.join(",");
    }
    body = string(body);
    magic::initializeFunction(this, __ES4__::global, parameters, body);
}


 The magic function initializeFunction initializes the function
object this from the list of parameters and the body, as specified
in section translation:FunctionExpression.  The global
object is passed in as the scope parameter.

A prototype object is automatically created for every function,
to provide for the possibility that the function will be used as a
constructor.

NOTE    It is permissible but not necessary to have one argument for
each formal parameter to be specified. For example, all three of the
following expressions produce the same result:

new Function("a", "b", "c", "return a+b+c")

new Function("a, b, c", "return a+b+c")

new Function("a,b", "c", "return a+b+c")



Function (p1, p2, … , pn, body)

Description 
 When the Function class object is called as a function it
creates and initialises a new Function object.  Thus the function
call Function(…) is equivalent to the object creation
_expression_ new Function(…) with the same arguments.

Returns 
 The Function class object called as a function returns a
new Function instance.

Implementation

meta static function invoke(...args)
    new Function(...args);


FIXME    Ticket #357: That particular definition makes use of the
prefix "spread" operator, which has not yet been formally accepted
into the language.

apply ( fn, thisArg=…, argArray=… )

Description 
 The static apply method takes arguments fn,
thisArg, and argArray and invokes fn in the standard
manner, passing thisArg as the value for this and the members
of argArray as the individual argument values.

Returns 
 The apply method returns the value returned by fn.

Implementation
static public function apply(fn /*: function*/, thisArg: Object=null, argArray: Object=null) {
    if (thisArg === null)
        thisArg = global;
    if (argArray === null)
        argArray = [];
    return magic::apply(fn, thisArg, argArray);
}


NOTE    The magic apply function performs the actual invocation (see
magic::apply).  This code will eventually change to
use the prefix "spread" operator.



bind ( fn, thisArg=…, ...args )

Description 
 The static bind method takes arguments fn, thisArg,
and optionally some args.

Returns 
 The bind method returns a Function object that accepts
some arguments moreargs and which calls fn with thisArg as
the this object and the values of args and moreargs as
actual arguments.

Implementation
static public function bind(method /*: function*/, thisObj: Object=null, ...args)
    helper::bind(method, thisObj, args);

static helper function bind(method, thisObj, args)
    function (...moreargs)
        method.apply(thisObj, args.concat(moreargs));



call ( fn, thisArg=…, ...args )

Description 
 The static call method takes arguments fn and
thisArg and optionally some args and invokes fn in the
standard manner, passing thisArg as the value for this and the
members of args as the individual argument values.

Returns 
 The call method returns the value returned by fn.

Implementation
static public function call(fn /*: function*/, thisObj: Object=null, ...args)
    Function.apply(fn, thisObj, args);




Methods on Function instances

meta::invoke ( … )

Description 
 The meta method invoke is specialized to the individual
Function object.  When called, it evaluates the executable code
for the function.

 The meta method invoke is typically called by the ECMAScript
implementation as part of the function invocation and object
construction protocols.  When a function or method is invoked, the
invoke method of the function or method object provides the code
to run.  When a function is used to construct a new object, the
invoke method provides the code for the constructor function.

 The signature of the meta method invoke is determined when the
Function instance is created, and is determined by the text that
defines the function being created.

NOTE    The meta method invoke is final; therefore subclasses
can add properties and methods but can't override the function calling
behavior.

FIXME    (Ticket #173.)  While it is necessary that the invoke
method is completely magic in Function instances, it's not clear
it needs to be magic for instances of subclasses of Function,
because these can be treated like other objects that have invoke
methods (and which already work just fine in the reference
implementation).  Therefore it should not be final.

Returns 
 The meta method invoke returns the value produces by the
first return statement that is evaluated during the evaluation of
the executable code for the function represented by this Function
object.

Implementation 

The implementation of the meta function invoke is
implementation-dependent.

intrinsic::toString ( )

Description 
 The intrinsic toString method converts the executable code
of the function to a string representation.  This representation has
the syntax of a FunctionDeclaration or FunctionExpression.
Note in particular that the use and placement of white space, line
terminators, and semicolons within the representation string is
implementation-dependent.

COMPATIBILITY NOTE   ES3 required the syntax to be that of a FunctionDeclaration
only, but that made it impossible to produce a string representation
for functions created from unnamed function expressions.

Returns 
 The intrinsic toString method returns a string.

Implementation
override intrinsic function toString() : string
    Private::toString();


 The private function toString is implementation-dependent.


intrinsic::apply ( thisObj=…, args=… )

Description 
 The intrinsic apply method calls the static apply method
with the value of this as the first argument.

Returns 
 The intrinsic apply method returns the result of the static
apply method.

Implementation
intrinsic function apply(thisArg: Object=null, argArray: Object=null)
    Function.apply(this, thisArg, argArray);



intrinsic::bind ( thisObj=…, ...args)

Description 
 The intrinsic bind method calls the static bind method
with the value of this as the first argument.

Returns 
 The intrinsic bind method returns the result of the static
bind method.

Implementation
intrinsic function bind(thisObj: Object=null, ...args)
    Function.helper::bind(this, thisObj, args);



intrinsic::call ( thisObj=…, ...args)

Description 
The intrinsic call method calls the static apply method
with the value of this as the first argument.

Returns 
 The intrinsic call method returns the result of the static
call method.

Implementation
intrinsic function call(thisObj: Object=null, ...args)
    Function.apply(this, thisObj, args);



Value properties of Function instances

length

 The value of the constant length property is the number of
non-rest arguments accepted by the function.

 The value of the length property is an integer that indicates
the "typical" number of arguments expected by the function.  However,
the language permits the function to be invoked with some other number
of arguments.  The behaviour of a function when invoked on a number of
arguments other than the number specified by its length property
depends on the function.


prototype

 The initial value of the prototype property is a fresh
Object instance.

 The value of the prototype property is used to initialise the
internal [[Prototype]] property of a newly created object before the
Function instance is invoked as a constructor for that newly
created object.


Invoking the Function prototype object

 When the Function prototype object is invoked it accepts any
arguments and returns undefined:

prototype meta function invoke(...args)
    undefined;



Methods on the Function prototype object

 The methods on the Function prototype object perform simple
type adjustments and then perform the same actions as their intrinsic
counterparts:

prototype function toString(this:Function)
    this.Private::toString();

prototype function apply(/*this: function, */ thisArg=undefined, argArray=undefined)
    Function.apply(this,
                   thisArg === undefined ? null : thisArg,
                   argArray === undefined ? null : argArray);

prototype function bind(/*this: function, */ thisObj=undefined, ...args)
    Function.helper::bind(this,
                          thisObj === undefined ? null : thisObj,
                          args);

prototype function call(/*this: function, */ thisObj=undefined, ...args)
    Function.apply(this,
                   thisObj === undefined ? null : thisObj,
                   args);



Value properties on the Function prototype object

length 

 The initial value of the length prototype property is 0.

Implementation 

prototype var length : uint = 0;


COMPATIBILITY NOTE    The "length" property of the prototype is not obviously
required by the 3rd Edition of this Standard, but MSIE, Firefox,
Opera, and Safari all provide it.







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