Objects

Objects
================================================================================
(define-struct object (call proto class table))

call :  (evector -> value)

    For callable objects (i.e., functions and constructors), the
    representation of the function, which takes a vector of arguments
    and returns the result of evaluating the JavaScript function.

proto : (optional object)

    The internal prototype link, or #f if the object has no prototype.

class : string

    The string representation of the ``class'' of an object.

table : (hash-table-of string property)

    The table of properties, indexed by strings.


Properties
================================================================================
property-value ::= value | ref
property       ::= property-value | (cons property-value attributes)
attributes     ::= (bit-field-of READ-ONLY? DONT-ENUM? DONT-DELETE?)

property-value : (union value ref)

    The current value of the property, or a getter/setter pair for the value.

attributes : bit-field

    The attributes of the property.

READ-ONLY? : bit-flag

    Are attempts to update the value of the property ignored?

DONT-ENUM? : bit-flag

    Should the property be skipped in for-in enumerations?

DONT-DELETE? : bit-flag

    Are attempts to delete the property ignored?


Arrays
================================================================================
(define-struct (array object) (vector))

vector : (evector-of (optional value))

    The internal vector of integer-indexed values. For sparse arrays,
    undefined elements are represented as #f. This allows for the
    distinction between undefined elements and defined elements with
    the `undefined' value.


Internal Properties and Methods
================================================================================

The following describes the runtime representation of the internal properties
and methods as described by section 8.6.2 in the EcmaScript spec.


Field              Representation
--------------------------------------------------
[[Prototype]]      js:object field
[[Class]]          meta-function
[[Value]]          meta-function
[[Get]]            meta-function*
[[Put]]            meta-function*
[[CanPut]]         meta-function
[[HasProperty]]    meta-function
[[Delete]]         meta-function
[[DefaultValue]]   meta-function
[[Construct]]      js:object procedure field
[[Call]]           js:object procedure field
[[HasInstance]]    meta-function
[[Scope]]          environment (static, dynamic)
[[Match]]          meta-function
--------------------------------------------------

* will eventually be overrideable, so will
  eventually need to be a js:object field


Justification
================================================================================

[[Class]]

    From 8.6.2:

    "Note that this specification does not provide any means for
    a program to access that value except through
    Object.prototype.toString"

[[Value]]

    only implemented for a fixed set of primitive objects, so a
    case dispatch is sufficient

[[Get]], [[Put]]

    overrideable in the future, but for now fixed

[[CanPut]]

    just needs to check attributes, not overrideable

[[HasProperty]], [[Delete]], [[DefaultValue]]

    never overridden

[[Construct]]

    Array works differently depending on whether it's called as
    a function or a constructor

[[HasInstance]]

    only defined on Function objects
    only used to implement instanceof

[[Scope]]

    only used as internal spec mechanism
    when lexically scoped, just use Scheme closures
    when dynamically scoped, use dynamic environments and meta-functions

[[Match]]

    only used for regexps, never overridden