Provides a defer/when style promise API for JavaScript

- usable as a CommonJS module, in Node,
- usable as a <script> in all web browsers,
- inspired by Tyler Close's Waterken ref_send promises, and
- compliant with http://wiki.commonjs.org/wiki/Promises/B


For Node:

    $ curl http://npmjs.org/install.sh | sh
    $ npm install q
    $ node examples/test.js



THE HALLOWED API


when(value, callback_opt, errback_opt)

    Arranges for a callback to be called:
     - with the value as its sole argument
     - in a future turn of the event loop
     - if and when the value is or becomes a fully resolved
    Arranges for errback to be called:
     - with a value respresenting the reason why the object will
       never be resolved, typically a string.
     - in a future turn of the event loop
     - if the value is a promise and
       - if and when the promise is rejected
    Returns a promise:
     - that will resolve to the value returned by either the callback
       or errback, if either of those functions are called, or
     - that will be rejected if the value is rejected and no errback
       is provided, thus forwarding rejections by default.

    The value may be truly _any_ value.

    The callback and errback may be falsy, in which case they will not
    be called.
    

    Guarantees:

     - The callback will not be called before when returns.
     - The errback will not be called before when returns.
     - The callback will not be called more than once.
     - The errback will not be called more than once.
     - If the callback is called, the errback will never be called.
     - If the errback is called, the callback will never be called.
     - If a promise is never resolved, neither the callback or the
       errback will ever be called.


    THIS IS COOL

     - You can set up an entire chain of causes and effects in the
       duration of a single event and be guaranteed that any
       invariants in your lexical scope will not...vary.
     - You can both receive a promise from a sketchy API and return a
       promise to some other sketchy API and, as long as you trust
       this module, all of these guarantees are still provided.
     - You can use when to compose promises in a variety of ways:


    INTERSECTION

    function and(a, b) {
        return when(a, function (a) {
            return when(b, function (b) {
                // ...
            });
        })
    }


defer()

    Returns a "Deferred" object with a:

     - promise property
     - resolve(value) function
     - reject(reason) function

    The promise is suitable for passing as a value to
    the "when" function.

    Calling resolve with a promise notifies all observers
    that they must now wait for that promise to resolve.

    Calling resolve with a rejected promise notifies all
    observers that the promise will never be fully resolved
    with the rejection reason.  This forwards through the
    the chain of "when" calls and their returned "promises"
    until it reaches a "when" call that has an "errback".

    Calling resolve with a fully resolved value notifies
    all observers that they may proceed with that value
    in a future turn.  This forwards through the "callback"
    chain of any pending "when" calls.

    Calling reject with a reason is equivalent to
    resolving with a rejection.

    In all cases where the resolution of a promise is set,
    (promise, rejection, value) the resolution is permanent
    and cannot be reset.  All future observers of the
    resolution of the promise will be notified of the
    resolved value, so it is safe to call "when" on 
    a promise regardless of whether it has been or will
    be resolved.


    THIS IS COOL

    The Deferred separates the promise part from the resolver
    part. So:

     - You can give the promise to any number of consumers
       and all of them will observe the resolution independently.
       Because the capability of observing a promise is separated
       from the capability of resolving the promise, none of the
       recipients of the promise have the ability to "trick"
       other recipients with misinformation.

     - You can give the resolver to any number of producers
       and whoever resolves the promise first wins.  Furthermore,
       none of the producers can observe that they lost unless
       you give them the promise part too.

    
    UNION

    function or(a, b) {
        var union = defer();
        when(a, union.resolve);
        when(b, union.resolve);
        return union.promise;
    }

    
ref(value)

    If value is a promise, returns the value.

    If value is not a promise, returns a promise that has
    already been resolved with the given value.


reject(reason)

    Returns a promise that has already been rejected
    with the given reason.
    
    This is useful for conditionally forwarding a rejection through an
    errback.

        when(API.getPromise(), function (value) {
            return doSomething(value);
        }, function (reason) {
            if (API.stillPossible())
                return API.tryAgain();
            else
                return reject(reason);
        })
    
    Unconditionally forwarding a rejection is equivalent to omitting
    an errback on a when call.


isPromise(value)

    Returns whether the given value is a promise.


isResolved(value)

    Returns whether the given value is fully resolved.
    The given value may be any value, including
    but not limited to promises returned by defer() and
    ref().


promise.valueOf()

    Promises override their valueOf method such that if the
    promise is fully resolved, it will return the fully
    resolved value.


defined(value)

    Accepts a value or a promise for a value.

    Returns a promise that will only resolve to a defined value.

    If the given promise is resolved to "undefined", rejects
    the returned promise.

error(reason)

    Accepts a reason and throws an error.  This is a convenience for
    when calls where you want to trap the error clause and throw it
    instead of attempting a recovery or forwarding.

enqueue(callback Function)

    Calls "callback" in a future turn.


Copyright 2009, 2010 Kristopher Michael Kowal
MIT License (enclosed)