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/A
   - http://wiki.commonjs.org/wiki/Promises/B
   - http://wiki.commonjs.org/wiki/Promises/D


For Node:

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


APPLIED INTRODUCTION
--------------------

Skipping past what an asynchronous promise is and how to use
them directly for a moment, compare the usage of this
library to Tim Caswell's excellent `step` library.

    https://github.com/creationix/step

The `q/util` module, included here, provides a `step`
function similar to Tim's.  It takes any number of functions
as arguments and runs them in serial order.  Each function
returns a promise to complete its step.  When that promise
is deeply resolved (meaning there are no more unfinished
jobs in its object graph), the resolution is passed as the
argument to the next step.

    var Q = require("q/util");
    var FS = require("q-fs");

    Q.step(
        function () {
            return FS.read(__filename);
            // __filename is NodeJS-specific
        },
        function (text) {
            return text.toUpperCase();
        },
        function (text) {
            console.log(text);
        }
    );

In Node, this example reads itself and writes itself out in
all capitals.  Notice that any value can be treated as an
already resolved promise, since the second and third steps
return a string and `undefined` respectively.

You can also perform actions in parallel.  This example
reads two files at the same time and returns an array of
promises for the results.  Since the second step has more
than one argument, the results array gets unpacked into the
variadic arguments.

    var Q = require("q/util");
    var FS = require("q-fs");

    Q.step(
        function () {
            return [
                FS.read(__filename),
                FS.read("/etc/passwd")
            ];
        },
        function (self, passwd) {
            console.log(__filename + ':', self.length);
            console.log('/etc/passwd:', passwd.length);
        }
    );

The number of tasks performed in each step is not limited.
You can just as well return an array of promises of
indefinite length.  This example reads all of the files in
the same directory as the program and notes the length of
each.

    var Q = require("q/util");
    var FS = require("q-fs");

    Q.step(
        function () {
            return FS.list(__dirname);
        },
        function (fileNames) {
            return fileNames.map(function (fileName) {
                return [fileName, FS.read(fileName)];
            });
        },
        function (files) {
            files.forEach(function (pair) {
                var fileName = pair[0];
                var file = pair[1];
                console.log(fileName, file.length);
            });
        }
    );

All of these examples use the `q-fs` module, which is
packaged separately.  You can try these programs,
`step{1,2,3}.js` in the `examples/` directory of this
package.

When working with promises, exceptions are generally only
thrown to indicate programmer errors.  Promise-returning
API`s generally `reject` their promises to indicate that the
promise will never be resolved/fulfilled.  As such, the
above programs will terminate when the first step rejects a
the returned promise, which can happen if there is an error
while reading or listing a file.  The rejection can be
observed because the `step` function returns a `promise`
that will be eventually resolved by the return value of the
last step.

    var completed = Q.step(...);

We use the `when` method to observe either the resolution or
the rejection of the promise.

    Q.when(completed, function callback(completion) {
        // ok
    }, function errback(reason) {
        // error
    });

If a rejection is not explicitly observed, it gets
implicitly forwarded to the promise returned by `when`.

This is the implementation of `step` in terms of the `when`
method and the `deep` resolver method.

    function step() {
        return Array.prototype.reduce.call(
            arguments,
            function (value, callback) {
                return Q.when(deep(value), function (value) {
                    if (callback.length > 1) {
                        return callback.apply(undefined, value);
                    } else {
                        return callback(value);
                    }
                });
            },
            undefined
        );
    }


Thenables
---------

The Q API supports CommonJS/Promises/A, Kris Zyp's proposal
for "thenable" promises.  A thenable is any object with a
"then(callback, errback)" method.  The "then" method, in
turn, returns a promise for whatever value the callback's
eventually return.  Thus, promises are chainable.

Let's review Tim's example.  This illustrates the same
concept as the first example.  We asynchronously read our
own program and print it out in all capitals.

    var Q = require("q");
    var FS = require("q-fs");

    Q.when(FS.read(__filename))
    .then(function (text) {
        return text.toUpperCase();
    }).then(function (text) {
        console.log(text);
    });

/!\ IMPORTANT

The call to "Q.when" is not necessary but provides many
assurances that, even if the FS API is poorly written or
even if it is *maliciously* written, that the promise
returned will behave consistently.  That means that your
callbacks will all occur in future turns of the event loop
(so the state in your closure doesn't change), and that one
callback for each "thenable" will ever be called (so that
the state in your closure doesn't change more than once).

This example uses the "q/util" module again, because it has
that lovely "deep" method for turning objects with promises
inside-out (to a promise for an object).

    var Q = require("q/util");
    var FS = require("q-fs");

    Q.when(Q.deep({
        "self": FS.read(__filename),
        "passwd": FS.read("/etc/passwd")
    })).then(function (texts) {
        console.log(__filename + ":" + texts.self.length);
        console.log("/ext/passwd:" + texts.passwd.length);
    });

In this case, we've simultaneously read the text of our own
program, and the Unix user database, and then printed out
their corresponding file sizes.

Finally, to read all of the files in the examples directory
and note the lengths of each one, we can use a three step
thenable:

    var Q = require("q/util");
    var FS = require("q-fs");

    Q.when(FS.list(__dirname))
    .then(function (fileNames) {
        return Q.deep(fileNames.map(function (fileName) {
            return {
                "name": fileName,
                "text": FS.read(FS.join(__dirname, fileName))
            };
        }));
    }).then(function (files) {
        files.forEach(function (file) {
            console.log(file.name, file.text.length);
        });
    });

Again, all of these examples are in the `examples` directory
with the names `then{1,2,3}.js`.


Quacks Like a Duck:

Any object with a "then(callback, errback)" method will be
treated as a promise, and all promises provided by the Q API
have "then" methods so they can be used by any API that
accepts thenables.


The Q Ecosystem
---------------

    q-fs      https://github.com/kriskowal/q-fs
              basic file system promises
    q-http    https://github.com/kriskowal/q-http
              http client and server promises
    q-util    https://github.com/kriskowal/q-util
              promise control flow and data structures
    q-comm    https://github.com/kriskowal/q-comm
              remote object communication
    teleport  https://github.com/gozala/teleport
              browser-side module promises
    ...

    All available through NPM.


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 promise.

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


def(value)

    Annotates a value, wrapping it in a promise, such that
    that it is a local promise object which cannot be
    serialized and sent to resolve a remote promise.

    A def'ed value will respond to the `isDef` message
    without a rejection so remote promise communication
    libraries can distinguish it from non-def values.


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(). Rejected promises are not considered
    resolved.


isRejected(value)

    Returns whether the given value is a rejected
    promise.


promise.valueOf()

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


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.


ADVANCED API
------------

The "ref" promise constructor establishes the basic API for
performing operations on objects: "get", "put", "post", and
"del".  This set of "operators" can be extended by creating
promises that respond to messages with other operator names,
and by sending corresponding messages to those promises.


makePromise(descriptor, fallback_opt, valueOf_opt)

    Creates a stand-alone promise that responds to messages.
    These messages have an operator like "when", "get",
    "put", and "post", corresponding to each of the above
    methods for sending messages to promises.

    The descriptor is an object with function properties
    (methods) corresponding to operators.  When the made
    promise receives a message and a corresponding operator
    exists in the descriptor, the method gets called with
    the variadic arguments sent to the promise.  If no
    descriptor exists, the fallback method is called with
    the operator, and the subsequent variadic arguments
    instead.  These functions return a promise for the
    eventual resolution of the promise returned by the
    message-sender.  The default fallback returns a
    rejection.

    The `valueOf` function, if provided, overrides the
    `valueOf` method of the returned promise.  This is
    useful for providing information about the promise in
    the same turn of the event loop.  For example, resolved
    promises return their resolution value and rejections
    return an object that is recognized by `isRejected`.


send(value, operator, ...args)

    Sends an arbitrary message to a promise.

    Care should be taken not to introduce control-flow
    hazards and secuirity holes when forwarding messages to
    promises.  The methods above, particularly "when", are
    carefully crafted to prevent a poorly crafted or
    malicious promise from breaking the invariants like not
    applying callbacks multiple times or in the same turn of
    the event loop.



THE UTIL MODULE
---------------

The Q utility module exports all of the Q module's API but
additionally provides the following functions.

    var Q = require("q/util");


step(...functions)

    Calls each step function serially, proceeding only when
    the promise returned by the previous step is deeply
    resolved (see: `deep`), and passes the resolution of the
    previous step into the argument or arguments of the
    subsequent step.
    
    If a step accepts more than one argument, the resolution
    of the previous step is treated as an array and expanded
    into the step's respective arguments.

    `step` returns a promise for the value eventually
    returned by the last step.


delay(timeout, eventually_opt)

    Returns a promise for the eventual value after `timeout`
    miliseconds have elapsed.  `eventually` may be omitted,
    in which case the promise will be resolved to
    `undefined`.  If `eventually` is a function, progress
    will be made by calling that function and resolving to
    the returned value.  Otherwise, `eventually` is treated
    as a literal value and resolves the returned promise
    directly.


shallow(object)

    Takes any value and returns a promise for the
    corresponding value after all of its properties have
    been resolved.  For arrays, this means that the
    resolution is a new array with the corresponding values
    for each respective promise of the original array, and
    for objects, a new object with the corresponding values
    for each property.


deep(object)

    Takes any value and returns a promise for the
    corresponding value after all of its properties have
    been deeply resolved.  Any array or object in the
    transitive properties of the given value will be
    replaced with a new array or object where all of the
    owned properties have been replaced with their
    resolution.


reduceLeft(values, callback, basis, this)
reduceRight(values, callback, basis, this)
reduce(values, callback, basis, this)

    The reduce methods all have the signature of `reduce` on
    an ECMAScript 5 `Array`, but handle the cases where a
    value is a promise and when the return value of the
    accumulator is a promise.  In these cases, each reducer
    guarantees that progress will be made in a particular
    order.
    
    `reduceLeft` guarantees that the callback will be called
    on each value and accumulation from left to right after
    all previous values and accumulations are fully
    resolved.

    `reduceRight` works similarly from right to left.

    `reduce` is opportunistic and will attempt to accumulate
    the resolution of any previous resolutions.  This is
    useful when the accumulation function is associative.


THE QUEUE MODULE
----------------

The `q/queue` module provides a `Queue` object where
infinite promises for values can be dequeued before they are
enqueued.


put(value)

    Places a value on the queue, resolving the next gotten
    promise in order.

get()

    Returns a promise for the next value from the queue.  If
    more values have been enqueued than dequeued, this value
    will already be resolved.

close(reason_opt)

    Causes all promises dequeued after all already enqueued
    values have been depleted will be rejected for the given
    reason.

closed

    A promise that, when resolved, indicates that all
    enqueued values from before the call to `close` have
    been dequeued.


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