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


EXAMPLES
--------


Promises from callbacks

    function delay(ms) {
        var deferred = Q.defer();
        setTimeout(deferred.resolve, ms);
        return deferred.promise;
    }


Timeout

    function timeout(promise, ms) {
        var deferred = Q.defer();
        Q.when(promise, deferred.resolve);
        Q.when(delay(ms), function () {
            deferred.reject("Timed out");
        });
        return deferred.promise;
    }


Promises with rejection from Node callbacks

    function wrap(wrapped) {
        var deferred = Q.defer();
        wrapped(function (error, result) {
            if (error) {
                deferred.reject(error);
            } else {
                deferred.resolve(result);
            }
        });
        return deferred.promise;
    }


When Blocks

    var bPromise = Q.when(aPromise, function (aValue) {
        return bValue;
    });

    var bPromise = Q.when(aPromise, function (aValue) {
        return bValue;
    }, function (aReason) {
        throw bReason;
    });


Parallel Join

    var aPromise = aFunction();
    var bPromise = bFunction();
    var cPromise = Q.when(aPromise, function (aValue) {
        return Q.when(bPromise, function bValue) {
            return cValue;
        });
    });


Serial Join

    var aPromise = aFunction();
    var cPromise = Q.when(aPromise, function (aValue) {
        // bFunction will not proceed unless aPromise
        // is fulfilled
        var bPromise = bFunction();
        return Q.when(bPromise, function bValue) {
            return cValue;
        });
    });


Recovering from Failure

    function justDoIt(value) {
        var work = doIt(value);
        work = timeout(1000, work);
        return Q.when(work, function (work) {
            return work;
        }, function errback(reason) {
            // just do it again
            return justDoIt(value);
        });
    }


Array Parallel Join

    var done;
    array.forEach(function (value) {
        var work = doWork(value); 
        done = Q.when(done, function () {
            return work;
        });
    });
    return done;


Or

    return array.reduce(function (done, value) {
        var work = doWork(value);
        return Q.when(done, function () {
            return work;
        });
    }, undefined);


Array Serial Join

    var done;
    array.forEach(function (value) {
        done = Q.when(done, function () {
            return doWork(value); 
        });
    });
    return done;

Or

    return array.reduce(function (done, value) {
        return Q.when(done, function () {
            return doWork(value);
        });
    });


Conditional Array Serial Join

    var otherwise = function () {
        throw new Error("No dice");
    };
    array.forEach(function (value) {
        otherwise = (function (otherwise) {
            return function () {
                var work = doWork(value);
                return Q.when(work, function (work) {
                    if (keepGoing(work)) {
                        return work;
                    } else {
                        return otherwise();
                    }
                });
            };
        })(otherwise);
    });
    return otherwise();

Or

    array.reduce(function (otherwise, value) {
        var work = doWork(value);
        return Q.when(work, function (work) {
            if (keepGoing(work)) {
                return work;
            } else {
                return otherwise();
            }
        });
    }, function otherwise() {
        throw new Error("No dice");
    });


In Node, this example reads itself and writes itself out in
all capitals.

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

    var text = FS.read(__filename);
    Q.when(text, function (text) {
        console.log(text.toUpperCase());
    });


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.

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

    var self = FS.read(__filename);
    var passwd = FS.read("/etc/passwd");
    Q.join(self, passwd, function (self, passwd) {
        console.log(__filename + ':', self.length);
        console.log('/etc/passwd:', passwd.length);
    });

This example reads all of the files in the same directory as
the program and notes the length of each, in the order in
which they are finished reading.

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

    var list = FS.list(__dirname);
    var files = Q.when(list, function (list) {
        list.forEach(function (fileName) {
            var content = FS.read(fileName);
            Q.when(content, function (content) {
                console.log(fileName, content.length);
            });
        });
    });


This example reads all of the files in the same directory as
the program and notes the length of each, in the order in
which they were listed.

    var list = FS.list(__dirname);
    var files = Q.when(list, function (list) {
        return list.reduce(function (ready, fileName) {
            var content = FS.read(fileName);
            return Q.join(ready, content, function (ready, content) {
                console.log(fileName, content.length);
            });
        });
    });



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)