XQ - Reactive Promises

XQ is a hybrid between promises and reactive extensions. Its core is a Promises/A+ compliant promises implementation that can also function as a stream.

Think of it as a promise chain that you can push multiple values down. .then == .map

Get it

Installing with NPM

```bash` npm install -S xq ... X = require 'xq'

### Download Source

```bash
git clone https://github.com/algesten/xq.git

Example

XQ as a Promise

X = require 'xq'
X(42).then (v) -> console.log v
#... 42 

As a deferred

def = X.defer()
def.promise.then (v) ->
    v * 2
.then (v) ->
    console.log v
def.resolve 21
#... 42 

XQ as an Event Stream

Looping over an array.

X([1,2,3,4]).forEach(v) ->
  X(v * 2)
.then (v) ->
  console.log v
#... 2 
#... 4 
#... 6 
#... 8 

Pushing values

def = X.defer()
def.promise.map (v) ->
    v * 2
.then (v) ->
    console.log v
def.push 11
def.push 21
def.push 22
#... 22 
#... 42 
#... 44 

Deferred values and Event Streams

XQ deals with event streams. A stream can be thought of as a sequence of values, sometimes followed by an end-of-stream.

v1 - v2 - v3 - end

In XQ everything starts out as a non-ended stream.

def = X.defer()
def.promise.map((v) -> v*2).then (v) -> console.log v
def.push 2
def.push 4
def.push 8

2 - 4 - 8 -

Here we created an event stream and pushed 2, 4 and 8 down the stream. After running def.promise will hold the value 8. The following .map holds the value 16 (8*2) and the final .then also holds 16. The stream is not ended, so we could continue pushing more values to it. Each step would be executed for each new value pushed.

A resolved promise is an event + end

A promise is just a special stream that always is ended when the promise is resolved/rejected. The following examples are exactly equivalent.

X(2)

def = X.defer(); def.resolve(2); def.promise

def = X.defer(); def.push(2); def.end(); def.promise

OI .oi(f)

OI is a helper for passing multiple values through a promise/event chain. Read about OI.

API

Instantiation

• X(v) creates an instance resolved with value v. Equivalent to def.push(v); def.end().
• X.reject(v) creates an instance that is rejected with value v. Equivalent to def.pushError(v); def.end().
• X.resolver(f) f is synchronously called with resolve, reject which are functions used to resolve/reject the promise. X.resolve((resolve, reject) -> ... resolve(42))
• X.binder(f) f is synchronously called with sink, end The sink function is used to sink events/errors. The signature of the sink function is (v, isErr) ->. X.binder((sink) -> ... sink(42)... sink(err,true). The end function is to signal stream end. f can optionally return an unsubscribe function which will be called when the event stream ends.
• def = X.defer() creates a deferred value def.
• def.resolve(v) to resolve the deferred with value v.
• def.reject(e) to reject the deferred with reason e.
• def.push(v) to push a value down the chain.
• def.pushError(e) to push an error down the chain.
• p = def.promise to get the promise from the deferred.

State

• p.isEnded() tells whether the stream has been ended.
• p.isPending() tells whether the promise is pending. Equivalent to !p.isEnded().
• p.isFulfilled() tells whether the promise is resolved.
• p.isRejected() tells whether the promise is rejected.
• p.onEnd(f) will call f when stream is ended. Returns self.
• p.endOnError() makes stream stop on first encountered error. Returns self.
• p.stop() Immediately stops the stream. After this isEnded() will be true. Returns self.

Chaining

• p.then/map(fx[,fe]) attaches fx to receive value pushed down the chain. Optionally attaches fe to receive errors.
• p.fail/catch(fe) attaches fe to receive errors.
• p.always/finally/fin(f) attaches f to receive both values and errors. The signature for f is (v, isError) -> where the second argument is a boolean telling whether the received value was an error.
• p.serial(fx[,fe]) exactly like then/map but ensures only one argument is executed at a time. Additional events are buffered up and executed one by one. See section on everything being parallel.
• p.once(fx) promise for the first event/value from a stream/promise. Automatcially ends when first value is received.
• p.settle(fx[,fe]) promise for the last event/value from a stream/promise. will effectively block a stream to settle before releasing. arguments like .then

Arrays and Objects

• p.forEach/each(fx) attached fx to receive values. If the value is an array, it will invoke fx one by one. I.e. [a0,a1,a2] will invoke fx(a0), fx(a1), fx(a2)
• p.singly/oneByOne(fx) serialized version of forEach. Each value in the array is fed to the function only when the last value is finished. This mainly makes a difference for deferreds. See section explaining forEach and singly.
• p.spread(fx) attaches fx to receive values. If the value is an array, the array will be destructured to arguments in fx. I.e. [a0,a1,a2] will invoke fx(a0, a1, a2). Non-array values will be invoked as first argument (f(v)).
• p.all(fx) attaches fx to receive resolved arrays/objects. If the value to be executed is an array of promises [p1,p2,...], fx will only be invoked when all promises are resolved and will be receving an array with the first resolved values. For objects the function inspects each top level property (no deep inspection). {a:p1,b:p2,...} will result in an object with the resolved values bound to the same keys. Any promise failing will abort and reject with the error of that promise. For streams it ensures there is a pushed value, it keeps the first one received regardless of there being more.
• X.all(v) same as X(v).all().
• p.snapshot(fx) like .all, but uses current value instead of first. See section about the difference between all or snapshot.
• X.snapshot same as X(v).snapshot().
• X.oi(f) chaining helper function with signature (i,o). See oi doc.

Multiple

• X.merge(s1, s2, ...) merges the variable number of promises/streams to one. The resulting stream will end when all parts have ended.

Filtering

• p.filter(f) apply function f to each value. If f returns a truthy, the original value will be released down the chain.
• p.find(f) exactly like filter. but only the first value is released down the chain, and step is closed.

Everything is parallel

Every operation in XQ is potentially executed in parallel (in a process.nextTick). For non-deferred values this is mostly never noticable.

The result of this operation will come out in the order of the array.

X([0,1,2]).forEach (v) -> v*2

There is however one situation with endOnError where it may matter. A somewhat contrived example.

# This doesn't work as expected!!! 
 
X([0,1,2]).forEach (v) ->
    throw new Error('fail') if v == 1
.endOnError()
.then (v) ->
  #... will see 0 and 2 

The user may expect the last .then to never receive the 2. However since all values are fed into forEach in parallel, the error will happen too late to stop the 2. To fix this use forEach().serial().

Parallel deferreds

When using deferreds the order is not guaranteed.

url1 = 'http://www.google.com/'
url2 = 'http://github.com/'
url3 = 'http://www.reddit.com/'
X([url1,url2,url3]).forEach(doRequest) # returns a promise for result 
.map (result) ->
  # ... ? 

Depending on how slow the requests were, the .map operation will receive the result in any order. To fix it, we can use forEach().serial() which ensures that each url fed to doRequest will return a fulfilled promise before the result is passed on to .map. This however means each requests will run serially.

Strategy for unwrapping deferreds

The principle for unwrapping deferreds is to unwrap on exit of each step.

X(X(42)).then((v) -> X(v)).then (v) -> #... look ma, v is still 42! 

If we break down this sequence.

1. Each X() is a step like all others. It can be thought of as .then (x) -> x (a bit more involved since it handles errors).
2. X() wraps a deferred X(42).
3. On the exit of the outmost X(), the inner X(42) is unwrapped to 42.
4. 42 is therefor fed into the next .then-step and invoked for the function (v) -> X(v).
5. That function once again wraps v (42) into a X(v) which on the exit of that same .then-step is unwrapped again back to 42
6. The last .then-step is therefore also just fed 42.

forEach has a serial pitfall

When using forEach in combination with arrays of promises, there is a potential pitfall. forEach is also parallel and does not wait for one deferred to finish before feeding the next, which means the following code would execute doSomething in parallell for the values of the array.

# forEach does not work serially! 
p1 = makePromise()
p2 = makePromise()
p3 = makePromise()
 
X([p1,p2,p3]).forEach (p) -> p.then(doSomething)...

.forEach().serial() is not serial

A mistaken attempt at fixing this would be to use serial, as in .forEach().serial (p) ->... but this does not work. Having no function to forEach would be the equivalent to (x) -> x and any deferred would be unwrapped on the exit of that forEach-step. This means all deferred have been unwrapped in parallel already before the invocation of .serial().

.singly() does things serially.

.singly() (or alias .oneByOne())is a serialized version of .forEach().

# singly is serial 
p1 = makePromise()
p2 = makePromise()
p3 = makePromise()
 
X([p1,p2,p3]).singly (p) -> p.then(doSomething)... # is done one by one 
 

It will queue up each value of the array to be executed one after another. That means .singly (v) -> X(doSomething(v)) would wait with feeding another value to the function until the previous has been unwrapped. The same goes for the non-argument .singly().

.all or .snapshot

.all takes the first value .snapshot takes the current. This can be illustrated in beautiful yet informative ascii art.

                                          .all
  stream1       a3 - a2 - a1 ->           [  ]
  stream2  b3 - b2 - b1 ->                [  ]
  stream3                       c3 - c2 - [c1] ->

At this point .all has not resolved, only value c1 in stream3 has been. For the three streams moving into the .all array, only the first value will be used. Hence when the promise for .all resolves, we will get an array with the values [a1,b1,c1], the first three values of the three streams.

For snapshot however:

                    .snapshot
  stream1       a3 - [a2] - a1 ->
  stream2  b3 - b2 - [b1] ->
  stream3            [c3]         c3 - c2 - c1 ->

At the point when all incoming streams have a value (stream2 being the last), both stream1 and stream3 have taken other values. Hence the snapshot when it resolves is the current state [a2,b1,c3].

Interoperability with other .then-ables

XQ tries to play nice with other promise packages. It can both wrap and receive other promises.

X(Q(42)).then (v) -> ...42

X().then(-> Q.reject(42)).fail (v) -> ...42

Why a hybrid?

I like promises such as Q I also like reactive extensions (FRP). However I don't like the API that comes with libraries such as RxJS, Bacon.js etc. My biggest beef is with something rx-people call flatMap.

Comparison of Q and Bacon.js

Q(42).then((v) -> Q(2*v)).then (v) -> ...v is 84

Bacon.once(42).map((v) -> Bacon.once(2*v)).flatMap().onValue (v) -> ...v is 84

For promises we continue a chain with .then .then .then. It doesn't matter whether the returned value in a step is a promise for a value Q(2*v) or a non-deferred.

With regards to the second .then, these two chains are equivalent.

.then(-> 4).then (v) ->

.then(-> Q(4)).then (v) ->

In the rx-world things are not so easy. As long as you just transform simple (non-deferred) values, you keep using .map, however if you dare returning a deferred value (observable or event stream) you probably want .flatMap.

In XQ then == map, there is no difference and a deferred value is just a special case of an event stream.

Another weirdness is the idea of lazy streams. It is very important to end a rx-style chain with something that "subscribes" (i.e. .onValue(), .subscribe(), .onError() etc). It could be argued that this makes a more obvious distinction between functions and side effects, but I'm still not convinced. I really don't find myself ever creating random streams that I end up not using (i.e. not subscribing to). The point of laziness seems pedantic and unnecessary.