ajax coffeescript purely-functional bacon.js

Cope with Bacon.js's side effects

I'm a newbie to the Bacon.js, usually write programs in Haskell. By my experience with Haskell, I want to describe some situations in Bacon.js as purely-functional-like approach.

Here is an example situation.

triggerStream is a source stream.
resultStream tries ajax access when triggerStream's events occur.
resultStream2 also tries ajax access after completion of resultStream's ajax access.

This is my approach:

### =======
# Streams
# ======= ###
triggerStream = () ->
  Bacon.fromArray([1,2,3])

resultStream = 
  triggerStream()
    .flatMap((n) -> Bacon.fromPromise($.ajax(toAjax n)))
    .zip(triggerStream(), (r,t) ->
      {result: r, trigger: t}

resultStream2 =
  resultStream     # (*)
    .flatMap((o) -> Bacon.fromPromise($.ajax(toAjax2 o)))
    .zip(resultStream, (r2,r1) ->
      {result: r2, trigger: r1.trigger}

### =======
# Assignments
# ======= ###

triggerStream()
  .onValue(beforeAjax1) # (a)
resultStream
  .onValue(afterAjax1)  # (b)
resultStream2
  .onValue(afterAjax2)  # (c)

(a) is supporsed to be executed after each trigger event, in other words it's executed before resultStream's ajax access.

(b) is supporsed to be fired after resultStream's ajax access.

I know Bacon.js's streams or properties have side effects on themselves, and so my code can't work well. In (b), resultStream's events are removed from resultStream object, which causes the empty stream in (*).

The approach which changes resultStream to a function (like triggerStream) seams work well, but it causes independent two times resultStream's ajax access when (b) and (c).

Is there any idea to realize my approach?

Solution

The problem is in that you use Bacon.fromArray as the source. This method returns a synchrously responding stream that practicaly spits its contents out to the first subscriber.

You might try a more realistic source such as Bacon.sequentially to make this work. Or you may do stream.delay(0) to change it into an asynchronously responding one.