RxJS Promise Composition (passing data)

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I did not read all of it, but if you want to achieve the same as pl().then(p2).then(p3).then(console.log);, with p being function returning promises, you could do something like (example here)

Rx.Observable.fromPromise(p1())
             .flatMap(function(p1_result){return p2(p1_result);})
             .flatMap(function(p2_result){return p3(p2_result);})

Or the more symmetric :

 var chainedPromises$ = 
     Rx.Observable.just()
             .flatMap(p1)
             .flatMap(p2)
             .flatMap(p3);

Now if you want to execute sequentially callback wrapped through fromCallback or fromNodeCallback, you could do something like :

function rename (flag){
  return flag
          ? rename(file,dest).flatMap(return Rx.Observable.just(dest))
          : Rx.Observable.throw(new Error('File does not exist.'));
}

Rx.Observable.just(file)
             .flatMap(exists)
             .flatMap(rename)
             .flatMap(stat)

The latter code is untested, so keep me updated if that works.
Last comment, this should work if at each point you only have one value produced (like a promise). If you would have several files instead of one, with flatMap you might get ordering issues (if order matters to you), so in that case, you could use concatMap as a replacement.

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