concatMap with deferreds / chaining promises

Any ideas on how I would do the equivalent of this in most?

http://jsbin.com/kodoni/2/edit?js,console

var source = Rx.Observable.of(1,2,3,4)
  .concatMap(function(x) {
    return Rx.Observable.defer(function() {
      return new Promise(function(resolve, reject) {
        setTimeout(function() {
          resolve(x);
        }, 1000);
      });
    });
  })
  .subscribe(function(i) {
    console.log(i);
  });

[briancavalier]

Yay! 👍

[rektide]

Hey all, I had a similar but slightly different issue that this ticket helped me through- rather than trying to run fetch() multiple times on a list of urls, I was getting a stream of domainNames, and had a function that returned a promise with a list of records in the name. Finding out a way to flatten the results of that promise was a bit of a challenge for me- naively doing concatMap(n => lookupRecords(n)) returned the whole array as an element, but I wanted to flatten the resulting records out into the stream.

I used @briancavalier’s suggestion of unfolding to get by this, in slightly different form. It took me a pass or three to make right/not-ugly, so I built a library for the final version- most-promise-spread.

From the example there, I got mostPromiseSpread(Promise.resolve([1,2,3,4]).forEach(x => console.log(x*2)). For domainNames/records, it looks something like: domainNames.concatMap(n => mostPromiseSpread(lookupRecords(n)).