[charliesome]

> Pointless abstraction is one of my pet peeves in this industry. This looks like it has gone from a fairly straightforward, if kludgy, piece of code to something far more complex. Why not just: [code]

Your example code works just fine for promises of course, but not all monads support a coalescing operation like Promise.all.

So even though this article only discusses folding over Promises, the core idea here can be generalised to any monad type (such as Promise, Result, Option, or anything else)

[yorwba]

> not all monads support a coalescing operation like Promise.all.

Actually, they do. Haskell calls it sequence :: (Traversable t, Monad m) => t (m a) -> m (t a) [1]

It works by consuming the structure outside the monad and rebuilding it inside. A possible implementation specialized for lists is

  sequence [] = return []
  sequence (h:t) = do
    h' <- h
    t' <- sequence t
    return (h':t')

[1] http://hackage.haskell.org/package/base-4.10.0.0/docs/Prelud...

[charliesome]

Sorry, I should've been more clear. You're right - you can absolutely build sequence out of the bind operation for any monad.

Promise.all is not just sequence though, there's some additional subtleties to it. In particular the fail-fast behaviour:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

That's the kind of fundamental coalescing operation that you cannot implement with bind on plain monads.