Async computation with Validation in Scala using Scalaz

Question

Being writing a completely async library to access a remote service (using Play2.0), I\'m using Promise and Validation to create non-blocking call, whi

Answer 1

The concept you are looking for here is monad transformers. In brief, monad transformers compensate for monads not composing by allowing you to "stack" them.

You didn't mention the version of Scalaz you are using, but if you look in the scalaz-seven branch, you'll find ValidationT. This can be used to wrap any F[Validation[E, A]] into a ValidationT[F, E, A], where in your case F = Promise. If you change f and g to return ValidationT, then you can leave your code as

for {
  x ← f(a)
  y ← g(b)
} yield y

This will give you a ValidationT[Promise, E, B] as a result.