Future[Option] in Scala for-comprehensions

Question

I have two functions which return Futures. I\'m trying to feed a modified result from first function into the other using a for-yield comprehension.

This approach w

Answer 1

This answer to a similar question about Promise[Option[A]] might help. Just substitute Future for Promise.

I'm inferring the following types for getUserDetails and getSchool from your question:

getUserDetails: UserID => Future[Either[??, UserDetails]]
getSchool: SchoolID => Future[Option[School]]

Since you ignore the failure value from the Either, transforming it to an Option instead, you effectively have two values of type A => Future[Option[B]].

Once you've got a Monad instance for Future (there may be one in scalaz, or you could write your own as in the answer I linked), applying the OptionT transformer to your problem would look something like this:

for {
  ud  <- optionT(getUserDetails(user.userID) map (_.right.toOption))
  sid <- optionT(Future.successful(ud.schoolID))
  s   <- optionT(getSchool(sid))
} yield s

Note that, to keep the types compatible, ud.schoolID is wrapped in an (already completed) Future.

The result of this for-comprehension would have type OptionT[Future, SchoolID]. You can extract a value of type Future[Option[SchoolID]] with the transformer's run method.