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scalafunctional-programmingfor-comprehension

Composing Futures and Options

I have four types A,B, C and D, an initial value x of the type Future[Option[A]] and three functions: f1: A => Option[B] , f2: B => Future[Option[C]] and f3: C => D.

How can I write a for comprehension starting with x that results in a value of the type Future[Option[D]] that would be the "composition" of the three functions?

Solution

  • You can use monad transformers (from Scalaz) for this:

    import scalaz.OptionT
import scalaz.std.option._
import scalaz.syntax.monad._
val result: Future[Option[D]] = (for {
  a <- OptionT(x)
  b <- OptionT(f1(a).point[Future])
  c <- OptionT(f2(b))
} yield f3(c)).run

    You'll need a monad instance for Future; there's one in scalaz-contrib.