How can I (best) convert an Option into a Try?

Question

How can I (best) convert an Option returned by a method call into a Try (by preference, although an Either or a scalaz \/ or even a Validation might be OK) including specifying a Failure value if appropriate?

For example, I have the following code, which feels kludgy, but does at least do (most of) the job:

import scala.util._

case class ARef(value: String)
case class BRef(value: String)
case class A(ref: ARef, bRef: BRef)
class MismatchException(msg: String) extends RuntimeException(msg)

trait MyTry {

  // Given:
  val validBRefs: List[BRef]

  // Want to go from an Option[A] (obtained, eg., via a function call passing a provided ARef)
  // to a Try[BRef], where the b-ref needs to be checked against the above list of BRefs or fail:

  def getValidBRefForReferencedA(aRef: ARef): Try[BRef] = {

    val abRef = for {
      a <- get[A](aRef) // Some function that returns an Option[A]
      abRef = a.bRef
      _ <- validBRefs.find(_ == abRef)
    } yield (abRef)

    abRef match {
      case Some(bRef) => Success(bRef)
      case None => Failure(new MismatchException("No B found matching A's B-ref"))
    }
  }
}

It feels like there should be a way for the final match to be morphed into a map or flatMap or similar construct and incorporated into the preceeding for comprehension.

Also, I would prefer to be able to specify a different failure message if the call to return an Option[A] from the ARef failed (returned None) compared to the BRef check failing (I only care about knowing one reason for the failure, so a scalaz Validation doesn't feel like the ideal fit).

Is this a suitable place to use a monad transformer? If so, does scalaz provide a suitable one, or can someone give an example of what it would look like?

Answer 1

If you start out with a Try from the get go with your for-comp then you can eliminate the match at the end. You can do this by forcing the Option to a Try via fold. Here's what that could look like:

def getValidBRefForReferencedA(aRef: ARef): Try[BRef] = {
  for {
    a <- get[A](aRef).fold[Try[A]](Failure[A](new OtherException("Invalid aRef supplied")))(Success(_))
    abRef = a.bRef
    _ <- validBRefs.find(_ == abRef).fold[Try[BRef]](Failure(new MismatchException("No B found matching A's B-ref")))(Success(_))
  } yield abRef
}

With this approach, you can get different exceptions for the two different checks. It's not perfect, but maybe it will work for you.