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.

Answer 2

You can use an implicit conversion

  implicit class OptionOps[A](opt: Option[A]) {

    def toTry(msg: String): Try[A] = {
      opt
        .map(Success(_))
        .getOrElse(Failure(new NoSuchElementException(msg)))
    }
  }

Scala standard lib uses this type of approach. See http://docs.scala-lang.org/tutorials/FAQ/finding-implicits.html#companion-objects-of-a-type

Answer 3

Short and simple

Try(option.get)

no need for fancy mapping. In case the option is empty you get an error like:

java.util.NoSuchElementException: None.get

Answer 4

myOption
  .toRight(new Exception("y"))
  .toTry

This code will return either a Success(x) if myOption is Some(x) or Failure(Exception("y")) if it is a None.

Answer 5

If you want to use an Either, you can use Option.toRight:

def getValidBRefForReferencedA(aRef: ARef): Either[Throwable,BRef] = {    
  for {
    a <- get[A](aRef).toRight[Throwable](new Exception("Invalid ARef")).right
    bRef <- validBRefs.find(_ == a.bRef).toRight(new MismatchException("No B found matching A's B-ref")).right
  } yield bRef
}

Using a Try, you can simply write your code in a very procedural way, by throwing appropriate exceptions where needed and wrapping the whole with Try.apply (which will catch the exception and present them as Failure instances).

def getValidBRefForReferencedA(aRef: ARef): Try[BRef] = Try {
  val a = get[A](aRef).getOrElse(throw new Exception("Invalid ARef"))
  validBRefs.find(_ == a.bRef).getOrElse(throw new MismatchException("No B found matching A's B-ref"))
}

Answer 6

[Edited to identify different failures]

Tried to simplify

def getValidBRefForReferencedA(aRef: ARef): Try[BRef] = {
  val abRef = for {
    a <- get[A](aRef)
    bRef = a.bRef
    result = Either.cond(validBRefs.contains(bRef), bRef, "Invalid B Reference")
  } yield result

  abRef.map {
    case Right(bRef) => Success(bRef)
    case Left(error) => Failure(new InvalidReferenceException(error))
  }.getOrElse(Failure(new MismatchException("No B found matching A's B-ref")))
}

Answer 7

I have worked out an alternative solution, although it still doesn't allow me to specify a different failure message for the case that the Option[A] is None versus the BRef not being valid:

def getValidBRefForReferencedA(aRef: ARef): Try[BRef] = 
  Try {
    (for {
      a <- get[A](aRef)
      bRef = a.bRef
      _ <- bs.find(_ == bRef)
    } yield (bRef)) getOrElse (throw new MismatchException("No B found matching A's B-ref"))
  }

I guess I am expecting it should be possible to have a way to quickly convert the returned Option[A] into a Try (in a suitably idiomatic Scala way - eg. inside a for-comprehension), then continue the processing (obtaining and checking the b-ref) while setting any appropriate Failures along the way.