Scala: Making implicit conversion A->B work for Option[A] -> Option[B]

Question

I\'m trying to write a function which re-uses the implicit conversions which I have for Object A -> Object B when they are wrapped in an Option in a generic way so that Opti

Answer 1

Here's clue:

scala> val fooOpt: Option[Bar] = Option(Foo(1))
fooOpt: Option[Bar] = Some(Bar(1))

And another:

scala> implicit def foobar(x: String): Int = augmentString(x).toInt
foobar: (x: String)Int

scala> val y: Option[String] = Option(1)
y: Option[String] = Some(1)

scala> val y: Option[Int] = Option("1")
y: Option[Int] = Some(1)

Looks like a legitimately odd bug. I'd pop open a smaller test case and open an issue (or search for one in JIRA).

As an aside:

You could use some category theory to handle lots of different types of "Option-ish" things.

package object fun {
  trait Functor[Container[_]] {
    def fmap[A,B](x: Container[A], f: A => B): Container[B]
  }
  object Functor {
     implicit object optionFunctor extends Functor[Option] {
       override def fmap[A,B](x: Option[A], f: A => B): Option[B] = x map f
     }
     // Note: With some CanBuildFrom magic, we can support Traversables here.
  }
  implicit def liftConversion[F[_], A, B](x: F[A])(implicit f: A => B, functor: Functor[F]): F[B] = 
    functor.fmap(x,f)

}

That's a bit more advanced, as you're mapping some category theory FP onto the problem, but it's a more general solution to lift implicit conversations into containers as needed. Notice how they chain by using one implicit conversation method that takes a more limited implicit argument.

ALSO, this should make the examples work:

scala> val tmp = Option(Foo(1))
tmp: Option[Foo] = Some(Foo(1))

scala> val y: Option[Bar] = tmp
y: Option[Bar] = Some(Bar(1))

And make your usage of Some more dangerous:

scala> val tmp = Some(Foo(1))
tmp: Some[Foo] = Some(Foo(1))

scala> val y: Option[Bar] = tmp
:25: error: could not find implicit value for parameter functor: fun.Functor[Some]
       val y: Option[Bar] = tmp
                            ^

That's telling you that variance is critical, and interacts with implicits. My guess is you ran into a very rare, probably hard to fix bug that can be avoided using other techniques.