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

Indeed it's a very strange problem. I tried to use another type than Option, and it turns out that the problem is that Option is covariant in its type parameter. This works all:

case class A[B](value: B)  // invariant in B

case class X()
case class Y()

implicit def xtoy(x: X): Y = Y()
implicit def ytox(x: Y): X = X()
implicit def movea[U, V](from: A[U])(implicit view: U => V): A[V] =  A[V](from.value)

def test(a: A[Y]) = "ok"
test(A(X()))   // (1)
val f = A(X())
test(f)        // (2)

But if instead I define A as

case class A[+B](value: B)  // covariant in B

The case (2) fails. Case (1) always succeeds, because Scala already converts X to Y before wrapping it in an A.

Now that we know the problem source, you need to wait for a type guru to explain why this is actually a problem... The conversion is still valid, you see:

askForY(movea(f))  // succeeds, even with A[+B]