Assume the existence of the following types and method:
trait X[A <: X[A]]
case class C extends X[C]
def m(x: PartialFunction[X[_], Boolean])
I want to be able to create a PartialFunction to be passed into m.
A first attempt would be to write
val f: PartialFunction[X[_], Boolean] = {
case c: C => true
}
m(f)
This fails with type arguments [_$1] do not conform to trait X's type parameter bounds [A <: X[A]]. So, it seems we have to constraint X's type parameters.
A second attempt:
val f: PartialFunction[{type A <: X[A]}, Boolean] = {
case c: C => true
}
m(f)
This fails on the application of m because PartialFunction[AnyRef{type A <: X[this.A]},Boolean] <: PartialFunction[X[_],Boolean] is false.
Is there any way not involving casting that actually satisfies the compiler both on the definition of the partial function and on the application of m?
I'm not sure what exactly you want, but since you are using an existential type (in disguise of the _ syntax), this is how you can make that work:
val f: PartialFunction[X[A] forSome {type A <: X[A]}, Boolean] = {
case c : C => true
}
The _ syntax isn't good enough here, since you need to give the existential type the right upper bound. That is only possible with the more explicit forSome syntax.
What I find surprising, though, is that Scala accepts the declaration
def m(x: PartialFunction[X[_], Boolean])
in the first place. It seems weird that it even considers X[_] a well-formed type. This is short for X[A] forSome {type A <: Any}, which should not be a valid application of X, because it does not conform to the parameter bounds.