In Scala 3, I'm able to write a poly-function of type 1:
val y = [C <: Int] => (x: C) => x * 2
When I try to generalise it into type 2:
val z = [C <: Int] => ([D <: Int] => (x: C, y: D) = x * y)
I got the following error:
DependentPoly.scala:19:37: Implementation restriction: polymorphic function literals must have a value parameter
So is this feature not implemented? Or I'm not writing it properly?
Implementation restriction: polymorphic function literals must have a value parameter means that
val y = [C <: Int] => foo[C]
is illegal (for example for def foo[C <: Int]: C => Int = _ * 2) while
val y = [C <: Int] => (x: C) => x * 2
is legal.
Similarly,
val z = [C <: Int] => [D <: Int] => (x: C, y: D) => x * y
val z = [C <: Int] => [D <: Int] => (x: C) => (y: D) => x * y
are illegal while
val z = [C <: Int, D <: Int] => (x: C, y: D) => x * y
val z = [C <: Int, D <: Int] => (x: C) => (y: D) => x * y
val z = [C <: Int] => (x: C) => [D <: Int] => (y: D) => x * y
val z = [C <: Int] => (_: C) => [D <: Int] => (x: C, y: D) => x * y
val z = [C <: Int] => (_: C) => [D <: Int] => (x: C) => (y: D) => x * y
are legal.
This is because of
trait PolyFunction:
def apply[A](x: A): B[A]
https://docs.scala-lang.org/scala3/reference/new-types/polymorphic-function-types.html