Description
The documentation of kind-projector provides valuable information as to what is and what isn't possible to cross-compile. This is greatly appreciated, however, here's a usecase that doesn't seem to have a non-projection-based way of cross-compilng with Scala 2.
Compiler version
3.2.1
Minimized example
The problem can be described as a formulation a type lambda that depends on a method bound and that expresses partial application where of a parametric type, where the free type variable is higher-kinded.
We're trying to express when its F paramerter is fixed, PolyFunction forms a FunctorK
Scala 3 :
//> using option "-Ykind-projector"
object Test {
trait PolyFunction[F[_], G[_]]
trait FunctorK[Alg[_[_]]]
def polyFunctionFunctorK[F[_]] : FunctorK[[G[_]] =>> PolyFunction[F, G]] = ???
}Scala 2 (using the kind-projector compiler plugin)
//> using plugin "org.typelevel:::kind-projector:0.13.2"
//> using scala "2.13.10"
object Test {
trait PolyFunction[F[_], G[_]]
trait FunctorK[Alg[_[_]]]
def polyFunctionFunctorK[F[_]] : FunctorK[PolyFunction[F, *[_]]] = ???
}The only way I'm aware of cross-compiling this is to resort to the de-sugared Scala 2 encoding :
Desugared encoding, which compiles on both Scala 2 and Scala 3
object Test {
trait PolyFunction[F[_], G[_]]
object PolyFunction {
type Curried[F[_]] = {
type λ[G[_]] = PolyFunction[F, G]
}
}
trait FunctorK[Alg[_[_]]]
def polyFunctionFunctorK[F[_]] : FunctorK[PolyFunction.Curried[F]#λ] = ???
}Expectation
I'd personally appreciate if the support for λ in Scala 3 was not limited to *-kinded type parameters, and if we could write the following, which compiles in Scala 2 using kind-projector.
object Test {
trait PolyFunction[F[_], G[_]]
trait FunctorK[Alg[_[_]]]
def polyFunctionFunctorK[F[_]] : FunctorK[λ[G[_] => PolyFunction[F, G]]] = ???
}