polymorphic-functions.scala

object Main {

  trait ~>[F[_], G[_]] {
    def apply[T](t: F[T]): G[T]
  }

  // ERROR: Compiler doesn't think this implements `def apply[T](t: F[T]): G[T]` :(
  class Poly1[F[_], G[_]] extends ~> {

    private def implicitApply[T](t: F[T])(implicit f: F[T] ~> G[T]): G[T] = f(t)

    def apply[T](t: F[T]): G[T] = implicitApply(t)
  }

  def main(args: Array[String]): Unit = {
    type Id[T] = T
    type Const[C] = {
      type I[T] = C
    }

    def singleton = new (Id ~> Set) {
      def apply[T](t: T): Set[T] = Set(t)
    }

    def identity = new (Id ~> Id) {
      def apply[T](t: T): T = t
    }

    def headOption = new (List ~> Option) {
      def apply[T](t: List[T]): Option[T] = t.headOption
    }

    def size = new (Id ~> Const[Int]#I) {
      def apply[T](t: T): Int = 0
    }

    println(singleton(List(1, 2, 3)))
    println(singleton("7"))

    println(identity(List(1, 2, 3)))
    println(identity("7"))

    println(headOption(List(1, 2, 3)))

    println(size(List(1, 2, 3)))
    println(size("7"))
  }
}