typeclass fu

gistfile1.scala

// reifies the assertion that type A is an instance of typeclass F
final class instance[A, F[_]](value: A, typeclass: F[A]) {
  def apply[B](f: (F[A], A) => B): B = f(typeclass, value)
}

// provides an implicit conversion from any A to the above "instance" wrapper
object instance {
  implicit def anyToInstance[A, F[_]](a: A)(implicit fa: F[A]) = new instance(a, fa)
}

object Hoho {

  trait Foo[A] {
    def phooey(a: A): Unit
  }

  implicit object IntFoo extends Foo[Int] {
    def phooey(a: Int): Unit = println("int " + a.toString)
  }

  implicit object StringFoo extends Foo[String] {
    def phooey(a: String): Unit = println("string " + a.toString)
  }

  def main(args: Array[String]): Unit = {
    val l: List[_ instance Foo] = List(1, "hoho")
    l foreach (_(_ phooey _))
  }

}