Is it possible in Scala to force the caller to specify a type parameter for a polymorphic method?

Question

//API
class Node
class Person extends Node

object Finder
{
  def find[T <: Node](name: String): T = doFind(name).asInstanceOf[T]
}

//Call site (correct)
val per         


        

Answer 1

Miles Sabin posted a really nice solution for this problem on the scala-user mailing list. Define a NotNothing type class as follows:

sealed trait NotNothing[T] { type U }                                          
object NotNothing {
   implicit val nothingIsNothing = new NotNothing[Nothing] { type U = Any }
   implicit def notNothing[T] = new NotNothing[T] { type U = T }           
}

Now you can define your Finder as

object Finder {
   def find[T <: Node : NotNothing](name: String): T = 
      doFind(name).asInstanceOf[T]
}

If you try to invoke Finder.find without a type parameter, you'll get a compile-time error:

error: ambiguous implicit values: both method notNothing in object $iw of type [T]java.lang.Object with NotNothing[T]{type U = T} and value nothingIsNothing in object $iw of type => java.lang.Object with NotNothing[Nothing]{type U = Any} match expected type NotNothing[T] Finder.find("joe")

This solution is far more general than the ones proposed in my other answers. The only drawback I can see is that the compile-time error is pretty opaque, and the @implicitNotFound annotation doesn't help.