Scala-Prolog

grandchild.scala

trait John
trait Carl
trait Tom
trait Child[T,U]
trait GrandChild[T,U]

implicit val john_carl = new Child[John,Carl]{}
implicit val carl_tom  = new Child[Carl,Tom ]{}

implicit def grandChild[X,Y,Z](
	implicit 
		xy: Child[X,Y],
		yz: Child[Y,Z]
	) = new GrandChild[X,Z] {}


> implicitly[ GrandChild[John, Tom] ]
// (compiles; returns the fact instance)

> implicitly[ GrandChild[John, Carl] ]
Compilation Failed

// both compile:
> implicitly[ GrandChild[John, _]] 
> implicitly[ GrandChild[John, X forSome { type X }] ] 

> implicitly[ GrandChild[_, Tom] ] // does not compile 

implicits.scala

trait LowPriority {
  implicit def grandChildFallback[X,Y,Z](
    implicit 
      xy: Child[X,Y],
      yz: Child[Y,Z]
  ) = new GrandChild[X,Z]{}
}

object Implicits extends LowPriority {
  implicit def grandChild[X,Y,Z](
    implicit 
      yz: Child[Y,Z],
      xy: Child[X,Y]
  ) = new GrandChild[X,Z]{}
}

import Implicits._