Rough sketch

gistfile1.scala

object TestCase {
   trait MYtype[A]
   case class Bob(i: Int) extends MYType[Int]
   case class Wibble[A,B](t1: MYType[A], t2: MYType[B]) extends MYType[A\/B]

   // MYType => ( List[(A,B)] => List[B] )
   // So we define myfun for each member of the MYType ADT
   object myfun extends Poly1 {
      implicit def caseOne = at[Bob]( i => { ... somefn } )
      implicit def caseTwo[A,B] = at[Wibble[A,B]]( i => { { xs => myfun(as) ++ myfun(bs) } } )
   }

   object somefn extends Poly1 {
      implicit def caseOne[A] = at[List[(Int,A)]]( ... )
   }

   val t1 = myfun(Bob(42))
   val t2 = myfun(Wibble(Bob(5),Bob(101)))
}

I'm still not getting it ... too many '...'s in critical places. What here are you either trying to close over? Or, alternatively, what is the binary function that you'd like to partially apply?