1 def distinctBy[B, C](xs: List[B], f: B => C, acc: List[C] = Nil): List[B] = xs match {

     2   case Nil => Nil

     3   case (x::xs) => {

     4     val res = f(x)

     5     if (acc.contains(res)) distinctBy(xs, f, acc)  

     6     else x::distinctBy(xs, f, res::acc)

     7   }

     8 } 

     9 

    10 

    11