implicit_magic_quiz.scala

implicit def IterableW[F[_], T](implicit w: W[T], ev: F[T] <:< Iterable[T]) = {
  new W[F[T]] {
    def write(xs: F[T]) = {
      ev(xs).map(x => w.write(x)).mkString(", ")
    }
  }
}

def writeIterable[F[_], T](xs: F[T])(implicit w: W[T], ev: F[T] <:< Iterable[T]) = {
  ev(xs).map(x => w.write(x)).mkString(", ")
} 

implicitly[W[Seq[(Int,Int)]]].write(Seq((1 -> 2), (2 -> 3), (3 -> 4))) 
// "1:2, 2:3, 3:4"

implicitly[W[Map[Int, Int]]] 
// error: could not find implicit value for parameter e: W[Map[Int,Int]]

// But

writeIterable(Map(1 -> 2, 2 -> 3, 3 -> 4)) 
// "1:2, 2:3, 3:4"

// ------------------------

trait W[T] { 
  def write(x: T): String
} 

implicit object IntW extends W[Int] { 
  def write(x: Int) = x.toString
}

implicit def TupleW[A, B](implicit wa: W[A], wb: W[B]) = new W[(A, B)] {
  def write(tpl: (A, B)) = wa.write(tpl._1) + ":" + wb.write(tpl._2)
}