First attempt at a Bind[M[X] <: Traversable[X]]

traversable-bind.scala

import collection._
import collection.generic._

trait Bind[Z[_]] {
  def bind[A, B](a: Z[A], f: A => Z[B]): Z[B]
}

def TraversableBind[M[X] <: Traversable[X]] = new Bind[M] {
  def bind[A, B](r: M[A], f: A => M[B]): M[B] = {
    implicit object cbf extends CanBuildFrom[Traversable[A], B, M[B]] {
      def apply(from: Traversable[A]) = r.genericBuilder[B].asInstanceOf[collection.mutable.Builder[B, M[B]]]

      def apply() = error("undefined")
    }    
    r.flatMap[B, M[B]](f)
  }
}

val bound = TraversableBind[List].bind(List(1, 2, 3), (x: Int) => List(x, x))
println(bound)