Bind Type Class instance for scala.collection.traversable. It ain't pretty.

improved-traversable-bind.scala

import collection._
import collection.generic._

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

implicit 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]) = apply

      def apply() = r.companion.newBuilder[B].asInstanceOf[collection.mutable.Builder[B, M[B]]]
    }
    r.flatMap[B, M[B]](f)
  }
}

assert(implicitly[Bind[List]].bind(List(1, 2, 3), (x: Int) => List(x, x)) == List(1, 1, 2, 2, 3, 3))
assert(implicitly[Bind[Stream]].bind(Stream.continually(1), (x: Int) => Stream(x, -x)).take(3).toList == List(1, -1, 1))