How to define an implicit class for Traversable with CanBuildFrom?

Question

I am trying to add some methods to a trait Traversableby pimping through implicit class.

But I seemed to be lost with the dash CanBuildFrom. Consider the following:

implicit class TraversableExt[+A, +Repr <: Traversable[A]](traversable: Repr) {
  def debug[That](name: String)(implicit bf: CanBuildFrom[Repr, A, That]): That =
    traversable.map{ a => println(name + ": " + a); a }(bf)
}

This is not an error:

Error: (21, 59) type mismatch
  found: scala.collection.generic.CanBuildFrom [Repr, A, That]
  required: scala.collection.generic.CanBuildFrom [Traversable [A], A, That]
      traversable.map {a => println (name + ":" + a); a} (bf) ^

I assume that since Reprin CanBuildFrom[-Repr, -Elem, +To]is contravariant, and therefore mine Repr, whose upper limit Traversable[A]may not work.

, . - ?

Answer 1

TraversableLike , inferencer CanBuildFrom:

implicit class TraversableExt[A, C[X] <: TraversableLike[X, C[X]]](traversable: C[A]) {
  def debug[That](name: String)(implicit bf: CanBuildFrom[C[A], A, That]): That =
    traversable.map{ a => println(name + ": " + a); a }(bf)
}

C[A], CanBuildFrom[Traversable[A],...]

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