Question

I am trying to write a generic fill method, and following is what I have come up with so far:

scala> import collection.generic.{GenericTraversableTemplate => GTT}
import collection.generic.{GenericTraversableTemplate=>GTT}

scala> import collection.generic.{TraversableFactory => TF}
import collection.generic.{TraversableFactory=>TF}

scala> def fill[A, CC[X] <: Traversable[X] with GTT[X, CC]]
     |   (n: Int)(elem: => A)(tf: TF[CC]) = tf.fill(n)(elem)
fill: [A, CC[X] <: Traversable[X] with scala.collection.generic.GenericTraversab
leTemplate[X,CC]](n: Int)(elem: => A)(tf: scala.collection.generic.TraversableFa
ctory[CC])CC[A]

scala> fill(3)('d')(List)
res42: List[Char] = List(d, d, d)

This works with all traversable collections except arrays. How do I make this code work with arrays?

Answer 1

If you don’t mind creating an extra object, there’s

def fill[CC[_]](n: Int) = new {
  def apply[A](elem: => A)(implicit cbf: CanBuildFrom[Nothing, A, CC[A]]) = {
    val b = cbf()
    1 to n foreach { _ => b += elem }
    b.result
  }
}

It doesn’t get around objection (2), but the usage is nice:

scala> fill[List](3)("wish")
res0: List[java.lang.String] = List(wish, wish, wish)

scala> fill[Array](3)("wish")
res1: Array[java.lang.String] = Array(wish, wish, wish)