How to extend GenSeqLike?

Question

I have difficulties understanding Scalas type system. What's the correct way to write this code?

implicit class ExtSeq[A <: GenSeqLike[B, A], B](seq: A) {
  def prePadTo(len: Int, elem: B) = seq.reverse.padTo(len, elem).reverse
}

Answer 1

Simplest way - just use Seq:

implicit class ExtSeq[T](seq: Seq[T]) {
  def prePadTo(len: Int, elem: T) = seq.reverse.padTo(len, elem).reverse
}

Generic way - use IsTraversableOnce:

import scala.collection.GenTraversableOnce
import scala.collection.generic.{IsTraversableOnce, CanBuildFrom}

class PrePadTo[T, Repr](coll: GenTraversableOnce[T]) {
  def prePadTo[That](len: Int, elem: T)
                    (implicit cbf: CanBuildFrom[Repr, T, That]): That = {
    val b = cbf()
    val tmp = coll.toSeq
    b ++= Iterator.fill(len - tmp.size)(elem)
    b ++= tmp.iterator
    b.result
  }
}

implicit def toPrePadTo[Repr](coll: Repr)
                             (implicit traversable: IsTraversableOnce[Repr]) =
    new PrePadTo[traversable.A, Repr](traversable.conversion(coll))

Usage:

scala> "abc".prePadTo(5, '-')
res0: String = --abc

Answer 2

There are two issues with your implicit class:

• B isn't constrained, so it will end up as Nothing. You need a way to infer it from the input collection.

• You need an implicit CanBuildFrom on your prePadTo method so that the padTo method knows how to build a new instance of the input collection.

Hence:

import scala.collection.{GenSeq, GenSeqLike}
import scala.collection.generic.CanBuildFrom

implicit class ExtSeq[A <: GenSeqLike[B, A], B](seq: A with GenSeq[B]) {
  def prePadTo(len: Int, elem: B)(implicit cbf: CanBuildFrom[A, B, A]) =
    seq.reverse.padTo(len, elem).reverse
}

And then:

scala> List(1,2,3).prePadTo(10, 4)
res1: List[Int] = List(4, 4, 4, 4, 4, 4, 4, 1, 2, 3)

scala> Vector('1','2','3').prePadTo(10, '4')
res2: Vector[Char] = Vector(4, 4, 4, 4, 4, 4, 4, 1, 2, 3)

Note that it will not work for classes that are only implicit convertible to a Seq, like String for example.