distinctby

ListOps.scala

final class ListOps[A](val l: List[A]) extends AnyVal {
  def distinctBy[K](f: A => K): List[A] = {
    val seen = collection.mutable.HashSet.empty[K]

    @tailrec
    def loop(l: List[A], distinctAccum: List[A]): List[A] = l match {
      case Nil => distinctAccum
      case t :: ts =>
        val k = f(t)
        if (seen.contains(k)) loop(ts, distinctAccum)
        else {
          seen += k
          loop(ts, t :: distinctAccum)
        }
    }
    loop(l, Nil)
  }
}