scala - Returning original collection type in generic method

Question

Say we want to make a function like minBy that returns all elements of equal minimalism in a collection:

def multiMinBy[A, B: Ordering](xs: Traversable[A])(f: A => B) = {
  val minVal = f(xs minBy f)
  xs filter (f(_) == minVal)
}

scala> multiMinBy(List("zza","zzza","zzb","zzzb"))(_.last)
res33: Traversable[java.lang.String] = List(zza, zzza)

So far, so good, except that we have a Traversable back instead of our initial List.

So I tried changing the signature to

def multiMinBy[A, B: Ordering, C <: Traversable[A]](xs: C)(f: A => B)

in the hope I might get a C back rather than a Traversable[A]. However, I don't get anything back:

scala> multiMinBy(List("zza","zzza","zzb","zzzb"))(_.last)

<console>:9: error: inferred type arguments [Nothing,Nothing,List[java.lang.String]] 
do not conform to method multiMinBy's type parameter bounds [A,B,C <: Traversable[A]]

I think this is because we have C appearing in the arguments before A has been inferred? So I flipped the order of the arguments, and added a cast:

def multiMinBy[A, B: Ordering, C <: Traversable[A]](f: A => B)(xs: C) = {
  val minVal = f(xs minBy f)
  (xs filter (f(_) == minVal)).asInstanceOf[C]
}

which works, except we have to call it like this:

multiMinBy((x: String) => x.last)(List("zza","zzza","zzb","zzzb"))

Is there a way to retain the original syntax, while getting the right collection type back?

Answer 1

I think Miles Sabin solution is way too complex. Scala's collection already have the necessary machinery to make it work, with a very small change:

import scala.collection.TraversableLike
def multiMinBy[A, B: Ordering, C <: Traversable[A]]
              (xs: C with TraversableLike[A, C])
              (f: A => B): C = {
  val minVal = f(xs minBy f)
  xs filter (f(_) == minVal)
}