How to find all maximum elements in a scalaz.Foldable container - scalaz

Question

scalaz.Foldable has a maximumBy method that finds a maximum element in a container. But is there an elegant way to find them all using scalaz? ie:
Vector(Person("Ben", 1), Person("Jil", 3), Person("Bob", 3)).maximumsBy(_.age)
== Vector(Person("Jil", 3), Person("Bob", 3))
I have a problem where, if there are several equal maximum values, I want to select among these candidates randomly.

Answer 1

You can do something like that
implicit def MaxNonEmptyListSemigroup[A : Order]:
Semigroup[NonEmptyList[A]] = new Semigroup[NonEmptyList[A]] {
def append(l1: NonEmptyList[A], l2: =>NonEmptyList[A]): NonEmptyList[A] =
Order[A].apply(l1.head, l2.head) match {
case GT => l1
case LT => l2
case EQ => l1 append l2
}
}
// returns None if the list is empty
// otherwise returns Some(non-empty-list of maximum elements)
list.foldMap1Opt(a => NonEmptyList.nels(a)) :: Option[NonEmptyList[A]]

Answer 2

Ideally, maximumsBy will return the maximums in the same type of container as was provided. To do this efficiently seems to require scalaz.Reducer, a typeclass that models append- and prepend- to a container.
import scalaz._
import Ordering._
import std.AllInstances._
object Maximums extends App {
def maximumsBy[F[_]: Foldable, A, B: Order](fa: F[A])(f: A => B)
(implicit r: Reducer[A, F[A]]): Option[F[A]] =
Foldable[F].foldMapLeft1Opt(fa)(a => (f(a), r.unit(a))) {
case (curr#(max, maxes), a) => {
val next = f(a)
Order[B].apply(next, max) match {
case GT => (next, r.unit(a))
case LT => curr
case EQ => (max, r.snoc(maxes, a))
}
}
}.map(_._2)
println(maximumsBy(Vector(("a", 1), ("c", 3), ("c", 3)))(_._2))
println(maximumsBy(List(("a", 1), ("c", 3), ("c", 3)))(_._2))
//Output:
//Some(Vector((c,3), (c,3)))
//Some(List((c,3), (c,3)))
}
I was slightly dismayed by how complex maximumsBy ended up. Are there any ways to simplify it, while keeping the same behavior?