(* ML examples *)

fun filter f [] = []
  | filter f (x::xs) = if f x then x::(filter f xs) else filter f xs

fun append [] ys = ys
  | append (x::xs) ys = x::(append xs ys)

fun qsort [] = []
  | qsort (p::xs) =
    let
      val lesser = filter (fn x => x < p) xs
      val greater = filter (fn x => x >= p) xs
    in
      append (qsort lesser) (p::(qsort greater))
    end