Map, Reduce, Filter

base.sml

(* map *)
fun map f nil     = nil
  | map f (x::xs) = (f x)::(map f xs);

(* reduce *)
fun reduce f x0 nil      = x0
  | reduce f x0 (x::nil) = f x x0
  | reduce f x0 (x::xs)  = f x (reduce f x0 xs);

(* filter *)
fun filter p nil = nil
  | filter p (x::xs) = let
    val ys = filter p xs
  in
    if (p x) then x::ys else ys
  end;