(*
   FILTER (P,L) -- P is a predicate (i.e. a function that returns true or false)
                   L is a list
                   FILTER (P,L) returns the elements in L for which P is true
              e.g.  Given: fun P X = ((X mod 2) = 0);
                           FILTER (P,L) would return the even numbers in L
                           e.g. FILTER (P,[1,2,3,4,5]) = [2,4]
*)

fun FILTER (P,[])         = []
 |  FILTER (P,head::tail) = if (P head)
                            then head::(FILTER (P,tail))
                            else (FILTER (P,tail))
;

use "FIRST_ARG";
local
     fun P X = ((X mod 2) = 0)
in
val evens = FIRST_ARG (FILTER,P)
end;