(* higher order functions in ML *)

fun mymap(f,[]) = []
  | mymap(f,(h::t)) = ((f h)::mymap(f,t));

fun filter(f,[]) = []
  | filter(f,(h::t)) = if (f h) then (h::filter(f,t)) else filter(f,t);

(* filter( (fn x => (x < 5)), [5, 2, 7, 4, 8] )  -> [2,4] *)

fun fold(f,ident,[]) = ident
  | fold(f,ident,(h::t)) = f(h, (fold(f,ident,t)));

fun evens(L) = filter (fn x => x mod 2 = 0) L;

fun myand(x,y) = x andalso y; 
fun g5(x) = x>5;

fun ag5(L) = fold(myand,true,mymap(g5,L));

(*  

fun f1(x,y) = x+y;
fold(f1,0,[2,4,6,8]) => 20


cell fold(int (*f)(int,int),int ident, cell L)
{ 
   if (L == NULL) return L; 
   else return (*f)(L->head,fold(f,ident,L->tail));
}


*)