(* simple HOF dictionary *)
exception NotFound;

(* empty dictionary *)
fun null_dict(key:'a) = raise NotFound;

(*
 * update receives an entry (key, data), 
 * an old dictionary (dict) and
 * returns a new dictionary (dict')
 *) 
fun update (key:''a, data:'b) (dict:''a -> 'b) =
  let fun dict' (k':''a) =
    if (k' = key) 
    then data     (* this is the entry we were looking for *)
    else dict(k') (* it was not, look further down in the old dict *)
  in dict'
  end;
  
(* adds a new entry that raises "NotFound for the given key *)
fun delete (key:''a) dict =
  fn (k') => if (k' = key) 
             then raise NotFound
             else dict(k');

(* try it *)
(* create 2 entries *)
val dict1 = (update ("pi", 3.141)
               (update ("e", 2.718) null_dict));
(* get some entries *)
dict1("pi");
dict1("e");
(* add another entry *)
val dict2 = update ("one", 1.0) dict1;
dict2("one");
dict2("pi");
(* dict2("cat"); *)
val dict2 = delete "pi" dict2;
(*dict2("pi");*)
dict2("one");