compiler and vm: added subtraction

lib/vm/native.ml

@@ -23,20 +23,30 @@ let numeric_add = function
   | NDouble x -> (function
                   | NInt y -> NDouble (x+.(float_of_int y))
                   | NDouble y -> NDouble (x+.y))
-
+let numeric_sub = function
+  | NInt x -> (function
+               | NInt y -> NInt (x - y)
+               | NDouble y -> NDouble ((float_of_int x) -. y))
+  | NDouble x -> (function
+                  | NInt y -> NDouble (x-.(float_of_int y))
+                  | NDouble y -> NDouble (x-.y))
 
 let builtin_print (v : Types.value ref list) =
   List.iter (fun r -> print_endline (print_value !r)) v;
   Types.Nil
 
-
-
 let builtin_add (vs : Types.value ref list) =
   of_numeric (List.fold_left numeric_add (NInt 0) (List.map (fun r -> to_numeric !r) vs))
+let builtin_sub (vs : Types.value ref list) =
+  match vs with
+  | f :: rest -> of_numeric (List.fold_left numeric_sub (to_numeric !f) (List.map (fun r -> to_numeric !r) rest))
+  | [] -> failwith "invalid number of arguments for subtraction"
+
 
 let table = [|
     builtin_print;
-    builtin_add
+    builtin_add;
+    builtin_sub
   |]