Add var table

eval.ml

@@ -21,13 +21,13 @@ let rec binop op l r =
     end
   | Some f -> f l r
 
-let eval ans ast =
+let eval vars ast =
   let rec aux = function
     | Value v -> v
-    | Var v ->
-      if v = "ans"
-      then ans
-      else raise @@ No_such_variable v
+    | Var v -> begin match Hashtbl.find_opt vars v with
+        | None -> raise @@ No_such_variable v
+        | Some v -> v
+      end
     | Binop (l, op, r) ->
       let l = aux l and r = aux r in
       binop op l r

main.ml

@@ -18,8 +18,10 @@ let error_to_string e =
 let print_error e =
   printf "error: %s\n" @@ error_to_string e
 
+let vars = Hashtbl.create 100
+
 (* read-eval-print *) 
-let rep ans : unit =
+let rep vars : unit =
   printf "> ";
   let line = read_line () in
   if line = "quit" then raise Exit;
@@ -27,27 +29,28 @@ let rep ans : unit =
     line
     |> Lex.tokenize
     |> Parser.parse
-    |> Eval.eval !ans
+    |> Eval.eval vars
   in
   match v with
   | Nop -> ()
   | _ ->
-    ans := v;
+    Hashtbl.replace vars "ans" v;
     printf "%s\n" @@ Ast.Value.to_string v
 
 let init_repl () =
+  Hashtbl.replace vars "ans" (Ast.Value.Int 0);
+  (* treat Ctrl-C as to reset line *)
   let sigintf _ = raise Reset_line in
   Sys.(set_signal sigint (Signal_handle sigintf))
 
 (* simple REPL with error handling *)
-let rec repl ans : unit =
-  try rep ans; repl ans with
+let rec repl vars : unit =
+  try rep vars; repl vars with
   | Exit | End_of_file -> ()
-  | Reset_line -> printf "\n"; repl ans
-  | e -> print_error e; repl ans
+  | Reset_line -> printf "\n"; repl vars
+  | e -> print_error e; repl vars
 
 let () =
-  let ans = ref @@ Ast.Value.Int 0 in
   init_repl ();
   printf "Configurable Evaluator %s\n" version; (* banner *)
-  repl ans
+  repl vars