Add Env module

env.ml

@@ -0,0 +1,33 @@
+module Value = Ast.Value
+
+type t = {
+  vars : (string, Value.t) Hashtbl.t;
+  parent : t option;
+}
+
+let init_global () = {
+  vars = Hashtbl.create 100;
+  parent = None;
+}
+
+let make parent = {
+  vars = Hashtbl.create 100;
+  parent = Some parent;
+}
+
+exception Not_found
+
+let rec get e name =
+  match Hashtbl.find_opt e.vars name with
+  | None -> begin match e.parent with
+      | None -> raise Not_found
+      | Some p -> get p name
+    end
+  | Some value -> value
+
+let get_opt e name =
+  try Some (get e name)
+  with Not_found -> None
+
+let set e name value =
+  Hashtbl.replace e.vars name value

eval.ml

@@ -57,10 +57,10 @@ let apply f args =
   | "rad", [n] -> floatfun rad n
   | _ -> raise @@ No_such_function f
 
-let eval vars ast =
+let eval env ast =
   let rec aux = function
     | Value v -> v
-    | Var v -> begin match Hashtbl.find_opt vars v with
+    | Var v -> begin match Env.get_opt env v with
         | None -> raise @@ No_such_variable v
         | Some v -> v
       end
@@ -73,7 +73,7 @@ let eval vars ast =
       binop op l r
     | Let (var, e) ->
       let v = aux e in
-      Hashtbl.replace vars var v;
+      Env.set env var v;
       v
     | Apply (v, args) ->
       let args = List.map aux args in

main.ml

@@ -18,10 +18,10 @@ let error_to_string e =
 let print_error e =
   printf "error: %s\n" @@ error_to_string e
 
-let vars = Hashtbl.create 100
+let g = Env.init_global ()
 
 (* read-eval-print *) 
-let rep vars : unit =
+let rep env : unit =
   printf "> ";
   let line = read_line () in
   if line = "quit" then raise Exit;
@@ -29,30 +29,30 @@ let rep vars : unit =
     line
     |> Lex.tokenize
     |> Parser.parse
-    |> Eval.eval vars
+    |> Eval.eval env
   in
   match v with
   | Nop -> ()
   | _ ->
-    Hashtbl.replace vars "ans" v;
+    Env.set env "ans" v;
     printf "%s\n" @@ Ast.Value.to_string v
 
 exception Reset_line (* used to indicate ^C is pressed *)
 
 let init_repl () =
-  Hashtbl.replace vars "ans" (Ast.Value.Int 0);
+  Env.set g "ans" (Ast.Value.Int 0);
   (* treat Ctrl-C as to reset line *)
   let reset_line _ = raise Reset_line in
   Sys.(set_signal sigint (Signal_handle reset_line))
 
 (* simple REPL with error handling *)
-let rec repl vars : unit =
-  try rep vars; repl vars with
+let rec repl env : unit =
+  try rep env; repl env with
   | Exit | End_of_file (* Ctrl-D *) -> ()
-  | Reset_line -> printf "\n"; repl vars
-  | e -> print_error e; repl vars
+  | Reset_line -> printf "\n"; repl env
+  | e -> print_error e; repl env
 
 let () =
   init_repl ();
   printf "Configurable Evaluator %s\n" version; (* banner *)
-  repl vars
+  repl g