Implement Ocaml-style return

eval.ml

@@ -213,7 +213,7 @@ let rec binop op l r =
 
 exception Unbound of string
 
-let rec eval env ast : value =
+let rec eval env ast : string * value =
   let rec aux = function
     | Nint n -> Int n
     | Nfloat n -> Float n
@@ -232,9 +232,6 @@ let rec eval env ast : value =
     | Binop (l, op, r) ->
       let l = aux l and r = aux r in
       binop op l r
-    | Let (var, e) ->
-      let v = aux e in
-      Env.set env var v; v
     | Apply (v, args) ->
       begin match aux v with
         | Function (vars, e) ->
@@ -243,7 +240,7 @@ let rec eval env ast : value =
           let nenv = Env.make env in
           List.combine vars args
           |> List.iter (fun (v, a) -> Env.set nenv v a);
-          eval nenv e
+          snd @@ eval nenv e
         | External f ->
           let args = List.map aux args in
           External.apply f args
@@ -263,8 +260,15 @@ let rec eval env ast : value =
       Hashtbl.replace Parser.oper_assoc op @@ Parser.assoc_of_string a;
       Nop
     | Get_binop_aso op ->
-      match Hashtbl.find_opt Parser.oper_assoc op with
-      | None -> String "left"
-      | Some a -> String (Parser.assoc_to_string a)
+      (match Hashtbl.find_opt Parser.oper_assoc op with
+       | None -> String "left"
+       | Some a -> String (Parser.assoc_to_string a))
+    | _ -> failwith "Eval.eval"
   in
-  aux ast
+
+  match ast with
+  | Let (var, e) ->
+    let v = aux e in
+    Env.set env var v;
+    var, v
+  | ast -> "-", aux ast

main.ml

@@ -36,7 +36,7 @@ let rep env : unit =
   printf "> ";
   let line = read_line () in
   if line = "quit" then raise Exit;
-  let v =
+  let var, v =
     line
     |> Lex.tokenize
     |> Parser.parse
@@ -46,7 +46,8 @@ let rep env : unit =
   | Nop -> ()
   | _ ->
     Env.set env "ans" v;
-    printf "%s\n" @@ Value.to_string v
+    printf "%s: %s = %s\n"
+      var (Type.to_string @@ Value.typeof v) (Value.to_string v)
 
 exception Reset_line (* used to indicate ^C is pressed *)
 

parser.ml

@@ -139,17 +139,24 @@ let more f seq =
   let xs, seq = aux [] seq in
   List.rev xs, seq
 
+(* decl := let_value
+ *       | expr
+ *)
+let rec decl seq =
+  seq |> oneof [
+    let_value;
+    expr min_int;
+  ]
+
 (* expr := level
  *       | assoc
- *       | let
  *       | apply
  *       | value binop_right
 *)
-let rec expr pre seq =
+and expr pre seq =
   seq |> oneof [
     level;
     assoc;
-    let_value;
     lambda;
     extern_value;
     apply;
@@ -267,6 +274,6 @@ and binop pre left seq =
 
 (* parse tokens *)
 let parse ts =
-  let ast, rest = expr min_int ts in
+  let ast, rest = decl ts in
   if rest () <> Seq.Nil then failwith "Parser.parse";
   ast