Add ans

ast.ml

@@ -88,6 +88,7 @@ end
 
 type t =
   | Value of Value.t
+  | Var of string
   | Binop of t * Binop.t * t
   | Set_binop_pre of Binop.t * t
   | Get_binop_pre of Binop.t
@@ -103,6 +104,7 @@ let print ast =
   let pv v = pr "%s" @@ Value.to_string v in
   let rec aux = function
     | Value n -> pv n
+    | Var v -> pr "%s" v
     | Binop (left, op, right) -> begin
         let op = Binop.to_string op in
         pr "(%s " op; aux left; pr " "; aux right; pr ")";

eval.ml

@@ -2,6 +2,7 @@ open Ast
 open Ast.Value
 
 exception No_operation
+exception No_such_variable of string
 
 let rec binop op l r =
   let tl = typeof l and tr = typeof r in
@@ -20,18 +21,25 @@ let rec binop op l r =
     end
   | Some f -> f l r
 
-let rec eval = function
-  | Value v -> v
-  | Binop (l, op, r) ->
-    let l = eval l and r = eval r in
-    binop op l r
-  | Set_binop_pre (op, l) ->
-    let l =
-      match eval l with
-      | Int n -> n
-      | v -> raise @@ Invalid_type (typeof v)
-    in
-    Hashtbl.replace Parser.precedence op l;
-    Nop
-  | Get_binop_pre op ->
-    Int (Hashtbl.find Parser.precedence op)
+let eval ans ast =
+  let rec aux = function
+    | Value v -> v
+    | Var v ->
+      if v = "ans"
+      then ans
+      else raise @@ No_such_variable v
+    | Binop (l, op, r) ->
+      let l = aux l and r = aux r in
+      binop op l r
+    | Set_binop_pre (op, l) ->
+      let l =
+        match aux l with
+        | Int n -> n
+        | v -> raise @@ Invalid_type (typeof v)
+      in
+      Hashtbl.replace Parser.precedence op l;
+      Nop
+    | Get_binop_pre op ->
+      Int (Hashtbl.find Parser.precedence op)
+  in
+  aux ast

main.ml

@@ -10,6 +10,7 @@ let error_to_string e =
   | Parser.Expected t -> sprintf "expected %s" t
   | Parser.Unexpected_token t -> sprintf "unexpected token \"%s\"" t
   | Ast.Invalid_type t -> sprintf "invalid type %s" (Ast.Type.to_string t)
+  | Eval.No_such_variable v -> sprintf "no such variable %s" v
   | Failure f -> sprintf "error on %s" f
   | Division_by_zero -> "cannot divide by zero"
   | _ -> raise e
@@ -18,32 +19,35 @@ let print_error e =
   printf "error: %s\n" @@ error_to_string e
 
 (* read-eval-print *) 
-let rep () : unit =
+let rep ans : unit =
   printf "> ";
   let line = read_line () in
   if line = "quit" then raise Exit;
-  let ans =
+  let v =
     line
     |> Lex.tokenize
     |> Parser.parse
-    |> Eval.eval
+    |> Eval.eval !ans
   in
-  match ans with
+  match v with
   | Nop -> ()
-  | _ -> printf "%s\n" @@ Ast.Value.to_string ans
+  | _ ->
+    ans := v;
+    printf "%s\n" @@ Ast.Value.to_string v
 
 let init_repl () =
   let sigintf _ = raise Reset_line in
   Sys.(set_signal sigint (Signal_handle sigintf))
 
 (* simple REPL with error handling *)
-let rec repl () : unit =
-  try rep (); repl () with
+let rec repl ans : unit =
+  try rep ans; repl ans with
   | Exit | End_of_file -> ()
-  | Reset_line -> printf "\n"; repl ()
-  | e -> print_error e; repl ()
+  | Reset_line -> printf "\n"; repl ans
+  | e -> print_error e; repl ans
 
 let () =
+  let ans = ref @@ Ast.Value.Int 0 in
   init_repl ();
   printf "Configurable Evaluator %s\n" version; (* banner *)
-  repl ()
+  repl ans

parser.ml

@@ -90,6 +90,7 @@ let parse ts =
     | Seq.Cons (x, seq) -> begin match x with
         | Token.Int n -> Value (Int n), seq
         | Float n -> Value (Float n), seq
+        | Ident id -> Var id, seq
         | LParen -> expr seq
         | _ -> unexpected_token x
       end