Add unit type, remove Nop

ast.ml

@@ -1,6 +1,7 @@
 (* simple, untyped AST. *)
 type t =
   | Nothing
+  | Nunit
   | Nint of int
   | Nfloat of float
   | Nbool of bool
@@ -29,6 +30,7 @@ let print ast =
   let pr = Printf.printf in
   let rec aux = function
     | Nothing -> pr ""
+    | Nunit -> pr "()"
     | Nint n -> pr "%d" n
     | Nfloat n -> pr "%f" n
     | Nbool b -> pr "%b" b

eval.ml

@@ -2,6 +2,7 @@ open Ast
 
 (* resulting value of eval *)
 type value =
+  | Unit
   | Int of int
   | Float of float
   | Bool of bool
@@ -10,7 +11,6 @@ type value =
   (* (name), bound variables, expression, environment *)
   | Function of string option * string list * expr * env
   | External of string
-  | Nop (* return of system operations (will be deprecated) *)
 
 and expr = Ast.t
 
@@ -20,6 +20,7 @@ and env = Env of (string * value) list
 (* TODO: add proper type system *)
 module Type = struct
   type t =
+    | Unit
     | Int
     | Float
     | Bool
@@ -33,6 +34,7 @@ module Type = struct
   exception Expected of t
 
   let to_string = function
+    | Unit -> "unit"
     | Int -> "int"
     | Float -> "float"
     | Bool -> "bool"
@@ -55,6 +57,7 @@ module Value = struct
   type t = value
 
   let to_string = function
+    | Unit -> "()"
     | Int n -> string_of_int n
     | Float n -> string_of_float n
     | Bool b -> string_of_bool b
@@ -62,17 +65,16 @@ module Value = struct
     | Symbol s -> "#" ^ s
     | Function _ -> "<fun>"
     | External f -> "external " ^ f
-    | Nop -> "nop"
 
   let typeof = function
-    | Int _ -> Type.Int
-    | Float _ -> Type.Float
-    | Bool _ -> Type.Bool
-    | String _ -> Type.String
-    | Symbol _ -> Type.Symbol
-    | Function _ -> Type.Function
-    | External _ -> Type.External
-    | Nop -> failwith "Value.typeof"
+    | Unit -> Type.Unit
+    | Int _ -> Int
+    | Float _ -> Float
+    | Bool _ -> Bool
+    | String _ -> String
+    | Symbol _ -> Symbol
+    | Function _ -> Function
+    | External _ -> External
 
   let promote = function
     | Int n -> Float (float n)
@@ -148,7 +150,7 @@ module Primitive = struct
     | [Symbol op; Int l] ->
       let op = symbol_to_op op in
       Hashtbl.replace Parser.precedence op l;
-      Nop
+      Unit
     | _ -> failwith "set_op_pre"
 
   let get_op_pre = function
@@ -161,7 +163,7 @@ module Primitive = struct
     | [Symbol op; String a] ->
       let op = symbol_to_op op in
       Hashtbl.replace Parser.oper_assoc op @@ Parser.assoc_of_string a;
-      Nop
+      Unit
     | _ -> failwith "set_op_assoc"
 
   let get_op_assoc = function
@@ -183,12 +185,12 @@ module Primitive = struct
     in
     List.map to_string args
     |> List.iter (Printf.printf "%s");
-    Nop
+    Unit
 
   let println args =
     ignore @@ print args;
     Printf.printf "\n";
-    Nop
+    Unit
 
 
   let methods =
@@ -288,11 +290,13 @@ let extern f args =
     f vs
 
 exception Unbound of string
+exception Noop
 
 let rec eval global env ast =
   let aux = eval global env in (* eval with current env *)
   match ast with
-    | Nothing -> Nop
+    | Nothing -> raise Noop
+    | Nunit -> Unit
     | Nint n -> Int n
     | Nfloat n -> Float n
     | Nbool b -> Bool b

main.ml

@@ -49,9 +49,6 @@ let rep () : unit =
   let ast = line |> Lex.tokenize |> Parser.parse in
   if !debug then Ast.print ast;
   let var, v = Eval.eval_top g ast in
-  match v with
-  | Nop -> ()
-  | _ ->
   Hashtbl.replace g "ans" v;
   printf "%s: %s = %s\n"
     var (Type.to_string @@ Value.typeof v) (Value.to_string v)
@@ -67,6 +64,7 @@ let init_repl () =
 let rec repl () : unit =
   try rep (); repl () with
   | Exit | End_of_file (* Ctrl-D *) -> ()
+  | Noop -> repl ()
   | Reset_line -> printf "\n"; repl ()
   | e -> print_error e; repl ()
 

parser.ml

@@ -271,9 +271,14 @@ and value seq =
         let _, t, seq = any seq in
         Nsymbol (Token.to_string t), seq
       | LParen ->
+        seq |> either
+          (fun seq ->
+             let _, seq = token RParen seq in
+             Nunit, seq)
+          (fun seq ->
              let e, seq = mustbe (expr min_int) seq in
              let _, seq = mustbe (token RParen) seq in
-        e, seq
+             e, seq)
       | _ -> unexpected_token col x
     end