Add error handling

main.ml

@@ -1,14 +1,30 @@
+open Printf
+
+let error_to_string e =
+  try raise e with
+  | Parser.Expected t -> sprintf "expected %s" t
+  | Parser.Unexpected_token t -> sprintf "unexpected token \"%s\"" t
+  | Failure f -> f
+  | Division_by_zero -> "cannot divide by zero"
+  | _ -> raise e
+
+let print_error e =
+  printf "error: %s\n" @@ error_to_string e
+
 (* simple REPL *)
 let rec repl () : unit =
-  Printf.printf "> ";
+  printf "> ";
   let line = read_line () in
   if line <> "quit" then begin
-    line
-    |> Lex.tokenize
-    |> Parser.parse
-    |> Eval.eval
-    |> Ast.typ_to_string
-    |> Printf.printf "%s\n";
+    try
+      line
+      |> Lex.tokenize
+      |> Parser.parse
+      |> Eval.eval
+      |> Ast.typ_to_string
+      |> printf "%s\n"
+    with
+    | e -> print_error e;
     repl ()
   end