Code from Friday, September 6th

Arith.ml

open Arith_syntax

let rec subst (x : id) (v : int) (e : exp) : exp = 
  match e with
  | Int _ -> e
  | Add (e1, e2) -> Add (subst x v e1, subst x v e2)
  | Mul (e1, e2) -> Mul (subst x v e1, subst x v e2)
  | Let (y, e1, e2) ->
    Let (y, subst x v e1, if x = y then e2 else subst x v e2)
  | Id y -> if x = y then Int v else Id y

let rec eval (e : exp) : int =
  match e with
  | Int n -> n
  | Add (e1, e2) -> eval e1 + eval e2
  | Mul (e1, e2) -> eval e1 * eval e2
  | Let (x, e1, e2) -> eval (subst x (eval e1) e2)
  | Id x -> failwith ("unbound identifier " ^ x)

let rec repl () = 
  print_string "> ";
  let str = read_line () in
  match Arith_util.parse str with
  | Arith_util.Exp exp -> 
    let _ = print_int (eval exp) in
    let _ = print_newline () in
    repl ()
  | Arith_util.ParseError msg ->
    let _ = print_string msg in
    let _ = print_newline () in
    repl ()

let _ =  
  match Array.to_list Sys.argv with
  | [ exe; "repl" ] -> repl ()
  | _ -> ()