open Sexplib.Sexp

let rec parse_bindings bindings =
  match bindings with
    | [] -> []
    | first::rest ->
      match first with
        | List([Atom(name); e]) ->
          [(name, parse e)] @ (parse_bindings rest)

let rec parse sexp : expr =
  match sexp with
    | Atom(s) ->
      (match int_of_string_opt s with
        | Some(num) -> ENumber(num)
        | None -> if mem s reserved_words then failwith "s is a reserved keyword" else EId(s))
    | List(sexps) ->
      match sexps with
        | [Atom("add1"); arg] -> EPrim1(Add1, parse arg)
        | [Atom("sub1"); arg] -> EPrim1(Sub1, parse arg)
        | [Atom("+"); arg1; arg2] -> EPrim2(Plus, parse arg1, parse arg2)
        | [Atom("-"); arg1; arg2] -> EPrim2(Minus, parse arg1, parse arg2)
        | [Atom("*"); arg1; arg2] -> EPrim2(Times, parse arg1, parse arg2)
        | [Atom("let"); List(bindings_sexp); e2] ->
          let binding_expr = parse_bindings bindings in
            ELet(binding_expr, parse e2)
        | _ -> failwith "foo"