module Num =
mixin body
  type term = Const of int | Add of term * term
  type value = Num of int
  type env
  let rec eval tm (e:env) = match tm with
    | Const i -> Num i
    | Add (t1, t2) -> begin match eval t1, eval t2 with
      | Num i, Num j -> Num (i + j)
      | _ -> failwith "Num.eval: type mismatch"
      end
    | tm -> inner tm e
end (* Num *)

module Func =
mixin
  let bind (x, v, e) = fun y ->
    if x=y then v else e y
body
  type term = Var of string
    | Abs of string * term | App of term * term
  type value = Clos of term * env
  and env = string -> value
  let eval tm (e:env) = match tm with
    | Var x -> e x
    | Abs _ as f -> Clos (f, e)
    | App (rator, rand) e -> begin
      match eval rator e with
      | Clos (Abs (x, b), e') ->
        eval b (bind (x, eval rand e, e'))
      | _ -> failwith "Func.eval: type mismatch"
      end
    | _ -> inner tm e
end (* Func *)