eval

(* 型の宣言 *)
type t = Num of int
          | Plus of t * t
type v = VNum of int

(* 値を文字列に変換する関数 *)
let v_to_string v = match v with
    VNum (n) -> string_of_int n

(* インタープリタ *)
let rec eval t e = match t with
    Num (n) -> VNum (n)
  | Plus (t0, t1) -> (let v1 = eval t1 e
      in let v0 = eval t0 e
      in (match (v1, v0) with (VNum (n1), VNum (n0))
            -> VNum (n1 + n0)))
let f t = eval t []