test.ml

type _ expr =
  | Num : int -> int expr
  | Add : int expr * int expr -> int expr
  | Lt  : int expr * int expr -> bool expr

let rec eval (type a) (expr : a expr) : a =
  match expr with
  | Num n -> n
  | Add (a, b) -> eval a + eval b
  | Lt (a, b) -> eval a < eval b