test2.ml

type _ t =
  | IntLit : int -> int t
  | BoolLit : bool -> bool t;;

let eval : type a . a t -> (a * a) list =
  fun term ->
    List.map (fun x -> x, x) ((match term with
        | IntLit x -> [x]
        | BoolLit x -> [x]) : a list)