OCaml GADT's usage example

gadt.ml

type _ t =
  | Const : int -> int t
  | Add : int t * int t -> int t
  | Less : int t * int t -> bool t
  | IfThenElse : bool t * int t * int t -> int t

let rec eval : type a. a t -> a = function
  | Const x -> x
  | Add (l, r) -> (eval l) + (eval r)
  | Less (l, r) -> (eval l) < (eval r)
  | IfThenElse (cond, then_, else_) ->
      eval (if eval cond then then_ else else_)

let int_term =
  Add (Const 10,
       IfThenElse (Less (Add (Const 20,
                              Const 30),
                         Const 40),
                   Const 0,
                   Add (Const 30,
                        Const 2)))

let bool_term = Less (int_term, Const 50)

let () =
  assert (eval int_term = 42);
  assert (eval bool_term)