[Caml-list] Calling a single function on every member of a GADT?

[rixed@happyleptic.org]

I'm basically trying to do the equivalent of this simple `fold` function:

---
module Simple =
struct
  type term =
     | Int of int
     | Add
     | App of term * term

  let rec fold i f = function
    | Int _ as t -> f i t
    | Add -> f i Add
    | App (x, y) as t -> f (fold (fold i f x) f y) t
end
---

... but using a GADT:

---
module Gadt =
struct
  type _ term =
     | Int : int -> int term
     | Add : (int -> int -> int) term
     | App : ('b -> 'a) term * 'b term -> 'a term

  let rec fold : type a. 'r -> ('r -> _ term -> 'r) -> 'r = fun i f -> function
    | Int _ as t -> f i t
    | Add -> f i Add
(*
     ^ Error: This pattern matches values of type (int -> int -> int) term
        but a pattern was expected which matches values of type int term
        Type int -> int -> int is not compatible with type int
*)
    | App (x, y) as t -> f (fold (fold i f x) f y) t
end
---

I've tried other variants of the syntax and got many encouragements but no green flag from the type-checker.
Why is the compiler expecting an int term in there? I though the whole point of the `type a. ...` syntax was to allow the matched type to vary from one pattern to the next?
Is there a way to do this?