OCaml GADTs and avoiding "type constructor would escape its scope" errors

test.ml

(* GADT list that exposes the type of the head element *)
type _ hlist =
 | Nil: 'a hlist
 | Cons: ('a * 'b hlist) -> 'a hlist

(* let rec len = function *)
(* let rec len (type a) (l: a hlist): int = match l with *)
(* both of the above result in a "type constructor would escape its scope" error *)
(* correct version: *)
let rec len : type a. a hlist -> int = function
(* also correct: *)
(* let rec len : 'a. 'a hlist -> int = function *)
 | Nil -> 0
 | Cons (h, t) -> 1 + len t

(* NB: this only works if you don't extract your elements.
   if you need to extract the elements then the error is real, and you either need to redesign your code
   so that the inner types (to be extracted) are contained in the outer type param *)