Pass module's existential type to universal argument

m.ml

module type S = sig
  type t

  val of_string : string -> t
  val to_string : t -> string
end


let deser : 'a. (module S with type t = 'a) -> (_ -> 'a) * _ = fun m ->
  let module M = (val m) in
  M.(of_string, to_string)

(* Error:
 *   The type of this packed module contains variables:
 *   (module Convert.CONVERTER with type t = 'a0)
 *)
 

Found the answer:

module type S = sig
  type t

  val of_string : string -> t
  val to_string : t -> string
end

type 'a c = (module S with type t = 'a)

let deser = fun (type a) ((module M) : a c) ->
  M.(of_string, to_string)

(* ==>
 *   val deser : 'a c -> (string -> 'a) * ('a -> string) = <fun>
 *)