Type constructor would escape its scope... How to express return type?

Learning
4

Thanks for the comments. My objective was to abstract over the Zip compression library for portability reasons. I would like to target both native and JavaScript backends. I ended up with something like this:

module type Z = sig
    type in_file
    type entry
end

module type ContentType = sig
    type t
    type in_file
    val make : in_file -> t
    val do_something_with_t : t -> unit
    val from_archive : in_file -> t
end

module MakeContent(Zip : Z) : (ContentType with type in_file = Zip.in_file) = struct
    type in_file = Zip.in_file
    type t = unit
    let make _in_f = ()
    let do_something_with_t _t = print_string "done"
    let from_archive = make
end

(* In executable with a concrete implementation of Zip *)
let () =
    let module ZipContent = MakeContent(Zip) in
    let content = ZipContent.from_archive (Zip.open_in "archive.zip") in
    ZipContent.do_something_with_t content
;;
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