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Date: Tue, 03 Apr 2012 13:23:27 +0200
From: Dawid Toton <d0@wp.pl>
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Subject: [Caml-list] When functor yields many types - include a module with extra type
 equality?

Consider the following:

--- a.mli
module type B = sig
   type t
end

module type S = sig
   type t
   type other_lenghty_definitions
   val not_much_code : t
end

module Make (B : B) : S
   with type t = B.t
---
--- a.ml
module type B = sig
   type t
end

module type S = sig
   type t
   type other_lenghty_definitions
   val not_much_code : t
end

module Make (B : B) = struct
   type t = B.t
   type other_lenghty_definitions
   let not_much_code = assert false
end
---

So we have to keep 3 copies of other_lenghty_definitions. I can have it 
isolated in a separate file and insert into a.ml and a.mli in an extra 
preprocessing step. But I believe there should be some clean solution, 
something like the following:

--- c_sig.ml
module S = struct
   type t
   type other_lenghty_definitions
end

module type S = sig
   include (module type of S) (* 1 *)
   val not_much_code : t
end
---
--- c.mli
module type B = sig
   type t
end

module Make (B : B) : C_sig.S
   with type t = B.t
---
--- c.ml
module type B = sig
   type t
end

module Make (B : B) = struct
   include C_sig.S
     with type t = B.t (* 2 *)

   let not_much_code = assert false
end
---

How to do what I mark with (* 1 *) and (* 2 *) correctly? Would it help 
if I upgrade my toolchain from 3.11.2 to some more recent version?
Dawid