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Date: Sat, 21 Sep 2013 01:17:44 +0100
Message-ID: <CAAxsn=EQR8og6Hr1TXj4CV=NnxCHA0a_o_EYf20stjHVpFv68Q@mail.gmail.com>
From: Jeremy Yallop <yallop@gmail.com>
To: Peter Frey <pjfrey@sympatico.ca>
Cc: Caml List <caml-list@inria.fr>
Content-Type: text/plain; charset=UTF-8
Subject: Re: [Caml-list] Packing/Unpacking modules

On 21 September 2013 00:22, Peter Frey <pjfrey@sympatico.ca> wrote:
> I am showing my code (which does not work) first; the rest follows:
>
> let q = let module M = (val s2:Stream with type a = char) in
>                   (module  struct
>                    include (val s2: Stream  with type a = char)
>                    let stream = (M.tail (M.stream))
>                    end : Stream with type a = char )

The root problem here is that there are two types in the Stream
signature, of which one (a) is exposed and one (t) is opaque.  Each
unpacking of s2 gives you a new instance of t which is incompatible
with the instances that come from other unpackings.

The solution is to unpack s2 only once:

 let q = let module M = (val s2:Stream with type a = char) in
                   (module  struct
                    include M
                    let stream = (M.tail (M.stream))
                    end : Stream with type a = char )