[Caml-list] include two module implementations sharing the same type definitions

[Caml-list] include two module implementations sharing the same type definitions

[Yann Régis-Gianas]

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Dear OCaml module hackers,

the recently polished "substitution inside type signature" (described in
Sec 8.11 in the manual) is a real improvement to modularize module
signature developments.

Yet, it seems to me that "include" at the level of module implementations
is preventing us to unleash the full composability power of "include" at
the level of signature. Consider the following example:

``
module type A = sig type t end

module type B = sig type t end

type u = U

module AI : A with type t = u = struct
  type t = u
end

module BI : B with type t = u = struct
  type t = u
end

module ABI : sig
  include A
  include B with type t := t
end = struct
  include AI
  include BI (* This include unfortunately triggers:

Error: Multiple definition of the type name t.
       Names must be unique in a given structure or signature.

  *)
end
``

I have two questions:

Would it make sense to allow module implementation inclusion to introduce
multiple definitions of a given type name as long as these definitions are
equivalent?

Is there an idiom to work around this (apparent) limitation?

Cheers,
-- 
Yann Régis-Gianas

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Re: [Caml-list] include two module implementations sharing the same type definitions

[Danny Gratzer]

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Hello,

Perhaps I'm overlooking something but replacing

    include BI

with

    include (BI : B with type t := t)

seems like a reasonable fix to me.

Danny

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Re: [Caml-list] include two module implementations sharing the same type definitions

[Yann Régis-Gianas]

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Hello Danny,

and thank you! That is clearly a nice work around even though it may
require to introduce unnecessary module signatures for the only purpose of
composing module implementations.

Cheers,

Le jeu. 29 nov. 2018 à 10:02, Danny Gratzer <danny.gratzer@gmail.com> a
écrit :

> Hello,
>
> Perhaps I'm overlooking something but replacing
>
>     include BI
>
> with
>
>     include (BI : B with type t := t)
>
> seems like a reasonable fix to me.
>
> Danny
>
-- 
Yann Régis-Gianas

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Re: [Caml-list] include two module implementations sharing the same type definitions

[Frédéric Bour]

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1) In all generality,

  include (BI : module type of struct include BI end with type t := t)

will work if you haven't defined `module type B` or if `BI` exports other
types (say u) and you need to preserve equalities (BI.u = B.u).

2) Changes have been made to the module system in the trunk version of
OCaml. The original code should be accepted without any change.

On Thu, Nov 29, 2018 at 10:03 AM Danny Gratzer <danny.gratzer@gmail.com>
wrote:

> Hello,
>
> Perhaps I'm overlooking something but replacing
>
>     include BI
>
> with
>
>     include (BI : B with type t := t)
>
> seems like a reasonable fix to me.
>
> Danny
>

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Re: [Caml-list] include two module implementations sharing the same type definitions

[Yann Régis-Gianas]

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Thank you Frédéric!

Le jeu. 29 nov. 2018 à 10:08, Frédéric Bour <defree@gmail.com> a écrit :

> 1) In all generality,
>
>   include (BI : module type of struct include BI end with type t := t)
>
> will work if you haven't defined `module type B` or if `BI` exports other
> types (say u) and you need to preserve equalities (BI.u = B.u).
>
> 2) Changes have been made to the module system in the trunk version of
> OCaml. The original code should be accepted without any change.
>
> On Thu, Nov 29, 2018 at 10:03 AM Danny Gratzer <danny.gratzer@gmail.com>
> wrote:
>
>> Hello,
>>
>> Perhaps I'm overlooking something but replacing
>>
>>     include BI
>>
>> with
>>
>>     include (BI : B with type t := t)
>>
>> seems like a reasonable fix to me.
>>
>> Danny
>>
> --
Yann Régis-Gianas

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