Type equalities in sub modules

Type equalities in sub modules

[Daniel Bünzli]

Hello,

In the example below, is there any way to achieve M.B.t = M.t without  
introducing the intermediate mt type ?

> module M = struct
>   type t = int
>   module B = struct
>     type mt = t
>     type t = mt
>   end
> end

This doesn't work :

> module M = struct
>   type t = int
>   module B = struct
>     type t = M.t
>   end
> end

This doesn't work :

> module M = struct
>   type t = int
>   module B = struct
>     type t = t
>   end
> end

It seems type definitions should have been designed like value  
definitions by having both a 'type' and 'type rec' construct (though  
one can argue this is sufficently rare to justify the above  
inconvenience).

Thanks for your answers,

Daniel

Re: [Caml-list] Type equalities in sub modules

[Hendrik Tews]

Daniel Bünzli <daniel.buenzli@epfl.ch> writes:

   In the example below, is there any way to achieve M.B.t = M.t without
   introducing the intermediate mt type ?

   > module M = struct
   >   type t = int
   >   module B = struct
   >     type mt = t
   >     type t = mt
   >   end
   > end

How about

module M = struct
  type t = int
  module BF(T : sig type mt end) = struct
    type t = T.mt
  end
  module B = BF(struct type mt = t end)
end

??

Hendirk

Re: [Caml-list] Type equalities in sub modules

[Till Varoquaux]

You seem to be looking for recursive modules, this might prove quite a
painful solution since you have to explicitly write the signature
(there's no type inference on recursive modules, this is a rather
complicated issue. Information can be found in [1])...:

module rec  M :
sig
 type t = int
  module B : sig type t = M.t end
end= struct
  type t = int
  module B = struct
    type mt = t
    type t = mt
  end
end;;

Happy new year,
Till
[1]:http://perso.ens-lyon.fr/tom.hirschowitz/
On 1/2/07, Hendrik Tews <H.Tews@cs.ru.nl> wrote:
> Daniel Bünzli <daniel.buenzli@epfl.ch> writes:
>
>    In the example below, is there any way to achieve M.B.t = M.t without
>    introducing the intermediate mt type ?
>
>    > module M = struct
>    >   type t = int
>    >   module B = struct
>    >     type mt = t
>    >     type t = mt
>    >   end
>    > end
>
> How about
>
> module M = struct
>   type t = int
>   module BF(T : sig type mt end) = struct
>     type t = T.mt
>   end
>   module B = BF(struct type mt = t end)
> end
>
> ??
>
> Hendirk
>
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Re: [Caml-list] Type equalities in sub modules

[Daniel Bünzli]

You are right, that's one way to solve the problem I didn't think about.

> module rec M : sig
>   type t = int
>   module B : sig type t = M.t end
> end = struct
>   type t = int
>   module B = struct type t = M.t end\x13
> end ;;

Thanks,

Daniel