Re: [Caml-list] First-class modules in functor bodies

[Markus Mottl <markus.mottl@gmail.com>]

In my example "module A" should be bound to "struct let x = 42 end",
not to a module containing a first-class module binding "a".

But this reminds me of a further question.  The following is allowed:

-----
module M (U : sig end) = struct
  module type S = sig val x : int end
  let a = ((module struct let x = 42 end : S))
  module A = struct
    let x =
      let module A = (val a) in
      A.x
  end
end
-----

Wouldn't the above be essentially equivalent to what I want to do?
Maybe the current restriction covers some weird corner-case that I
can't think of right now.

On Sun, May 19, 2013 at 7:07 PM, Philippe Wang <mail@philippewang.info> wrote:
> I'm curious because I don't quite understand: could you tell the
> supposed semantic differences between your code and the following
> code?
> module M (U : sig end) = struct
>   module type S = sig val x : int end
>   let a = ((module struct let x = 42 end : S))
>   module A = struct let a = a end
> end
>
> On Sun, May 19, 2013 at 4:14 PM, Markus Mottl <markus.mottl@gmail.com> wrote:
>> Hi,
>>
>> I've been wondering why the following is disallowed:
>>
>> -----
>> module M (U : sig end) = struct
>>   module type S = sig val x : int end
>>   let a = ((module struct let x = 42 end : S))
>>   module A = (val a)
>> end
>> -----
>>
>> The error message is:
>>
>> -----
>> File "foo.ml", line 4, characters 13-20:
>> Error: This kind of expression is not allowed within the body of a functor.
>> -----
>>
>> Making M a module by removing the functor argument works as expected.
>>
>> Is there some inherent unsoundness issue with allowing this kind of
>> use of first-class modules within functor bodies, or would it just be
>> hard adding sound support for the above to the current type system?
>>
>> Regards,
>> Markus
>>
>> --
>> Markus Mottl        http://www.ocaml.info        markus.mottl@gmail.com
>>
>> --
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>
>
>
> --
> Philippe Wang
>    mail@philippewang.info



-- 
Markus Mottl        http://www.ocaml.info        markus.mottl@gmail.com