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

[Philippe Wang <mail@philippewang.info>]

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