Re: [Caml-list] strange error with packed modules and module type of

["Milan Stanojević" <milanst@gmail.com>]

If I just do
ocamlopt.opt -for-pack G -c foo.mli foo.ml
ocamlopt.opt -pack -o g.cmx foo.cmx

that is, forget all the fancyness with std and specifying interface
for G, the resulting G module has the type
sig module Foo : sig type t = G.Foo.t val x : t end end

So somehow the resulting interface of G is defined in terms of itself?!

Should I just file this problem on ocaml mantis?

p.s the only way I was able to get interface for G is to make a helper
program that uses G with a wrong signature and get the compiler to
give me the type in the error message. Is there a better way?

main:ml
module F : sig type t end = G
let () = ()

$ ocamlopt.opt g.cmx main.ml
File "main.ml", line 1, characters 28-29:
Error: Signature mismatch:
       Modules do not match:
         sig module Foo : sig type t = G.Foo.t val x : t end end
       is not included in
         sig type t end
       The field `t' is required but not provided



On Tue, May 29, 2012 at 10:30 PM, Markus Mottl <markus.mottl@gmail.com> wrote:
> On Tue, May 29, 2012 at 8:25 PM, Milan Stanojević <milanst@gmail.com> wrote:
>> g.mli:
>> module Std : sig include module type of Std end
>
> Btw., this is equivalent and slightly shorter:
>
>  module Std : module type of Std
>
>> and compile similarly like before
>> ocamlopt.opt -for-pack G -c foo.mli foo.ml
>> ocamlopt.opt -for-pack G -c foo.cmx std.ml
>> ocamlopt.opt -c g.mli
>> ocamlopt.opt -pack -o g.cmx foo.cmx std.cmx
>>
>> But now I get this helpful error message
> [snip]
>> Does anyone know what is going on here?
>
> This seemingly identical construction compiles:
>
> g.mli:
> module Std : sig module Foo : module type of Foo end
>
> I guess this also points to what is going wrong.  If I understand this
> correctly, the workaround basically says: "Std contains a submodule
> Foo which just happens to have the same signature as the module
> implemented by foo.ml".  But the broken version says: "Std contains a
> submodule Foo which is equivalent to the module implemented by
> foo.ml".  This equivalence is somehow lost with packing, hence the
> unhelpful error message.  Not sure, but I guess this can be fixed in
> the compiler?
>
> Regards,
> Markus
>
> --
> Markus Mottl        http://www.ocaml.info        markus.mottl@gmail.com