Excerpts from Dario Teixeira's message of Sat Oct 24 18:11:26 +0200 2009:
> Hi,
> 
> I've come across another issue with Ocamlbuild, and I'm not sure if this one
> is a bug or a feature.  Consider the project tree below; note that everything
> under lib/ is supposed to produce a foo.cma library, while cmd/ is a command
> line application that uses foo.cma (and which can be safely ignored for now).
> 
> |-- _tags
> |-- lib/
> |   |-- _tags
> |   |-- foo.mllib
> |   |-- foo_core.mlpack
> |   |-- foo_core/
> |   |   |-- (...)
> |   |
> |   |-- foo_ext.mlpack
> |   |-- foo_ext/
> |       |-- (...)
> |
> |-- cmd/
>     |-- (...)
> 
> The file lib/_tags just contains "<foo_core> or <foo_ext>: include".  If I issue
> "ocamlbuild foo.cma" inside the lib/ directory, then foo.cma gets built as expected.
> However, if I move to the parent directory and issue "ocamlbuild lib/foo.cma",
> I get an error:
> 
> ocamlc -pack lib/foo_ext/beta.cmo lib/foo_ext/alpha.cmo -o lib/foo_ext.cmo
> File "_none_", line 1, characters 0-1:
> Error: The implementation (obtained by packing)
>        does not match the interface lib/foo_ext.mli:
>        Modules do not match:
>          sig val num : int end
>        is not included in
>          Foo_core.Zero.S
>        Unbound module type Foo_core.Zero.S
> Command exited with code 2.
> 
> 
> The problem is of course that "-I lib" should be added to the invocation of ocamlc.
> For this purpose I've created a myocamlbuild.ml in the root directory, with the
> following contents:

What about adding «"lib": tnclude» into the main _tags file?

BTW «<foo_core> or <foo_ext>: include» could be written
  «"foo_core" or "foo_ext": include»

Best regards,

-- 
Nicolas Pouillard
http://nicolaspouillard.fr