[Haskell-beginners] Friend Modules?

[Brent Yorgey]

On Tue, Nov 09, 2010 at 07:00:59PM +0200, John Smith wrote:
> I have a module which defines data structures and their functions for
> a program. I have another module which defines show methods for the
> structures. Both modules have a substantial amount of code, and I
> would prefer to keep them separate. However, the show methods require
> access to internal data structures, which would not normally be
> exported. Furthermore, many modules do not require the show
> functions. Is there any solution to this, other than an intermediate
> module which imports the data module, and only exports the required
> names?

I have wanted this, too (in the context of writing tests, where I want
to test the unexported internals of a module).  But unfortunately
there is no way to do it other than the method you cite (to have a
module which just imports the other module and only exports certain
things).  In several of my projects I follow the pattern of having a
bunch of modules that export everything, and then a single module that
imports all the other modules and only re-exports selected things; the
intention is that users will just import the single module.

-Brent