[Haskell-cafe] How to selectively export internal entities from a module for testing?

[Thorkil Naur]

Hello,

It is common practice to export only selected entities from a Haskell module 
and refrain from exporting other entities so that they are only available for 
internal use. There are many reasons for wanting to do this, such as reducing 
the number of importable entities, avoiding the exposure of internal details 
that may therefore more easily be changed, and to enable certain 
optimizations (http://haskell.org/haskellwiki/Performance/Modules).

However, for one particular use, namely automated testing, it often seems 
useful, even necessary, to allow even entities considered internal to be 
exported: If internal entities cannot be imported by the testing code, they 
can only be tested indirectly, via the exported entities, and that may turn 
out to be troublesome. A possibility would be to include the testing code in 
the module itself, but that seems clumsy and wasteful.

For possible guidance, I have looked at 
http://darcs.haskell.org/packages/filepath and System/FilePath/Internal.hs 
contains

>     -- Note: leave this section to enable some of the tests to work
> #ifdef TESTING
>     -- * Drive methods
>     splitDrive, joinDrive,
>     takeDrive, replaceDrive, hasDrive, dropDrive, isDrive,
> #endif

precisely to allow automated testing. And that, of course, solves the problem.

But I am wondering whether other solutions to this problem could be found, 
perhaps even solutions that stay within the limits of the Haskell language 
itself? If anybody know of alternative solutions, I would very much like to 
hear about them.

Thanks and best regards
Thorkil