cabal: question on compiling build prerequisites

[Sven Panne]

2013/12/7 Henning Thielemann <lemming at henning-thielemann.de>:
> I have not tried this solution but I would guess that it is a good idea to
> put the preprocessor in a separate package anyway, since it might have its
> own build-depends.

I thought about a similar scenario recently, and I would like to hear
if there is a clean cabal solution for this, too. Ripping out a helper
program which has only a single use case (support for the "real"
package) and which totally has to stay in sync with that "real"
package would not be very nice. It would just be an artifact of a
(missing?) cabal feature.

Note that e.g. Template Haskell is not a solution for all needs
(portability, XML=>Haskell translations, etc.), so support for some
preprocessing step would be nice.