Thomas Tuegel
What exactly does the pre-process phase do, anyways?
It runs the appropriate pre-processor (Alex, Happy, c2hs, etc.) for modules that require it. It's slow because of the way the process is carried out: For each module in the package description, Cabal tries to find an associated .hs source file in the hs-source-dirs. If it cannot, it looks for a file with an extension matching one of the pre-processors it knows about. If it finds one, it runs the corresponding program if the output files are missing or outdated.
Interesting! In the new Shake-based build system we also need to automagically generate .hs files using Alex et al. My first implementation was slow but then I realised that it is possible to scan the source tree only once and remember where all .hs/.x/etc files are. This brought down the complexity from quadratic to linear in my case -- maybe this could be reused in cabal too? By the way, there seem to be a fair amount of code & functionality overlap in cabal and the new build system. We might want to look into this once the build system becomes more stable. Cheers, Andrey