Henning Thielemann
I prefer the Cabal approach since it requires one (hacky) tool less in the tool chain. I use different Hs-Source-Dirs depending on a Cabal flag. With ghci you can choose the different modules using the -i option.
I see, but what about CABAL's version constraint solving algorithm: are there situations, where using the flag-based approach (to switch between disjunct version-ranges for a given dependency, thus requiring CABAL to backtrack on flag-settings iirc) leads to an observably different "solutions" than those which result when offering CABAL a contiguous version-range (and defer the version-range "discontinuity" to the CPP level)? Or are the two approaches "semantically" equivalent from the CABAL version constraint-solving point-of-view?