portability and .cabal files?

how is portability (meant to be) documented in .cabal files? the only thing i can find in Cabal/authors.html are 'os()' and 'arch()', but those are tests, rather than dependency fields. should .cabal files for non-portable packages consist of a big if with the tested and known-to-work os/arch combinations as test and an empty else-branch for unknown/untested/known-to-fail combinations? shouldn't hackage display portability of its packages? if i look through the system-dependent packages on hackage, hardly any of them indicate their platform limitations anywhere but in free-form comments and module names, so if you wanted to filter packages by supported platforms, you'd be stuck.. claus