portability and .cabal files?

[Claus Reinke]

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