InstalledPackageInfo, IPI641, IPI642

[Edward Z. Yang]

Hello all,

I am currently adding support for packages to reexport modules (proper)
from other packages, but I've run into a nasty bit of BC in Cabal that I
don't quite understand; namely, InstalledPackageInfo, IPI641 and IPI642.

So, I understand in principle why we might want these data structures:
GHC is compiled with a specific version of Cabal, but one might want to
use a version of cabal-install with a different version of Cabal. What
I don't understand is why we have both InstalledPackageInfo and IPI642.
Additionally, what is the procedure for making changes to the IPI
format? This seems to have only been done once before, and I am not sure
how the current scheme generalizes.

Thanks,
Edward