cabal-install now very picky about install planning

[Duncan Coutts]

Hi all,

If you're following the progress of cabal-install this is a heads up.

I've switched it over to using this new InstallPlan stuff. So the
dependency resolver is now expected to produce a valid install plan. The
conditions for validity are quite strict and the current weedy
dependency resolver is often not able to make a valid install plan. It
is particularly bad with dependencies on inconsistent versions of
packages, especially if the already installed packages have such
inconsistencies.

So watch out for these and report the most common ones and we can think
about how to improve the resolver. It does produce fairly detailed error
messages describing the problems with the install plan.

There's a bit more work to do to make the dep resolver pluggable.

I suspect it's not working with hugs at the moment again (producing
valid install plans when you've no idea what's installed is more than a
little difficult).

Duncan