Question

I’ve found --avoid-reinstalls makes packages build where otherwise cabal fails. What exactly is it doing? Doesn’t cabal already try to avoid reinstalls, or shouldn’t it?

Answer 1

This mailinglist post explains it pretty well: http://www.haskell.org/pipermail/beginners/2011-June/007502.html

The dependency resolution algorithm will try to make a consistent
install plan that allows gitit to be run. However, in order to achieve
that, it may decide that it’s necessary to reinstall some existing
packages in a new configuration (with other dependency or flag
settings). Such reinstalls will then overwrite the previous package in
the store, and if that previous package was depended on by something
else, these other things will break.

The –avoid-reinstalls flag prevents exactly this behavior.