Upgrading packages/Updating to GHC 7
A list of common problems upgrading packages to GHC 7.
If you maintain packages, this is for you.
If you use -XGADTs or -XTypeFamilies (or -fglasgow-exts, which is deprecated) you get -XMonoLocalBinds, which says that local let/where bindings are not auto-generalised. There's an extensive discussion of the reason for this decision in our paper "Let should not be generalised" and the journal version
base 3 goes away
There is no base 3 now, after being deprecated for several years, so dependencies on base 3 won't compile with GHC 7.