Bulat Ziganshin
base isn't compatible between major ghc versions (6.6/6.8). so you may use only base 2.0 with 6.6 or base 3.0 with 6.8.
So is a lot of other packages.
situation is different for other libraries - you can keep to use fps 0.7 with any ghc version because your program developed against it,
With any? Are you sure? And even, perhaps, with future versions of GHC, 100 years after?
or you can upgrade to new version if you need new features
If you look at http://www.haskell.org/ghc/docs/latest/html/libraries/index.html you'll see, that base is just a buch of some libraries, maintained together. There's no practical reason why base can't be released separate from GHC. Base is used with other Haskell compilers as well, as you know. If it will be possible to automatically compile a cabalized package against appropriate version of it's dependencies, then i fail to see why it shouldn't be possible to compile against different releases of base as well.