On 6 October 2012 17:25, José Lopes
OK.
But, wouldn't it be possible for xmobar to use mtl-2.0.1.0 and for parsec to use mtl-2.1.1, while xmobar would use this parsec version? In this case, I am assuming that mtl-2.0.1.0 and mtl-2.1.1 are considered two different libraries.
Possibly, yes, but cabal doesn't know that. It has to make the conservative assumption that you might use them together and so they'd better be the same type. If cabal knew for sure that parsec did not expose types from mtl, then it'd be fine for it to use parsec built against a different version of mtl, because there would be no way to end up trying to equate types from two different package instances. This is the idea behind private or encapsulated dependencies: we would declare in .cabal files that our use of some dependency does not "leak out". But to be clear: this feature has not yet been implemented. But actually in this case I think parsec does expose the fact that it uses types from mtl. So it actually would not help here. Duncan