On Sun, Sep 12, 2010 at 20:46, Tillmann Rendel
Paolo Giarrusso wrote:
in a tracker entry you linked to, http://hackage.haskell.org/trac/hackage/ticket/704, duncan argues that "we also want to be able to do things like linking multiple versions of a Haskell package into a single application". [snip] Even with the technical ability to link all of foo, bar, pair-1 and pair-2 together, I don't see how this program could be reasonably compiled. Therefore, I think that the notion of consistent install plans is relevant semantically, not just to work around some deficiency in the linking system.
Your case is valid, but OTOH there other cases to support: if I link together two programs which use _internally_ different versions of regex packages, cabal should support that - and here I guess we agree. The issue is how to express or recognise the distinction. I had this kind of scenario in mind, and that's why I proposed using versioned typenames for typechecking - your example program would then be caught as ill-typed. However, that's not enough, because the correct solution is to use the same pair version. - OTOH, Program would probably have its own cabal file, which could maybe list a dependency on pair. But I don't like this solution - the developer shouldn't have to do this. - The nicer alternative would be to extract, from the types used in the .hi files, whether they mention pair at all - like here, and unlike the case when the different packages are used internally. This solution is perfect but takes extra work which I can't estimate. Actually, some more work would maybe be needed to cope with cross-module inlining, but I believe that this can be done by cabal, by just "looking at" .hi files, without further changes to GHC - after versioned typechecking is introduced if missing, anyway. And maybe some interface to .hi files should be exposed. -- Paolo Giarrusso - Ph.D. Student http://www.informatik.uni-marburg.de/~pgiarrusso/