Package authors, We are about to add an additional quality check in the Hackage upload process that will affect many packages. Hackage will require an upper bound on the version of the base package and reject packages that omit it. Lots of packages currently specify: build-depends: base or build-depends: base >= 3 This is bad. There is no way for the tools to discover if you mean base 3 or 4 and yet most packages break when we pick the wrong one. You must specify an upper bound so that your package does not break when the next major version of the base library is released. For example, if you've tested with base 3 and 4 then use: build-depends: base >= 3 && < 5 Or if it needs version 4 then use: build-depends: base == 4.* Background ---------- When base 4 was released with ghc-6.10.x the vast majority of packages would have broken were it not for a compatibility shim that we added to the cabal-install tool. It defaulted to picking base 3 when the version constraints did not otherwise specify the use of base 4. The same trick will not work when base 5 is released because it will likely not be possible to continue to ship base 3. It is also important that we start to move towards making the majority of packages compatible with base 4. cabal-install changes --------------------- We will shortly release a minor update to cabal-install to change the way it picks between base 3 and 4. Currently if it has the choice it picks version 3. However that is not optimal for cases like: build-depends: base >= 3 && < 5 where it is clear that it is supposed to work with both. In this case we should not be pessimistic in going with the old base 3 and should instead pick base 4. More precisely, we will only apply the compatibility shim when the version is unbounded above. When it is bounded above, as Hackage will soon require, the normal preference to use the latest version applies. If you'd like to help us check this feature is working properly then grab the latest darcs version of cabal-install (which needs the latest darcs version the Cabal lib). See also http://hackage.haskell.org/trac/hackage/ticket/485 Duncan