#220: detect inconsistent package dependencies -----------------------------+---------------------------------------------- Reporter: duncan | Owner: Type: enhancement | Status: new Priority: normal | Milestone: Component: Cabal library | Version: 1.2.3.0 Severity: normal | Keywords: Difficulty: hard (< 1 day) | Ghcversion: 6.8.2 Platform: | -----------------------------+---------------------------------------------- Now that packages are more fine grained and upgradeable we're commonly getting situations where we build a package that uses two other packages that were built against different versions of another common package. The most common example in the wild at the moment is people installing `bytestring-0.9.0.4` in addition to the `bytestring-0.9.0.1` that came with ghc-6.8.2 and then half their packages are built against one version and half against the other and when they try to use packages built against different versions we get a type error about `bytestring-0.9.0.1:Data.ByteString.ByteString` not being the same as `bytestring-0.9.0.4:Data.ByteString.ByteString`. Cabal could and should detect this situation, and `cabal-install` should plan around it in its dependency resolution algorithm. However in both cases we require the dependency information of the existing installed packages. See http://hackage.haskell.org/trac/ghc/ticket/1839 The simplest thing is to get the full `InstalledPackageInfo` for all the packages in the package databases. From that we can find all the dependencies. When we do find this situation it is not guaranteed to cause a compiler error, it's just rather likely. So it's not obvious if Cabal should warn or error. `cabal-install` could try to fix up inconsistencies but in general only by re-installing existing packages against different versions of their dependencies. The longer term solution is to allow installing multiple instances of the same versions of a package built against different versions of its dependencies. This is the functional Nix approach however it would probably also require support from ghc. -- Ticket URL: http://hackage.haskell.org/trac/hackage/ticket/220 Hackage http://haskell.org/cabal/ Hackage: Cabal and related projects