#615: cabal install attempts to reinstall "same version" of already-installed package ---------------------------------+------------------------------------------ Reporter: guest | Owner: Type: defect | Status: new Priority: normal | Milestone: Component: cabal-install tool | Version: HEAD Severity: normal | Resolution: Keywords: | Difficulty: very hard (<1 week) Ghcversion: | Platform: ---------------------------------+------------------------------------------ Changes (by duncan): * difficulty: unknown => very hard (<1 week) * platform: Linux => Comment: If you do a --dry-run with -v it'll probably tell you that it's reinstalling the package because it has different and incompatible dependencies. The solver only chooses relatively late between the installed and version of a package and the one from hackage. It does this because it is sometimes necessary to rebuild packages to get consistent dependencies. At the end, if it's possible it'll substitute installed instances in the plan instead of rebuilding things. However that's not possible if the dependencies of the installed one clash with the other versions of packages that the solver has already selected. In general it is a good idea to bump the version if there are substantial changes. Since the solver does rather rely on the assumption that the hackage and installed instances of the same package version are essentially the same. There are a few ways this could be improved. If we used a completely different solver implementation it might be able to try harder to pick installed package instances rather than falling back to rebuilding. There is also the possibility to use "installed constraints" where you tell the solver explicitly to only use an installed instance of a particular package. This would only be a workaround of course. Ideally you'd like it to try harder in the first place to avoid rebuilding. -- Ticket URL: http://hackage.haskell.org/trac/hackage/ticket/615#comment:1 Hackage http://haskell.org/cabal/ Hackage: Cabal and related projects