Hi.
$ cabal install persistent Resolving dependencies... In order, the following would be installed: aeson-0.6.0.2 (reinstall) changes: mtl-2.1.1 -> 2.0.1.0 persistent-0.9.0.3 (new package) cabal: The following packages are likely to be broken by the reinstalls: buildwrapper-0.5.2 Use --force-reinstalls if you want to install anyway.
Any ideas what is going on ? I believe aeson-0.6.0.2 is already installed.
Yes.
And as far as I can tell the build of aeson-0.6.0.2 doesn't require that specific version of mtl.
It doesn't, otherwise it couldn't already be installed. Something persistent depends on in the selected plan depends on mtl-2.0.1.0, and therefore implies the reinstall. It'd be interesting to see the trace of the solver (pass -v3 to the cabal command). You could also try what adding --constraint="mtl == 2.1.1" yields. Cheers, Andres