$ 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.
And as far as I can tell the build of aeson-0.6.0.2 doesn't require that specific version of mtl.