[Haskell-cafe] library conflicts and how to resolve them

[Andres Löh]

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