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

[Graham Berks]

Attempting 

$ 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.

Am I missing something ? Is there a way to force ghc to ignore this ??

Thanks

[Sjoerd Visscher]

My guess would be that persistent indirectly depends on transformers 0.2, since 0.3 is quite new and mtl 2.1.1 doesn't support transformers 0.2. But I can't find which package that is.

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Sjoerd Visscher

https://github.com/sjoerdvisscher/blog

[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