mtl library in HEAD?

Kathleen Fisher kathleen.fisher at
Thu Sep 2 00:21:09 EDT 2010

I'm trying to install the mtl package with the current HEAD version of Haskell, and I get the following error message:

> Control/Monad/Error.hs:76:10:
>     Duplicate instance declarations:
>       instance Error e => Monad (Either e)
>         -- Defined at Control/Monad/Error.hs:76:10-38
>       instance Monad (Either e) -- Defined in Control.Monad.Instances
> Control/Monad/Error.hs:87:10:
>     Duplicate instance declarations:
>       instance Error e => MonadFix (Either e)
>         -- Defined at Control/Monad/Error.hs:87:10-41
>       instance MonadFix (Either e) -- Defined in Control.Monad.Fix
> cabal: Error: some packages failed to install:
> mtl- failed during the building phase. The exception was:
> ExitFailure 1

The library installs fine with earlier versions of Haskell.  Is there something wrong with my installation, or is this a new incompatibility?  If the latter, is a corrected version of the mtl library going to be available soon?  Alternatively, how can I fix it?


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