[Haskell-beginners] problem installing checkers

[Brent Yorgey]

On Sun, Oct 23, 2011 at 06:56:57PM -0400, Britt Anderson wrote:
> I wanted to try reactive, but cabal install fails on checkers (a
> dependency). Here is a portion of the error message, there are other
> overlapping instances that follow. Can someone advise me how to
> proceed. Thank you.
> 
> [13 of 15] Compiling Test.QuickCheck.Classes (
> src/Test/QuickCheck/Classes.hs, dist/build/Test/QuickCheck/Classes.o )
> 
> src/Test/QuickCheck/Classes.hs:137:38:
>     Overlapping instances for Show (a -> b)
>       arising from a use of `property'
>     Matching instances:
>       instance Show (a -> b) -- Defined in Text.Show.Functions
>       instance Show base:System.Event.Manager.IOCallback
>         -- Defined in base:System.Event.Manager

This is very naughty of System.Event.Manager, to define IOCallback as
a *type synonym* for a certain function type, and then declare a Show
instance for it.  However, the instance shouldn't come into play
unless System.Event.Manager is imported, and I don't think it is. So
this error message seems rather odd to me.  What version of GHC do you
have?  What is the output of 'ghc-pkg list' and 'ghc-pkg check'?

-Brent