[Haskell-cafe] GHC 7/Cabal request for help

[John Goerzen]

Hi folks,

I don't have a GHC 7 environment running yet (it's on my list...) but I 
received a bug report pointing me at this build failure:

http://hackage.haskell.org/packages/archive/testpack/2.0.1/logs/failure/ghc-7.0

Among other things, this noted:

Dependency QuickCheck >=2.1.0.3: using QuickCheck-2.4.0.1

and the errors were:

[1 of 3] Compiling Test.QuickCheck.Instances ( 
src/Test/QuickCheck/Instances.hs, dist/build/Test/QuickCheck/Instances.o )

src/Test/QuickCheck/Instances.hs:39:10:
     Duplicate instance declarations:
       instance Arbitrary Word8
         -- Defined at src/Test/QuickCheck/Instances.hs:39:10-24
       instance Arbitrary Word8 -- Defined in Test.QuickCheck.Arbitrary

src/Test/QuickCheck/Instances.hs:42:10:
     Duplicate instance declarations:
       instance CoArbitrary Word8
         -- Defined at src/Test/QuickCheck/Instances.hs:42:10-26
       instance CoArbitrary Word8 -- Defined in Test.QuickCheck.Arbitrary

Now, that's fairly standard, and in fact, in my code, is wrapped with:

#if MIN_VERSION_QuickCheck(2,3,0)
     -- we have Word8 instances here
#else
instance Arbitrary Word8 where
     arbitrary = sized $ \n -> choose (0, min (fromIntegral n) maxBound)

instance CoArbitrary Word8 where
     coarbitrary n = variant (if n >= 0 then 2 * x else 2 * x + 1)
                 where x = abs . fromIntegral $ n
#endif

And that code has been working to support modern QuickCheck versions for 
some time.

It would appear that something in Cabal, GHC 7, or QuickCheck is 
breaking this check.

Ideas?

-- John