[Haskell-beginners] Quickcheck2 - writing a modifier for test data

[Brent Yorgey]

On Sun, Mar 31, 2013 at 12:21:48PM +0200, Nathan Hüsken wrote:
> 
>     No instance for (QuickCheck-2.4.2:Test.QuickCheck.Arbitrary.Arbitrary
>                        ValidPos)

The fact that it actually lists the package and version number in the
error message strongly suggests that the problem is conflicting
versions of the QuickCheck package.  Do 'ghc-pkg list QuickCheck' to
see if you have multiple versions installed, and unregister all but
one of them.

-Brent