[Haskell-cafe] Error when trying to use Hashable

Brandon Allbery allbery.b at gmail.com
Mon May 12 01:03:46 UTC 2014

On Sun, May 11, 2014 at 8:58 PM, John Ky <john at gocatch.com> wrote:

> I have some code that compiles when running on GHC on OS X, but not on
> Ubuntu:
>     No instance for (hashable-
>                        (GHC.Generics.Rep Point))
>       arising from a use of
> `hashable-$gdmhashWithSalt'
>     Possible fix:
>       add an instance declaration for
>       (hashable-
>          (GHC.Generics.Rep Point))
>     In the expression:
>       (hashable-$gdmhashWithSalt)
>     In an equation for `hashWithSalt':
>         hashWithSalt
>           = (hashable-$gdmhashWithSalt)
>     In the instance declaration for `Hashable Point'

The fully qualified (with package name/version) typeclass makes me think
you have multiple versions of the hashable package installed, and the one
your source is using is different from one pulled in by something else.

ghc makes it dangerous to have multiple versions of a package installed.
Use `ghc-pkg list` to make sure you have only one; if there is one in the
global section and another in the local section, remove the local one and
reinstall anything broken by that with `cabal install $thepackage
--constraint='hashable installed'`.

brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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