Hi Johan,

The error message is not ideal, but it does say that adding a Generic (Foo a)
instance might solve the problem.

I generally do not export classes like GHashable because they are closed; users
should never need to provide more instances themselves. Also, exporting the class
won't make the error message any better, and the solution to this error doesn't really
involve the GHashable class...


Cheers,
Pedro

On Wed, Dec 12, 2012 at 4:28 AM, Johan Tibell <johan.tibell@gmail.com> wrote:
Hi,

I noticed that you're not required to export the types mentioned in
the default method signature. For example, you could have:

    default hashWithSalt :: (Generic a, GHashable (Rep a)) => Int -> a -> Int
    hashWithSalt salt = ghashWithSalt salt . from

and not export the GHashable class. However, if users try to define an
instance of Hashable but forget to derive Generic:

    data Foo a = Foo a String
                 deriving (Eq)  -- Oops, forgot Generic here

    instance (Hashable a) => Hashable (Foo a)

they get a pretty bad error message:

Test.hs:10:10:
    Could not deduce (Generic (Foo a),
                      Data.Hashable.Class.GHashable (GHC.Generics.Rep (Foo a)))
      arising from a use of `Data.Hashable.Class.$gdmhashWithSalt'
    from the context (Hashable a)
      bound by the instance declaration at Test.hs:10:10-41
    Possible fix:
      add (Generic (Foo a),
           Data.Hashable.Class.GHashable
             (GHC.Generics.Rep (Foo a))) to the context of
        the instance declaration
      or add instance declarations for
         (Generic (Foo a),
          Data.Hashable.Class.GHashable (GHC.Generics.Rep (Foo a)))
    In the expression: (Data.Hashable.Class.$gdmhashWithSalt)
    In an equation for `hashWithSalt':
        hashWithSalt = (Data.Hashable.Class.$gdmhashWithSalt)
    In the instance declaration for `Hashable (Foo a)'

Exporting GHashable would help a little bit in that users would at
least know what this GHashable class that the error talks about is.

What's best practice?

-- Johan

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