[Haskell-cafe] How is default method signatures supposed to be used together with Generics

[dag.odenhall at gmail.com]

The practice seems to be to not export it, but maybe it would be a better
practice to export it. That way it can work without DefaultSignatures too,
and if you use the generic-deriving package it could work with zero
extensions or GHC-specific dependencies.

Are you adding support to hashable after all? I'd love that, but thought
you decided against it because of the clash with the existing defaults.


On Wed, Dec 12, 2012 at 5:28 AM, Johan Tibell <johan.tibell at 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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