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

[dag.odenhall at gmail.com]

To be clear, I think DefaultSignatures should still be an option, but
possibly guarded by some CPP.


On Wed, Dec 12, 2012 at 6:31 AM, dag.odenhall at gmail.com <
dag.odenhall at gmail.com> wrote:

> 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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>
>
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