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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