> data Bar f a = Foo f => Bar {bar :: f a}

The class context on the data constructor buys you nothing extra in terms of 
expressivity in the language.  All it does is force you to repeat the context 
on every function that uses the datatype.  For this reason, the language 
committee has decided that the feature will be removed in the next revision of 
Haskell.

Regards,
    Malcolm


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