Malcolm Wallace <malcolm.wallace <at> me.com> writes:
>
>
> > 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
>
"All it does is force you to repeat the context on every function that uses the
datatype"
I think that's exactly what the original poster is complaining about. As a real-
life example, consider
data Graph a = Ord a => G [a] [[a]]
My intention is that whenever I have a Graph a, I want to be able to use the
Ord
instance on a.
So suppose I now define
automorphisms :: (Ord a) => Graph a -> [Permutation a]
On the basis of the "don't repeat yourself" principle, it seems redundant to
have to specify the (Ord a) context here, since I already specified it in the
data constructor for Graph a.
So this is a proposal for a change to the language: don't require a context on
a
function if it is already implied by a context on a data type.
(This would make even more sense for newtypes, where we know there is exactly
one data constructor, so no possibility of different data constructors
requiring
different contexts.)
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