[Haskell-cafe] Type Constraints on Data Constructors

[DavidA]

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.)