Re: [Haskell-cafe] [Haskell] Higher types in contexts

moved to haskell-cafe On Mon, 5 Mar 2012, Barney Hilken wrote:

Is there any deep reason why I can't write a polymorphic type in a context? I think the record update problem can be (sort of) solved if you could write:

class Has r Rev (forall a. [a] -> [a]) => HRClass r where setHRClass :: (forall a.[a] -> [a]) -> r -> r

but polymorphic types are not allowed in contexts. Is this one of the problems SPJ considers "Hard" or is it a feasible extension?

I don't know what you want to do, but you may wrap the (forall a. [a] -> [a]) in an existential type: data ListFunc = forall a. ListFunc ([a] -> [a]) class Has r Rev ListFunc => HRClass r where setHRClass :: ListFunc -> r -> r