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

Henning Thielemann lemming at henning-thielemann.de
Mon Mar 5 13:39:36 CET 2012

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

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