Hello Andrew, Tuesday, May 27, 2008, 1:02:06 AM, you wrote:
Today I was reading a potentially interesting paper, and I stumbled across something referred to as a "rank-2 type". Specifically,
existentials and polymorphic values are complement each other. existential contains *some* type of given class. polymorphic function can deal with *any* value of given class. so the only thing you can do with existential value is to apply polymorphic function to it f :: Class x => x -> Int is [order-1] polymorphic function which may be applied to any type of given class. higher-order function that receives order-n polymorphic function as its argument called polymorphic function of order n+1: ff :: (Class x => x -> Int) -> Int its first argument is a *function*. polymorphic function. so you need to call it with polymorphic function which is able to deal with *any* value of given class. it's very different to polymorphic function: g :: Class x => x -> Int -> Int which accepts any value of given class. in the last case you have polymorphic function that you can use with any value, in the first case you need to provide yourself polymorphic function as argument look at http://haskell.org/haskellwiki/GADTs_for_dummies it should be helpful to understand type extensions machinery -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com