Re: [Haskell-cafe] A curios monad

David Menendez wrote:

On Thu, Dec 11, 2008 at 1:48 PM, Andrew Coppin wrote:

...

BTW, does anybody know how rank-N types are different from existential types?

You mean the Haskell extensions?

ExistentialQuantification lets you define types such as,

data SomeNum = forall a. Num a => SomeNum a

RankNTypes lets you nest foralls arbitrarily deep in type signatures,

callCC :: ((forall b. a -> m b) -> m a) -> m a -- this is rank-3

RankNTypes implies ExistentialQuantification (among others).

So how is foo :: ((forall b. a -> m b) -> m a) -> m a different from bar :: forall b. ((a -> m b) -> m a) -> m a then?