Re: [Haskell-cafe] A curios monad

Am Sonntag, 14. Dezember 2008 00:00 schrieb Andrew Coppin:

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

Here, the argument of foo's argument must be a polymorphic function, capable of returning an (m b) whatever b is.

different from

bar :: forall b. ((a -> m b) -> m a) -> m a

Here, the argument of bar's argument can have any monomorphic type (a -> m b)

then?