Am Sonntag, 14. Dezember 2008 00:00 schrieb Andrew Coppin: > David Menendez wrote: > > On Thu, Dec 11, 2008 at 1:48 PM, Andrew Coppin > > > > <andrewcop...@btinternet.com> 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? > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
