Re: [Haskell-cafe] what is a difference between existential quantification and polymorhic field?

[apologies to Bulat for the repeat posting] On Thu, Sep 21, 2006 at 12:05:23PM +0400, Bulat Ziganshin wrote:

now i'm reading Haskell' proposals and found that these two things considered as different:

http://hackage.haskell.org/trac/haskell-prime/wiki/ExistentialQuantification http://hackage.haskell.org/trac/haskell-prime/wiki/PolymorphicComponents

can you please explain me what is the difference between

data Ex = forall a. Num a => Ex a

and

data Po = Po (forall a. Num a => a)

Consider the types of the constructors: Ex :: forall a. (Num a) => a -> Ex Po :: (forall a. (Num a) => a) -> Po