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

Hello haskell-cafe, 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) ? -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com