[Haskell-cafe] Re: Existencial quantification and polymorphic datatypes (actually, components...)

[Gleb Alexeyev]

I just thought that the shorter explanation could do better: the 
difference is in the types of the constructor functions.

Code:
 > {-# LANGUAGE ExistentialQuantification #-}
 > {-# LANGUAGE RankNTypes #-}
 > data SomeNum1 = forall a. SN1 a
 > data SomeNum2 = SN2 (forall a. a)

ghci session:
*Main> :t SN1
SN1 :: a -> SomeNum1
*Main> :t SN2
SN2 :: (forall a. a) -> SomeNum2

This is not the whole story, types of the bound variables you get on 
pattern matching differ too, but this makes the short explanation a bit 
longer :).