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 :).
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