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

Mauricio briqueabraque at yahoo.com
Tue Jan 20 11:56:30 EST 2009

>> 4294967296 :: Integer
> (...)
> In the above you can see the polymorphism of the return type of 
> fromInteger, it returns a Int8 or a Int32.
> You can see the polymorphism of the argument of "show", it takes an Int8 
> or Int32 or Integer.
> The latest ghc-6.10.1 also allows avoiding use of SomeNum, see 
> impredicative-polymorphism:
> http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#impredicative-polymorphism 

Great, thanks! I'm enlightened :)

But how is this:

data SomeNum = forall a. SN a

different from:

data SomeNum = SN (forall a. a)



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