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

[Bruno Oliveira]

Hello Bullat,

>> also, ghc66 adds impredicative polymorphism. how it differs from
>> unqualified existentials?

> I have not tried ghc66, but I think one of the things you should be able to do and that 
> is perhaps helpful for understanding existencial is:
> 
> > myList :: [forall a . Num a => a]
> > myList = [3 :: Int, 4 :: Float, 6 :: Integer]

> which in previous versions of GHC would need to be written as:

> > myList :: [Ex]
> > myList = [Ex (3 ::Int), Ex (4 :: Float), Ex (6 :: Integer)]

I took a look at the documentation and I think I told you the wrong thing here.
I think this should be equivallent to:

> myList :: [Po]
> myList = [Po 3, Po 4, Po 6]

which, in this case wouldn't be too useful (would it?). Having structures with polymorphic 
components is more useful when you have functions. The example they give is:

> f :: Maybe (forall a. [a] -> [a]) -> Maybe ([Int], [Char])
>  f (Just g) = Just (g [3], g "hello")
>  f Nothing  = Nothing

However, the following (which would be the right way to express my example) 
would be handy to have as well:

> myList :: [exists a . Num a => a]
> myList = [3 :: Int, 4 :: Float, 6 :: Integer]

but I don't think this is available in GHC 6.6.

Can anyone confirm this?

Cheers,

Bruno Oliveira