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

[ChrisK]

> Great, thanks! I'm enlightened :)

And no one had to hit you with a stick first!

> 
> But how is this:
> 
> data SomeNum = forall a. SN a
> 
> different from:
> 
> data SomeNum = SN (forall a. a)
> 
> ?

At a glance they look the same to me — but only the first is accepted by ghc.

There is also the GADT syntax:

> data SomeNum where SomeNum :: forall a. (Typeable a, Num a) => a -> SomeNum

which is accepted by ghc with the LANGUAGE GADTs extension.

The GADT is more than simple syntactic sugar, it allows for easier use this kind 
of existential type.

-- 
Chris