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

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