Re: [Haskell-cafe] What does the `forall` mean ?

2009/11/12 Ryan Ingram :

On Thu, Nov 12, 2009 at 2:50 AM, Eugene Kirpichov wrote:

...

But that's not an issue of semantics of forall, just of which part of the rather broad and universal semantics is captured by which language extensions.

The forall for existential type quantification is wierd.

...

data Top = forall a. Any a   -- existential data Bottom = All (forall a. a) -- rank 2

Hm, you're right. I didn't even remember you could define existential types without GADT syntax. I also find the GADT syntax much better for teaching people what an ADT is.

I think it makes much more sense in GADT syntax:

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data Top where    Any :: forall a. a -> Top data Bottom where    All :: (forall a. a) -> Bottom

where it's clear the forall is scoping the type of the constructor.

 -- ryan

-- Eugene Kirpichov Web IR developer, market.yandex.ru