Existentials

[Jon Fairbairn]

On 2003-04-17 at 09:02BST "Simon Peyton-Jones" wrote:
> Many of you have grown to love existential data types, which we current
> write like this:
> 
> 	data T = forall a. Foo a (a -> Int)
> 
> Mark and I chose 'forall' rather than 'exists' to save grabbing another
> keyword from the programmer.  And indeed, the type of the constructor
> Foo is
> 	Foo :: forall a. a -> (a->Int) -> T
> 
> But every single time I explain this to someone, I find I have to make
> excuses for using the term 'forall'.  I say "it really means 'exists',
> but we didn't want to lose another keyword". 
> 
> I have gradually concluded that our decision was a mistake.

I strongly agree. Suppose at some point we wanted to
allow something like

data T = exists t. Foo t (t -> Int) (forall x . x -> x)

the present notation would be /really/ confusing, especially
since any real example would be more complex than the above.

> 	Allow 'exists'	

Yes.

> 	Deprecate 'forall' (for defining existentials, that is)
> 	Eventually allow only 'exists'

Is there really a need for an overlap? If, at the next
release you were to emit a special error message, rather
than a "deprecated" warning, wouldn't that prompt people to
make the change more swiftly? I suppose this would only be
acceptable if an automatic translation were provided.

  Jón


-- 
Jón Fairbairn                                 Jon.Fairbairn@cl.cam.ac.uk