Existentials

[Matt Fairtlough]

>From: "Simon Peyton-Jones" <simonpj@microsoft.com>
>To: "GHC users" <glasgow-haskell-users@haskell.org>, >
>Dear GHC users, Hugs users, and Hugs implementors
>
>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.  

looks like it!

>(In
>fairness to Mark, I think I was the primary advocate for it.)  I reckon
>that we should
>
>	Allow 'exists'	
>	Deprecate 'forall' (for defining existentials, that is)
>	Eventually allow only 'exists'
>
>Does anyone have any opinions on this topic?  It's a small point, but
>one that bites quite frequently.  It might even be possible to arrange
>that 'forall' and 'exists' were only keywords in types, and not in
>terms, but I'm not sure it's worth the bother.

I would have said it _was_ worth the bother.  Both forall and exists are very 
useful function names, in my opinion.

>I don't think it affects NHC, because it's not H98 anyway.
>
>
>Simon
>
>
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>

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