On Wed, Oct 27, 2010 at 07:57:59PM +0000, Simon Peyton-Jones wrote:
| Drifting off-topic, but wouldn't we want to be able to use similar | syntax to bind types too? e.g. | | f ((Just @ t) x) = (Right @ String @ t) x | | but @ is unavailable in patterns.
Oh yes, good point. It'd be particularly useful in existential patterns:
data T where MkT :: forall a. a -> (a -> Int) -> T
f (MkT @ a x g) = g (x::a)
The idea is that the pattern (MkT @ a x g) brings the type variable 'a' into scope. As you point out, though, '@' is already used in patterns, but perhaps this use is unambiguous. Confusing though f (MkS @ a x@(p,q) z) = ....
Maybe someone else can think of good syntax.
The difference in what "Just" means in (Just 'c') and ((Just @ Char) 'c') feels a bit wrong to me. Maybe it would be a better to have a syntax to get something with its real type, and then use normal application, e.g. currently we have Just :: forall a . a -> Maybe a Just :: Char -> Maybe Char And if #... is the new syntax then: #Just :: /\ a . a -> Maybe a #Just Char :: Char -> Maybe Char Just 'a' == #Just Char 'a' and also map ord "foo" == #map Char Int ord "foo" Thanks Ian