Add extension for type application

[igloo]

In  http://www.haskell.org/pipermail/libraries/2010-October/014761.html we discussed adding an extension for explicit type application.

The main stumbling block is syntax.

For example, we would like to be able to write something to the effect of

(Just @ Char) 'a'

and also to be able to pattern match types, e.g.

f ((Just @ t) x) = (Right @ String @ t) x

For a more useful example:

data T where
    MkT :: forall a. a -> (a -> Int) -> T

f (MkT @ a x g) = g (x::a)

Possible suggested syntaxes to make something of type

Maybe (forall a. a->a)

were:

Just @ (forall a. a->a) id    (@ has another meaning in patterns)
Just[forall a. a->a] id       (pvs, opal, HasCASL with paramterized modules; conflicts with Haskell list syntax)
Just {forall a. a->a} id      (Agda)
#Just (forall a. a->a) id
@Just (forall a. a->a) id     (coq)

In the last 2 cases we would presumably have something like

Just :: forall a . a -> Maybe a
Just :: Char -> Maybe Char
#Just :: /\ a . a -> Maybe a
#Just Char :: Char -> Maybe Char

and similarly

#map :: /\ a b . (a -> b) -> [a] -> [b]

[igloo]

See also #5296.