Hello,
let's also start the discussion on feature request 126. The idea here is that we allow the @ notation for explicit type applications to also be used on constructors in patterns. Using @ with a constructor in a pattern has the same meaning as it does it an expression: the provided type is used to instantiate the corresponding type parameter of the constructor. If the type contains variables, those are treated in the same way as in #128, where "unbound" variables name the matching types. Here are some examples:
f1 (Just @Int x) = x -- This has type `Maybe Int -> Int`
f2 (Just @[a] x) = x == "c" -- `a` is an alias for `Char`
f3 (SomeException @e ex) = ... -- `e` is a name for the existentially hidden exception type
Overall I think that is a simple and natural extension to the way @ already works, and I propose that we accept it.