Async: Arguments and return values to blocks

Currently Async supports passing parameters through an external var like:

var date: NSDate?
Async.background {
    date = NSDate()
}.main {
    // Access date 
}

Possible syntax for a better solution:

Async.background {
    let date = NSDate()
}.main(date) {
    // Access date
}

I have no idea if or how this is possible – please let me know if you do!