final class Box { var value = 1 }
final class Queue {
    private var action: (() -> Int)?
    func enqueue(_ action: @escaping () -> Int) { self.action = action }
    func run() -> Int? {
        let current = action
        action = nil
        return current?()
    }
}

func experiment() {
    var number = 1
    let live = { number }
    let snapshot = { [number] in number }
    number = 2
    precondition(live() == 2 && snapshot() == 1)

    let box = Box()
    let capturedReference = { [box] in box.value }
    box.value = 3
    precondition(capturedReference() == 3)

    var temporary: Box? = Box()
    let weakRead = { [weak box = temporary] in box?.value }
    precondition(weakRead() == 1)
    temporary = nil
    precondition(weakRead() == nil)

    let queue = Queue()
    queue.enqueue(snapshot)
    precondition(queue.run() == 1 && queue.run() == nil)
    print("variable=2 snapshot=1 captured-object=3 weak-after-release=nil queued=1 then=nil")
}
experiment()