Why do I get error: cannot convert value of type 'Int' to expected argument type 'XX' on line 10?

gistfile1.txt

class XX {

	func a(b: Int) -> Int {
		return b
	}
	
	var functionA = a

	func run() {
		print("\(functionA(1))")   // Needs (self) after functionA
	}
}
var x = XX()
x.run()


// What I really need is as below

// Default is nil
class P {
	var a: (()->())!
	func run() {
		if let f = a {
			f()
		}
	}
}

var p = P()
p.run()
p.a = { print("PP") }
p.run()


// Default is empty closure
class Q {
	var a = { ()->() in }
	func run() {
		a()
	}
}

var q = Q()
q.run()
q.a = { print("QQ") }
q.run()