Why does Swift's optional binding succeed with 'nil' in certain cases?

Question

Apple\'s Swift language documentation says that optional binding (a.k.a. if let) will \"check for a value inside an optional\" and \"extra

Answer 1

I interpret this differently:

var x: Int? = 1

if let y1: Int = x {
    println("y1 = \(y1)") 
}

//prints y = 1, the optional was checked, contains a value and passes it

var x: Int? = nil

if let y1: Int = x {
    println("y1 = \(y1)") 
}

//does not execute because x does not contain value that can be passed to a non optional y

var x: Int? = nil

if let y1: Int? = x {
    println("y1 = \(y1)")
}
// y = nil, since y is optional and can hold a value of x which is nil, then it passes nil

Optional binding is for checking if an optional contains a value to pass to a non optional parameter.

Answer 2

You assigned an optional Int to an optional Int. The assignment did succeed. It will always succeed, whether the optional Int contained an Int or not.