[Solved] Why use an extra let statement here? [duplicate]

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The if-let construction is sort of superfluous in a simple case like this, but in a more complicated piece of code it can be useful. You can use it in a lot of the same ways you’d use an assignment in a conditional in (Obj)C (remember if (self = [super init])).

For example, if the optional being tested is a computed property:

var optionalName: String? {
get {
    if checkTouchID() {
        return "John Appleseed"
    } else {
        return nil
    }
}
}
var greeting = "Hello!"
if optionalName {
    greeting = "Hello, \(optionalName)"
}

Paste that into a playground, along with a stub implementation of checkTouchID() that returns true, and you’ll immediately see in the results area that the optionalName getter is executing twice. If you use an if-let construction instead, you’ll only execute the getter once.

This also true — and perhaps more commonly useful — if you have a series of chained optional calls, as in the if let johnsStreet = john.residence?.address?.street example from the docs. You don’t want to rewrite the whole chain in the body of the if statement, much less recompute it.

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solved Why use an extra let statement here? [duplicate]