use of let in optional value along with if

Question

From the Swift book, that Apple has released we can make optional value by putting ? to a variable like

var optionalString: String? = "Hello"
optionalString == nil

and also written that “You can use if and let together to work with values that might be missing.”

var optionalName: String? = "John Appleseed"
var greeting = "Hello!"
if let name = optionalName {
   greeting = "Hello, \(name)"
}

written above code for checking that optional value is nil or not, it will not go inside if optionalName is nil.

but same can be done without using the let like

if optionalName {
     greeting = "Hello, \(optionalName!)"
}

why in book it is suggested to use the let?

Answer 1

Your confusion arises because you're using println to display optionalName, and println is smart enough to handle optional values and unwrap them as necessary. If you used the hypothetically similar code:

if optionalName {
    greeting = "Hello, " + optionalName
}

you would get an error because you can't concatenate a String and a String? You have three ways around that

First, you can use conditional unwrapping after you do the check, but this is inefficient because if you reference optionalName more than once, since you have to unwrap it each time.

if optionalName {
    greeting = "Hello, " + optionalName?
}

You can improve that by using forced unwrapping, since you've already done the test and know that optionalName can never be nil. This is still inefficient if you reference optionalName multiple times, as each time the unwrapping must be done.

if optionalName {
    greeting = "Hello, " + optionalName!
}

Lastly, you can use the if let syntax to test and unwrap all at the same time. Name is safe to use because it will be of type String, not String?

if let name = optionalName {
    greeting = "Hello, " + name
}

Answer 2

Your second code snippet is functionally equivalent to the one from the book. However, the book suggests using let to avoid multiple unwrapping of the optional value.

Since you are checking optionalName for being empty, it is safe to assume that you plan to use it inside the conditional block. When you do so, Swift would need to unwrap the optional value from optionalName again, because the variable remains optional.

When you do the assignment of optionalName to name in the let declaration, the name constant that you get back is not optional. This saves you the unwrap inside the if statement.