It is sometimes required for the methods with optional arguments to be able to differentiate between its default value and the value passed from the caller. Passing nil might initially sound like a good idea since it represents “nothingness”. However, it might turn out that nil is a legit value and there might be cases where it is desirable for the caller to pass nil. In such a case, we cannot use it as a default value if we want to implement a special logic for the case of not providing that value.

Fortunately, there is an easy way to deal with it – use a special constant. Read more