Are nil and Optional<T>.None the same thing in Swift?

Question

I'm a bit confused by Swift tutorials.

Is nil just a convenient shortcut for Optional<T>.None?
Is there an implicit conversion from one to another?

A couple of observations:

• Optional<String>.None == nil
• nil literal seems to have a NilType

If this is an implicit conversion, can I define my own type that “accepts” nil, or is Optional somehow special in this regard? I don't think defining custom convertible-to-nil types is a good idea—I'm just trying to understand how the type system works in this case.

Answer 1

If you don’t provide an initial value when you declare an optional variable or property, its value automatically defaults to nil.

They have same value, both are nil.

Actually Optional<T>.None is a polymorphic primitive value, and nil is a constant having this value. Optional<T> is a polymorphic type. The type of nil is Optional<T>. That's why you cannot assign nil to anything else but an Optional<T>. For the same reason you cannot assign true to anything but a Bool.

 

For now, according to the documentation you can not use nil for any custom and arbitrary type but optionals.

nil cannot be used with non-optional constants and variables. If a constant or variable in your code needs to be able to cope with the absence of a value under certain conditions, always declare it as an optional value of the appropriate type.