The type of the null value. Any union type of form Null|T is considered an optional type, whose values include null. Any type of this form may be written as T? for convenience.

The if (exists ... ) construct, or, alternatively, the assert (exists ...) construct, may be used to narrow an optional type to a definite type, that is, a subtype of Object:

String? firstArg = process.arguments.first;
if (exists firstArg) {
    print("hello " + firstArg);
}

The else operator evaluates its second operand if and only if its first operand is null:

String name = process.arguments.first else "world";

The then operator evaluates its second operand when its first operand evaluates to true, and to null otherwise:

Float? diff = x>=y then x-y;