A type whose values are distinct from valid values of other types. Unlike many languages in which null is a value, in ideal null is type, and values are specific--missing, not_applicable, unknown, not_a_number and so on. The motivation behind this is to allow the programmer to specify the nature of the exceptional value. Developers can subtype null if necessary, though the need for this should be quite rare.

The check for null should be of the form if (x is null). The program shouldn't depend on what type of null value is being returned.