15. Tribooleans: Three state logic¶
Through the package GNATCOLL.Tribooleans, GNATColl provides a type that extends the classical Boolean type with an Indeterminate value.
There are various cases where such a type is useful. One example we have is when a user is doing a search (on a database or any set of data), and can specify some optional boolean criteria (“must the contact be french?”). He can choose to only see french people (“True”), to see no french people at all (“False”), or to get all contacts (“Indeterminate”). With a classical boolean, there is no way to cover all these cases.
Of course, there are more advanced use cases for such a type. To support these cases, the Tribooleans package overrides the usual logical operations “and”, “or”, “xor”, “not” and provides an Equal function.
See the specs of the package to see the truth tables associated with those operators.