In theory, computer scientists and programmers only admit the existence of 0, 1, and infinity.
In practice, I've encountered the "tri-state boolean" at least three times, completely separately, and one coworker once joyfully produced a quint-state boolean, which is to say, in practice programmers will use without restriction all sorts of strange mathematical extensions to numbers.
(A "tri-state boolean" is a boolean field that probably started life with two values but grew a third on the side instead of being upgraded cleanly. A common case is in SQL, where you can make a bool value NULL and thus have true, false, and NULL, which may end up being treated differently by only some parts of the code. Happy debugging!)
This can be all sorts of fun in Objective-C, which for historical reasons has a commonly-used BOOL type which is an eight-bit integer. You're supposed to only store 0 and 1 in it and pretend the other possible values don't exist. Every so often I see some code that uses 2 as well. Once I saw some code, written far too late at night, which used a BOOL as a counter.
In practice, I've encountered the "tri-state boolean" at least three times, completely separately, and one coworker once joyfully produced a quint-state boolean, which is to say, in practice programmers will use without restriction all sorts of strange mathematical extensions to numbers.
(A "tri-state boolean" is a boolean field that probably started life with two values but grew a third on the side instead of being upgraded cleanly. A common case is in SQL, where you can make a bool value NULL and thus have true, false, and NULL, which may end up being treated differently by only some parts of the code. Happy debugging!)