Short-circuit boolean operators

Two additional boolean operators are proposed: orElse and andAlso. These have the same effect as or and and, except that there is a guarantee that the second operand will not be evaluated unnecessarily. This makes it possible to ensure that evaluating the second operand does not produce an error. For example, the expression:

will never fail with a divide-by-zero error, because the division will never be executed if the value of pages is zero.

When the ordinary or and and operators are used, the W3C specification gives XPath processors license to re-arrange the order of execution, and Saxon will occasionally do so. Using these operators is therefore advised when the first condition is a guard to prevent the second condition failing.

The same effect can be achieved without relying on extensions, by writing A orElse B as if (A) then true() else boolean(B), and rewriting X andAlso Y as if (X) then boolean(Y) else false().