predicate is a typedef name of exprn_mapper<bool>. We know that any exprn_mapper<T> can be implicitly constructed from a value of type T, so there are constants predicate(true) and predicate(false). We also saw that the relational operators produce predicates, and we are about to meet other devices that produce them. In this section it's the logical operators, i.e. operators that produce predicates from other predicates.

Quince defines overloads of the infix binary operators && and ||, and of the prefix unary operator !.

The overloads of && and || apply when one of the operands is of type predicate and the other is of type predicate or bool. The overload of ! applies when the operand is of type predicate.

There are also overloads of the infix binary operators == and !=, which apply when one of the operands is of type predicate and the other is of type predicate or bool. Whenever they can apply, they do apply: these logic overloads of == and != take priority over the more general overloads.

In all cases, logic operators return a predicate. Usually it represents a server-side expression that, when executed, evaluates the subexpression(s) represented by the operand(s), and then applies the appropriate SQL operator (AND, OR, NOT, = or !=) to the result(s).

Some of those steps might be skipped, however, if the expression is optimized. Quince applies the following optimizations recursively:

where T is either true or predicate(true), F is either false or predicate(false), and X is any operand.

If the result of any optimization is a bool rather than a predicate, it is converted to a predicate(true) or predicate(false).