xpath - XPath 是否对逻辑表达式进行短路评估？

Question

我的问题是关于 XPath 中的执行顺序。

我有一个表达式，例如:

//person[@created and customFunction(.)]

我的问题是我的自定义函数计算量很大，我只希望它在具有已创建属性集的节点上运行。威尔@created总是在 customFunction 之前评估?我可以编写一个程序来测试这一点，但实际上并不能保证这样一个实验的成功，至少从长远来看不能。

如果这是 XPath 实现的问题，我使用的是 .NET 4.0。

Answer 1

XPath 1.0 进行短路评估，在 XPath 2.0 和 XPath 3.0 中它依赖于实现 .

根据 XPath 1.0 specs, section 3.4: booleans :

An or expression [...] The right operand is not evaluated if the left operand evaluates to true.

An and expression [...] The right operand is not evaluated if the left operand evaluates to false.

根据 XPath 2.0 specs, section 3.6: Logical Expressions和 XPath 3.0 specs, section 3.8: Logical Expressions :

If XPath 1.0 compatibility mode is true [...] it is defined that when there is no need to evaluate the second operand in order to determine the result, then no error can occur as a result of evaluating the second operand.

If XPath 1.0 compatibility mode is false, the order in which the operands of a logical expression are evaluated is implementation-dependent. In this case, an or-expression can return true if the first expression evaluated is true, and it can raise an error if evaluation of the first expression raises an error. Similarly, an and-expression can return false if the first expression evaluated is false, and it can raise an error if evaluation of the first expression raises an error. As a result of these rules, a logical expression is not deterministic in the presence of errors, as illustrated in the examples below.

使用 XPath 2.0 或 XPath 3.0 时，您可以通过评估以下示例表达式来确定当前实现是否进行短路评估:

true() or name(1234)

函数name返回节点参数的名称，或者如果您传递它，例如一个数字，则会引发错误，因此:

如果它返回 true 而不引发错误，则实现会进行短路评估。

如果出现错误，则实现不会进行短路评估(因为它已经评估了不必要的正确操作数)。