10.3 Define errors out of existence 定义错误不存在

The best way to eliminate exception handling complexity is to define your APIs so that there are no exceptions to handle: define errors out of existence. This may seem sacrilegious, but it is very effective in practice. Consider the Tcl unset command discussed above. Rather than throwing an error when unset is asked to delete an unknown variable, it should have simply returned without doing anything. I should have changed the definition of unset slightly: rather than deleting a variable, unset should ensure that a variable no longer exists. With the first definition, unset can’t do its job if the variable doesn’t exist, so generating an exception makes sense. With the second definition, it is perfectly natural for unset to be invoked with the name of a variable that doesn’t exist. In this case, its work is already done, so it can simply return. There is no longer an error case to report.

消除异常处理复杂性的最好方法是定义您的 API，以便没有异常要处理：定义错误而已。这似乎是牺牲品，但在实践中非常有效。考虑上面讨论的 Tcl unset 命令。而不是在要求 unset 删除未知变量时引发错误，它应该只是返回而无需执行任何操作。我应该稍微修改一下 unset 的定义：与其删除一个变量，不应该删除 unset 来确保一个变量不再存在。根据第一个定义，如果变量不存在，则 unset 不能执行其工作，因此生成异常是有意义的。使用第二个定义，使用不存在的变量名调用 unset 是很自然的。在这种情况下，它的工作已经完成，因此可以简单地返回。