The second issue with returning special values is that it can lead to awkward
code. If a piece of code calls promptNumber 10 times, it has to check 10 times
whether null was returned. And if its response to finding null is to simply
return null itself, callers of the function will in turn have to check for it, and
so on.
Exceptions
When a function cannot proceed normally, what we would like to do is just
stop what we are doing and immediately jump to a place that knows how to
handle the problem. This is what exception handling does.
Exceptions are a mechanism that makes it possible for code that runs into
a problem to raise (or throw) an exception. An exception can be any value.
Raising one somewhat resembles a super-charged return from a function: it
jumps out of not just the current function but also its callers, all the way down
to the first call that started the current execution. This is called unwinding
the stack. You may remember the stack of function calls that was mentioned
in Chapter 3. An exception zooms down this stack, throwing away all the call
contexts it encounters.
If exceptions always zoomed right down to the bottom of the stack, they
would not be of much use. They’d just provide a novel way to blow up your
program. Their power lies in the fact that you can set “obstacles” along the
stack to catch the exception as it is zooming down. Once you’ve caught an
exception, you can do something with it to address the problem and then
continue to run the program.
Here’s an example: