Why not use exceptions as regular flow of control?

Question

To avoid all standard-answers I could have Googled on, I will provide an example you all can attack at will.

C# and Java (and too many others) have with plenty of ty

Answer 1

The standard anwser is that exceptions are not regular and should be used in exceptional cases.

One reason, which is important to me, is that when I read a try-catch control structure in a software I maintain or debug, I try to find out why the original coder used an exception handling instead of an if-else structure. And I expect to find a good answer.

Remember that you write code not only for the computer but also for other coders. There is a semantic associated to an exception handler that you cannot throw away just because the machine doesn't mind.

Answer 2

Exceptions are basically non-local goto statements with all the consequences of the latter. Using exceptions for flow control violates a principle of least astonishment, make programs hard to read (remember that programs are written for programmers first).

Moreover, this is not what compiler vendors expect. They expect exceptions to be thrown rarely, and they usually let the throw code be quite inefficient. Throwing exceptions is one of the most expensive operations in .NET.

However, some languages (notably Python) use exceptions as flow-control constructs. For example, iterators raise a StopIteration exception if there are no further items. Even standard language constructs (such as for) rely on this.

Answer 3

I don't really see how you're controlling program flow in the code you cited. You'll never see another exception besides the ArgumentOutOfRange exception. (So your second catch clause will never be hit). All you're doing is using an extremely costly throw to mimic an if statement.

Also you aren't performing the more sinister of operations where you just throw an exception purely for it to be caught somewhere else to perform flow control. You're actually handling an exceptional case.

Answer 4

You might be interested in having a look at Common Lisp's condition system which is a sort of generalization of exceptions done right. Because you can unwind the stack or not in a controlled way, you get "restarts" as well, which are extremely handy.

This doesn't have anything much to do with best practices in other languages, but it shows you what can be done with some design thought in (roughly) the direction you are thinking of.

Of course there are still performance considerations if you're bouncing up and down the stack like a yo-yo, but it's a much more general idea than "oh crap, lets bail" kind of approach that most catch/throw exception systems embody.

Answer 5

I feel that there is nothing wrong with your example. On the contrary, it would be a sin to ignore the exception thrown by the called function.

In the JVM, throwing an exception is not that expensive, only creating the exception with new xyzException(...), because the latter involves a stack walk. So if you have some exceptions created in advance, you may throw them many times without costs. Of course, this way you can't pass data along with the exception, but I think that is a bad thing to do anyway.

Answer 6

I don't think there is anything wrong with using Exceptions for flow-control. Exceptions are somewhat similar to continuations and in statically typed languages, Exceptions are more powerful than continuations, so, if you need continuations but your language doesn't have them, you can use Exceptions to implement them.

Well, actually, if you need continuations and your language doesn't have them, you chose the wrong language and you should rather be using a different one. But sometimes you don't have a choice: client-side web programming is the prime example – there's just no way to get around JavaScript.

An example: Microsoft Volta is a project to allow writing web applications in straight-forward .NET, and let the framework take care of figuring out which bits need to run where. One consequence of this is that Volta needs to be able to compile CIL to JavaScript, so that you can run code on the client. However, there is a problem: .NET has multithreading, JavaScript doesn't. So, Volta implements continuations in JavaScript using JavaScript Exceptions, then implements .NET Threads using those continuations. That way, Volta applications that use threads can be compiled to run in an unmodified browser – no Silverlight needed.