Comments on: Exceptions versus Return Values

By: admin

admin — Wed, 18 Mar 2009 00:43:23 +0000

Tom,
Thanks for commenting.

I agree that exceptions should only be used for exceptional conditions. The debate lies in what constitutes an exceptional condition. My own definition is a little looser than some other peoples.

Exceptions definitely shouldn’t be used or normal control flow, but I do think it is OK to use them for dealing with ‘contingency’ situations e.g. to signal a situation that stops a method from completing in its normal way, and that the method itself doesn’t know how to deal with – but that a calling method may know how to deal with and recover from. If that contingency is important enough that it shouldn’t be (accidentally) ignored, then checked exceptions are a good fit.

I think I did a better job of explaining this in a recent blog posting:

http://www.shaunabram.com/checked-exceptions-article/

Maybe it will change your mind If not, it would be great to hear your counter views…

Shaun

By: admin

admin — Wed, 18 Mar 2009 00:29:38 +0000

Thanks Esteve…

I think I understand what you mean – that when using a return value or Special Result Class, you always get that result back, regardless. But you may well then need do do a series of if blocks for example,

if (result.hasErrors()) {…}
else {…}

which isn’t really too different from the catch blocks involved with the exceptions approach?

Shaun

By: admin

admin — Tue, 17 Mar 2009 23:58:12 +0000

Hi Ricky,
Thanks for posting, but I’m not sure I follow it entirely. I think what you are proposing is a new, custom, parameterized type, called “Either” which has a single method called effect(). As well as a new class called Failure? Or am I being slow and it’s actually a standard class/library that I haven’t came across before?

Seems like an interesting idea. If you could point us to any complete code, or posting/docs on it, I’d like to understand it a little better…

And I assume by ‘a better language’ you mean Scala?

Thanks,

Shaun

By: Tom Palmer

Tom Palmer — Tue, 17 Mar 2009 12:35:08 +0000

I mostly have a simple rule most of the time: Exceptions are for exceptional cases. That is, if you are likely to ignore an error condition and it shouldn’t be ignored, you should make sure to throw an exception rather than use a return value. This covers a great majority of error conditions, I think. Use a special class if you determine that specific identification is valuable, but any exceptions is better than none. Never use checked exceptions.

By: SeanJA

SeanJA — Tue, 17 Mar 2009 12:29:20 +0000

One thing that frustrates me is when people throw exceptions for error checking and you only get one error back after say… filling out a form, and then you resubmit it to get another error back… and so on…

By: Esteve

Esteve — Tue, 17 Mar 2009 11:57:23 +0000

Good job!
Another problem on using exceptions is that on caller method you don’t know when an exception may be thrown. Using return values/class you know explicitly what may happen.

By: Ricky Clarkson

Ricky Clarkson — Tue, 17 Mar 2009 11:43:39 +0000

Either[Integer, Failure] result = getUserValue();

result.effect(new E[Integer] { public void e(Integer i) { whatever you want if getUserValue succeeded } },
new E[Failure] { public void e(Failure f) { and if it failed } });

I used []s instead of angle brackets to avoid problems with comment formatting.

The above can be prettier if you use a better language.

val result = getUserInput

result match { case Left(i) => println(“The user specified ” + i)
case Right(f) => println(“The user is an eejit because ” + f)
}