I have seen some serious problems caused by the handling of the Error with
VSE.  The main source of problems I have seen is caused by the fact that
MessageNotUnderstood is a subclass of Error.  This can be a real problem in
code that is poorly tested, and exception handling has been added early in
the development cycle. I would favor a class hierarchy that kept
MessageNotUnderstood in a different branch than the generic error that would
be handled as proposed below.
>    ["some risky code"] on: Error do: ["clean up after my code died"].

Brett Taylor

>-----Original Message-----
>From: Norton, Chris [mailto:chrisn at Kronos.com]
>Sent: Monday, August 16, 1999 1:29 PM
>To: 'squeak at cs.uiuc.edu'
>Subject: RE: Re: Exception Hierarchies
>
>
>Hi Folks.
>
>This response was written with the philosophy: "1) open mouth. 2) insert
>foot to kneecap."  So, Alan, I am NOT trying to criticize you.  :-)
>
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>Alan Lovejoy [sourcery at pacbell.net] wrote:
>[...snip...]
>> Ah, then perhaps the problem is simply that "Error" is misnamed.
>>
>> I suggest the following hierarchy:
>>
>> Exception (the root class, takes no action if no handler is found,
>> proceedable by default)
>> 	DefaultHandlerException (takes some default action if no handler is
>> found)
>> 		HandlerRequiredException (raises NoHandlerException if no
>> handler)
>> 			ErrorException (root of all errors, not proceedable
>> by default)
>> 			WarningException
>> 		NoHandlerException	(raised when no handler is found for
>> a
>> 					HandlerRequiredException, opens
>> 					default "UnhandledException"
>> notifier
>> 					if no handler found for NoHandler
>> exception)
>> 		ControlException
>> 			HaltException
>> 			UserInterruptException
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>I'm not an expert on Exception handling, but I sure find it handy to say:
>
>    ["some risky code"] on: Error do: ["clean up after my code died"].
>
>I think that Error is a good "catch all" name for (near-)fatal exceptions.
>Having said that, I'd like to point out that in VSE, there is an exception
>handler called KeyboardInterrupt() that happens when you hit Ctrl-Break.
>This interrupt is subclassed from Notification(), which is not subclassed
>from Error.  Perhaps the Halt and UserInterrupt should be subclassed from
>Notification() in Squeak.  After all, they aren't really errors.  They are
>simply interruptions that can (usually) be resumed.
>
>Just my $0.02.  Cheers to Craig Latta, the folks at TFEI and to Steven Pope
>for making Exceptions happen in Squeak!!
>
>---==> Chris
>