[Pharo-project] Improving Pharo's Exception Hierarchy

Dale Henrichs dhenrich at vmware.com
Wed Apr 13 23:31:36 CEST 2011

By managing the class hierarchy:

   Imagine trying to find meaningful exception classes in a class
   hierarchy of 10,000 classes with significant nesting vs finding a
   meaningful class in a hierarchy of 10 classes ... at what point does
   the hierarchy get out of hand?

I think GemStone had 600 some unique errors and that was pared down to 
<30 Exception classes with reason codes...

It's a matter of practicality...I would think that somewhere around 100 
classes the hierarchy starts to become very tedious to understand even 
for those trying to find the right place to insert there brand spanking 
new exception class.

99% of exception handlers will _use_ Error_ anyway, so the exception 
hierarchy will be largely ignored by most folks, especially if it is 
large an unwieldly.

100 small classes that serve a real purpose and have explicit exception 
handlers written using every one of them, well then by god! 100 classes 
are absolutely called for.

To me this is different than 100 places where I signal an exception ... 
do really need a different exception class for each of the 100 
signalling sites? What if there are no handlers written for any of them? 
Like I said, if there are legitimate need to handle an exception, then 
there is a legitimate need to create an exception class, but if I have 5 
different error conditions, but I will have a single exception handler 
for all 5 conditions, _should_ I create 5 classes or 1 class with some 
state to disambiguate?

If your answer is "It depends...", then you will understand what I am 


On 04/13/2011 02:01 PM, Toon Verwaest wrote:
> I find the "managing the exception hierarchy" a bit strange... Do you
> really have to manage anything more than just normal classes? Every
> exception is related to a specific part of your code, just like other
> classes. What's wrong with creating hundreds of small classes wherever
> it's necessary? You might save a tiny bit of memory by using error codes
> or symbols, but I don't really see much more of a gain there, while you
> do gain from proper objects.
> Being clear and modeling properly generally pays off in the long run.
> And the bit of runtime memory you save isn't really worth your while...
> Or am I missing something else? I didn't really follow the whole
> conversation in-depth.
> cheers,
> Toon
> On 04/13/2011 09:54 PM, Dale Henrichs wrote:
>> Camillo,
>> Hey, I _am_ old, but not _that_ old:) ... There are a couple of things
>> that were invented in the Stone Age that have survived to today, so
>> old ideas are not immediately bad, because they are old:)
>> It feels like you are creating classes that are not much more than
>> symbols ...
>> I am not one to shy away from using classes when they are called for,
>> but I am just making a practicality point ... we don't have a unique
>> class for each character in the alphabet, but we could....we could
>> have a unique class for every possible error condition or not ...
>> I think it is simply a practical answer...
>> I do maintain that you _should_ use some sort of test along the lines
>> of: "Will anyone ever need to write a handler for the exception?" in
>> your criteria for deciding when to create a class and when to use
>> something like a "reason code" to disambiguate the signalling site...
>> Dale
>> On 04/13/2011 12:32 PM, Camillo Bruni wrote:
>>> Perfect, I ll be there to bang heads ;).
>>> So without "Exception" pre- or suffix seems to be nice. However I
>>> don't see the need of using symbols over real classes. This feels
>>> indeed like going to stone age of error handling, thats what you have
>>> polymorphism and ExceptionSets for.
>>> Anyway, the main idea is to make single exceptions recognizable and
>>> not just use one single, basically meaningless, exception type.
>>> best regards, Camillo Bruni
>>> On 2011-04-13, at 21:22, Sven Van Caekenberghe wrote:
>>>> Thanks a lot everybody for the reactions, this could become a nice
>>>> discussion next Friday. All points raised are valid, I would like
>>>> simple names and a compact multipurpose hierarchy too.
>>>> On 13 Apr 2011, at 19:39, Dale Henrichs wrote:
>>>>> Some thoughts from an old man (started programming before
>>>>> exceptions of any kind were available:) ...
>>>>> In the old days, error numbers had a place in the universe ...
>>>>> error numbers of a certain range indicated specific errors and
>>>>> the "error handlers" could check for a range or a specific error
>>>>> ...
>>>>> Today I think there is still a place for the notion of "error
>>>>> numbers".
>>>>> In Smalltalk I would use Symbols instead of numbers, but the idea
>>>>> would be to use a concrete exception class to identify broad
>>>>> categories of error conditions (i.e., FileStreamError) and a
>>>>> symbolic "reason code" to indicate the specific error (i.e.,
>>>>> #fileDoesNotExist, #fileExists, #cannotDelete, etc.), that way an
>>>>> error handler can be written for FileStreamError and then
>>>>> specific action take with respect to which "reason code" is
>>>>> involved, if such action is needed.
>>>>> The main advantage of using reasonCodes over using a "class per
>>>>> error condition" is that you can reduce the size of the Exception
>>>>> hierarchy to a manageable size (GemStone has hundreds of error
>>>>> conditions, so we've resorted to using "reason codes" to manage
>>>>> the size of the hierarchy).
>>>>> As Hernan hints, more often than not it is important to be very
>>>>> specific about the error condition when signalling an error (a
>>>>> unique error message per "per reason code" would be desirable),
>>>>> but the there are very few places where the handler is going to
>>>>> be that specific ...
>>>>> In other words, if it is likely that programmers in the course of
>>>>> using an application will be writing specific error handlers to
>>>>> distinguish between the KeyNotFound and ValueNotFound condition,
>>>>> then classes should be created, otherwise, the NotFoundException
>>>>> could be implemented with three reason codes: #keyNotFound,
>>>>> #valueNotFound, and #elementNotFound and you'd get the best of
>>>>> both worlds, explicit information at the signalling site and a
>>>>> much smaller and more manageable Exception class hierarchy.
>>>>> Dale
>>>>> On 04/13/2011 10:15 AM, Hernan Wilkinson wrote:
>>>>>> I think it is not a good idea to use the prefix Exception. We
>>>>>> do not use the word "exception" in real life, so we should not
>>>>>> do it on our systems. About the proposed hierarchy, the problem
>>>>>> with having specific exceptions is that they are important for
>>>>>> those who catch them, not for those who signal them. For
>>>>>> example, besides the name, what is the difference between
>>>>>> KeyNotFound or ValueNotFound? none. So, I think that the
>>>>>> exception hierarchy should be grown from it uses, not created
>>>>>> based on how or where they are signaled.
>>>>>> my 2 cents :-)
>>>>>> On Wed, Apr 13, 2011 at 1:55 PM, Miguel
>>>>>> Cobá<miguel.coba at gmail.com<mailto:miguel.coba at gmail.com>>
>>>>>> wrote:
>>>>>> El mié, 13-04-2011 a las 14:52 +0200, Camillo Bruni escribió:
>>>>>>> And as Mariano pointed out, there should be a convention on
>>>>>>> the naming: I am still not sure about suffixing the exception
>>>>>>> classes
>>>>>> with
>>>>>>> "Exception", but I guess this is a good thing to do. Though
>>>>>>> I
>>>>>> must say
>>>>>>> that I omitted it so far ;) and just put the verb there, but
>>>>>>> that can be easily changed.
>>>>>> I would say no to suffixes. Analogous to announcements, they
>>>>>> shouldn't have the suffix. The name should be descriptive
>>>>>> enough and intention revealing that the suffix isn't needed in
>>>>>> most cases. For example, I think that
>>>>>> DividedByZero
>>>>>> is better than
>>>>>> DividedByZeroException
>>>>>> and no information is lost with the sorter name. Instead,
>>>>>> DivideByZero isn't clear enough to indicate that is a event
>>>>>> that happened.
>>>>>> What do you think?
>>>>>> -- Miguel Cobá http://twitter.com/MiguelCobaMtz
>>>>>> http://miguel.leugim.com.mx
>>>>>> -- *Hernán Wilkinson Agile Software Development, Teaching&
>>>>>> Coaching Mobile: +54 - 911 - 4470 - 7207 email:
>>>>>> hernan.wilkinson at 10Pines.com site:
>>>>>> http://www.10Pines.com<http://www.10pines.com/>*

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