[Pharo-project] Improving Pharo's Exception Hierarchy

Dale Henrichs dhenrich at vmware.com
Wed Apr 13 19:39:56 CEST 2011

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 


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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