12.14: Specific Exceptions

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The class Exception in Pharo has ten direct subclasses, as shown in Figure \(\PageIndex{1}\). The first thing that we notice from this figure is that the Exception hierarchy is a bit of a mess; you can expect to see some of the details change as Pharo is improved.

Pharo exception hierarchy diagram. — Figure \(\PageIndex{1}\): A part of Pharo exception hierarchy.

The second thing that we notice is that there are two large sub-hierarchies: Error and Notification. Errors tell us that the program has fallen into some kind of abnormal situation. In contrast, Notifications tell us that an event has occurred, but without the assumption that it is abnormal. So, if a Notification is not handled, the program will continue to execute. An important subclass of Notification is Warning; warnings are used to notify other parts of the system, or the user, of abnormal but non-lethal behavior.

The property of being resumable is largely orthogonal to the location of an exception in the hierarchy. In general, Errors are not resumable, but 10 of its subclasses are resumable. For example, MessageNotUnderstood is a subclass of Error, but it is resumable. TestFailures are not resumable, but, as you would expect, ResumableTestFailures are.

Resumability is controlled by the private Exception method isResumable. For example:

Exception new isResumable               →    true
Error new isResumable                   →    false
Notification new isResumable            →    true
Halt new isResumable                    →    true
MessageNotUnderstood new isResumable    →    true

As it turns out, roughly 2/3 of all exceptions are resumable:

Exception allSubclasses size    →    160
(Exception allSubclasses select: [:each | each new isResumable]) size    →    79

If you declare a new subclass of exceptions, you should look in its protocol for the isResumable method, and override it as appropriate to the semantics of your exception.

In some situations, it will never make sense to resume an exception. In such a case you should signal a non-resumable subclass — either an existing one or one of your own creation. In other situations, it will always be OK to resume an exception, without the handler having to do anything. In fact, this gives us another way of characterizing a notification: a Notification is a resumable Exception that can be safely resumed without first modifying the state of the system. More often, it will be safe to resume an exception only if the state of the system is first modified in some way. So, if you signal a resumable exception, you should be very clear about what you expect an exception handler to do before it resumes the exception.

When defining a new exception. It is difficult to decide when it is worth defining a new exception instead of reusing an existing one. Here are some heuristics: you should evaluate whether

you can have an adequate solution to the exceptional situation,
you need a specific default behavior when the exceptional situation is not handled, and
if you need to store more information to handle the exception case.