14 - Exceptions

Stéphane Ducasse
Stéphane Ducasse
stephane.ducasse@inria.fr
http://stephane.ducasse.free.fr/
1
Exceptions...a simple
introduction

S.Ducasse 2
Standardized by ANSI in 1996
Exception is the root of the exception
hierarchy: 84 predefined exceptions. The two
most important classes are:
Error
Notification
Specialised into predefined exceptions ->
subclass them to create your own exceptions
Some methods of Exception:
defaultAction is executed when an exception occurs
description string describing the actual exception
Exceptions

S.Ducasse 3
|x y|
x := 7.
y := 0.
[x/y]
on: ZeroDivide
do: [:exception| Transcript show:
exception description, cr.
0....]
Catching an Exception

S.Ducasse 4
Signaling an Exception
Error signal
Warning signal: ‘description of the exception’

S.Ducasse 5
Exception
an Exception Handler is defined using on:do: and is
composed of
an exception class (ZeroDivide) and
a handler block [:theException| Transcript show: ‘ division
by zero’]
An Exception Handler completes by returning
the value of the handler block in place of the
value of the protected block (here [x/y]).
We can exit the current method by putting an
explicit return inside the handler block

S.Ducasse 6
The Main Exceptions of VW
Exception class Exceptional Event Default Action
Error Any program error Open a Notifier a Notifier
ArithmeticError
Any error evaluating an
arithmetic
Inherited from Error
MessageNotUnderstood
Any unusual event that
does not impair
continued execution of
the program
Inherited from Error
Notification
Notification Any
unusual event that does
not impair continued
execution of the
program Do nothing
continuing executing
Do nothing continuing
executing
An unusual event that Display Yes/No dialog

S.Ducasse 7
Exception Sets
[do some work]
on: ZeroDivide, Warning
do: [ :ex| what you want]
Or
|exceptionSets|
exceptionSets := ExceptionSet with: ZeroDivide
with: Warning.
[do some work]
on: exceptionSets
do: [ :ex| what you want]
Exception Sets

S.Ducasse 8
Each process has its own exception environment: an
ordered list of active handlers.
Process starts -> list empty
[aaaa] on: Error do: [bbb] -> Error,bbb added to the
beginning of the list
When an exception is signaled, the system sends a
message to the first handler of the exception handler.
If the handler cannot handle the exception, the next
one is asked
If no handler can handle the exception then the default
action is performed
Exception Environment
8

S.Ducasse 9
A handler block completes by executing the last
statement of the block. The value of the last statement
is then the value returned by the handler block.
Where this value should be returned depends:
Nonresumable (Error)
Sq: ([Error signal. ‘Value from protected
block’]
on: Error
do: [:ex|ex return: ‘Value from handler’])
> ‘Value from handler’
Resumable and Non-Resumable (i)

S.Ducasse 10
Resumable (Warning, Notification)
In this case Notification signal raises an exception,
then the context is restored and the value returned
normally
[Notification raiseSignal. ‘Value from protected block’]
on: Notification
do: [:ex|ex resume: ‘Value from handler’]
[Notification signal. 'Value from protected block']
on: Notification
do: [:ex|ex resume: 'Value from handler']
> ‘Value from protected Block’.
Resumable and Non-Resumable (ii)

S.Ducasse 11
Transcript show:
[Notification raiseSignal. 'Value from protected block']
on: Notification
do: [:ex| Transcript show: 'Entering handler '.
'Value from handler'. '5']
-> Entering handler 5
Resume:/Return:

S.Ducasse 12
Transcript show: [Notification raiseSignal. 'Value from
protected block']
on: Notification
ex resume: 'Value from handler'. '5']
> Entering handler Value from protected block
Transcript show: [Notification raiseSignal. 'Value from
protected']
on: Notification
ex return: 'Value from handler'. '5']
-> Entering handler Value from handler
Resume:/Return:

S.Ducasse 13
exit or exit: (VW specific) Resumes on a resumable
and returns on a nonresumable exception
resume or resume: Attempts to continue processing
the protected block, immeditely following the message
that triggered the exception.
return or return: ends processing the protected block
that triggered the exception
retry re-evaluates the protected block
retryUsing: evaluates a new block in place of the
protected block
Exiting Handlers Explicitly

S.Ducasse 14
resignalAs: resignal the exception as another on
pass exit the current handler and pass to the next
outer handler, control does not return to the passer
outer as with pass, except will regain control if the
outer handler resumes
exit:, resume: and return: return their argument as the
return value, instead of the value of the final statement
of the handler block
Exiting Handlers Explicitly (ii)

S.Ducasse 15
Look in Exception class examples categories
-2.0 to: 2.0 do: [ :i |
[ 10.0 / i. Transcript cr; show: i printString ]
on: Number divisionByZeroSignal do:
[:ex | Transcript cr; show: 'divideByZero abort'.
ex return ]
]
-2.0
-1.0
divideByZero abort
1.0
2.0
Examples

S.Ducasse 16
retry recreates the exception environment of active
handlers
[ x /y]
on: ZeroDivide
do: [:exception|
y := 0.00001.
exception retry]
Examples

14 - Exceptions

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14 - Exceptions