This document discusses common mistakes and debugging techniques in VisualWorks. It covers issues such as: confusing true and True, redefining classes, modifying self and super, redefining basic methods, iterating over and modifying collections, using breakpoints, and printing for debugging. The document provides examples and recommendations to avoid common bugs and debug issues efficiently.

1. S.Ducasse 1
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Stéphane Ducasse
Stephane.Ducasse@univ-savoie.fr
http://www.listic.univ-savoie.fr/~ducasse/
Common Mistakes and
Debugging in VW

2. S.Ducasse 2
• Preventing: Most Common Mistakes
• Curing: Debugging Fast (from ST Report July 93)
• Extras
Roadmap

3. S.Ducasse 3
• true is the boolean value, True its class.
• Book>>initialize
• inLibrary := True
• Book>>initialize
• inLibrary := true
• nil is not an acceptable receiver for ifTrue:
Common Beginner Bugs (I)

4. S.Ducasse 4
• whileTrue: and whileTrue receivers must be a block
•
[x<y] whileTrue: [x := x + 3]
• Redefining a class:
• Before creating a class, check if it already exists.This is
(sigh) a weakness of the system
Object subclass: #View (Squeak)
• VisualWorks 7.0 has namespaces so less likely to
redefine a class
Common Beginner Bugs (II)

5. S.Ducasse 5
• In a method self is returned by default. Do not forget ^
for returning something else.
• Packet>>isAddressedTo: aNode
• ^ self addressee = aNode name
Common Beginner Bugs (III)

6. S.Ducasse 6
• Do not try to access instance variables to initialize
them in a class method. It is impossible!
• A class method can only access class instance variables
and classVariables.
• -> Define and invoke an initialize method on
instances.
InstanceVariable Access in Class Method

7. S.Ducasse 7
Example
Packet class>>send: aString to: anAddress
contents := aString.
addressee := anAddress
Instead create an instance and invoke instance methods
Packet class>>send: aString to: anAddress
^ self new
contents: aString;
addressee: anAddress

8. S.Ducasse 8
• Do not try to assign a value to a method argument.
Arguments are read only
setName: aString
aString := aString, 'Device'.
name := aString
Method Argument Are Read-Only

9. S.Ducasse 9
self and super are Read-Only
• Do not try to modify self and super

10. S.Ducasse 10
• Never redefine basic-methods (==, basicNew,
basicNew:, basicAt:, basicAt:Put:...)
• Never redefine class
• Never redefine name on the class side!
basic* Method Redefinition

11. S.Ducasse 11
The hash and = Pair
• Redefine hash when you redefine = so that if a = b
then a hash = b hash
•
Book>>=aBook
^self title = aBook title & (self author = aBook
author)
Book>>hash
^self title hash bitXor: self author hash

12. S.Ducasse 12
• add: returns the argument and not the receiver, so use
yourself to get the collection back.
• Do not subclass Collection classes.
Common Beginner Bugs - Collections

13. S.Ducasse 13
• Never iterate over a collection which the iteration
somehow modifies.
•
timers do: [:aTimer| aTimer isActive
ifFalse: [ timers remove: aTimer]]
• First copy the collection
• timers copy do: [:aTimer| aTimer isActive
• ifFalse: [ timers remove:
aTimer]
• Take care, since the iteration can involve various
methods and modifications which may not be obvious!
Don’t iterate over collection and modify it

14. S.Ducasse 14
• Basic Printing
• Transcript cr; show:‘The total= ’, self total printString.
• Use a global or a class to control printing information
Debug
ifTrue:[Transcript show: self total printString]
Debug > 4
ifTrue:[Transcript show: self total printString]
Debug print:[Transcript show: self total printString]
Debugging - Hints

15. S.Ducasse 15
• Breakpoints
• self halt.
• self error:‘ invalid’
• Conditional halt
i > 10 ifTrue:[self halt]
i haltIfNil
• In Squeak 3.8: haltIf
•self haltIf: (i > 10)
•i haltIf: [:o | o >10]
•self haltIf: #doIt
BreakPoints

16. S.Ducasse 16