S.Ducasse Stéphane Ducasse 1 Common Mistakes and Debugging in VW

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

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)

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)

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)

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. Instance Variable Access in Class Method

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

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

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

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

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

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

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

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

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

S.Ducasse 16