Dispatch
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This document discusses implementing boolean logic and conditional logic using messages instead of primitive types or conditionals in an object-oriented programming language. It shows how true and false can be implemented as objects that understand messages like not, | (or), and ifTrue:ifFalse: by delegating to subclasses. This approach avoids conditionals and uses message dispatching instead, which can be optimized by the virtual machine. The key ideas are that messages act as a dispatcher, subclasses create the vocabulary, and conditionals should be avoided in favor of using objects and messages.
![S.Ducasse 4
Booleans
3 > 0
ifTrue: ['positive']
ifFalse: ['negative']](https://image.slidesharecdn.com/stoop-112-essenceofdispatch-101026131731-phpapp01/85/Dispatch-4-320.jpg)
![S.Ducasse 5
Booleans
3 > 0
ifTrue: ['positive']
ifFalse: ['negative']
'positive'](https://image.slidesharecdn.com/stoop-112-essenceofdispatch-101026131731-phpapp01/85/Dispatch-5-320.jpg)
![S.Ducasse 6
Yes ifTrue:ifFalse: is a message!
Weather isRaining
ifTrue: [self takeMyUmbrella]
ifFalse: [self takeMySunglasses]
ifTrue:ifFalse is sent to an object: a boolean!](https://image.slidesharecdn.com/stoop-112-essenceofdispatch-101026131731-phpapp01/85/Dispatch-6-320.jpg)
![S.Ducasse 8
Lazy Logical Operators
false and: [1 error: 'crazy']
PrIt-> false and not an error](https://image.slidesharecdn.com/stoop-112-essenceofdispatch-101026131731-phpapp01/85/Dispatch-8-320.jpg)
![S.Ducasse 12
Implementing ifTrue:ifFalse:
Now you are good and you should implement it
Propose an implementation of not in a world
where you do not have Booleans
false ifTrue: [ 3 ] ifFalse: [ 5 ]
true ifTrue: [ 3 ] ifFalse: [ 5 ]](https://image.slidesharecdn.com/stoop-112-essenceofdispatch-101026131731-phpapp01/85/Dispatch-12-320.jpg)
Dispatch
- 1. Stéphane Ducasse 1 Stéphane Ducasse [email protected] http://stephane.ducasse.free.fr/ Objects to the Roots: Learning from beauty Learning from beauty
- 2. S.Ducasse 2 Really?! No primitive types No hardcoded constructs for conditional Only messages Only objects and this works? I mean really? Not even slow? Can’t be real!
- 3. S.Ducasse 3 Motto Let’s open our eyes, look, understand, and deeply understand the underlying design aspects of object- oriented programming...
- 4. S.Ducasse 4 Booleans 3 > 0 ifTrue: ['positive'] ifFalse: ['negative']
- 5. S.Ducasse 5 Booleans 3 > 0 ifTrue: ['positive'] ifFalse: ['negative'] 'positive'
- 6. S.Ducasse 6 Yes ifTrue:ifFalse: is a message! Weather isRaining ifTrue: [self takeMyUmbrella] ifFalse: [self takeMySunglasses] ifTrue:ifFalse is sent to an object: a boolean!
- 7. S.Ducasse 7 Booleans & | not or: and: (lazy) xor: ifTrue:ifFalse: ifFalse:ifTrue: ...
- 8. S.Ducasse 8 Lazy Logical Operators false and: [1 error: 'crazy'] PrIt-> false and not an error
- 9. S.Ducasse 9 Yes! ifTrue:ifFalse: is a message send to a Boolean. But optimized by the compiler :)
- 10. S.Ducasse 10 Implementing not Now you are good and you should implement it Propose an implementation of not in a world where you do not have Booleans false not -> true true not -> false
- 11. S.Ducasse 11
- 12. S.Ducasse 12 Implementing ifTrue:ifFalse: Now you are good and you should implement it Propose an implementation of not in a world where you do not have Booleans false ifTrue: [ 3 ] ifFalse: [ 5 ] true ifTrue: [ 3 ] ifFalse: [ 5 ]
- 13. S.Ducasse 13 Boolean Objects false and true are objects described by classes Boolean,True and False
- 14. S.Ducasse 14 Let’s the receiver decide!
- 15. S.Ducasse 15 Boolean>>not “Class Boolean is an abstract class that implements behavior common to true and false. Its subclasses are True and False. Subclasses must implement methods for logical operations &, not, controlling and:, or:, ifTrue:, ifFalse:, ifTrue:ifFalse:, ifFalse:ifTrue:” Boolean>>not "Negation. Answer true if the receiver is false, answer false if the receiver is true." self subclassResponsibility
- 16. S.Ducasse 16 Not false not -> true true not -> false Boolean>>not "Negation. Answer true if the receiver is false, answer false if the receiver is true.” self subclassResponsibility False>>not "Negation -- answer true since the receiver is false." ^true True>>not "Negation--answer false since the receiver is true." ^false
- 17. S.Ducasse 17 | (Or) • true | true -> true • true | false -> true • true | anything -> true • false | true -> true • false | false -> false • false | anything -> anything
- 18. S.Ducasse 18 Boolean>> | aBoolean Boolean>> | aBoolean "Evaluating disjunction (OR). Evaluate the argument. Answer true if either the receiver or the argument is true." self subclassResponsibility
- 19. S.Ducasse 19 False>> | aBoolean false | true -> true false | false -> false false | anything -> anything False>> | aBoolean "Evaluating disjunction (OR) -- answer with the argument, aBoolean." ^ aBoolean
- 20. S.Ducasse 20 True>> | aBoolean true | true -> true true | false -> true true | anything -> true True>> | aBoolean "Evaluating disjunction (OR) -- answer true since the receiver is true." ^ self
- 21. S.Ducasse 21 Boolean,True and False
- 22. S.Ducasse 22 Implementation Note Note that theVirtual Machine shortcuts calls to boolean such as condition for speed reason. Virtual machines such asVisualWorks introduced a kind of macro expansion, an optimisation for essential methods and Just In Time (JIT) compilation.A method is executed once and afterwards it is compiled into native code. So the second time it is invoked, the native code will be executed.
- 23. S.Ducasse 23
- 24. S.Ducasse 24 Ok so what? You will probably not implement another Boolean classes So is it really that totally useless?
- 25. S.Ducasse 25 Ternary logic Boolean: true, false, unknown
- 26. S.Ducasse 26 More important... Message sends act as case statements
- 27. S.Ducasse 27 OOP: the art of dispatching Subclasses create your vocabulary
- 28. S.Ducasse 28 Avoid Conditional Use objects and messages VM dispatch is a conditional switch: Use it! AntiIfCampaign
- 29. S.Ducasse 29 Summary Messages act as a dispatcher Avoid conditional