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Uniform and Safe Metaclass Composition

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Uniform and Safe Metaclass
Composition. Stéphane Ducasse, Nathanael Schaerli and Roel Wuyts (ESUG 2004, Koethen)

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  • 1. Uniform and Safe Metaclass Composition Stéphane Ducasse, Nathanael Schaerli and Roel Wuyts ducasse@iam.unibe.ch http://www.iam.unibe.ch/~ducasse/ S. Ducasse 1
  • 2. Why Metaclasses? • Classes are objects too • A class is an instance of a class called a metaclass • Uniformity: Classes are just objects • Control: Instance creation behavior • Reuse of class behavior S. Ducasse 2
  • 3. Implicit vs. Explicit • Implicit: programmer cannot specify the class • Safe but • Limited or no reuse • Explicit: • Composition metaclass is not safe • Ad-hoc composition mechanisms (for example: strings manipulation of neoclasstalk and dynamic class change) • Safe?: upward and downward compatible S. Ducasse 3
  • 4. Upward Compatible instance of Meta A Meta B inherits from c-bar ? A>>i-foo A B ^ self class c-bar i-foo • aB i-foo => no error S. Ducasse 4
  • 5. Downward Compatible Meta A Meta B Meta A>>c-bar c-bar ^ self new i-bar instance of A B inherits from i-bar ? • B c-bar => no error S. Ducasse 5
  • 6. Smalltalk-80 • Implicit • Safe • Limited reuse Meta A>>c-foo ^ self new i-bar Meta A Meta B c-bar c-foo A>>i-foo ^ self class c-bar A B i-foo instance of i-bar inherits from S. Ducasse 6
  • 7. Existing Solutions • Smalltalk-80 • Implicit, safe • But limited reuse • Clos • Explicit but help yourself! • NeoClasstalk • Dynamic class changes • Metaclass behavior specified as strings • MetaClasstalk • Double meta-layer with mixin composition S. Ducasse 7
  • 8. Our approach: traits • Implicit metaclasses • But traits to compose classes • Reuse of traits Meta A Meta B Class Class Property2 Property1 A B instance of inherits from S. Ducasse 8
  • 9. What are Traits? • Traits are parameterized behaviors • Traits provide a set of methods • Traits require a set of methods • Traits are purely behavioral • Traits do not specify any state TCircle area radius bounds radius: diameter center hash center: S. Ducasse 9
  • 10. How are Traits Used? • Traits are the behavioral building blocks of classes • Class = Superclass + state + traits + glue methods Object ColoredCircle TColor TCircle • Class methods take precedence over trait methods S. Ducasse 10
  • 11. Conflicts Explicitly Resolved ColoredCircle TCircle hash -> circleHash hash TColor hash -> colorHash I) Override the conflict with a glue method  Aliases provide access to the conflicting methods II) Avoid the conflict  Exclude the conflicting method from one trait S. Ducasse 11
  • 12. Hierarchies Revisited... Behavior Boolean class TCreator new False class basicNew TAbstract TSingleton Boolean class True class TAbstract TSingleton new Boolean False class TSingleton False new basicNew default uniqueInstance reset uniqueInstance: True S. Ducasse 12
  • 13. Metalevel Reengineering TAllocate basicNew TInstantiator TInitInstantiator new new initialize nonRememberingNew ! new sharedNew ! new TRememberInstances new rememberedInstances TAbstract TSharedInstance rememberInstance rememberedInstances: new reset directSharedInstance instances sharedInstance directSharedInstance: reset default ! sharedInstance uniqueInstance ! TFinal sharedInstance subclass: TDefault TSingleton new Legend TSingleton m1 ! m2 is composed from new creates an alias Trait m2 referring to m1 S. Ducasse 13
  • 14. Traits Implementation • Available in Squeak 3.7 • Bootstrapping the kernel PureBehavior TraitBehavior Behavior Trait Class Description Description Trait Class Metaclass S. Ducasse 14
  • 15. Conclusion • Uniform • Traits are instance and class level! • Explicit Composition • Reuse but Safe S. Ducasse 15