First-class undefined classes for Pharo
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This document proposes a design for representing undefined classes in Pharo to allow code loading when class references are unknown or missing. It evaluates three designs before selecting design three, where there is one subclass of UndefinedClass per undefined class. This allows undefined classes to behave like normal classes while ensuring loading correctness and integration with tools like Monticello and CodeImporter. The design is tested by loading old Seaside code and Seaside packages in random order.
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First-class undefined classes for Pharo
- 1. First-Class Undefined Classes for Pharo From Alternative Designs to a Unified Practical Solution Guille Polito, Luc Fabresse, Stéphane Ducasse @guillepolito
- 2. Context Package Dependency Object Platform workingDirectory Windows workingDirectory Platform workingDirectory Unix call:library: FFI Platform >> workingDirectory ^ self subclassResponsibility Windows >> workingDirectory ^ FFI call: ’_getcwd’ library: LibC Unix >> workingDirectory ^ FFI call: ’getcwd’ library: LibC
- 4. Problem Statement • Loading code with unresolved class references happens when: • Migrating code to new Pharo versions • Loading code with unknown or cyclic dependencies
- 6. Undefined Classes • First-class entity representing non-existing classes to load: • subclasses with unknown superclasses • methods extending unknown classes • Single and uniform mechanism for all tools
- 7. Design 1 Object Class UndefinedClass UndefinedClass class Windows Windows class Unix Unix class Shared for all non-existent classes (name lost, …) A unique UndefinedClass class
- 8. Design 2 One instance of UndefinedClass per undefined class Object Class UndefinedClass Windows Windows class WindowsFFI WindowsFFI class Platform FFI UndefinedClass class Class class Complex and tools issue that must deal with UndefinedClass instances in addition to Classes and Traits
- 9. Design 3 One subclass of UndefinedClass per undefined class Object Class UndefinedClass UndefinedClass class Windows Windows class WindowsFFI WindowsFFI class Platform Platform class FFI FFI class UndefinedClass will behave as a normal defined class (can be instantiated? …)
- 10. Design selection for Pharo
- 11. Ensuring Loading Correctness • Only one UndefinedClass subclass per missing class (identity) • UndefinedClass subclass automatic migration, and destruction when loading the actual class • System uniformity i.e., all code loading mechanism relies on our system
- 12. UndefinedClasses MOP • an UndefinedClass cannot be instantiated • UndefinedClass>>doesNotUnderstand: throws a notifying error • prevent misunderstanding with sub-instances
- 13. Tools Integration • Monticello • create undefined classes at load time if needed • create the extended class as UndefinedClasses if non-existing • CodeImporter • parse class definition expressions and create undefined superclasses if needed • create the extended class as UndefinedClasses if non-existing
- 15. Migrating Old Code • Seaside 2.8 (2011) • subclasses of PackageInfo (removed in Pharo 4) • extension methods in BlockContext (removed ~Pharo 2) and ContextPart (renamed Context in Pharo 4) ConfigurationOfSeaside28 load • Omni Browser (2015) • subclasses + extension methods of undefined classes • class-side initialize with references to undefined classes ConfigurationOfOmniBrowser project lastVersion load
- 16. Loading Order • Loading Seaside 3 and its 59 monticello packages dependencies in a random order Seaside3LoadingTest>> testLoadingSeasidePackagesInRandomOrder | maxNbOfUndefinedClasses | self assert: UndefinedClass allSubclasses isEmpty. maxNbOfUndefinedClasses := self loadSeasidePackagesInRandomOrder. self assert: maxNbOfUndefinedClasses > 0. self assert: UndefinedClass allSubclasses isEmpty. self executeInitializeClassMethodsInCorrectOrder. self assertAllSeasideUnitTestsAreGreen.
- 17. Conclusion • An UndefinedClass • is a first-class entity representing one missing class • stores all information (name, extension methods, …) • can be subclassed and referenced • is destroyed when loading actual class definition after introducing all infos in the real class • has a specific MOP • is compliant with tools (MC, CodeImporter)
- 18. Future Work • Integration of Undefined Classes into Pharo 7.0 • Towards a module system: • when loading a module, class references inside a module may not be resolved until we bound them
- 19. Conclusion • An UndefinedClass • is a first-class entity representing one missing class • stores all information (name, extension methods, …) • can be subclassed and referenced • is destroyed when loading actual class definition after introducing all infos in the real class • has a specific MOP • is compliant with tools (MC, CodeImporter)