1. Encoding Ownership Types in Java Nicholas Cameron James Noble Victoria University of Wellington, New Zealand

2. Ownership types for real life Ownership types are great! – (More later...)

3. Ownership types for real life But ownership type systems are big and complex – And writing compilers is hard – And the type systems are not well-understood

4. Ownership types for real life There is another way...

5. Ownership Types Are a facilitating type system: – Effects Parallelisation Optimisation – Concurrency – Memory management – Security –...

6. Ownership Types When the heap gets large, reasoning gets hard Solution: break it up into smaller regions – BUT, we don’t program this way Nest the regions – Welcome to ownership types!

7. Ownership Types owner:ClassName – this:C – world:D owner keyword names the owner of this – owner:C Context parameters add flexibility

8. Java Generics – List

9. Java Wildcards – List

10. End of Background...

11. Basic idea We use type parameters to mimic ownership parameters (OGJ)

12. An object’s owner (and the ‘world’ context) class C {...} world:C class C {...} C – class World {}

13. Context parameters Become type parameters

14. Bounds

15. The ‘this’ context This* is where it gets interesting We depart from OGJ – (OGJ does this with magic) Must correspond with the this variable *no pun intended

16. The ‘this’ context Kind of like another context parameter – class C {... } We can name This within the class

17. The ‘this’ context But this cannot be named outside the class – So neither should This Use a wildcard to hide This

18. The ‘this’ context class E world:E class E E

19. The ‘this’ context But, what about nesting?

20. The ‘this’ context Use bounds – class C Wildcards inherit declared bounds – C

21. The ‘this’ context class E world:E class E<C1, C2, Owner, This extends Owner> E

22. The ‘this’ context class E world:E class E<C1, C2, Owner, This extends Owner> E (E ) new

23. The ‘this’ context

24. The type system thinks there is a hierarchy –  X inside  Y inside  Z inside... But in reality all owners are World

25. Nice...

26. Existential Owners and variant ownership Use wildcards

27. Inner Classes Require inner classes to be able to name surrounding This parameter – Comes naturally with Java generics

28. Type Parameters Work alongside translated context parameters class F {... } world:F class F {... } F

29. Universes rep CC peer CC any CC

30. and... Ownership Domains Context-parametric methods Dynamic aliases Fields as contexts Existential downcasting

31. Owners-as-Dominators Most of the work is done by the hiding of This using wildcards Must ensure it cannot be named indirectly Works with the extensions too – Including inner classes

32. Owners-as-Dominators Cannot be enforced by translating compiler Requires enforcing well-formedness of intermediate types

33. Contributions Prototype compilers – Ownership types++ – Universes How to leverage existing compiler technology for OTs Formalisation of OTs in Java – Proved sound – Ownership hierarchy is preserved and enforced at runtime Better understanding of OTs

34. Thank you!