1. 1 Variant Ownership with Existential Types Nicholas Cameron Sophia Drossopoulou Imperial College London

2. 2 Outline Variant ownership Existential types for variance Combine with generics for expressiveness

3. ncameron@doc.ic.ac.uk 3 class List { Object datum; List next; } List

4. 4 GUI e.g. window panel1 panel2 button1 button2 button3 button4

5. ncameron@doc.ic.ac.uk 5 Subtype Variance class Window {... void render(Panel p) { List allWidgets =... ;... }... }

6. ncameron@doc.ic.ac.uk 6 Subtype Variance class Window {... void render(Panel p) { List allWidgets =... ;... }... }

7. 7 GUI e.g. window panel1 panel2 button1 button2 button3 button4

8. ncameron@doc.ic.ac.uk 8 Subtype Variance class Window {... void render(Panel p) { List allWidgets =... ;... }... } Panel

9. ncameron@doc.ic.ac.uk 9 Subtype Variance Already exists! – On Ownership and Accessibility (Lu and Potter, ECOOP 06)‏ – any owner in universes (Müller, Dietl,...)‏ – ? in MOJO (Cameron, Drossopoulou, Noble, Smith, OOPSLA 07)‏ –...

10. ncameron@doc.ic.ac.uk 10 Existential Types List

11. ncameron@doc.ic.ac.uk 11 Existential Types List

12. ncameron@doc.ic.ac.uk 12 Existential Types List  o.List

13. ncameron@doc.ic.ac.uk 13 Owner Polymorphic Methods Useful for iterators, etc. Require existential types Expressible but not denotable types Similar problem to modelling wildcards - e.g., Wed 10:00am

14. ncameron@doc.ic.ac.uk 14 Generics and Ownership class List { Object datum; } List vs Class List { X datum; } List > [Potanin, Noble, Clarke, Biddle, OOPSLA 06]

15. ncameron@doc.ic.ac.uk 15...and Existential Owners  o.List > List >  o.List > List >>

16. ncameron@doc.ic.ac.uk 16 Benefits Expressive and uniform Easy to reason about Well understood mechanism – Existential types – Wildcards Todo Encapsulation properties Application/relation to other systems

17. ncameron@doc.ic.ac.uk 17 Thank You!