1. Multiple Ownership Nicholas Cameron Sophia Drossopoulou James Noble Matthew Smith

2. ncameron@doc.ic.ac.uk2 Background – Ownership Types Multiple Ownership Objects in Boxes MOJO Variance - ? Effects and Disjointness

3. ncameron@doc.ic.ac.uk3 Background

4. ncameron@doc.ic.ac.uk4 Ownership Types The heap is messy:

5. ncameron@doc.ic.ac.uk5 Ownership Types Organise it:

6. ncameron@doc.ic.ac.uk6 Ownership Types Give each object an owner:

7. ncameron@doc.ic.ac.uk7 Ownership Types class Project { Task t1; Task t2; List clients; }

8. ncameron@doc.ic.ac.uk8 Our Work

9. ncameron@doc.ic.ac.uk9 The Objects In Boxes Model

10. ncameron@doc.ic.ac.uk10 A Box is a Set of Objects

11. ncameron@doc.ic.ac.uk11 Objects in Boxes Single owner interpretation: An object is in its owner’s box

12. ncameron@doc.ic.ac.uk12 But, Programs are not Trees “We’re tired of trees… We should stop believing in trees, roots, radicles” Deleuze and Guattari, A Thousand Plateaus

13. ncameron@doc.ic.ac.uk13 One Owner is not Enough 75% of ownership structures require multiple ownership [34] Mitchell, ECOOP, ’06 Trees can not describe non-hierarchical structures

14. ncameron@doc.ic.ac.uk14 We Propose: Objects in Multiple Boxes Multiple Ownership: objects may be in more than one box An object with multiple owners is in the intersection of their boxes:

15. ncameron@doc.ic.ac.uk15 Objects in Multiple Boxes

16. ncameron@doc.ic.ac.uk16 MOJO

17. ncameron@doc.ic.ac.uk17 A Descriptive System Describes the heap Does not restrict references etc.

18. ncameron@doc.ic.ac.uk18 MOJO Class declarations have the usual, formal ownership parameters: class C …

19. ncameron@doc.ic.ac.uk19 MOJO Class declarations have the usual, formal ownership parameters: class C … Types may have multiple corresponding actual owner parameters: C

20. ncameron@doc.ic.ac.uk20 MOJO Class declarations have the usual, formal ownership parameters: class C … Types may have multiple corresponding actual owner parameters: C

21. ncameron@doc.ic.ac.uk21 MOJO Class declarations have the usual, formal ownership parameters: class C … Types may have multiple corresponding actual owner parameters: C

22. ncameron@doc.ic.ac.uk22 MOJO class Connection { … } class Client { Connection c; } class Server { Connection c; }

23. ncameron@doc.ic.ac.uk23 Variance

24. ncameron@doc.ic.ac.uk24 Variance Connection Some owner

25. ncameron@doc.ic.ac.uk25 Variance Connection Some box May be the intersection of several boxes Variance in the number of owners

26. ncameron@doc.ic.ac.uk26 Subtyping Subtyping with wildcards is variant with respect to owners: C

27. ncameron@doc.ic.ac.uk27 Subtyping Subtyping with wildcards is variant with respect to owners: C

28. ncameron@doc.ic.ac.uk28 Subtyping Subtyping with wildcards is variant with respect to owners: C

29. ncameron@doc.ic.ac.uk29 Subtyping Subtyping with wildcards is variant with respect to owners: C But: C

30. ncameron@doc.ic.ac.uk30 Strict typing – field assignment Strict method sends and assignments to deal with variance of owners: class D { D f; } D da; D d1; D d2; da.f = da;

31. ncameron@doc.ic.ac.uk31 Strict typing – field assignment Strict method sends and assignments to deal with variance of owners: class D { D f; } D da; D d1; D d2; da.f = da; d1.f = d2; 

32. ncameron@doc.ic.ac.uk32 Strict typing – field assignment Strict method sends and assignments to deal with variance of owners: class D { D f; } D da; D d1; D d2; D db; D dc; d1 = db; d2 = dc; da.f = da; d1.f = d2; 

33. ncameron@doc.ic.ac.uk33 Strict typing – field assignment

34. ncameron@doc.ic.ac.uk34 Constraints intersects disjoint

35. ncameron@doc.ic.ac.uk35 Constraints class C a intersects b, b disjoint c { … }

36. ncameron@doc.ic.ac.uk36 Constraints class D a intersects b { D } class E a disjoint b { }

37. ncameron@doc.ic.ac.uk37 Constraints class D a intersects b { D } class E a disjoint b { D  }

38. ncameron@doc.ic.ac.uk38 Constraints class D a intersects b { D Object } class E a disjoint b { D  }

39. ncameron@doc.ic.ac.uk39 Effects

40. ncameron@doc.ic.ac.uk40 Effects

41. ncameron@doc.ic.ac.uk41 Effects Task t1; Task t2; … t1.f; // reads x / writes ε

42. ncameron@doc.ic.ac.uk42 Effects Task t1; Task t2; … t1.f; // reads x / writes ε t1.f = t2.f; // reads x,y / writes x

43. ncameron@doc.ic.ac.uk43 Effects with Multiple Owners Task t1; Task t2; … t1.f; // reads x & ? / writes ε

44. ncameron@doc.ic.ac.uk44 Effects with Multiple Owners Task t1; Task t2; … t1.f; // reads x & ? / writes ε t1.f = t2.f; // reads x & ?,y & z / writes x & ?

45. ncameron@doc.ic.ac.uk45 Effects with Multiple Owners t1.f = t2.f; // reads x & ?,y & z/ writes x & ?

46. ncameron@doc.ic.ac.uk46 Disjointness Two expressions are disjoint if their effects do not overlap Complicated by ? – but & = intersection

47. ncameron@doc.ic.ac.uk47 Disjointness cl1 disjoint cl2 Connection c1; Connection c2;

48. ncameron@doc.ic.ac.uk48 Disjointness cl1 disjoint cl2 Connection c1; Connection c2; cl.drop(); //rd cl1 & ? / wr cl1 & ? c2.send(…); //rd cl2 & ? / wr cl2 & ?

49. ncameron@doc.ic.ac.uk49 Disjointness cl1 disjoint cl2 Connection c1; Connection c2; cl.drop(); //rd cl1 & ? / wr cl1 & ? c2.send(…); //rd cl2 & ? / wr cl2 & ? cl1 & ? # cl2 & ?

50. ncameron@doc.ic.ac.uk50 Disjointness cl1 disjoint cl2 Connection c1; Connection c2; cl.drop(); //rd cl1 & ? / wr cl1 & ? c2.send(…); //rd cl2 & ? / wr cl2 & ? cl1 & ? # cl2 & ? c2.send() does not affect c1.drop()

51. ncameron@doc.ic.ac.uk51 Future Work

52. ncameron@doc.ic.ac.uk52 Future Work Explore variant owners using existential types class TaskList { Task datum; TaskList next; }

53. ncameron@doc.ic.ac.uk53 Future Work Reference and modification control

54. ncameron@doc.ic.ac.uk54 Future Work Constraints Topology - inside Unary - may read, may write Binary – may point to, may modify

55. ncameron@doc.ic.ac.uk55 Summary Multiple Owners Objects in Boxes Variance Formalism - MOJO Effects Disjointness Thank you! Any questions? “structures like the city, which do require overlapping sets within them, are nevertheless persistently conceived as trees” Christopher Alexander, A City is not a Tree