1. AN EXCEPTION AWARE BEHAVIORAL TYPE SYSTEM FOR OBJECT-ORIENTED PROGRAMS Filipe Militão and Luís Caires CITI / Departamento de Informática Universidade Nova de Lisboa - Portugal CoRTA 2009

2. TravelOrder Class class TravelOrder { void flight(){ … } void hotel(){ … } void buy(){ … } void cancel(){ … } string show(){ … } }

3. TravelOrder Documentation /** * A TravelOrder requires the selection of * a flight and an hotel before it can be bought * or canceled. */

4. TravelOrder Class class TravelOrder { void flight(){ … } void hotel(){ … } void buy(){ … } void cancel(){ … } string show(){ … } } only available on specific situations can be called freely

5. Background  Increasing software complexity  Effort to eliminate bugs  Faster feedback on errors (static detection)

6. Objectives and Contributions  Statically check the externally observable behavior of objects (based on formal description)  Guarantee termination (reaches stop)  Exceptions (checked)  Linear ownership + Borrowing (call-by-ref)  Class body consistent with class behavior  Working prototype

7. Checking call sequences TravelOrder object TravelOrder object flight(…) hotel(…) buy() flight. hotel. buy → Hidden internal state (like in regular expressions) → Tracks observable calls (i.e., the behavior), not states → Hidden internal state (like in regular expressions) → Tracks observable calls (i.e., the behavior), not states

8. What is a Behavioral Type? void hotel() void flight() void order() void cancel() string show() 1 st flight 2 nd hotel 3 rd buy or cancel T RAVEL O RDER T YPE T RAVEL O RDER B EHAVIOR + B EHAVIORAL T YPE

9. Describing the Behavior  What about “ string show() ” ? free / non-behavioral, can be called at any time... 1 st flight 2 nd hotel 3 rd buy or cancel T RAVEL O RDER B EHAVIOR. (buy + cancel)  Regular expression-like protocol: flight. hotel

10. TravelOrder Class class TravelOrder { usage flight.hotel.(buy+cancel) void flight(){ … } void hotel(){ … } void buy(){ … } void cancel(){ … } string show(){ … } }

11. Example let t in t = new TravelOrder(); t.flight(); t.show(); t.hotel(); if( … ) t.buy() else t.cancel() flight. hotel. ( buy + cancel )stop

12. The rest of this talk… Introduction  Type System  Exceptions  Linear Ownership  Borrowing  Subtyping  Consistency check  Related work  Future work

13. Basic typing judgement environment (before) expression type of expression environment (after)

14. Sequence e0e0 e1e1 T0T0 T1T1 Will be lost!

15. Example (try branch) void method(N#buy[Error: undo] v){ try v.buy() catch(Error excp) v.undo() } buy [Error: undo] v  stop on Error  { v  undo }

16. Example (catch branch) void method(N#buy[Error: undo] v){ try v.buy() catch(Error excp) v.undo() } undo v  stop on Error  { v  undo }

17. Exceptions - throw expression produces compatible environment Runtime is behavior agnostic, so thrown object must be stopped Exception handler for N

18. Exceptions - try catch Try branch with new exception handler Catch branch restores old handler e try e catch T throw “N”

19. Linear Ownership  Only one owner at a time* ( *unless the object was borrowed… ) let t0 in let t1 in t0 = new TravelOrder(); t1 = t0 ; t1.flight(); t0.hotel() t0  t1  stop flight.hotel.(buy+cancel) Type Error !

20. Borrowing  Checking call-by-reference let t in t = new TravelOrder(); orderStuff(t); t.buy() void orderStuff(TravelOrder#flight.hotel x)  Storing values requires owned permission t  flight.hotel.(buy+cancel)

21. The rest of this talk… Introduction Type System Exceptions Linear Ownership Borrowing  Subtyping  Consistency check  Related work  Future work

22. Subtyping  Structural subtyping  Requires compatible behavior  Subtype must accept same behavior as super… flight.hotel (flight+swim).hotel <: <: flight[Full: cancel].hotel

23. Consistency check class ABC { usage a.b.c TravelOrder t; void a(){ t = new TravelOrder(); } void b(){ t.flight(); } void c(){ t.buy(); } } Type Error !  Server code must also obey protocols

24. Related work  Resource usage analysis (2002) Atsushi Igarashi and Naoki Kobayashi  Lightweight object specification with typestates (2007) Kevin Bierhoff and Jonathan Aldrich  Dynamic interfaces (2009) Vasco T. Vasconcelos, Simon Gay, António Ravara, Nils Gesbert, and Alexandre Z. Caldeira.  …

25. Future work  Concurrency

26. Questions? AN EXCEPTION AWARE BEHAVIORAL TYPE SYSTEM FOR OBJECT-ORIENTED PROGRAMS Filipe Militão and Luís Caires http://ctp.di.fct.unl.pt/yak/ YAK PROTOTYPE  Paper:  Web site: