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An Exception Aware Behavioral Type System for Object-Oriented Programs

This paper introduces a type system that statically checks the observable behavior of objects, ensuring termination and handling exceptions in object-oriented programs.

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An Exception Aware Behavioral Type System for Object-Oriented Programs

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  1. An Exception Aware Behavioral Type System for Object-Oriented Programs CITI / Departamento de Informática Universidade Nova de Lisboa - Portugal Filipe Militãoand LuísCaires CoRTA 2009

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

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

  4. TravelOrderClass 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 flight(…) hotel(…) buy() . buy flight . hotel • Hidden internal state (like in regular expressions) • Tracks observable calls (i.e., the behavior), not states

  8. What is a Behavioral Type? TravelOrder Type Type TravelOrder Behavior Behavioral void hotel() void flight() void order() void cancel() string show() 1stflight 2ndhotel 3rdbuy or cancel +

  9. Describing the Behavior • What about “string show()” ? free / non-behavioral, can be called at any time... TravelOrder Behavior 1stflight 2ndhotel 3rdbuy or cancel • Regular expression-like protocol: flight . hotel . (buy + cancel)

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

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

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

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

  14. Sequence e0 e1 T0 T1 Will be lost!

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

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

  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 etry T throw “N” ecatch

  19. Linear Ownership • Only one owner at a time* ( *unless the object was borrowed… ) let t0in let t1 in t0 = new TravelOrder(); t1 = t0 ; t1.flight(); t0.hotel() Type Error! t1  stop t0  flight. hotel.(buy+cancel)

  20. Borrowing • Checking call-by-reference let tin t= new TravelOrder(); orderStuff(t); t.buy() voidorderStuff(TravelOrder#flight.hotelx) t flight.hotel. (buy+cancel) • Storing values requires ownedpermission

  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+swim).hotel <: flight.hotel flight.hotel <: flight[Full: cancel].hotel

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

  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, AntónioRavara, Nils Gesbert, and Alexandre Z. Caldeira. • …

  25. Future work • Concurrency

  26. Questions? Filipe Militãoand LuísCaires • Paper: • Web site: An Exception Aware Behavioral Type System for Object-Oriented Programs yak prototype http://ctp.di.fct.unl.pt/yak/

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