Reflections on the Work of C.A.R. Hoare, 2010

ThisvolumehasitsoriginsinameetingheldatMicrosoftResearch,Cambridge,in April2009tocelebrateTonyHoare’s75thBirthday(actually11Jan2009). Allthe technicalpapersexceptforthosewrittenbyAbramsky,Jackson,JonesandMeyer arebased–sometimesclosely,sometimesnot–onpresentationsgivenatthatme- ing. TheideaforthemeetingaroseinconversationsbetweenourselvesandAndrew HerbertofMicrosoft,whohostedatrulymemorableandhappyevent. ThemeetingwasorganisedbyourselvesandKenWood,withthe?nancials- portofMicrosoftResearchandFormalSystems(Europe)Ltd,andheldovertwo days. We wouldlike to recordparticularthanksto Angela Still of Microsoftfor makingallthelocalarrangementsatCambridgeandmuchmore:themeetingwould nothavehappenedwithouther. Whilethemajorityofthepapersinthisvolumearetechnical,weaskedauthorsto re?ectonthein?uenceofHoare’sworkontheirown?eldsandtomakeappropriate remarksonit. Allthetechnicalpaperswererefereed. DiscussionswithWayneWheelerofSpringerinspiredthetwoofustowritethe scienti?cbiographyofHoarethatisthe?rstpaperinthisvolume. Thoughwehave bothknownTonywellformanyyears,wewereamazedathowmanydiscoveries abouthimwemadeduringtheprocessofwritingthisarticle. WewouldlikethankWayneandhisassistantSimonReesfortheirhelpinprep- ingthisvolumeaswellastheirpatience. Muchoftheworkingatheringthepapers, ensuringconsistencyofLaTeXstyles,etc. ,wasdonebyLucyLiofOxfordUniv- sityComputingLaboratoryandwethankherwarmly. Tragically,KenWood’swifeLisadiedafteralongillnessinSeptember2009. Wededicatethisvolumetohermemory. January2010 CliffJones BillRoscoe ix Contents 1 Insight,InspirationandCollaboration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 C. B. JonesandA. W. Roscoe 2 FromCSPtoGameSemantics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 SamsonAbramsky 3 OnMereologiesinComputingScience. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 DinesBjørner 4 Roles,Stacks,Histories:ATripleforHoare. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 Johannes Borgstrom, ¨ Andrew D. Gordon, andRiccardoPucella 5 ForwardwithHoare. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 MikeGordonandHel ´ene ` Collavizza 6 ProbabilisticProgrammingwithCoordination. . . . . . . . . . . . . . . . . . . . . . . . . . . 123 HeJifeng 7 TheOperationalPrincipleandProblemFrames. . . . . . . . . . . . . . . . . . . . . . . . . 143 MichaelJackson 8 TheRoleofAuxiliaryVariablesintheFormal DevelopmentofConcurrentPrograms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 C. B. Jones 9 AvoidaVoid:TheEradicationofNullDereferencing. . . . . . . . . . . . . . . . . . . . 189 BertrandMeyer,AlexanderKogtenkov,andEmmanuelStapf 10 UnfoldingCSP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213 MikkelBundgaardandRobinMilner xi xii Contents 11 Quicksort:CombiningConcurrency,Recursion, andMutableDataStructures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 DavidKitchin,AdrianQuark,andJayadevMisra 12 TheThousand-and-OneCryptographers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 A. K. McIverandC. C. Morgan 13 On Process-AlgebraicExtensions of Metric TemporalLogic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 ChristophHaase,Joel ¨ Ouaknine,andJamesWorrell 14 FunwithTypeFunctions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301 OlegKiselyov,SimonPeytonJones,andChung-chiehShan 15 OnCSPandtheAlgebraicTheoryofEffects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 RobvanGlabbeekandGordonPlotkin 16 CSPisExpressiveEnoughfor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371 A. W. Roscoe 17 TheTokeneerExperiments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405 JimWoodcock,EmineGokc ¨ ¸eAydal,andRodChapman Chapter1 Insight,InspirationandCollaboration C. B. JonesandA. W. Roscoe Abstract TonyHoare’smanycontributionstocomputingsciencearemarkedby insightthatwasgroundedinpracticalprogramming. Manyofhispapershavehada profoundimpactontheevolutionofour?eld;theyhavemoreoverprovidedasource ofinspirationtoseveralgenerationsofresearchers. Weexaminethedevelopmentof hisworkthroughareviewofthedevelopmentofsomeofhismostin?uentialpieces ofworksuchasHoarelogic,CSPandUnifyingTheories. 1. 1 Introduction To many who know Tony Hoare only through his publications, they must often looklikepolishedgemsthatcomefromamindthatrarelymakesfalsesteps,nor evenperhapshastoworkattheircreation. Assooften,thisimpressionisafurther complimenttosomeonewhoactuallyaddstoveryhardworkandmanydiscarded attempts the ?nal polish thatmakes complexideas relatively easy for the reader tocomprehend. Asindicatedonpagexiof[HJ89],hisideastypicallygothrough manyrevisions. ThetwoauthorsofthecurrentpapereachhadthehonourofTonyHoaresuperv- ingtheirdoctoralstudiesinOxford. Theyknowat?rsthandhiskindandgenerous styleandwillcountitasanachievementifthispapercanconveysomethingofthe workingmethodsofsomeonebigenoughtoeschewcompetitionandpointscoring. Indeedit willbe apparentfromthe followingsectionshowoften,havingstarted somenewwayofthinkingorexcitingideas,hehappilyleavestheirexplorationand developmenttoothers. Wehavebothbene?tedpersonallyfromthis. C. B. Jones( ) SchoolofComputingScience,NewcastleUniversity,UK e-mail:cliff. jones@ncl. ac. uk A. W. Roscoe OxfordUniversityComputingLaboratory,UK e-mail:Bill. Roscoe@comlab. ox. ac. uk C. B. Jonesetal. (eds. ),Re?ectionsontheWorkofC. A. R.

Insight, Inspiration and Collaboration C. B. Jones and A. W. Roscoe From CSP to Game Semantics Samson Abramsky On Mereologies in Computing Science Dines Bjørner Roles, Stacks, Histories: A Triple for Hoare Johannes Borgstrom, Andrew D. Gordon and Riccardo Pucella Forward with Hoare Mike Gordon and Helene Collavizza Probabilistic Programming with Coordination He Jifeng The Operational Principle and Problem Frames Michael Jackson The Role of Auxiliary Variables in the Formal Development of Concurrent Programs Cliff B. Jones Avoid a Void: The Eradication of Null Dereferencing Bertrand Meyer, Alexander Kogtenkov and Emmanuel Stapf Unfolding CSP Mikkel Bundgaard and Robin Milner Quicksort: Combining Concurrency, Recursion, and Mutable Data Structures David Kitchin, Adrian Quark and Jayadev Misra The Thousand-and-One Cryptographers AK McIver and CC Morgan On Process-algebraic Extensions of Metric Temporal Logic Christoph Haase, Joel Ouaknine and James Worrell Fun with Type Functions Oleg Kiselyov, Simon Peyton Jones and Chung-chieh Shan On CSP and the Algebraic Theory of Effects Rob van Glabbeek and Gordon Plotkin CSP is Expressive Enough for ? A.W. Roscoe The Tokeneer Experiments Jim Woodcock, Emine Gokce Aydal and Rod Chapman