Adding Logic Programming Behaviour to the World Wi

AddingLogicProgrammingBehaviourtotheWorldWideWebSengWaiLoke1998SubmittedintotalfulfilmentoftherequirementsofthedegreeofDoctorofPhilosophyDepartmentofComputerScienceSchoolofElectricalEngineeringandComputerScienceTheUniversityofMelbourneAUSTRALIAAbstractImperativeprogramminghaslargelydominatedbothaspectsofWebprogram-ming:addingsophisticatedinteractivebehaviourstotheWebandconstructingprogramswhichinteractwiththeWeb.MostmobilecodelanguagessuchasJavaarebasedontheimperativeprogrammingparadigm.ImperativelanguagesarewidelyusedforbuildingWebbrowsersandinformationgatheringtools.Thefocusofmuchprogramminglanguageresearchhasbeenonraisingthelevelofabstraction.Logicprogramminglanguages,whichviewcomputationasde-ductionfromasetofaxioms,isatahigherlevelofabstractionthanimperativeprogramminglanguagesenablingaproblemorsubjectdomaintobemodelledwithoutfocusingonthecomputer’sVonNeumannarchitecture.Logicprogram-mingwithprogramstructuringabstractionshasshownitsutilityinavarietyofapplicationsincludingexpertsystems,ArtificialIntelligenceproblemsolving,anddeductivedatabases.ImplementationsoflogicprogrammingsuchasPro-loghavefeaturesnotfoundintraditionalimperativelanguagesincludingeaseofmeta-programming,backtrackingsearch,anddynamicdatabasemanipulation,andfavourablefeaturessuchasautomaticmemorymanagementandmodular-ity.Comparedtoimperativeprogramminglanguages,therehasbeenlittleexplo-rationoflogicprogramminglanguagesforWebprogramming.OnlyinthelastfewyearshasworkbegunontherelationshipbetweenlogicprogrammingandtheWeb.ThisthesisinvestigatesLogicWeb,amodeloftheWebasacollectionofinter-relatedlogicprograms.LogicWebaddslogicprogrammingbasedinterac-tivebehaviourstotheWeb,andenablesthemanipulationofWebinformationwithoutfocusingondetailsofnetworkinganddataextraction.WithLogicWeb,aWebdocumentisviewedasaliveinformationentityabletorespondtouserqueriesusingitsownrulesandcanhavethebehavioursofitslinksdeterminedbyrule-basedreasoning.ProcessingofuserqueriesandlinkibehaviourscaninvolvecollectingandmanipulatingdatafromotherWebdoc-uments.LogicWebalsoaddstoWebdocumentsinformationintheformofde-ductivedatabasesandknowledge-bases.Anewlanguage(extendingProlog)isdevelopedbasedonLogicWebforcod-ingthelogicprogrammingbehaviours,andforrepresentingandqueryingtheWeb-situateddatabasesandknowledge-bases.ThislanguageoffersanewWebprogrammingparadigm,wherecomputingwiththeWebisequatedtoevaluat-inggoalsincompositionsofprograms.ThepracticalresultisthatprogrammingwiththeWebinthislanguageisaidedbyfamiliarmodularityabstractions,andtheprogrammerneednotexplicitlydealwithlow-levelissuessuchasdocumentretrieval,caching,andparsing.Anoperationalsemanticsisgivenforthelan-guage.AsystemrealisingLogicWebisimplementedbyintegratingapublicdomainPrologsystemwithanoff-the-shelfWebbrowser.SecurityisanimportantissueintheLogicWebsystembecauselogicprogramsdownloadedfromforeignhostsareexecutedlocally.Aflexibleandpreciselyspecifiedsecuritymodelforthesys-temisdeveloped.ArangeofexamplesillustratesLogicWeb-basedprogrammablebehaviours,anddemonstratesthefeasibilityandadvantagesoftheLogicWeblanguageforcodingWebsearchtools,Web-situateddatabasescalledlightweightdeductivedatabases,andextensionstothesemanticsofWeblinks.iiAcknowledgementsThisthesiswouldnothaveexistedwithouttheinvaluableguidance,support,continualencouragement,andwritinglessons(forPrologandEnglish)frommytwosupervisors-LeonSterlingandAndrewDavison.Theirconfidenceinmehelpedwhenminewaned.Leonledmetothinkaboutcitationsforthemainthe-sisapplications,andprovidedhelpfulfeedbackandthought-provokingques-tionsonLogicWeb.InlateMarch1995,atmyfirstvisittohisoffice,AndrewsuggestedthatIcouldperhapsexploreanideahehadinmind,whichwastoviewWebpagesaslogicprogrammingmodulesandWeblinksasrelationships.ThisviewlaterbecamethebasicideaofLogicWeb.IamgratefulforthemanyinterestingdiscussionswithAndrewthroughouttheyears,thefirstoneandahalfyearsovercappuccinoandtea,andtherestovertheInternet,asthedetails,applications,andrealisationofLogicWebwerebeingthoughtout.Leongaveinstructiveandpromptfeedbackonthethesisdrafts.Andrewprovidedpromptfeedbackonmanydetailsinthethesisdraftsandsuggestedmanyimprovementstothepresentation.IamalsogratefultoLeeNaishforanumberofimportantpointsonadraftofthethesis.Leon,Andrew,andLeeformedmyadvisorycom-mittee.Iamthankfulfortheenjoyableconversationswith(pastandpresent)mem-bersoftheIntelligentAgentLaboratoryovertechnicalandnon-technicalmat-ters,particularlyAndrewCassin,SharonGao(whocommentedondraftsofsev-eralchaptersofthethesis),DinhQueTranh,StewartBaillie,HoonKim,andYiHan.ThanksgotoLeonforencouragingmyforayintotheexcitingareaofsoft-wareagents,andtoAndrewCassinforsystemshelponnumerousoccasions.IamgladtohavesharedmanypostgraduatestudentexperienceswithVin-centTam(whosoonwillnotbeastudent!)overthemanyrelaxinglunchandteabreaks.ThanksgotoMichaelBieberandanonymousreferees(ofpapersonLog-iiiicWeb,particularly[127,128,126,130,129])fortheencouragingfeedbackonLog-icWeb.RolandYapperformedoneofthefirstinstallationsofanearlyversionoftheLogicWebsystemoutsideAustralia,andhelpedwithdebu