cognitive agent programming a semantic approach SIKS Dissertation Series No. 2006-19 The research reported in this thesis has been carried out under the auspices of SIKS, the Dutch Research School for Information and Knowledge Systems. 2006 M. Birna van Riemsdijk Printed by Gildeprint drukkerijen B.V., Enschede ISBN-10: 90-393-4355-1 ISBN-13: 978-90-393-4355-5 Cognitive Agent Programming A Semantic Approach Programmeren van Cognitieve Agenten Een Semantische Benadering (met een samenvatting in het Nederlands) Proefschrift ter verkrijging van de graad van doctor aan de Universiteit Utrecht op gezag van de rector magnificus, Prof. Dr. W.H. Gispen, ingevolge het besluit van het college voor promoties in het openbaar te verdedigen op woensdag 25 oktober 2006 des middags te 12.45 uur door Maria Birna van Riemsdijk geboren op 29 oktober 1978, te Wageningen promotoren: Prof. Dr. John-Jules Ch. Meyer co-promotoren: Dr. Frank S. de Boer Dr. Mehdi Dastani Het hoogste doel in het leven is de groei van het ik, de vergroting van het bewustzijn. Het hoogste geluk ligt in de vreugde over deze groei, over het gevoel meer mens te worden. The ultimate goal in life is the growth of self, the expansion of con- sciousness. The ultimate happiness lies in the joy about this growth, in the feeling of becoming more human. - Translated by Targetranslations, Apeldoorn. Bertus Mulder, Gerrit Thomas Rietveld: Leven Denken Werken Contents Preface xi 1 Introduction 1 1.1 CognitiveAgentProgramming . 2 1.2 FormalSemantics........................... 4 2 Setting the Stage 7 2.1 Syntax................................. 7 2.1.1 Beliefs,Goals,andPlans. 7 2.1.2 ReasoningRules ....................... 8 2.1.3 AnAgent ........................... 9 2.2 Semantics ............................... 10 2.3 Example................................ 14 2.3.1 BuildingaTower. .. .. .. .. .. .. .. .. .. 15 2.3.2 Goals and Plan Revision: Programming Flexible Agents . 17 2.4 OverviewofThesis .......................... 18 2.4.1 PartI:Goals ......................... 19 2.4.2 PartII:PlanRevision . 19 2.4.3 PartIII: SoftwareEngineeringAspects . 20 2.5 ImportantIssuesWeDoNotAddress . 21 I Goals 23 3 Semantics of Subgoals 25 3.1 Syntax................................. 26 3.2 Semantics ............................... 28 3.3 Comparisonwith3APL . .. .. .. .. .. .. .. .. .. 29 3.3.1 SyntaxandSemantics . 29 3.3.2 3APLandSubgoals . .. .. .. .. .. .. .. .. 32 3.4 ConclusionandRelatedWork . 36 vii 4 Goals in Conflict 39 4.1 Preliminaries ............................. 40 4.1.1 CognitiveAgentProgramming . 40 4.1.2 DefaultLogic ......................... 43 4.2 GoalBase............................... 44 4.2.1 Semantics........................... 44 4.2.2 Properties........................... 47 4.3 GoalBaseandGoalAdoptionRules . 51 4.3.1 Semantics........................... 51 4.3.2 Properties........................... 56 4.4 DynamicsofGoalsandIntentions . 60 4.4.1 CommitmentStrategiesforGoals. 60 4.4.2 IntentionGeneration. 62 4.5 RelatedWork............................. 68 4.5.1 VanFraassenandHorty . 68 4.5.2 BOIDandRelatedApproaches . 73 4.6 Conclusion .............................. 78 5 Putting Goals in Perspective 79 5.1 WhatIsaGoal? ........................... 80 5.1.1 Philosophy: DennettandBratman . 80 5.1.2 Formalizing Motivational Attitudes . 81 5.2 WhyGoalsinAgentProgramming? . 84 5.2.1 BridgingtheGap. .. .. .. .. .. .. .. .. .. 84 5.2.2 ProgrammingProactiveAgents . 85 5.2.3 GoalsasaModelingConcept . 86 5.3 Representation ............................ 87 5.3.1 RepresentingGoalsSeparatelyorNot . 87 5.3.2 Logic-Based and Non-Logic-Based Approaches . 89 5.3.3 InteractingGoals. 92 5.4 Behavior................................ 95 5.4.1 ProceduralandDeclarativeGoals. 95 5.4.2 DroppingandAdopting . 99 5.5 Conclusion .............................. 102 II Plan Revision 105 6 Semantics of Plan Revision 107 6.1 Syntax.................................109 6.1.1 Object-Level . .. .. .. .. .. .. .. .. .. .. 109 6.1.2 Meta-Level .......................... 110 6.2 OperationalSemantics . 111 6.2.1 Object-LevelTransitionSystem . 111 viii 6.2.2 Meta-LevelTransitionSystem. 111 6.2.3 OperationalSemantics . 113 6.3 Equivalence of Object- and Meta-Level Operational Semantics . 114 6.4 DenotationalSemantics . 118 6.4.1 Preliminaries . 118 6.4.2 Definition of Meta-Level Denotational Semantics . 120 6.4.3 Continuityof Φ ....................... 127 6.5 Equivalence of Operational and Denotational Semantics . .. 129 6.5.1 EquivalenceTheorem . 130 6.5.2 Denotational Semantics of Object-Level 3APL . 134 6.6 RelatedWorkandConclusion . 135 7 DynamicLogicforPlanRevision 137 7.1 RelatedWork............................. 138 7.2 3APL .................................140 7.2.1 Syntax............................. 140 7.2.2 Semantics ........................... 141 7.3 PlanRevisionDynamicLogic . 143 7.3.1 Syntax............................. 144 7.3.2 Semantics ........................... 144 7.4 TheAxiomSystem.......................... 145 7.4.1 Soundness........................... 147 7.4.2 Completeness . .. .. .. .. .. .. .. .. .. .. 149 7.5 Proving Properties of Non-Restricted Plans . 155 7.5.1 From Restricted to Non-Restricted Plans . 155 7.5.2 Examples ........................... 156 7.6 PlanRevisionRulesversusProcedures . 162 7.6.1 ReasoningaboutProcedures . 163 7.6.2 Induction ........................... 165 7.7 Conclusion .............................. 166 8 CompositionalSemanticsofPlanRevision 169 8.1 3APL .................................170 8.1.1 Syntax............................. 170 8.1.2 Semantics ........................... 170 8.2 3APLandNon-Compositionality . 171 8.2.1 Compositionality of Procedural Languages. 171 8.2.2 Non-Compositionalityof3APL . 172 8.2.3 Reasoningabout3APL . 172 8.3 Compositional3APL. 173 8.3.1 RestrictedPlanRevisionRules . 173 8.3.2 CompositionalityTheorem . 176 8.3.3 Reasoningabout Compositional 3APL . 179 ix III Software Engineering Aspects 181 9 Goal-Oriented Modularity 183 9.1 Goal-OrientedModularity . 184 9.1.1 RelatedWork......................... 184 9.1.2 OurProposal ......................... 186 9.1.3 Discussion........................... 187 9.2 Goal-OrientedModularityin3APL . 188 9.2.1 Syntax............................. 188 9.2.2 Semantics ........................... 191 9.2.3 Example............................ 195 9.3 FutureResearch ........................... 197 10 Prototyping 3APL in the Maude Term Rewriting Language 199 10.13APL .................................200 10.1.1 Syntax............................. 200 10.1.2 Semantics ........................... 202 10.2Maude................................. 203 10.3 Implementationof3APLinMaude . 205 10.3.1 Object-Level . 205 10.3.2 Meta-Level .. .. .. .. .. .. .. .. .. .. .. 209 10.4 DiscussionandRelatedWork . 211 10.4.1 AdvantagesofMaude . 211 10.4.2 Extending the Implementation . 213 10.4.3 RelatedWork. 214 11 Conclusion 215 11.1PartI:Goals ............................. 216 11.2 PartII:PlanRevision . 217 11.3 Part III: SoftwareEngineeringAspects . 217 11.4FinalRemarks ............................ 218 Programmeren van Cognitieve Agenten 235 Curriculum Vitae 237 SIKS Dissertation Series 239 x Preface Simplicity is my holy grail. Only through a striving for simplicity can we make progress. Striving for simplicity is important in the sense of identifying an issue and simplifying it such that its essence can be studied in isolation. This is what the famous Dutch computer scientist Dijkstra (1930 - 2002) has referred to as “separation of concerns”: “Let me try to explain to you, what to my taste is characteristic for all intelligent thinking. It is, that one is willing to study in depth an aspect of one’s subject matter in isolation for the sake of its own consistency, all the time knowing that one is occupying oneself only with one of the aspects. [. .] It is what I sometimes have called “the separation of concerns”, which, even if not perfectly possible, is yet the only available technique for effective ordering of one’s thoughts, that I know of.” [?] Contrary to what one might think, this is not a trivial undertaking. One should not oversimplify matters and ignore essential aspects of the issue. That is, it is not the case that simpler is always better. Rather, I argue that when proposing a certain approach, one should ideally be able to argue for each aspect of the approach why this aspect is incorporated. Through this, one can achieve a clarity of presentation and a true understanding of the subject matter. Unfortunately, simplicity doesn’t always sell, as illustrated by the following quote from Dijkstra: “When you give for an academic audience a lecture that is crystal clear from alpha to omega, your audience feels cheated and leaves the lecture hall commenting to each other: ‘That was rather trivial, wasn’t it?’ The sore truth is that complexity sells better.” [?] A solution to this problem is suggested in the following poem, which is attributed to1 the Danish artist and scientist Piet Hein (1905 - 1996). If you want the world to hear Write your papers crystal clear, then add some ingenuities to show how hard to do it is... 1I found this poem on the website of computer scientist Sophia Drossopoulou: http://www.doc.ic.ac.uk/~scd/. xi Besides the scientific progress that can be achieved through a striving for sim- plicity, a simple and clear theory can be enjoyed as a thing of beauty. This is where science and art meet. And this is the work of an artist. The artist, who thanks to his limited talents is able to make some clear distinctions,