Logic programming with Prolog

257 Pages·2013·1.17 MB·English

by  Max Bramer (auth.)

Max Bramer
Logic
Programming
with Prolog
Second Edition
Logic Programming with Prolog

Max Bramer
Logic Programming
with Prolog
Second Edition
123
MaxBramer
SchoolofComputing
UniversityofPortsmouth
Portsmouth,UK
ISBN978-1-4471-5486-0 ISBN978-1-4471-5487-7(eBook)
DOI10.1007/978-1-4471-5487-7
SpringerLondonHeidelbergNewYorkDordrecht
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Contents
Introduction ...................................................................... ix
1 GettingStarted.............................................................. 1
1.1 StartingProlog ....................................................... 1
1.2 PrologPrograms...................................................... 3
1.3 DataObjectsinProlog:PrologTerms............................... 8
PracticalExercise1.......................................................... 11
2 ClausesandPredicates..................................................... 13
2.1 Clauses................................................................ 13
2.2 Predicates............................................................. 15
2.3 LoadingClauses...................................................... 18
2.4 Variables.............................................................. 22
PracticalExercise2.......................................................... 27
3 SatisfyingGoals............................................................. 29
3.1 Introduction........................................................... 29
3.2 Unification............................................................ 31
3.2.1 UnifyingCallTerms........................................ 31
3.3 EvaluatingGoals ..................................................... 35
3.4 Backtracking.......................................................... 39
3.5 SatisfyingGoals:ASummary....................................... 48
3.6 RemovingCommonVariables....................................... 50
3.7 ANoteonDeclarativeProgramming................................ 51
3.8 ImportantNoteonUser-ControlledBacktracking.................. 52
PracticalExercise3.......................................................... 53
4 OperatorsandArithmetic................................................. 55
4.1 Operators ............................................................. 55
4.2 Arithmetic ............................................................ 58
4.3 EqualityOperators ................................................... 61
v
vi Contents
4.4 LogicalOperators .................................................... 64
4.5 MoreAboutOperatorPrecedence................................... 65
PracticalExercise4.......................................................... 68
5 InputandOutput........................................................... 69
5.1 Introduction........................................................... 69
5.2 OutputtingTerms..................................................... 69
5.3 InputtingTerms....................................................... 71
5.4 InputandOutputUsingCharacters.................................. 72
5.5 OutputtingCharacters................................................ 72
5.6 InputtingCharacters.................................................. 73
5.7 UsingCharacters:Examples......................................... 74
5.8 InputandOutputUsingFiles........................................ 76
5.9 FileOutput:ChangingtheCurrentOutputStream ................. 77
5.10 FileInput:ChangingtheCurrentInputStream..................... 77
5.10.1 ReadingfromFiles:EndofFile ........................... 78
5.10.2 ReadingfromFiles:EndofRecord........................ 78
5.11 UsingFiles:Examples ............................................... 79
PracticalExercise5.......................................................... 81
6 Loops......................................................................... 85
6.1 Introduction........................................................... 85
6.2 LoopingaFixedNumberofTimes.................................. 85
6.3 LoopingUntilaConditionIsSatisfied.............................. 89
6.3.1 Recursion ................................................... 89
6.3.2 Usingthe'repeat'Predicate................................. 91
6.4 BacktrackingwithFailure ........................................... 94
6.4.1 SearchingthePrologDatabase............................. 94
6.4.2 FindingMultipleSolutions................................. 96
PracticalExercise6.......................................................... 97
7 PreventingBacktracking .................................................. 99
7.1 Introduction........................................................... 99
7.2 TheCutPredicate .................................................... 99
7.3 CutwithFailure ...................................................... 105
PracticalExercise7.......................................................... 107
8 ChangingthePrologDatabase............................................ 109
8.1 ChangingtheDatabase:AddingandDeletingClauses............. 109
8.2 AddingClauses....................................................... 110
8.3 DeletingClauses...................................................... 111
8.4 ChangingtheDatabase:Example ................................... 112
8.5 MaintainingaDatabaseofFacts..................................... 114
PracticalExercise8.......................................................... 117
9 ListProcessing.............................................................. 119
9.1 RepresentingDataasLists........................................... 119
9.2 NotationforLists..................................................... 120
Contents vii
9.3 DecomposingaList.................................................. 122
9.4 Built-inPredicate:member .......................................... 124
9.5 Built-inPredicate:length ............................................ 125
9.6 Built-inPredicate:reverse ........................................... 126
9.7 Built-inPredicate:append ........................................... 127
9.8 ListProcessing:Examples........................................... 128
9.9 Usingfindall/3toCreateaList...................................... 132
PracticalExercise9.......................................................... 134
10 StringProcessing ........................................................... 137
10.1 ConvertingStringsofCharactersToandFromLists............... 137
10.2 JoiningTwoStrings.................................................. 138
10.3 TrimmingaString.................................................... 139
10.4 InputtingaStringofCharacters ..................................... 141
10.5 SearchingaString.................................................... 142
10.6 DividingaStringintoItsComponentParts......................... 144
PracticalExercise10......................................................... 146
11 MoreAdvancedFeatures.................................................. 147
11.1 Introduction........................................................... 147
11.2 ExtendingProlog:Arithmetic ....................................... 147
11.3 ExtendingProlog:OperationsonStrings ........................... 153
11.4 ExtendingProlog:Sets............................................... 155
11.5 ProcessingTerms..................................................... 157
PracticalExercise11......................................................... 163
12 UsingGrammarRulestoAnalyseEnglishSentences .................. 165
12.1 Introduction........................................................... 165
12.2 ParsingEnglishSentences........................................... 165
12.3 ConvertingSentencestoListForm.................................. 181
PracticalExercise12......................................................... 186
13 ProloginAction............................................................. 187
13.1 ImplementinganArtificialLanguage ............................... 187
13.2 DevelopinganExpertSystemShell................................. 200
PracticalExercise13......................................................... 210
Appendix1 Built-inPredicates............................................... 211
Appendix2 Built-inOperators ............................................... 217
Appendix3 SpecimenSolutionstoPracticalExercises.................... 221
Appendix4 Glossary........................................................... 243
Index............................................................................... 251

Introduction
LogicProgrammingisthenamegiventoadistinctivestyleofprogramming,very
differentfromthatofconventionalprogramminglanguagessuchasCCCandJava.
Fans of Logic Programming would say that 'different' means clearer, simpler and
generallybetter!
AlthoughthereareotherLogicProgramminglanguages,byfarthemostwidely
usedisProlog.ThenamestandsforProgramminginLogic.Thisbookteachesthe
techniques of Logic Programming through the Prolog language. Prolog is based
on research by computer scientists in Europe in the 1960s and 1970s, notably at
the Universities of Marseilles, London and Edinburgh. The first implementation
was at the University of Marseilles in the early 1970s. Further development at the
University of Edinburgh led to a de facto standard version, now known as Edin-
burgh Prolog. Prolog has been widely used for developing complex applications,
especially in the field of Artificial Intelligence. Although it is a general-purpose
language,itsmainstrengthsareforsymbolicratherthanfornumericalcomputation.
Thedevelopers ofthelanguagewereresearchersworkingonautomatingmath-
ematicaltheoremproving.Thisfieldisoftenknownascomputationallogic.Butif
youarenotaComputerScientist,alogicianoramathematiciandonotletthisdeter
you! This book is aimed at the 99.9 % of the population who are none of these.
Thosewhoare,alreadyhaveanumberofexcellenttextbooksfromwhichtochoose.
Theideathatthemethodsdevelopedbycomputationallogicianscouldbeusedas
thebasisforapowerfulgeneralpurposeprogramminglanguagewasrevolutionary
30 years ago. Unfortunately most other programming languages have not yet
caughtup.
The most striking feature of Prolog for the newcomer is how much simpler
the programs look than in other languages. Many language designers started out
withgoodintentionsbutcouldnotresist,orperhapscouldnotavoid,makingtheir
creationsoverelaborate.Insomelanguagesevenwritingthecustomarytestprogram
to print out the words Hello World! to the user's screen is hard work. All the user
hastodoinPrologistoenterwrite('HelloWorld!').
Traditional programming languages have one feature in common. They all
containaseriesofinstructionsthataretobeperformed('executed')oneafteranother.
ix