Programming Languages: An Active Learning Approach

289 Pages·2008·3.92 MB·English

by  Kent Lee

PROGRAMMING LANGUAGES:
AN ACTIVE LEARNING APPROACH
PROGRAMMING
LANGUAGES:
AN ACTIVE LEARNING
APPROACH

Kent D. Lee
Luther College

123
Kent D. Lee
Luther College
Department of Computer Science
700 College Drive
Decorah, Iowa 52101-1045
[e-mail]

ISBN: 978-0-387-79421-1 e-ISBN: 978-0-387-79422-8
DO I: 10.1007/ 978-0-387-79421-1

Library of Congress Control Number: 2008924915

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To Denise.
Preface
ComputerSciencehasmaturedintoaverylargediscipline.Youwillnotbetaught
everything you will need in your career while you are a student. The goal of a
Computer Science education is to prepare you for a life of learning. The creativ-
ityencouragedbyalifetimeoflearningmakesComputerScienceoneofthemost
excitingfieldstodayaccordingtoMoneyMagazine’sBestJobsinAmericain2006.
ThewordsComputerSciencedon’treallyreflectwhatmostcomputerprogram-
mersdo.ThefieldreallyshouldbenamedsomethinglikeComputerProgramEngi-
neering.Itismoreofanengineeringfieldthanascience.That’swhyprogrammers
are often called Software Engineers. There is science in Computer Science gener-
ally relating to the gathering of empirical evidence pertaining to performance of
algorithmsandhardware.Therearealsotheoreticalaspectsofthediscipline,some
of which this text will explore. However, the bulk of what computer scientists do
is related to the creative process of building programs. As computer scientists we
knowsomethingsabouthowtowritegoodprograms,butitisstillaverycreative
process.
Given that programming is such a creative process, it is imperative that we be
abletopredictwhatourprogramswilldo.Topredictwhataprogramwilldo,you
mustunderstandhowtheprogramworks.Theprogramsofalanguageexecuteac-
cordingtoamodelofcomputation.Amodelmaybeimplementedinmanydifferent
waysdependingonthetargetedhardwarearchitecture.However,itisnotnecessary
to understand all the different architectures out there to understand the model of
computationusedbyalanguage.
Forseveralyearsinthelate1980’sand1990’sIworkedatIBMontheoperating
systemfortheAS/400(whatisnowcalledSystemiandthei5/OS).Myunderstand-
ing of compilers and language implementation helped me make better decisions
about how to write code and several times helped me find problems in code I was
testing.AnunderstandingofthemodelsofcomputationusedbythelanguagesIpro-
grammedinaidedmeinpredictingwhatmyprogramswoulddo.Aftercompletinga
courseinprogramminglanguages,youshouldunderstandsomeofthebasicsoflan-
guageimplementation.Thisbookisnotintendedtobeacompletetextoncompiler
orinterpreterimplementation,butthereareaspectsoflanguageimplementationthat
areincludedinit.Wearebetterusersoftoolswhenweunderstandhowthetoolswe
usework.
Ihopeyouenjoylearningfromthistextandthecourseyouareabouttotake.The
textismeanttobeusedinteractively.Youshouldreadasectionandasyoureadit,
dothepracticeexerciseslistedinthegrayboxes.Eachoftheexercisesaremeantto
giveyouagoalinreadingasectionofthetext.
The text has a website where code and other support files may be downloaded.
Seehttp://www.cs.luther.edu/˜leekent/ProgrammingLanguagesforallsupportfiles.
viii Preface
ForTeachers
This book was written to fulfill two goals. The first is to introduce students to
three programming paradigms: object-oriented/imperative, functional, and logic
programming.Tobereadyforthecontentofthisbookstudentsshouldhavesome
background in an imperative language, probably an object-oriented language like
PythonorJava.TheyshouldhavehadanintroductorycourseandacourseinData
Structuresasaminimum.Whilethepreparedstudentwillhavewrittenseveralpro-
grams, some of them fairly complex, most probably still struggle with predicting
exactlywhattheirprogramwilldo.Itisassumedthatideaslikepolymorphism,re-
cursion,andlogicalimplicationarerelativelynewtothestudentreadingthisbook.
Thefunctionalandlogicparadigms,whilenotthemainstream,havetheirplaceand
havebeensuccessfullyusedininterestingapplications.
TheObject-OrientedlanguagespresentedinthisbookareC++andRuby.Teach-
ersmaychoosebetweenthechapteronRubyorthechapteronC++,ormayassign
both.Itmightbeusefultoreadbothchaptersifyouwishtocompareandcontrast
astaticallytyped,compiledlanguagetoadynamicallytyped,interpretedlanguage.
The same project is presented in both chapters with the C++ chapter requiring a
little more explanation in terms of the compiler and organization of the code. Ei-
therlanguageisinterestingtochoosefromandthechaptersdocross-referenceeach
other to compare and contrast the two styles of programming so if you only have
timetocoveroneortheother,thatispossibletoo.
C++hasmanynuancesthatareworthyofseveralchaptersinaprogramminglan-
guagesbook.NotablythepassbyvalueandpassbyreferencemechanismsinC++
createconsiderablecomplexityinthelanguage.Polymorphismisanotherinterest-
ingaspectofObject-Orientedlanguagesthatisstudiedinthistext.
Ruby is relatively new to the programming language arena, but is widely ac-
ceptedandisalargelanguagewhencomparedtoPythonandJava.Inaddition,its
object-centeredapproachisverysimilartoSmalltalk.Rubyisalsointerestingdue
totherecentdevelopmentof“RubyonRails”asacodegenerationtool.
The text uses Standard ML as the functional language. ML has a polymorphic
type inference system to statically type programs of the language. In addition, the
typeinferencesystemofMLisformallyprovensoundandcomplete.Thishassome
implicationsinwritingprograms.WhileML’scrypticcompilererrormessagesare
sometimes hard to understand at first, once a program compiles it will often work
correctlythefirsttime.That’sanamazingstatementtomakeifyourpastexperience
isinadynamicallytypedlanguagelikeLisp,Scheme,Ruby,orPython.
ThelogiclanguageisProlog.WhilePrologisanArtificialIntelligencelanguage,
it originated as a meta-language for expressing other languages. The text concen-
trates on using Prolog to implement other languages. Students learn about logical
implicationandhowaproblemtheyarefamiliarwithcanbere-expressedinadif-
ferentparadigm.
Thesecondgoalofthetextistobeinteractive.Thisbookisintendedtobeusedin
andoutsideofclass.Itismyexperiencethatwealmostalllearnmorebydoingthan
byseeing.Tothatend,thetextencouragesteacherstoactivelyteach.Eachchapter
Preface ix
followsapatternofpresentingatopicfollowedbyapracticeexerciseorexercises
thatencouragestudentstotrywhattheyhavejustread.Theseexercisescanbeused
inclasstohelpstudentschecktheirunderstandingofatopic.Teachersareencour-
agedtotakethetimetopresentatopicandthenallowstudentstimetopracticewith
theconceptjustpresented.Inthiswaythetextbecomesalectureresource.Students
gettwothingsoutofthis.Itforcesthemtobeinteractivelyengagedinthelectures,
notjustpassiveobservers.Italsogivesthemimmediatefeedbackonkeyconcepts
tohelpthemdetermineiftheyunderstandthematerialornot.Thisencouragesthem
to ask questions when they have difficulty with an exercise. Tell students to bring
the book to class along with a pencil and paper. The practice exercises are easily
identified.Lookforthelightgraypracticeproblemboxes.
Thebookpresentsseveralprojectstoreinforcetopicsoutsidetheclassroom.Each
chapter of the text suggests several non-trivial programming projects that accom-
panytheparadigmbeingcoveredtodrivehometheconceptscoveredinthatchapter.
The projects described in this text have been tested in practice and documentation
andsolutionsareavailableuponrequest.
Finally, it is expected that while teaching a class using this text, lab time will
be liberally sprinkled throughout the course as the instructor sees fit. Reinforcing
lectures with experience makes students appreciate the difficulty of learning new
paradigmswhilemakingthemstrongerprogrammers,too.
Supplementarymaterialsincludingsamplelecturenotes,lectureslides,answers
to exercises, and programming assignment solutions are available to instructors
uponrequest.
Acknowledgments
I have been fortunate to have good teachers throughout high school, college, and
graduate school. Good teachers are a valuable commodity and we need more of
them. Ken Slonneger was my advisor in graduate school and this book came into
beingbecauseofhim.Heinspiredmetowriteatextthatsupportsthesameteaching
style he uses in his classroom. Encouraging students to interact during lecture by
givingthemshortproblemstosolvethatreflectthematerialjustcoveredisavery
effectivewaytoteach.Itmakestheclassroomexperienceactiveandenergizesthe
students. Ken graciously let me use his lecture notes on Programming Languages
when I started this book and some of the examples in this text come from those
notes. He also provided me with feedback on this text and I appreciate all that he
did.Thankyouverymuch,Ken!
OthergreatteacherscometomindaswellincludingDennisTackwhotaughtme
thebeautyofagoodproof,KarenNancewhotaughtmetowrite,AlanMacdonald
who introduced me to programming languages as a field of study, Walt Will who
taughtmehowtowritemyfirstassembler,andlastbutnotleastSteveHubbardwho
still inspires me with his ability to teach complex algorithms and advanced data
structurestoComputerSciencemajorsatLutherCollege!Thankstoyouall.
Contents
Preface............................................................ vii
1 Introduction................................................... 1
1.1 HistoricalPerspective ...................................... 2
1.2 ModelsofComputation..................................... 4
1.3 TheOriginsofaFewProgrammingLanguages ................. 7
1.4 LanguageImplementation................................... 12
1.5 Wheredowegofromhere? ................................. 17
1.6 Exercises................................................. 18
1.7 SolutionstoPracticeProblems ............................... 19
1.8 AdditionalReading ........................................ 20
2 SpecifyingSyntax .............................................. 21
2.1 Terminology .............................................. 21
2.2 BackusNaurForm(BNF)................................... 23
2.3 TheEWELanguage........................................ 24
2.4 Context-FreeGrammars .................................... 28
2.5 Derivations ............................................... 29
2.6 ParseTrees ............................................... 30
2.7 Parsing................................................... 31
2.8 ParserGenerators.......................................... 32
2.9 Bottom-UpParsers......................................... 33
2.10 Top-DownParsers ......................................... 33
2.11 OtherFormsofGrammars .................................. 33
2.12 AbstractSyntaxTrees ...................................... 37
2.13 Infix,Postfix,andPrefixExpressions.......................... 38
2.14 LimitationsofSyntacticDefinitions........................... 38
2.15 Exercises................................................. 40
2.16 SolutionstoPracticeProblems ............................... 41
2.17 AdditionalReading ........................................ 46
xii Contents
3 Object-OrientedProgrammingwithC++ ......................... 47
3.1 ApplicationDevelopment ................................... 48
3.2 TheTokenClass........................................... 55
3.3 ImplementingaClass....................................... 57
3.4 InheritanceandPolymorphism............................... 59
3.5 AHistoricalLookatParameterPassing ....................... 62
3.6 ConstinC++.............................................. 67
3.7 TheASTClasses .......................................... 70
3.8 TheScanner .............................................. 73
3.9 TheParser................................................ 75
3.10 PuttingItAllTogether...................................... 81
3.11 Exercises................................................. 84
3.12 SolutionstoPracticeProblems ............................... 87
3.13 AdditionalReading ........................................ 90
4 Object-OrientedProgrammingwithRuby ........................ 91
4.1 DesigningCalc............................................ 93
4.2 TheTokenClass........................................... 94
4.3 ParameterPassinginRubyvsC++ ........................... 95
4.4 AccessorandMutatormethodsinRuby ....................... 96
4.5 Inheritance ............................................... 97
4.6 TheASTClasses .......................................... 97
4.7 PolymorphisminRuby .....................................100
4.8 TheScanner ..............................................102
4.9 TheParser................................................103
4.10 PuttingItAllTogether......................................107
4.11 StaticvsDynamicTypeChecking ............................109
4.12 Exercises.................................................112
4.13 SolutionstoPracticeProblems ...............................114
4.14 AdditionalReading ........................................116
5 FunctionalProgramminginStandardML ........................117
5.1 ImperativevsFunctionalProgramming........................118
5.2 TheLambdaCalculus ......................................119
5.3 GettingStartedwithStandardML ............................122
5.4 Expressions,Types,Structures,andFunctions ..................123
5.5 RecursiveFunctions........................................125
5.6 Characters,Strings,andLists ................................127
5.7 PatternMatching ..........................................129
5.8 Tuples ...................................................130
5.9 LetExpressionsandScope ..................................131
5.10 Datatypes.................................................133
5.11 ParameterPassinginStandardML............................136
5.12 EfficiencyofRecursion .....................................136
5.13 TailRecursion.............................................138