The Art of Functional Programming

205 Pages·2022·4.668 MB·English

by  Minh Quang Tran

The Art of
Functional
Programming
with examples in OCaml, Haskell, and Java
Minh Quang Tran, PhD
The Art of
Functional
Programming
with examples in OCaml, Haskell, and Java
Minh Quang Tran, PhD
The Art of Functional Programming
Copyright © 2022 Minh Quang Tran. All rights reserved.
About the author:
Minh Quang Tran has more than 20 years of experience studying so�ware development and working in
the so�ware industry. He has worked in various tech startups and big so�ware companies in Europe.
He received his M.Sc. in Computer Science from McMaster University, Canada, and his Ph.D. in
Computer Science from Technical University of Berlin, Germany. His interest lies in understanding and
mastering the fundamentals and principles that cut across programming languages, frameworks, and
tools.
Contents
Contents
1 Introduction 3
1.1 AboutThisBook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 ABiteofFunctionalProgramming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 WhyFunctionalProgrammingMatters? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.4 RequiredTools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.5 QuizonImperativevs.FunctionalProgramming . . . . . . . . . . . . . . . . . . . . . . . 16
1.6 AnswerstoQuizonImperativevs.FunctionalProgramming . . . . . . . . . . . . . . . . . 17
2 Expressions–BuildingBlocksofFunctionalPrograms 18
2.1 FunctionalProgrammingisAllAboutExpressions . . . . . . . . . . . . . . . . . . . . . . 18
2.2 SyntaxofExpressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.3 ParsingExpressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.4 TypesofExpressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.5 ValuesofExpressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2.6 AssignNamestoExpressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
2.7 ProgrammingChallenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
2.8 SolutionstoProgrammingChallenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
2.9 QuizonExpressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
2.10 AnswerstoQuizonExpressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3 BuildingAbstractionswithFunctions 56
3.1 LambdaCalculus:FoundationofFunctionalProgramming . . . . . . . . . . . . . . . . . 56
3.2 FunctionAbstractionandFunctionApplication . . . . . . . . . . . . . . . . . . . . . . . . 60
3.3 UseCurryingforFunctionChaining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
3.4 RecursiveFunctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
3.5 GeneralComputationMethodsasHigher-OrderFunctions . . . . . . . . . . . . . . . . . . 76
3.6 ProgrammingChallenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
3.7 SolutionstoProgrammingChallenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
3.8 QuizonFunctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
3.9 AnswerstoQuizonFunctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
4 CompoundDataTypes 93
4.1 GroupDataObjectsintoTuples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
4.2 Destructtupleswithpatternmatching . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
TheArtofFunctionalProgramming 1
Contents
4.3 StoreSequencesofDatawithLists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
4.4 DeclareUser-definedTypeswithAlgebraicDataTypes . . . . . . . . . . . . . . . . . . . . 103
4.5 ProgrammingChallenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
4.6 SolutionstoProgrammingChallenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
4.7 QuizonCompoundDatatypes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
4.8 AnswerstoQuizonCompoundDatatypes . . . . . . . . . . . . . . . . . . . . . . . . . . 121
5 CommonComputationPatterns 123
5.1 ThemapFunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
5.2 Thefilterfunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
5.3 Thefoldfunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
5.4 ThezipFunction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
5.5 ProgrammingChallenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
5.6 SolutionstoProgrammingChallenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
5.7 QuizonCommonComputationPatterns . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
5.8 AnswerstoQuizonCommonComputationPatterns . . . . . . . . . . . . . . . . . . . . . 156
6 DataflowProgrammingwithFunctions 159
6.1 List-basedDataflowProgramming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
6.2 Stream-basedDataflowProgramming . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
6.3 ProgrammingChallenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
6.4 SolutionstoProgrammingChallenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
6.5 QuizonDataflowProgrammingwithFunctions . . . . . . . . . . . . . . . . . . . . . . . . 182
6.6 AnswerstoQuizonDataflowProgrammingwithFunctions . . . . . . . . . . . . . . . . . 185
7 ApplyingFunctionalProgramminginPractice 187
7.1 HandleCollectionsinDataProcessingApplications . . . . . . . . . . . . . . . . . . . . . 187
7.2 HandleJSON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
8 Conclusion 201
8.1 WrapUp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201
8.2 WheretoGofromHere? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
TheArtofFunctionalProgramming 2
1 INTRODUCTION
1 Introduction
1.1 AboutThisBook
1.1.1 Bookdescription
WelcometoTheArtofFunctionalProgrammingbook!
Functionalprogrammingisapowerfulandelegantprogrammingparadigm.Initiallyonlypopularamong
universityresearchers,it’sgainedmuchtractioninthesoftwareindustryinthelastfewyears. Frombig
companies to start-ups, engineers and managers have realized that functional programming excels at
abstractionandcomposition.Functionalprogrammingallowsforhighlyconcisesolutionswithincreased
safety.Thishasledtorisingdemandforsoftwareengineerswithfunctionalprogrammingskills.Thisbook
willhelpyoumoveyourprogrammingskillstothenextlevel.
Thisbookisgroundedonmybeliefsaboutsoftwaredevelopmentresultingfrommyreflectionaftermany
yearsofstudyingandworkinginthesoftwareindustry. First,therearetonsofprogramminglanguages,
frameworks,andtoolsoutthere–withmanymorecominginthefuture. Theonlywaytostayaheadof
thegameinthisvastandquicklychangingsoftwareindustryistomasterthefundamentalsandprinciples
thatcutacrossprogramminglanguages,frameworks,andtools. Inthecaseoffunctionalprogramming,
learningtoadoptthefunctionalwayofsolvingproblemsismuchmoreproductivethanmemorizinghow
towritefunctionalcodeinaparticularlanguage.Thisbookteachesthisfunctionalwayofthinking.We’ll
also learn many fundamental techniques from programming languages, such as parsing, compilation,
andtypechecking.
Second,Ibelievethatatechniqueortoolisnotveryusefulifitdoesnothelpussolvereal-worldprob-
lemsandmakeourliveseasier. Inthisbook,we’llonlylookatexamplesandexercisesthataretypically
encounteredinaprogrammer’sday-to-dayjob. Furthermore,anentirechapterisdedicatedtoapplying
whatwe’velearnedtoreal-worldscenarios. Inparticular,we’llusefunctionalprogrammingtoprocess
collectionsofdataforane-commerceapplicationandhandletheJSONdatatype.
Iliketothinkofthedualityofstrivingtograspthefundamentalprinciplesofthefunctionalparadigmwhile
applyingittoreal-worldproblemspragmaticallyastheyinandyangofthejourneytomasterfunctional
programming.
TheArtofFunctionalProgramming 3
1 INTRODUCTION
Figure1:Yinandyangasasymbolofduality
Hereisthesummaryofthechaptersinthisbook:
• In Chapter 1: Introduction, we’ll start the book with an introduction to functional programming.
In particular, we’ll see how it can overcome some of the inherent weaknesses of the imperative
programmingparadigm. We’llalsodiscusswhyfunctionalprogrammingmatterstoanysoftware
engineer.
• InChapter2: Expressions–TheBuildingBlocksofFunctionalPrograms,weexamineexpressions
andhowtobuildcomplexexpressionsfromsimplerones. Threeaspectsofexpressions–syntax,
types,andsemantics—willbecovered. Alongtheway,we’llgainamuchdeeperunderstandingof
howprogramminglanguageswork,includingparsing,typechecking,interpretation,andcompila-
tion.
• InChapter3: BuildingAbstractionswithFunctions,we’llgettoknowlambdacalculus–amathe-
maticalmodelservingasthefoundationofallfunctionalprogramminglanguages.We’lllearnhow
tocapturecomputationpatternsasfunctions.Finally,we’lldiscussvarioustechniquesforworking
withfunctionssuchascurrying,recursion,andhigher-orderfunctions.
• InChapter4: ComplexDataTypes,we’llfocusonthecomplexdatatypestypicallyfoundinfunc-
tionalprogramminglanguages,suchastuplesandlists.Furthermore,we’llusealgebraicdatatypes
torepresenthierarchicaldata,andpatternmatchingtoextractdatafromcomplexdatatypes.
• InChapter5: CommonComputationPatterns,we’lldiveintosomeofthemostcommoncomputa-
tionpatterns,suchasmap,filter,fold,andzip.Thesefunctionscapturehighlygeneralcomputation
patternsonlistsandotherdatastructuresthatcanbereusedtoformulatemanyotherfunctions.
TheArtofFunctionalProgramming 4
1 INTRODUCTION
• InChapter6: DataflowProgrammingwithFunctions,we’llgooverdataflowprogramming,apro-
grammingparadigmthatemphasizescomposingprogramsfromexistingcomponents. We’lllearn
howfunctionalprogrammingallowsustododataflowprogrammingelegantlyandreapallitsben-
efits.
• InChapter7:ApplyingFunctionalProgrammingtoVariousDomains,we’llapplywhatwe’velearned
toprocesscollectionsofdatacommonlyfoundinmobileandwebapplications,aswellasbackend
services.Furthermore,we’llusefunctionalprogrammingtorepresentandhandleJSON.
We’llmainlyuseOCamlandoccasionallyHaskeltodemonstratethekeyconceptsandtechniquesoffunc-
tionalprogrammingthroughoutthebook.Tocontrastfunctionalprogrammingconceptswithimperative
ones,we’lluseJava.Yet,itisessentialtorepeatthatmostfunctionalprogrammingtechniquesintroduced
inthisbookareuniversal.Thatmeanswecanapplythemtoalmostanyprogramminglanguagethatsup-
portsthefunctionalprogrammingstyle. ThisincludesbutisnotlimitedtoSwift,Kotlin,JavaScript,Go,
Python,andevenJava(usingtheJavaStreamAPI).
1.1.2 Intendedaudience
Thisisabeginnerandintermediatebookaimedatsoftwareengineers, engineeringmanagers, orcom-
putersciencestudentsinterestedinunderstandingtheessenceoffunctionalprogramming. Itisalsoan
excellentfitforindividualswhoarecurrentlypreparingforcodinginterviewsandwanttoimprovetheir
problem-solvingskills.
1.2 ABiteofFunctionalProgramming
Functionalprogrammingisaprogrammingparadigm—astyleorawayofthinkingwhenwritingsoft-
wareprograms. Therearemanyprogrammingparadigmsavailable,someofwhicharepresentedinthe
followingdiagram.
TheArtofFunctionalProgramming 5
1 INTRODUCTION
Figure2:Programmingparadigms
Theimperativeprogrammingparadigmistheoldest,andstillthemostpopular,paradigmusedtoday.
Astheterm“imperative”indicates, thisparadigmmodelsaprogramasasequenceofcommandsthat
changeaprogram’sstate–“firstdothis,thendothat.”Evenifwefollowtheobject-orientedprogramming
paradigmbyencapsulatinglogicintoclasses,we’lllikelyimplementtheclassmethodsintheimperative
style.
Why has imperative programming dominated the programming world? In his Turing Award lecture ti-
tled CanProgrammingBeLiberatedfromthevonNeumannStyle? AFunctionalStyleandItsAlgebraof
Programs,thecomputerscientistJohnBackusgivesaninsightfulanswer. Accordingtohisobservation,
thereasoncanbetracedbacktotheVonNeumannarchitecture.NamedafterthemathematicianJohn
vonNeumann,whotookinspirationfromtheTuringmachine,itisacomputerarchitectureusedbymost
computersproducedtoday.
Inthefollowing,we’llreviewthevonNeumannarchitectureanddiscussseveralproblemsoftheimpera-
tiveprogrammingparadigmduetoitstightcouplingwiththisarchitecture. Thenwe’llseehowthefunc-
tionalprogrammingparadigmcanelegantlyovercomethoseproblems.
1.2.1 VonNeumannarchitecture
In its simplest form, a Von Neumann computer consists of a Central Processing Unit (CPU), a memory,
andabusthatconnectsthem. TheCPUactsasthebrainofthecomputerwiththeabilitytoexecutea
predefinedsetofmachinecodeinstructions. Theseinstructionsareinbinaryform,consistingof0sand
1s,anddoaveryprimitivething,suchasaddingtwonumbersortestingwhetheranumberequalszero
TheArtofFunctionalProgramming 6
1 INTRODUCTION
ornot.TheCPUhasahandfulofregisterstostoredataneededwhenexecutinganinstruction.
Thememoryistheplacewhereaprogramanditsdataarestored. Aprogramisasequenceofmachine
codeinstructions. TheCPUandthememoryareconnectedviaabus. Duetothis,asubsetofmachine
codeinstructionsisdedicatedtoloadingdatafrommemoryontotheregistersorstoringthedatafrom
theregisterontothememory.
ThefollowingdiagramillustratestheVonNeumannarchitecture.
Figure3:ThevonNeumannarchitecture
So,howdoesaprogramrun? It’squitesimple. TheCPUrunstheprogramfollowingamechanicalfetch-
executecycle. First, itfetchesthefirstinstructionintheprogramandexecutesit. Thenitfetchesthe
nextinstructionandexecutesit,andthiscyclecontinueson.Someinstructionsaffecttheorderofexecu-
tion. Forinstance,ajumpinstructiondirectstheCPUtojumpbacktoapreviouspointoftheinstruction
sequence. A branch instruction tells the CPU to branch to a particular instruction if some condition is
TheArtofFunctionalProgramming 7