Table Of Content

Chris Eidhof, Florian Kugler, and Wouter Swierstra Contents 1 Introduction 1 UpdatestotheBook . . . . . . . . . . . . . . . . . . . . . . 3 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 3 2 ThinkingFunctionally 5 Example:Battleship . . . . . . . . . . . . . . . . . . . . . . 5 First-ClassFunctions . . . . . . . . . . . . . . . . . . . . . . 11 Type-DrivenDevelopment. . . . . . . . . . . . . . . . . . . . 16 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3 WrappingCoreImage 18 TheFilterType . . . . . . . . . . . . . . . . . . . . . . . . . 19 BuildingFilters . . . . . . . . . . . . . . . . . . . . . . . . . 19 ComposingFilters. . . . . . . . . . . . . . . . . . . . . . . . 22 TheoreticalBackground:Currying. . . . . . . . . . . . . . . . 24 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 4 Map,Filter,Reduce 27 IntroducingGenerics . . . . . . . . . . . . . . . . . . . . . . 27 Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Reduce . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 PuttingItAllTogether . . . . . . . . . . . . . . . . . . . . . . 36 Genericsvs.theAnyType . . . . . . . . . . . . . . . . . . . . 37 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 5 Optionals 41 CaseStudy:Dictionaries . . . . . . . . . . . . . . . . . . . . 41 CombiningOptionalValues . . . . . . . . . . . . . . . . . . . 45 WhyOptionals? . . . . . . . . . . . . . . . . . . . . . . . . . 50 6 QuickCheck 54 BuildingQuickCheck . . . . . . . . . . . . . . . . . . . . . . 56 MakingValuesSmaller . . . . . . . . . . . . . . . . . . . . . 60 ArbitraryArrays . . . . . . . . . . . . . . . . . . . . . . . . . 62 UsingQuickCheck . . . . . . . . . . . . . . . . . . . . . . . . 66 NextSteps . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 7 TheValueofImmutability 69 VariablesandReferences . . . . . . . . . . . . . . . . . . . . 69 ValueTypesvs.ReferenceTypes . . . . . . . . . . . . . . . . 70 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 8 Enumerations 78 IntroducingEnumerations . . . . . . . . . . . . . . . . . . . 78 AssociatedValues . . . . . . . . . . . . . . . . . . . . . . . 81 AddingGenerics . . . . . . . . . . . . . . . . . . . . . . . . 84 OptionalsRevisited . . . . . . . . . . . . . . . . . . . . . . . 86 TheAlgebraofDataTypes. . . . . . . . . . . . . . . . . . . . 87 WhyUseEnumerations? . . . . . . . . . . . . . . . . . . . . 89 9 PurelyFunctionalDataStructures 90 BinarySearchTrees. . . . . . . . . . . . . . . . . . . . . . . 90 AutocompletionUsingTries . . . . . . . . . . . . . . . . . . . 96 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 10Diagrams 103 DrawingSquaresandCircles . . . . . . . . . . . . . . . . . . 103 TheCoreDataStructures . . . . . . . . . . . . . . . . . . . . 107 CalculatingandDrawing . . . . . . . . . . . . . . . . . . . . 110 CreatingViewsandPDFs . . . . . . . . . . . . . . . . . . . . 114 ExtraCombinators . . . . . . . . . . . . . . . . . . . . . . . 115 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 11GeneratorsandSequences 119 Generators . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 Sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 CaseStudy:TraversingaBinaryTree . . . . . . . . . . . . . . 128 CaseStudy:BetterShrinkinginQuickCheck . . . . . . . . . . 130 BeyondMapandFilter . . . . . . . . . . . . . . . . . . . . . 134 12ParserCombinators 138 TheCore . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 Choice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 ConvenienceCombinators . . . . . . . . . . . . . . . . . . . 149 ASimpleCalculator . . . . . . . . . . . . . . . . . . . . . . . 152 13CaseStudy:BuildingaSpreadsheetApplication 157 SampleCode . . . . . . . . . . . . . . . . . . . . . . . . . . 158 Parsing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 TheGUI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 14Functors,ApplicativeFunctors,andMonads 179 Functors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 ApplicativeFunctors . . . . . . . . . . . . . . . . . . . . . . 181 TheM-Word. . . . . . . . . . . . . . . . . . . . . . . . . . . 185 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 15Conclusion 190 FurtherReading. . . . . . . . . . . . . . . . . . . . . . . . . 190 WhatisFunctionalProgramming?. . . . . . . . . . . . . . . . 192 Closure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 AdditionalCode 194 StandardLibrary . . . . . . . . . . . . . . . . . . . . . . . . 194 Chapter6,QuickCheck . . . . . . . . . . . . . . . . . . . . . 196 Chapter10,Diagrams . . . . . . . . . . . . . . . . . . . . . . 197 Chapter11,Generators . . . . . . . . . . . . . . . . . . . . . 200 Chapter12,ParserCombinators . . . . . . . . . . . . . . . . 201 References 205 Chapter1 Introduction Whywritethisbook? ThereisplentyofdocumentationonSwiftreadily availablefromApple,andtherearemanymorebooksontheway. Why doestheworldneedyetanotherbookonyetanotherprogramminglan- guage? Thisbooktriestoteachyoutothinkfunctionally.WebelievethatSwift hastherightlanguagefeaturestoteachyouhowtowritefunctionalpro- grams. Butwhatmakesaprogramfunctional? Andwhybotherlearning aboutthisinthefirstplace? Itishardtogiveaprecisedefinitionoffunctionalprogramming—in thesameway,itishardtogiveaprecisedefinitionofobject-orientedpro- gramming,oranyotherprogrammingparadigmforthatmatter. Instead, wewilltrytofocusonsomeofthequalitiesthatwebelievewell-designed functionalprogramsinSwiftshouldexhibit: • Modularity: Ratherthanthinkingofaprogramasasequenceof assignments and method calls, functional programmers empha- sizethateachprogramcanberepeatedlybrokenintosmallerand smallerpieces;allthesepiecescanbeassembledusingfunction applicationtodefineacompleteprogram. Ofcourse,thisdecom- positionofalargeprogramintosmallerpiecesonlyworksifwe canavoidsharingstatebetweentheindividualcomponents. This bringsustoournextpoint. • ACarefulTreatmentofMutableState: Functionalprogrammingis sometimes(half-jokingly)referredtoas‘value-orientedprogram- ming.’ Object-oriented programming focuses on the design of classesandobjects,eachwiththeirownencapsulatedstate.Func- tionalprogramming,ontheotherhand,emphasizestheimportance ofprogrammingwithvalues,freeofmutablestateorothersideef- fects.Byavoidingmutablestate,functionalprogramscanbemore easilycombinedthantheirimperativeorobject-orientedcounter- parts. • Types: Finally,awell-designedfunctionalprogrammakescareful useoftypes.Morethananythingelse,acarefulchoiceofthetypes ofyourdataandfunctionswillhelpstructureyourcode.Swifthas apowerfultypesystemthat,whenusedeffectively,canmakeyour codebothsaferandmorerobust. WefeelthesearethekeyinsightsthatSwiftprogrammersmaylearnfrom thefunctionalprogrammingcommunity.Throughoutthisbook,wewillil- lustrateeachofthesepointswithmanyexamplesandcasestudies. Inourexperience,learningtothinkfunctionallyisnoteasy. Itchal- lengesthewaywe’vebeentrainedtodecomposeproblems.Forprogram- merswhoareusedtowritingforloops,recursioncanbeconfusing;the lackofassignmentstatementsandglobalstateiscrippling;andclosures, generics,higher-orderfunctions,andmonadsarejustplainweird. Throughoutthisbook,wewillassumethatyouhavepreviousprogram- mingexperienceinObjective-C(orsomeotherobject-orientedlanguage). Wewon’tcoverSwiftbasicsorteachyoutosetupyourfirstXcodeproject, butwewilltrytorefertoexistingAppledocumentationwhenappropriate. YoushouldbecomfortablereadingSwiftprogramsandfamiliarwithcom- monprogrammingconcepts,suchasclasses,methods,andvariables.If you’veonlyjuststartedtolearntoprogram,thismaynotbetherightbook foryou. Inthisbook,wewanttodemystifyfunctionalprogramminganddispel someoftheprejudicespeoplemayhaveagainstit.Youdon’tneedtohave aPhDinmathematicstousetheseideastoimproveyourcode!Functional programmingisnottheonlywaytoprograminSwift.Instead,webelieve thatlearningaboutfunctionalprogrammingaddsanimportantnewtool toyourtoolboxthatwillmakeyouabetterdeveloperinanylanguage. UpdatestotheBook AsSwiftevolves,we’llcontinuetomakeupdatesandenhancementsto thisbook. Shouldyouencounteranymistakes, orifyouwouldliketo sendanyotherkindoffeedbackourway,pleasefileanissueinourGitHub repository. Acknowledgements We’dliketothankthenumerouspeoplewhohelpedshapethisbook.We wantedtoexplicitlymentionsomeofthem: NatalyeChildressisourcopyeditor.Shehasprovidedinvaluablefeed- back,notonlymakingsurethelanguageiscorrectandconsistent,but alsomakingsurethingsareunderstandable. SarahLincolndesignedthecoverandgaveusfeedbackonthedesign andlayoutofthebook. WouterwouldliketothankUtrechtUniversityforlettinghimtaketime toworkonthisbook. Wewouldliketothankthebetareadersfortheirfeedbackduringthe writingofthisbook(listedinalphabeticalorder): Adrian Kosmaczewski, Alexander Altman, Andrew Halls, Bang Jun- young, Daniel Eggert, Daniel Steinberg, David Hart, David Owens II, Eugene Dorfman, f-dz-v, Henry Stamerjohann, J Bucaran, Jamie For- rest,JaromirSiska,JasonLarsen,JesseArmand,JohnGallagher,Kaan Dedeoglu,KareMorstol,KielGillard,KristopherJohnson,MatteoPiombo, NicholasOutram,OleBegemann,RobNapier,RonaldMannak,SamIsaac- son,SsuJenLu,StephenHorne,TJ,TerryLewis,TimBrooks,VadimSh- pakovski. Chris,Florian,andWouter Chapter2 ThinkingFunctionally FunctionsinSwiftarefirst-classvalues,i.e.functionsmaybepassedas argumentstootherfunctions,andfunctionsmayreturnnewfunctions. Thisideamayseemstrangeifyou’reusedtoworkingwithsimpletypes, suchasintegers,booleans,orstructs. Inthischapter,wewilltrytoex- plainwhyfirst-classfunctionsareusefulandprovideourfirstexampleof functionalprogramminginaction. Example:Battleship We’llintroducefirst-classfunctionsusingasmallexample:anon-trivial functionthatyoumightneedtoimplementifyouwerewritingaBattleship- likegame.Theproblemwe’lllookatboilsdowntodeterminingwhetheror notagivenpointisinrange,withoutbeingtooclosetofriendlyshipsorto us. Asafirstapproximation,youmightwriteaverysimplefunctionthat checkswhetherornotapointisinrange. Forthesakeofsimplicity,we willassumethatourshipislocatedattheorigin. Wecanvisualizethe regionwewanttodescribeinFigure2.1. Thefirstfunctionwewrite,inRange1,checksthatapointisinthegrey