Table Of ContentProduction Haskell
Succeeding in Industry with Haskell
Matt Parsons
Thisbookisforsaleathttp://leanpub.com/production-haskell
Thisversionwaspublishedon2023-02-01
ThisisaLeanpubbook.Leanpubempowersauthorsandpublishers
withtheLeanPublishingprocess.LeanPublishingistheactof
publishinganin-progressebookusinglightweighttoolsandmany
iterationstogetreaderfeedback,pivotuntilyouhavetherightbook
andbuildtractiononceyoudo.
©2020-2023MattParsons
Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i
AnOpinionatedTourGuide . . . . . . . . . . . . . . . . . . . . . . i
AbouttheAuthor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v
Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi
Complexity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi
Novelty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix
Cohesion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x
Empathy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvi
I Building Haskell Teams
. . . . . . . . . . . 1
1. SellingHaskell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 AssessingReceptiveness . . . . . . . . . . . . . . . . . . . . . 2
1.2 SoftwareProductivity . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 StatisticsofProductivity . . . . . . . . . . . . . . . . . . . . . 4
1.4 KnowYourCompetition . . . . . . . . . . . . . . . . . . . . . 6
2. LearningandTeachingHaskell . . . . . . . . . . . . . . . . . . . 7
2.1 ThePhilologyofHaskell . . . . . . . . . . . . . . . . . . . . . 7
2.2 ProgrammingIsHardToLearn . . . . . . . . . . . . . . . . . 8
2.3 PickLearningMaterials. . . . . . . . . . . . . . . . . . . . . . 9
2.4 WriteLotsofCode . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.5 Don’tFeartheGHC . . . . . . . . . . . . . . . . . . . . . . . . 10
2.6 StartSimple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.7 SolveRealProblems . . . . . . . . . . . . . . . . . . . . . . . . 13
CONTENTS
2.8 PairProgramming . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.9 ADialogue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.10 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3. HiringHaskellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.1 TheDouble-edgedSword . . . . . . . . . . . . . . . . . . . . . 23
3.2 JuniorsandSeniors . . . . . . . . . . . . . . . . . . . . . . . . 23
3.3 HiringSeniors . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.4 HiringJuniors . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4. EvaluatingConsultancies . . . . . . . . . . . . . . . . . . . . . . . 32
4.1 IdentifyingtheTarget . . . . . . . . . . . . . . . . . . . . . . . 32
4.2 Well-Typed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.3 FPComplete . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
II Application Structure
. . . . . . . . . . . . . 37
5. TheBeginning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
5.1 AbstractionforMocking . . . . . . . . . . . . . . . . . . . . . 42
5.2 ForwardCompatibility . . . . . . . . . . . . . . . . . . . . . . 42
5.3 AppEnvironment . . . . . . . . . . . . . . . . . . . . . . . . . 44
5.4 TheReaderTPattern . . . . . . . . . . . . . . . . . . . . . . . 49
5.5 Embed,don’tStack . . . . . . . . . . . . . . . . . . . . . . . . 51
6. ThreeLayerHaskellCake . . . . . . . . . . . . . . . . . . . . . . . 54
6.1 Layer1:ImperativeProgramming . . . . . . . . . . . . . . . 54
6.2 Layer2:ObjectOrientedProgramming . . . . . . . . . . . . 56
6.3 Layer3:FunctionalProgramming . . . . . . . . . . . . . . . 62
6.4 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
7. InvertYourMocks! . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
7.1 DecomposingEffects . . . . . . . . . . . . . . . . . . . . . . . 65
7.2 StreamingDecomposition . . . . . . . . . . . . . . . . . . . . 68
7.3 Plainol’abstraction . . . . . . . . . . . . . . . . . . . . . . . . 73
7.4 Decompose!!! . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
7.5 WhatifIneedto? . . . . . . . . . . . . . . . . . . . . . . . . . . 75
8. ProjectPreludes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
8.1 PreludeProblems . . . . . . . . . . . . . . . . . . . . . . . . . 78
8.2 CustomBenefits . . . . . . . . . . . . . . . . . . . . . . . . . . 84
CONTENTS
8.3 Off-The-ShelfPreludes . . . . . . . . . . . . . . . . . . . . . . 89
8.4 NestingPreludes . . . . . . . . . . . . . . . . . . . . . . . . . . 97
8.5 Downsides. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
8.6 UsingaCustomPrelude . . . . . . . . . . . . . . . . . . . . . 99
9. OptimizingGHCCompileTimes . . . . . . . . . . . . . . . . . . . 100
9.1 TheProject.TypesMegamodule . . . . . . . . . . . . . . . 100
9.2 PackageSplitting. . . . . . . . . . . . . . . . . . . . . . . . . . 102
9.3 BigOl’InstancesModule . . . . . . . . . . . . . . . . . . . . . 104
9.4 TemplateHaskell . . . . . . . . . . . . . . . . . . . . . . . . . 107
9.5 Somerandompartingthoughts . . . . . . . . . . . . . . . . . 117
III Domain Modeling
. . . . . . . . . . . . . . . .118
10.TypeSafetyBackandForth . . . . . . . . . . . . . . . . . . . . . . 119
10.1 TheRippleEffect . . . . . . . . . . . . . . . . . . . . . . . . . . 123
10.2 AskOnlyWhatYouNeed . . . . . . . . . . . . . . . . . . . . . 124
11.KeepYourTypesSmall… . . . . . . . . . . . . . . . . . . . . . . . 126
11.1 ExpansionandRestriction . . . . . . . . . . . . . . . . . . . . 127
11.2 ConstraintsLiberate. . . . . . . . . . . . . . . . . . . . . . . . 129
11.3 RestricttheRange . . . . . . . . . . . . . . . . . . . . . . . . . 130
11.4 Aperfectfit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
12.TheTroublewithTypedErrors . . . . . . . . . . . . . . . . . . . 132
12.1 Monolithicerrortypesarebad . . . . . . . . . . . . . . . . . 137
12.2 Boilerplatebegone! . . . . . . . . . . . . . . . . . . . . . . . . 138
12.3 TypeClassesToTheRescue! . . . . . . . . . . . . . . . . . . . 140
12.4 Thevirtueofuntypederrors . . . . . . . . . . . . . . . . . . . 141
13.Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
13.1 ExceptionsInFiveMinutes. . . . . . . . . . . . . . . . . . . . 142
13.2 BestPractices . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
13.3 Hierarchies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
13.4 Reinventing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
13.5 AsynchronousExceptions . . . . . . . . . . . . . . . . . . . . 162
13.6 TheTheory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
13.7 HasCallStack . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
14.EDSLDesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
CONTENTS
14.1 Trickswithdo . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
14.2 OverloadedLiterals . . . . . . . . . . . . . . . . . . . . . . . . 182
14.3 TypeInferenceTrick . . . . . . . . . . . . . . . . . . . . . . . 187
14.4 FluentInterfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 191
14.5 CaseStudy:WeightliftingLogging . . . . . . . . . . . . . . . 194
14.6 CaseStudy:rowdy . . . . . . . . . . . . . . . . . . . . . . . . . 200
14.7 CaseStudy:hspec . . . . . . . . . . . . . . . . . . . . . . . . . 205
15.GrowingPains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209
15.1 ATaxonomyofBreakingChanges . . . . . . . . . . . . . . . 209
15.2 AvoidingBreakingChanges . . . . . . . . . . . . . . . . . . . 214
15.3 CommunicatingToUsers. . . . . . . . . . . . . . . . . . . . . 232
IV Interfacing the Real
. . . . . . . . . . . . . .238
16.Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
16.1 LibrariesandTools . . . . . . . . . . . . . . . . . . . . . . . . 239
16.2 DesigningCodeforTesting . . . . . . . . . . . . . . . . . . . . 245
17.LoggingandObservability . . . . . . . . . . . . . . . . . . . . . . 259
17.1 OnDebug.Trace . . . . . . . . . . . . . . . . . . . . . . . . . . 259
17.2 PreferdoNotation . . . . . . . . . . . . . . . . . . . . . . . . . 263
17.3 LoggingContexts . . . . . . . . . . . . . . . . . . . . . . . . . . 265
17.4 LibrariesinBrief . . . . . . . . . . . . . . . . . . . . . . . . . . 269
18.Databases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272
18.1 SeparateDatabaseTypes . . . . . . . . . . . . . . . . . . . . . 273
18.2 Migrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284
18.3 AccessPatterns. . . . . . . . . . . . . . . . . . . . . . . . . . . 285
18.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302
V Advanced Haskell
. . . . . . . . . . . . . . . . .303
19.TemplateHaskellIsNotScary . . . . . . . . . . . . . . . . . . . . 304
19.1 ABeginnerTutorial . . . . . . . . . . . . . . . . . . . . . . . . 304
19.2 waitthisisn’thaskellwhatamidoinghere . . . . . . . . . . 306
19.3 ConstructinganAST . . . . . . . . . . . . . . . . . . . . . . . 306
19.4 BoilerplateBeGone! . . . . . . . . . . . . . . . . . . . . . . . . 312
CONTENTS
20.BasicTypeLevelProgramming . . . . . . . . . . . . . . . . . . . 323
20.1 TheBasicTypes . . . . . . . . . . . . . . . . . . . . . . . . . . 323
20.2 TheHigherKinds . . . . . . . . . . . . . . . . . . . . . . . . . 324
20.3 DynamicallyKindedProgramming . . . . . . . . . . . . . . . 328
20.4 DataKinds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329
20.5 GADTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330
20.6 Vectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331
20.7 TypeFamilies. . . . . . . . . . . . . . . . . . . . . . . . . . . . 334
20.8 ThisSucks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340
20.9 HeterogeneousLists . . . . . . . . . . . . . . . . . . . . . . . . 340
20.10InductiveTypeClassInstances . . . . . . . . . . . . . . . . . 342
20.11ExtensibleRecords . . . . . . . . . . . . . . . . . . . . . . . . 343
20.12Likewhatyouread? . . . . . . . . . . . . . . . . . . . . . . . . 347
21.FamilyValues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348
21.1 TypeFamilies. . . . . . . . . . . . . . . . . . . . . . . . . . . . 348
21.2 OpenorClosedTypeFamilies? . . . . . . . . . . . . . . . . . 354
21.3 TheBridgeBetweenWorlds . . . . . . . . . . . . . . . . . . . 355
21.4 DataFamilies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357
21.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360
22.Trade-offsinTypeProgramming . . . . . . . . . . . . . . . . . . 361
22.1 MPTCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361
22.2 MPTCs+Fundeps . . . . . . . . . . . . . . . . . . . . . . . . . 364
22.3 AssociatedTypes . . . . . . . . . . . . . . . . . . . . . . . . . 370
22.4 Comparisons . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371
23.CaseStudy:Prairie . . . . . . . . . . . . . . . . . . . . . . . . . . . 379
23.1 ProblemStatement: . . . . . . . . . . . . . . . . . . . . . . . . 380
23.2 PriorArt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382
23.3 TheGADTApproach. . . . . . . . . . . . . . . . . . . . . . . . 384
23.4 Improvements? . . . . . . . . . . . . . . . . . . . . . . . . . . 407
23.5 Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409
23.6 CompareandConstrast . . . . . . . . . . . . . . . . . . . . . . 423
23.7 IdentifytheIssue. . . . . . . . . . . . . . . . . . . . . . . . . . 425
23.8 GeneralizeaGADT . . . . . . . . . . . . . . . . . . . . . . . . . 425
23.9 Fundeps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426
23.10MoreClassPlease . . . . . . . . . . . . . . . . . . . . . . . . . 430
23.11RefiningtheClass . . . . . . . . . . . . . . . . . . . . . . . . . 431
23.12TemplateHaskell . . . . . . . . . . . . . . . . . . . . . . . . . 433
CONTENTS
23.13Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442
Introduction
An Opinionated Tour Guide
So you’ve learned Haskell. You’ve taught your friends about monads,
you’ve worked through some beginner textbooks, and maybe you’ve
played around with some open source projects. Now that you’ve had a
taste,youwantmore:youwanttowriteanapplicationinHaskellforfun!
MaybeyouwanttouseHaskellatwork!
Yousitdownatyourcomputer,andyou’restuck.
How does anyone actually get anything done with this lan-
guage?
Thisisacommonthingtowonder.
Haskellhasalwaysenjoyedawidevarietyofhighqualitylearningmate-
rialforadvancedpartsofthelanguage,ifyou’renotafraidofacademic
papers. Many people have created fantastic resources for beginners in
thelastfiveyears.However,thelanguagedoesnothavemanyresources
for using it in production. The Haskell ecosystems can be difficult to
navigate.Therearemanyresourcesofvaryingqualitywithambiguous
goalsandvalues.Identifyingtherightadviceisnearlyaschallengingas
findingitinthefirstplace.
Haskellisahugelydiverselandscape.Therearemanyregionalgroups:
UnitedKingdom,Scandinavia,mainlandEurope,Russia,theUSA,Japan,
China,andIndiaallhavethrivingHaskellecosystemsthathaveinterest-
ingdialectsanddifferencesincustomandpractice.
People come to Haskell with many backgrounds. Some people learned
Haskellwellintotheircareers,andhadalongcareerwritingJava,Scala,
orC#beforehand.SomepeoplecametoHaskellfromdynamicallytyped
languages,likeLISPorRuby.SomepeoplestartedlearningHaskellearly
i
Introduction ii
on in their programming career, and use it as the basis of comparison.
SomepeopleprimarilyuseHaskellinacademicresearch,whileothers
primarilyuseHaskellinindustrialapplications.Somepeoplearehobby-
istsandjustliketowriteHaskellforfun!
ThisbookisintendedforpeoplethatwanttowriteHaskellinindustry.
The trade-offs and constraints that industrial programmers face are
differentfromacademicorhobbyistprogrammers.Thisbookwillcover
not only technical aspects of the Haskell language, but also social and
engineeringconcernsthataren’t“really”aboutHaskell.
Part of this book will be objective. I will teach you how to use some
interestingtechniquesandideastomakedevelopingwithHaskellmore
productive.We’lllearnaboutTemplateHaskell,type-levelprogramming,
andotherfuntopics.
However,forthemostpart,thisbookisinherentlysubjective.Because
Haskell serves so many ecosystems, it is imperative to discern what
ecosystem a something is intended for. More than just giving out pre-
scriptions-“Thislibraryisproductionready!Thisisatoy!”-Ihopeto
show my thought process and allow you to make your own judgment
calls.
Ultimately,thisisabookaboutthesocialrealityofsoftwareengineering
inanichelanguage.
Afterreadingthisbook,youshouldfeelcomfortable:
• WritinglargesoftwareprojectsinHaskell
• Evaluatingcompetinglibrariesandtechniques
• ProductivelyreadingmaterialfromavarietyofHaskellusers
About the Author
I’mMattParsons.
IstartedlearningprogramminginJanuary2014withComputerScience
101 at the University of Georgia. At the time, I was working for the IT
department,installingWindowsandtroubleshootingprinters.Myman-
agerdislikedmeandmadeitclearthathe’dthrowmeunderthebusat
