Table Of ContentFunctional Programming for Mortals
with Scalaz
Sam Halliday
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Contents
AboutThisBook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii
CopyleftNotice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii
Thanks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv
Practicalities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 AbstractingoverExecution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 PureFunctionalProgramming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. ForComprehensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1 SyntaxSugar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2 Unhappypath . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3 Gymnastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.4 Incomprehensible . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3. ApplicationDesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.1 Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.2 Interfaces/Algebras . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.3 BusinessLogic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.4 UnitTests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.5 Parallel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4. DataandFunctionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.1 Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.2 Functionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.3 ModellingOAuth2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
5. ScalazTypeclasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
5.1 Agenda . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
5.2 AppendableThings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.3 ObjectyThings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
CONTENTS
5.4 MappableThings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
5.5 Variance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
5.6 ApplyandBind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
5.7 ApplicativeandMonad . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
5.8 DivideandConquer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
5.9 Plus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
5.10 LoneWolves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
5.11 Co-things . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
5.12 Bi-things . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
5.13 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
6. ScalazDataTypes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
6.1 TypeVariance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
6.2 Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
6.3 Memoisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
6.4 Tagging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
6.5 NaturalTransformations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
6.6 Isomorphism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
6.7 Containers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
6.8 Collections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
6.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
7. AdvancedMonads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
7.1 AlwaysinmotionistheFuture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
7.2 EffectsandSideEffects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
7.3 StackSafety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
7.4 MonadTransformerLibrary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180
7.5 AFreeLunch. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
7.6 Parallel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227
7.7 IO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231
7.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
8. TypeclassDerivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
8.1 RunningExamples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240
8.2 scalaz-deriving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
8.3 Magnolia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261
8.4 Shapeless . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271
8.5 Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297
8.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302
9. WiringuptheApplication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303
9.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304
9.2 Main . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
9.3 Blaze . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315
CONTENTS
9.4 ThankYou . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322
TypeclassCheatsheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323
ThirdPartyLicenses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324
ScalaLicense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325
ScalazLicense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326
CONTENTS i
“Love is wise; hatred is foolish. In this world, which is getting more and more closely
interconnected, we have to learn to tolerate each other, we have to learn to put up with
the fact that some people say things that we don’t like. We can only live together in that
way. But if we are to live together, and not die together, we must learn a kind of charity
andakindoftolerance,whichisabsolutelyvitaltothecontinuationofhumanlifeonthis
planet.”
―BertrandRussell
About This Book
ThisbookisforthetypicalScaladeveloper,probablywithaJavabackground,whoisbothsceptical
and curious about the Functional Programming (FP) paradigm. This book justifies every concept
withpracticalexamples,includingwritingawebapplication.
This book uses Scalaz 7.2¹, the most popular, stable, principled and comprehensive Functional
ProgrammingframeworkforScala.
This book is designed to be read from cover to cover, in the order presented, with a rest between
chapters. Earlier chapters encourage coding styles that we will later discredit: similar to how we
learn Newton’s theory of gravity as children, and progress to Riemann / Einstein / Maxwell if we
becomestudentsofphysics.
Acomputerisnotnecessarytofollowalong,butstudyingtheScalazsourcecodeisencouraged.Some
of the more complex code snippets are available with the book’s source code² and those who want
practicalexercisesareencouragedto(re-)implementScalaz(andtheexampleapplication)usingthe
partialdescriptionspresentedinthisbook.
WealsorecommendTheRedBook³asfurtherreading.ItteacheshowtowriteanFPlibraryinScala
fromfirstprinciples.
¹https://github.com/scalaz/scalaz
²https://github.com/fommil/fpmortals/
³https://www.manning.com/books/functional-programming-in-scala
ii
Copyleft Notice
This book is Libre and follows the philosophy ofFree Software⁴: you can use this book as you like,
thesourceisavailable⁵youcanredistributethisbookandyoucandistributeyourownversion.That
means you can print it, photocopy it, e-mail it, upload it to websites, change it, translate it, charge
forit,remixit,deletebits,anddrawalloverit.
ThisbookisCopyleft:ifyouchangethebookanddistributeyourownversion,youmustalsopass
thesefreedomstoitsrecipients.
This book uses the Creative Commons Attribution ShareAlike 4.0 International⁶ (CC BY-SA 4.0)
license.
All original code snippets in this book are separately CC0⁷ licensed, you may use them without
restriction. Excerpts from Scalaz and related libraries maintain their license, reproduced in full in
theappendix.
The example application drone-dynamic-agents is distributed under the terms of the GPLv3⁸:
onlythesnippetsinthisbookareavailablewithoutrestriction.
⁴https://www.gnu.org/philosophy/free-sw.en.html
⁵https://github.com/fommil/fpmortals/
⁶https://creativecommons.org/licenses/by-sa/4.0/legalcode
⁷https://wiki.creativecommons.org/wiki/CC0
⁸https://www.gnu.org/licenses/gpl-3.0.en.html
iii
Thanks
Diego Esteban Alonso Blas, Raúl Raja Martínez and Peter Neyens of 47 degrees, Rúnar Bjarnason,
Tony Morris, John de Goes and Edward Kmett for their help explaining the principles of FP. Kenji
YoshidaandJasonZauggforbeingthemainauthorsofScalaz,andPaulChuisano/MilesSabinfor
fixingacriticalbugintheScalacompiler(SI-2712⁹).
Thankstothereaderswhogavefeedbackonearlydraftsofthistext.
Some material was particularly helpful for my own understanding of the concepts that are in this
book. Thanks to Juan Manuel Serrano for All Roads Lead to Lambda¹⁰, Pere Villega for On Free
Monads¹¹, Dick Wall and Josh Suereth for For: What is it Good For?¹², Erik Bakker for Options in
Futures, how to unsuck them¹³, Noel Markham for ADTs for the Win!¹⁴, Sukant Hajra for Classy
MonadTransformers¹⁵,LukaJacobowitzforOptimizingTaglessFinal¹⁶,VincentMarquezforIndex
yourState¹⁷,GabrielGonzalezforTheContinuationMonad¹⁸,andYiLinWei/ZainabAlifortheir
tutorialsatHackTheTowermeetups.
The helpul souls who patiently explained things to me: Merlin Göttlinger, Edmund Noble, Fabio
Labella, Adelbert Chang, Michael Pilquist, Paul Snively, Daniel Spiewak, Stephen Compall, Brian
McKenna, Ryan Delucchi, Pedro Rodriguez, Emily Pillmore, Aaron Vargo, Tomas Mikula, Jean-
Baptiste Giraudeau, Itamar Ravid, Ross A. Baker, Alexander Konovalov, Harrison Houghton,
AlexandreArchambault,ChristopherDavenport,JoseCardona.
⁹https://issues.scala-lang.org/browse/SI-2712
¹⁰https://skillsmatter.com/skillscasts/9904-london-scala-march-meetup#video
¹¹http://perevillega.com/understanding-free-monads
¹²https://www.youtube.com/watch?v=WDaw2yXAa50
¹³https://www.youtube.com/watch?v=hGMndafDcc8
¹⁴https://www.47deg.com/presentations/2017/06/01/ADT-for-the-win/
¹⁵https://www.youtube.com/watch?v=QtZJATIPB0k
¹⁶https://lukajcb.github.io/blog/functional/2018/01/03/optimizing-tagless-final.html
¹⁷https://www.youtube.com/watch?v=JPVagd9W4Lo
¹⁸http://www.haskellforall.com/2012/12/the-continuation-monad.html
iv
Practicalities
To set up a project that uses the libraries presented in this book, use a recent version of Scala with
FP-specificfeaturesenabled(e.g.inbuild.sbt):
scalaVersion in ThisBuild := "2.12.6"
scalacOptions in ThisBuild ++= Seq(
"-language:_",
"-Ypartial-unification",
"-Xfatal-warnings"
)
libraryDependencies ++= Seq(
"com.github.mpilquist" %% "simulacrum" % "0.13.0",
"org.scalaz" %% "scalaz-core" % "7.2.26"
)
addCompilerPlugin("org.spire-math" %% "kind-projector" % "0.9.7")
addCompilerPlugin("org.scalamacros" % "paradise" % "2.1.1" cross CrossVersion.full)
Inordertokeepoursnippetsshort,wewillomittheimportsection.Unlesstoldotherwise,assume
thatallsnippetshavethefollowingimports:
import scalaz._, Scalaz._
import simulacrum._
v
