A companion booklet to ”Functional Programming in Scala” Chapter notes, errata, hints, and answers to exercises Rúnar Óli Bjarnason This book is for sale at http://leanpub.com/fpinscalacompanion This version was published on 2015-03-05 This is a Leanpub book. Leanpub empowers authors and publishers with the Lean Publishing process. Lean Publishing is the act of publishing an in-progress ebook using lightweight tools and many iterations to get reader feedback, pivot until you have the right book and build traction once you do. ©2015 Manning Publications Co., Rúnar Óli Bjarnason, and other contributors Contents About this booklet ......................................... 2 Why make this? ......................................... 2 License .............................................. 2 Errata ................................................ 3 Chapter notes ........................................... 6 Getting answers to your questions ............................... 6 Notes on chapter 1: What is functional programming? .................... 6 Notes on chapter 2: Getting started ............................... 7 Notes on chapter 3: Functional data structures ........................ 9 Notes on chapter 4: Handling errors without exceptions ................... 12 Notes on chapter 5: Strictness and laziness .......................... 14 Notes on chapter 6: Purely functional state .......................... 16 Notes on chapter 7: Purely functional parallelism ....................... 20 Notes on chapter 8: Property-based testing .......................... 21 Notes on chapter 9: Parser combinators ............................ 23 Notes on chapter 10: Monoids ................................. 26 Notes on chapter 11: Monads .................................. 29 Notes on chapter 12: Applicative and traversable functors . 38 Notes on chapter 13: External effects and I/O ......................... 42 Notes on chapter 14: Local effects and mutable state ..................... 44 Notes on chapter 15: Stream processing and incremental I/O . 46 Hints for exercises ......................................... 50 Hints for exercises in chapter 2 ................................. 50 Hints for exercises in chapter 3 ................................. 51 Hints for exercises in chapter 4 ................................. 53 Hints for exercises in chapter 5 ................................. 53 Hints for exercises in chapter 6 ................................. 54 Hints for exercises in chapter 7 ................................. 55 Hints for exercises in chapter 8 ................................. 56 Hints for exercises in chapter 9 ................................. 57 Hints for exercises in chapter 10 ................................ 58 CONTENTS Hints for exercises in chapter 11 ................................ 60 Hints for exercises in chapter 12 ................................ 62 Hints for exercises in chapter 13 ................................ 63 Hints for exercises in chapter 15 ................................ 64 Answers to exercises ....................................... 65 Answers to exercises for chapter 2 ............................... 65 Answers to exercises for chapter 3 ............................... 66 Answers to exercises for chapter 4 ............................... 76 Answers to exercises for chapter 5 ............................... 79 Answers to exercises for chapter 6 ............................... 85 Answers to exercises for chapter 7 ............................... 90 Answers to exercises for chapter 8 ............................... 95 Answers to exercises for chapter 9 ...............................100 Answers to exercises for chapter 10 ..............................105 Answers to exercises for chapter 11 ..............................112 Answers to exercises for chapter 12 ..............................121 Answers to exercises for chapter 13 ..............................128 Answers to exercises for chapter 14 ..............................130 Answers to exercises for chapter 15 ..............................132 A brief introduction to Haskell, and why it matters ...................... 138 About Haskell ..........................................138 Haskell syntax ..........................................139 Conclusion ............................................148 CONTENTS 1 Copyright (c) 2012, Manning Publications, Co., Rúnar Óli Bjarnason, and other contributors Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWAREOR THE USE OR OTHER DEALINGS IN THE SOFTWARE. About this booklet This booklet contains chapter notes, hints, answers to exercises, addenda, and errata for the book Functional Programming in Scala¹ by Paul Chiusano and Rúnar Bjarnason. There is no original material here, except for this brief introduction. This booklet is just a collection of material that is freely available online². Why make this? Functional Programming in Scala is a book that relies heavily on exercises. This can make it difficult to consume by reading rather than doing. Since the exercise answers, hints, and chapter notes are all online, a number of people have expressed difficulty studying the book on a train, on flights, in a classroom setting, or generally where there is no internet connection or where it’s inconvenient to use a laptop. So what I’ve done is literally print all of that material in a single document, as a companion for the book itself. Its purpose is to be convenient to use as a hardcopy, with an eBook reader, a tablet, or a phone. License This booklet, like the material it aggregates, is released under a permissive MIT license³. A copy of the license is provided at the front of this booklet. You are free to make copies of this booklet and distribute them as widely as you want. ¹http://manning.com/bjarnason ²http://github.com/fpinscala/fpinscala ³http://en.wikipedia.org/wiki/MIT_License Errata This is a list of post-publication errata for the first published version of Functional Programming in Scala. To submit new errata please open an issue⁴ in the book’s GitHub repository. Pg xviii in About this Book: URL is incorrectly listed as https://github.com/fpinscala/fpinscla (note missing ‘a’), should be https://github.com/fpinscala/fpinscala Pg 7: In throw new Exception, Exception should not be bolded, in the code listing at the top of the page. Pg 10: URL in footnote is incorrectly listed as https://github.com/pchiusano/fpinscala, should be https://github.com/fpinscala/fpinscala Pg 19: Parenthetical phrase in footnote is missing a closing parenthesis. Pg 55: employeesByName.get("Joe") should be lookupByName("Joe") at the top of the page. Pg 82: Int.MaxValue is incorrectly capitalized as Int.maxValue in the exercise prompt for 6.1 Pg 86: Definition of nonNegativeLessThan at bottom of page incorrectly reuses rng variable for recursive call. Definition should be: def nonNegativeLessThan(n: Int): Rand[Int] = { rng => val (i, rng2) = nonNegativeInt(rng) val mod = i % n if (i + (n-1) - mod >= 0) (mod, rng2) else nonNegativeLessThan(n)(rng2) } Pg 74: Figure contains a typo, “strean elements” instead of “stream elements” Pg 150: Listing 9.1 has a gratuitous nonstrict function argument in the definition of | in ParserOps. The definition should be: case class ParserOps[A](p: Parser[A]) { def |[B>:A](p2: Parser[B]): Parser[B] = self.or(p,p2) def or[B>:A](p2: Parser[B]): Parser[B] = self.or(p,p2) } Pg 150: Examples given for listOfN are incorrect. Should be: ⁴https://github.com/fpinscala/fpinscala/issues Errata 4 run(listOfN(3, "ab" | "cad"))("ababcad") == Right(List("ab","ab","cad")) run(listOfN(3, "ab" | "cad"))("cadabab") == Right(List("cad","ab","ab")) run(listOfN(3, "ab" | "cad"))("ababab") == Right(List("ab","ab","ab")) Pg 184: Exercise 10.13 incorrectly defines Leaf as a case object. It should be case class: case class Leaf[A](value: A) extends Tree[A] Pg 187: Definition of signature for map on Option has incorrect return type, should be: def map[A,B](oa: Option[A])(f: A => B): Option[B] Pg 238: Definition of printLine has a typo, an extra Return constructor, should be: def printLine(s: String): IO[Unit] = Suspend(() => println(s)) Pg 240: REPL session has a typo, should be: val g = List.fill(100000)(f).foldLeft(f) { (a, b) => x => Suspend(() => ()).flatMap { _ => a(x).flatMap(b)} } Note: we could write a little helper function to make this nicer: def suspend[A](a: => IO[A]) = Suspend(() => ()).flatMap { _ => a } val g = List.fill(100000)(f).foldLeft(f) { (a, b) => x => suspend { a(x).flatMap(b) } } Pg 241: TrailRec should be TailRec at the top of the page. Pg 285: Code snippet has an incorrectly named type parameter, currently