Learning Functional Programming in Scala Alvin Alexander A logical, step by step approach to ...

619 Pages·2017·3.88 MB·English

Learning Functional Programming
in Scala
Alvin Alexander
A logical, step by step approach
to learning functional programming

Copyright
Learning Functional Programming in Scala
Copyright 2017 Alvin J. Alexander
Allrightsreserved. Nopartofthisbookmaybereproducedwithoutprior
written permission from the author.
Disclaimer: This book is presented solely for educational purposes, and
it’s also a work in progress. While best efforts have been used in prepar-
ing this book, the author makes no representations or warranties of any
kind and assume no liabilities of any kind with respect to the accuracy or
completeness of the contents, and specifically disclaim any implied war-
ranties of merchantability or fitness of use for a particular purpose. The
author shall not be held liable or responsible to any person or entity with
respect to any loss or incidental or consequential damages caused, or al-
leged to have been caused, directly or indirectly, by the information or
programs contained herein. Any use of this information is at your own
risk. The advice and strategies contained herein may not be suitable for
your situation.
Version 0.1.1, published May 15, 2017
(The cover photo was taken on the Dalton Highway in Alaska, some-
where between Fairbanks and Prudhoe Bay.)

Contents
1 Changelog 1
2 Preface 3
3 Introduction
(or, Why I Wrote This Book) 5
4 Who This Book is For 11
5 Goals 15
6 Question Everything 23
7 Rules for Programming in this Book 33
8 One Rule for Reading this Book 39
9 What is “Functional Programming”? 41
10 What is This Lambda You Speak Of? 55
i
ii CONTENTS
11 The Benefits of Functional Programming 73
12 Disadvantages of Functional Programming 99
13 The “Great FP Terminology Barrier” 123
14 Pure Functions 131
15 Grandma’s Cookies (and Pure Functions) 141
16 Benefits of Pure Functions 153
17 Pure Functions and I/O 163
18 Pure Function Signatures Tell All 173
19 Functional Programming as Algebra 181
20 A Note About Expression-Oriented Programming 197
21 Functional Programming is Like Unix Pipelines 203
22 Functions Are Variables, Too 225
CONTENTS iii
23 Using Methods As If They Were Functions 243
24 How to Write Functions That Take Functions as Input
Parameters 255
25 How to Write a ‘map’ Function 279
26 How to Use By-Name Parameters 287
27 Functions Can Have Multiple Parameter Groups 303
28 Partially-Applied Functions (and Currying) 327
29 Recursion: Introduction 345
30 Recursion: Motivation 347
31 Recursion: Let’s Look at Lists 351
32 Recursion: How to Write a ‘sum’ Function 359
33 Recursion: How Recursive Function Calls Work 367
34 Visualizing the Recursive sum Function 373
iv CONTENTS
35 Recursion: A Conversation Between Two Developers 383
36 Recursion: Thinking Recursively 387
37 JVM Stacks and Stack Frames 397
38 A Visual Look at Stacks and Frames 407
39 Tail-Recursive Algorithms 417
40 A First Look at “State” 431
41 A Functional Game (With a Little Bit of State) 437
42 A Quick Review of Case Classes 455
43 Update as You Copy, Don’t Mutate 465
44 A Quick Review of for Expressions 477
45 How to Write a Class That Can Be Used in a for Expres-
sion 487
46 Creating a Sequence Class to be Used in a for Compre-
CONTENTS v
hension 491
47 Making Sequence Work in a Simple for Loop 499
48 How To Make Sequence Work as a Single Generator in a
for Expression 503
49 Enabling Filtering in a for Expression 509
50 How to Enable the Use of Multiple Generators in a for
Expression 519
51 A Summary of the for Expression Lessons 531
52 Pure Functions Tell No Lies 535
53 Functional Error Handling (Option, Try, or Either) 543
54 Embrace The Idioms! 557
55 What to Think When You See That Opening Curly Brace561
A Explaining Scala’s val Function Syntax 581
vi CONTENTS
B for expression translation examples 599