Grokking Functional Programming

522 Pages·2022·16.664 MB·English

by  Michał Płachta

Michał Płachta
Learn functional programming in the
most practical way possible!
Hundreds of
code snippets... Like this one
All executable from
your computer!
def parseShows(rawShows: List[String]): List[TvShow] = {
rawShows // List( T h e W i r e ( 2 0 0 2 - 2 0 0 8 ) , C h e r n o b y l ( ) )
.map(parseShow) // List(So m e ( T v S h o w ( T h e W i r e , 2 0 0 2 , 2 0 0 8 ) ), None)
.map(_.toList) // List(Li s t ( T v S h o w ( T h e W i r e , 2 0 0 2 , 2 0 0 8 ) ), List())
.flatten // List( T v S h o w ( T h e W i r e , 2 0 0 2 , 2 0 0 8 ) )
}
...and accompanying illustrations
Like this one
flatten goes through each of the lists in this list and adds
all their elements into the resulting list in the same order.
List[List[A]].flatten: List[A]
List(List( ), List(), List( ), List( )).flatten
→ List( )
Twelve chapters, each including a
different real-world scenario
You will implement a travel guide, a planner, a
meeting scheduler, a TV show parser, a music
artists search engine, a currency exchange, and
many more applications!
(Continued on inside back cover)
grokking
functional
programming
Michał Płachta

MANNING
S I
helter Sland
For online information and ordering of this and other Manning books, please visit
www .manning .com. The publisher offers discounts on this book when ordered in quantity.
For more information, please contact
Special Sales Department
Manning Publications Co.
20 Baldwin Road, PO Box 761
Shelter Island, NY 11964
Email: [e-mail]
©2022 by Manning Publications Co. All rights reserved.
No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form
or by means electronic, mechanical, photocopying, or otherwise, without prior written permission of the
publisher.
Many of the designations used by manufacturers and sellers to distinguish their products are claimed
as trademarks. Where those designations appear in the book, and Manning Publications was aware of a
trademark claim, the designations have been printed in initial caps or all caps.
Recognizing the importance of preserving what has been written, it is Manning’s policy to have the books
we publish printed on acid-free paper, and we exert our best efforts to that end. Recognizing also our
responsibility to conserve the resources of our planet, Manning books are printed on paper that is at least
15 percent recycled and processed without the use of elemental chlorine.
The author and publisher have made every effort to ensure that the information in this book was correct
at press time. The author and publisher do not assume and hereby disclaim any liability to any party for
any loss, damage, or disruption caused by errors or omissions, whether such errors or omissions result
from negligence, accident, or any other cause, or from any usage of the information herein.
Manning Publications Co. Development editor: Jennifer Stout
20 Baldwin Road Technical development editor: Josh White
Shelter Island, NY 11964 Review editor: Aleksandar Dragosavljevic´
Production editor: Keri Hales
Copy editor: Christian Berk
Proofreader: Katie Tennant
Technical proofreader: Ubaldo Pescatore
Typesetter: Dennis Dalinnik
Cover designer: Leslie Haimes
ISBN: 9781617291838
Printed in the United States of America
To my dear family: Marta, Wojtek, and Ola,
for all the good vibes and inspiration.
To my parents: Renia and Leszek, for all the
opportunities you’ve given me.

contents
preface xix
acknowledgments xxi
about this book xxiii
about the author xxvii
P 1 t . . . . . . . . . . . . . . . . . . .1
art he functional toolkit
1 Learning functional programming 3
Perhaps you picked up this book because... 4
What do you need to know before we start? 5
What do functions look like? 6
Meet the function 7
When the code lies... 8
Imperative vs. declarative 9
Coffee break: Imperative vs. declarative 10
Coffee break explained: Imperative vs. declarative 11
How useful is learning functional programming? 12
Leaping into Scala 13
Practicing functions in Scala 14
Getting your tools ready 15
v
vi contents
Getting to know the REPL 16
Writing your first functions! 17
How to use this book 18
2 Pure functions 21
Why do we need pure functions? 22
Coding imperatively 23
Breaking the code 24
Passing copies of the data 25
Breaking the code . . . again 26
Recalculating instead of storing 27
Focusing on the logic by passing the state 28
Where did the state go? 29
The difference between impure and pure functions 30
Coffee break: Refactoring to a pure function 31
Coffee break explained: Refactoring to a pure function 32
In pure functions we trust 34
Pure functions in programming languages 35
Difficulty of staying pure... 36
Pure functions and clean code 37
Coffee break: Pure or impure? 38
Coffee break explained: Pure or impure? 39
Using Scala to write pure functions 40
Practicing pure functions in Scala 41
Testing pure functions 42
Coffee break: Testing pure functions 43
Coffee break explained: Testing pure functions 44
3 Immutable values 47
The fuel for the engine 48
Another case for immutability 49
Can you trust this function? 50
contents vii
Mutability is dangerous 51
Functions that lie... again 52
Fighting mutability by working with copies 53
Coffee break: Getting burned by mutability 54
Coffee break explained: Getting burned by mutability 55
Introducing shared mutable state 58
State’s impact on programming abilities 59
Dealing with the moving parts 60
Dealing with the moving parts using FP 61
Immutable values in Scala 62
Building our intuition about immutability 63
Coffee break: The immutable API 64
String
Coffee break explained: The immutable API 65
String
Hold on . . . Isn’t this bad? 66
Purely functional approach to shared mutable state 67
Practicing immutable slicing and appending 69
4 Functions as values 71
Implementing requirements as functions 72
Impure functions and mutable values strike back 73
Using Java Streams to sort the list 74
Function signatures should tell the whole story 75
Changing requirements 76
We just pass the code around! 77
Using Java’s values 78
Function
Using the syntax to deal with code duplication 79
Function
Passing user-defined functions as arguments 80
Coffee break: Functions as parameters 81
Coffee break explained: Functions as parameters 82
Problems with reading functional Java 83
Passing functions in Scala 84
Deep dive into
sortBy 85
Signatures with function parameters in Scala 86
viii contents
Passing functions as arguments in Scala 87
Practicing function passing 88
Embracing declarative programming 89
Passing functions to custom-made functions 90
Small functions and their responsibilities 91
Passing functions inline 92
Coffee break: Passing functions in Scala 93
Coffee break explained: Passing functions in Scala 94
What else can we achieve just by passing functions? 95
Applying a function to each element of a list 96
Applying a function to each element of a list using 97
map
Getting to know 98
map
Practicing 99
map
Learn once, use everywhere 100
Returning parts of the list based on a condition 101
Returning parts of the list using 102
filter
Getting to know 103
filter
Practicing 104
filter
Our journey so far... 105
Don’t repeat yourself? 106
Is my API easy to use? 107
Adding a new parameter is not enough 108
Functions can return functions 109
Using functions that can return functions 110
Functions are values 111
Coffee break: Returning functions 112
Coffee break explained: Returning functions 113
Designing functional APIs 114
Iterative design of functional APIs 115
Returning functions from returned functions 116
How to return functions from returned functions 117
Using the flexible API built with returned functions 118
Using multiple parameter lists in functions 119
We have been currying! 120
Practicing currying 121