Functional Programming in Scala in a Nutshell

Functional Programming
in Scala
in a Nutshell
1

Java is too Verbose
// construct a empty list
List<Integer> list = new ArrayList<Integer>();
list.add(1);
list.add(2);
list.add(3);
// introduce a counting index, i
for (int i = 0; i<list.size(); i++) {
Integer element = list.get(i);
System.out.println(element);
}
3

Scala is Concise
(1 :: 2 :: 3 :: Nil).foreach(println)
5

... in Elegant Functional Programming Way
val openerO = Some(Opener)
val wineO = Some(Wine(vintage = 1997))
val contentsO =
for {
opener ← openerO
wine ← wineO
} yield opener.open(wine)
// contentsO: Option[Contents] = Some(contentsOfWine)
// no null, no NullPointerException
7

... in Elegant Functional Programming Way
val openerO = Some(Opener)
val wineO = None
val contentsO =
for {
opener ← openerO
wine ← wineO
} yield opener.open(wine)
// contentsO: Option[Contents] = None
// no null, no NullPointerException
8

Scala supports OOP, too
// OOP polymorphism
val newOpener: Opener = new NormalOpener()
val oldOpener: Opener = new BrokenOpener()
val wine = new Wine()
println(newOpener.open(wine)) // contentsOfWine
println(oldOpener.open(wine)) // Exception occurs!
9

Scala is Compatible with Java
Compatible with Hadoop and Spark
→ Official language for the bigdata community
// joda time based on java
import org.joda.time.DateTime, java.util.Date
val jodaDt: DateTime = new DateTime(new Date())
val month = month = dt.getMonthOfYear()
10

… and Vice Versa
// scala code
object Person {
val MALE = "m";
}
// java code
public class App {
public static void main(String argv[]) {
Person$ person = Person$.MODULE$;
System.out.println(person.MALE());
}
}
11

What is Functional
Programming?
12

It’s NOT map and reduce
&
It’s NOT lambda functions
13

By Definition,
A function is called pure if all its inputs are declared as
inputs - none of them are hidden - and likewise all its
outputs are declared as outputs. 1
— Kris Jenkins
1
It is not a concrete definition but easy and intuitive.
14

Immutability
// value, not variable
val a = 1
a = 2 // error: reassignment to val
// list
val ints1: List[Int] = 1 :: 2 :: Nil
val ints2: List[Int] = ints1 :+ 3
println(ints1) // List(1, 2)
println(ints2) // List(1, 2, 3)
println(ints2 == 1 :: 2 :: 3 :: Nil) // true
15

Immutability
def fibonacci(n : Int): Int = {
// while loop requires temporary varables
var (a, b, i) = (0, 1, 0)
while( i < n ) {
val c = a + b
a = b
b = c
i = i + 1
}
return a
}
16

Immutability
// recursion doesn't requires temporary varables
def fibonacci(n : Int): Int = n match {
case 0 | 1 => n
case _ => fibonacci(n - 1) + fibonacci(n - 2)
}
17

Side Effect, by Definition
A function is said to have a side effect if it modifies
some state outside its scope or has an observable
interaction with its calling functions or the outside
world.
— Side effect (computer science), Wikipedia
18

Side Effect
// this def has a side effect
def currentProgram(guide: TVGuide, channel: Int): Program = {
val schedule = guide.getSchedule(channel)
schedule.programAt(new Date())
}
19

Side Effect
// now it has no more side effects
// and it is immutable
def program(guide: TVGuide, channel: Int, date: Date): Program = {
val schedule = guide.getSchedule(channel)
schedule.programAt(date)
}
20

Let's Start at the Beginning Again
A Program is functional iff it has no side effects.2
2
cf. Definition using Referential Transparency
21

Why Functional Programming?
4
!
Type declaration exposes its action
4
"
Short code & Fewer bugs
4
#
Extremely easy to write unit test
4
$
Easy to maintain
22

When should you use Functional
Programming?
4
!
If you want to develop a product that you need to
manage in the long run
4
"
If you want to write an elegant code
4
#
If you want to build up your project quickly
23

When shoud not you use Functional
Programming?
4
!
If you want to write a script to use once
4
⏳
If your app requires nano-fast performance
4
#
If you do not know about functional programming,
and you need to develop your app in two weeks
4
$
If you want to live by maintaining a program for
whole lifetime
24

Grammars
4 스칼라 학교!
4 스칼라 공식 도큐먼트 입문
4 Effective Scala by Twitter
4 Effective Scala by Heejong Lee
4 Hacker Rank for Tutorials
4 Exercism
28

What is Functional Programming?
4 함수형 프로그래밍이란 무엇인가?
4 어떤 프로그래밍 언어들이 함수형인가?
29

Textbooks & References
4
!
스칼라로 배우는 함수형 프로그래밍
4 Functional Programming Principles in Scala on
Coursera
4 Big Data Analysis with Scala and Spark on Coursera
4 Scala Excercises
30

Popular Scala Libraries
4
!
Scalaz: Scala library for functional programming
4 Cats: Lightweight, modular, and extensible library
for functional programming
4 Monix: Asynchronous Programming for Scala
4 Circe: A JSON library for Scala powered by Cats
31

Haskell
4 Eta Programming Language, Haskell on the JVM
4 (가장 쉬운) 하스켈 책 : 느긋하지만 우아하고 세련된 함수형 언어
32

Resources
4 Scastie, an interactive playground for Scala
33

Functional Programming in Scala in a Nutshell

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