Daniela Sfregola
May 14, 2021
120

# ScalaBase 2021- FP: the Good, the Bad, the Ugly

You are about to fall in love with Functional Programming, if not already. You are going to learn the good parts that are going to make your day-to-day life easier. But since nobody is perfect - not even FP -, you are also going to see its bad and ugly parts, and you'll discover how to deal with them: from learning challenges to performance issues on the JVM.

May 14, 2021

## Transcript

2021
2. ### Hellooooo • Software Engineer   living in London UK •

Former Java Developer • Scala Lover ❤ • OS Maintainer • Author of   "Get Programming with Scala" by Manning    35% OFF with code "ctwscalabase21"
3. ### • Rome, Cinecitta'   23 December 1966 • Director  Sergio

Leone • Music  Ennio Morricone
4. ### Everything in Life • Good Parts  the bits that we

like • Bad Parts  the traps to avoid • Ugly Parts  the parts that we need to live with

7. ### Immutability private var i: Int = 0 while (i <

10) { // do stuff here i = i + 1 } OOP
8. ### Immutability private var i: Int = 0 while (i <

10) { // do stuff here i = i + 1 } OOP (0 until 10).foreach { i => // do stuff here } FP
9. ### Immutability private var i: Int = 0 while (i <

10) { i = i * 2 println(i) i = i + 1 } OOP (0 until 10).foreach { i => println(i * 2) } FP

value
12. ### Is this Pure? import java.time.ZonedDateTime def plusDays(n: Int): ZonedDateTime =

ZonedDateTime.now().plusDays(n) - Referentially Transparent? - Total?
13. ### Is this Pure? import java.time.ZonedDateTime def plusDays(n: Int): ZonedDateTime =

ZonedDateTime.now().plusDays(n) - Referentially Transparent? ⛔ - Total? ✅
14. ### Is this Pure? def div(a: Int, b: Int): Int =

a / b - Referentially Transparent? - Total?
15. ### Is this Pure? def div(a: Int, b: Int): Int =

a / b - Referentially Transparent? ✅ - Total? ⛔
16. ### Is this Pure? def sum(a: Int, b: Int): Int =

a + b - Referentially Transparent? - Total?
17. ### Is this Pure? def sum(a: Int, b: Int): Int =

a + b - Referentially Transparent? ✅ - Total? ✅

} Impure
19. ### Purity def foo(s: String): Unit = { // implementation here

} Impure def foo(s: String): Int = { // implementation here } Pure

25. ### You do not need to know Category Theory to write

good   Functional code

Endofunctors

36. ### Case Class to Array def toArray(product: Product): Array[Any] = {

product // Product .productIterator // Iterator[Any] .toArray // Array[Any] } FP
37. ### Case Class to Array def toArray(product: Product): Array[Any] = {

product // Product .productIterator // Iterator[Any] .toArray // Array[Any] } def toArray(product: Product): Array[Any] = { val size = product.productArity val arr = new Array[Any](size) var i = 0 while (i < size) { arr(i) = product.productElement(i) i = i + 1 } arr } FP OOP
38. ### Benchmarking* import com.danielasfregola.randomdatagenerator.RandomDataGenerator._ def avgRunTime[A](f: => A, n: Int =

1000): Double = { val start = Instant.now.toEpochMilli (0 until n).foreach(_ => f) val end = Instant.now.toEpochMilli val diff = end - start diff / n.toDouble } case class BigExample(f1: String, f2: Int, f3: Long, f4: Char, f5: String, f6: String, f7: Int, f8: Long, f9: Char, f10: String, f11: String, f12: Int, f13: Long, f14: Char, f15: String, f16: String, f17: Int, f18: Long, f19: Char, f20: String, f21: String, f22: Int, f23: Long, f24: Char, f25: String, f26: String, f27: Int, f28: Long, f29: Char, f30: String) val big = random[BigExample] avgRunTime(toArrayFP(big)) avgRunTime(toArrayOOP(big)) *not a serious benchmarking, executed on my laptop
39. ### Benchmarking* import com.danielasfregola.randomdatagenerator.RandomDataGenerator._ def avgRunTime[A](f: => A, n: Int =

1000): Double = { val start = Instant.now.toEpochMilli (0 until n).foreach(_ => f) val end = Instant.now.toEpochMilli val diff = end - start diff / n.toDouble } case class BigExample(f1: String, f2: Int, f3: Long, f4: Char, f5: String, f6: String, f7: Int, f8: Long, f9: Char, f10: String, f11: String, f12: Int, f13: Long, f14: Char, f15: String, f16: String, f17: Int, f18: Long, f19: Char, f20: String, f21: String, f22: Int, f23: Long, f24: Char, f25: String, f26: String, f27: Int, f28: Long, f29: Char, f30: String) val big = random[BigExample] avgRunTime(toArrayFP(big)) avgRunTime(toArrayOOP(big)) *not a serious benchmarking, executed on my laptop toArrayOOP avg execution: 0.002 ms toArrayFP avg execution: 0.028 ms
40. ### JVM and FP do not always go along... OOP version

is much faster than the FP one!
41. ### My ideal implementation of map function for List[A] final override

def map[B](f: A => B): List[B] = { @tailrec def loop(l: List[A], res: List[B]): List[B] = { l match { case head :: tail => loop(tail, res :+ f(head)) case Nil => res } } loop(this, res = Nil) }
42. ### The real implementation of map function for List[A]* final override

def map[B](f: A => B): List[B] = { if (this eq Nil) Nil else { val h = new ::[B](f(head), Nil) var t: ::[B] = h var rest = tail while (rest ne Nil) { val nx = new ::(f(rest.head), Nil) t.next = nx t = nx rest = rest.tail } releaseFence() h } } *scala 2.13.0
43. ### If you have a really good reason, break the rules!

...but keep the ugly bits isolated
44. ### Thank You! •Twitter: @DanielaSfregola •"Get Programming with Scala"   by

Manning    35% OFF with code "ctwscalabase21"