Benjamin AUGUSTIN
June 05, 2015
250

# Let's get Functional

Ever tried functional languages ? Ever been annoyed that every time you see the same Fibonacci examples or yet another maths problem ?

So what can functional languages really teach us on our day to day life as Java devs ?
This talk will take some concrete examples and show you how some functional approaches will make your life easier.
We will see how we can try and bring functional and immutability to our "beloved" Android Java 7 and what this brings us.

Video of the presentation at Droidcon Berlin 2015 available here:

June 05, 2015

## Transcript

4. ### “In computer science, functional programming is a programming paradigm that

treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data.” - Wikipedia contributors, 2015

the point ?

8. ### • Not mutable, unchangeable, changeless • Modification -> new value

• Safe to cache Immutability
9. ### • Same input -> Same output • No side effect

• Referential transparency Pure functions
10. ### • Not a requirement but helps … a lot •

Building your domain • Dealing with absence of data Strongly typed
11. ### “I call it my billion-dollar mistake. It was the invention

of the null reference in 1965.” - Sir Tony Hoare, 2009

14. ### • RxJava • AutoValue • Guava • Or… Kotlin ?

For java 7 and lower
15. ### Kotlin Syntax fun aFunction(parameter: String): String { return parameter }

fun highOrder(param: Int, func: (Int) -> Int): Int { return func(param) } val lambda = { x: Int, y: Int -> x + y }

17. ### fun foo(words: Array<String>): Map<String, Int> { val result = mapOf<String,

Int>() for (word in words) { val value = result.get(word) if (value != null) { result.set(word, value + 1) } else { result.set(word, 1) } } return result } What does foo do ?
18. ### fun foo(words: Array<String>): List<Pair<String, Int>> { return words.groupBy { word

-> word }.map { Pair(it.key, it.value.count()) } } Can you tell now ?
19. ### • Code should be self explanatory • Not just the

method name • Keep things concise Expressive code
20. ### • Get items occurring at least 3 times • Sort

by occurrences • As a list of strings Extensible code
21. ### fun mostFrequents(occurences: Array<String>, threshold: Int): List<String> { return countOccurences(occurences).filter {

it.second > threshold }.sortedBy { it.second }.map { it.first } } Extended Code

23. ### • Article, Product, User... • Value objects -> Immutable •

Lists of value objects What is your data ?
24. ### • Fetching a list of users • Calculating the price

of a basket • Input List -> Output List What is your processing ?
25. ### Think like shell #!/bin/bash for jpg; do png="\${jpg%.jpg}.png" if convert

"\$jpg" jpg.to.png ; then mv jpg.to.png "\$png" else echo 'error' >&2 exit 1 fi done echo all conversions successful exit 0 du -h | grep M | sort -nr | head -15
26. ### • map, flatMap, filter, findAll, merge, zip….. • first, last,

find, fold..… • Decompose your processing using those functions Functions upon lists
27. ### • Display the saved articles in a list • Fetch

the items in a basket requiring age check • Any filtering logic -> list + filter Don’t trust me ?
28. ### • Get the total of a basket • Find an

item in a list • Any list traversal logic -> list + fold Another one ?
29. ### • f(x,y) applied on each element in a list •

x element in list • y accumulator Fold ?

33. ### data class Basket(val items: List<Item>) data class Item(val barcode: String,

val price: Double)

36. ### data class Basket(val items: List<Item>) data class Item(val barcode: String,

val price: Double) fun calculateBasket(basket: Basket): Double { return basket.items .fold(0.0, sum()) } private fun sum() = { total:Double, item: Item -> total + item.price }
37. ### data class Basket(val items: List<Item>) data class Item(val barcode: String,

val price: Double) fun calculateBasket(basket: Basket): Double { var total = 0.0 for (item in basket.items) { total = sum(item, total) } return total } private fun sum(item: Item, total: Double) = { total + item.price }
38. ### data class Basket(val items: List<Item>) data class Item(val barcode: String,

val price: Double) fun calculateBasket(basket: Basket): Double { return basket.items .fold(0.0, sum()) } private fun sum() = { total:Double, item: Item -> total + item.price }
39. ### Make your data immutable Start small Aim for pure functions

Validate your data Focus on what not how
40. ### Learn a functional language Keep learning Spend some time on

pet projects Have fun !
41. ### • Learn you a Haskell for great good by Miran

Lipovača • Functional Programming in Java by Venkat Subramaniam • Category Theory for Programmers by Bartosz Milewski Good reads