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

1. Let’s get Functional

2. @Dorvaryn
+BenjaminAugustin-Dorvaryn
Dorvaryn
Benjamin Augustin
Android Software Craftsman

3. What is Functional
Programming ?

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

5. ● Less code
● Expressive code
● Small units
What’s the point ?

6. Focus on what not how

7. Core concepts

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

12. ● Null object
● Optional
● Mandatory
Modeling absence

13. You can use most common languages

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 }

16. Let’s get Functional

17. fun foo(words: Array): Map {
val result = mapOf()
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): List> {
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, threshold: Int):
List {
return countOccurences(occurences).filter {
it.second > threshold
}.sortedBy {
it.second
}.map {
it.first
}
}
Extended Code

22. Focus on your data

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 ?

30. Fold ?

31. Maximum using fold
+
+
+
->
->
->

33. data class Basket(val items: List)
data class Item(val barcode: String, val price: Double)

34. data class Basket(val items: List)
data class Item(val barcode: String, val price: Double)
var total = 0.0
for (item in basket.items) {
total = total + item.price
}
}

35. data class Basket(val items: List)
data class Item(val barcode: String, val price: Double)
.fold(0.0, { total, item -> total + item.price })
}

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

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

38. data class Basket(val items: List)
data class Item(val barcode: String, val price: Double)
.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
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