The document discusses whether functional programming is possible on Android. It begins by introducing the speaker and includes their contact information. It then provides examples of functional programming concepts like mapping, filtering, reducing collections in Kotlin. It discusses concepts like immutability, pure functions, and function composition. It also covers functional programming operators in Kotlin and examples of transforming imperative code to functional code. Recursion and tail recursion are explained. The document concludes by providing a bibliography of resources for learning more about functional programming.

1. Functional Programming on Android: is it possible?

2. About
• Android Developer @ M4U
• Organizer @ Kotlin Rio Meetup
Github: https://github.com/lalbuquerque
Linkedin: www.linkedin.com/in/lucasalbuquerque
Email: lucas.albuquerque12@gmail.com

3. TALK “FUNCTIONAL PROGRAMMING ON ANDROID: IS IT POSSIBLE?”
someData.map(it + 2)
.filter(it > 10)
.sort(…)
.reduce(…)
.group(…)
.flatMap(…)
.just(…)
.jointTo(…)
.fold(…)
.slice(…)
.takeLastWhile(…)
EXPECTATION
data class MyObject(val name: String)
REALITY

6. https://msdn.microsoft.com/en-us/library/hh242985(v=vs.103).aspx

7. "Functional code is characterised by one thing: the absence of
side effects. It doesn’t rely on data outside the current
function, and it doesn’t change data that exists outside the
current function. Every other “functional” thing can be derived
from this property. Use it as a guide rope as you learn.”
- - Mary Rose Cook
- https://maryrosecook.com/blog/post/a-practical-introduction-to-functional-
programming

8. SO I CAN GO FUNCTIONAL
WITH JAVA?

9. Lisp | John McCarthy and Steve Russell | 1958

10. Immutability

11. Read-Modify-Write

12. class Car(var noOfDrivers: Int)

14. class Car(val noOfDrivers: Int)

16. Using functions right

18. "A pure function is a function that, given the same input, will
always return the same output and does not have any
observable side effect."
- Professor Franklin Risby's
- Mostly Adequate Guide to Functional Programming

19. fun add(x: Int): Int {
val y: Int = readNumFromFile()
return x + y
}
Impure

20. fun add(x: Int, y: Int) = x + y
Pure

21. fun plus2(x: Int): Int {
val y = x + 2
cache(y)
return y
}
Impure

22. fun plus2(x: Int) = x + 2
Pure

23. "The name of a variable, function or class should answer the
big questions such as why it exists, what it does, how it is
used."
Aiden Mc Raft commenting Robert C. Martin’s Clean Code: A
Handbook of Agile Software Craftsmanship

24. Demystifying functions

25. fun add(x: Int, y: Int) = x + y

26. val add = fun add(x: Int, y: Int) = x + y

27. fun doSomething(function: (Int, Int) -> Int) {
…
}

28. data class User(val name: String,
val amount: Int,
val onClick: (User) -> Unit)

29. with(user) {
holder.itemView.setOnClickListener { onClick(this) }
}

30. enum class Operation { ADD, SUB }

31. fun applyOperation(operation: Operation): (Int, Int) -> Int {
val add = fun (x: Int, y: Int) = x + y
val sub = fun (x: Int, y: Int) = x - y
when (operation) {
Operation.ADD -> return add
Operation.SUB -> return sub
}
}

32. applyOperation(Operation.ADD) (1, 3)

33. Functional Operators

34. public inline fun <T, R> …Iterable<T>.map(transform: (T) -> (R)): …List<R>
val myList = listOf(2, 4, 6, 8, 10, 12)
val myNewList = myList.map { it * 2 }
// [ 4, 8, 12, 16, 20, 24 ]

35. none(…)
val myList = listOf(2, 4, 6, 8, 10, 12)
myList.none { it % 7 == 0 }
// true

36. filter(…)
val myList = listOf(4, 8, 12, 16, 20, 24)
myList.filter { it > 20 }
// [ 24 ]

37. forEach(…)
val myList = listOf(2, 4, 6, 8, 10, 12)
myList.forEach { Log.i(TAG, it.toString() }

38. fun returnBiggerNames(names: MutableList<String>): MutableList<String> {
names
.filter { it.length < 10 }
.forEach { names.remove(it) }
return names
}
Impure

39. fun returnBiggerNames(names: List<String>) = names.filter { it.length > 10 }
Pure

40. public List<ContributorCount> getContributorCounts(List<ArticleView> articleViews) {
HashMap<String, Integer> contributorCounts = new HashMap<String, Integer>();
for (ArticleView articleView : articleViews) {
Integer count = contributorCounts.get(articleView.contributor);
if (count == null)
contributorCounts.put(articleView.contributor, 1);
else
contributorCounts.put(articleView.contributor, count + 1);
}
List<ContributorCount> result = new ArrayList<ContributorCount>();
for (String contributor : contributorCounts.keySet()) {
int count = contributorCounts.get(contributor);
if (count > 1)
result.add(new ContributorCount(contributor, contributorCounts.get(contributor)));
}
return result;
}

41. fun getContributorCounts(views: List<ArticleView>) = views.groupBy { it.contributor }
.mapValues { it.value.size }
.filter { it.value > 1 }
.map { ContributorCount(it.key, it.value) }

42. Recursion

43. fun cosFixpoint(x: Double = 1.0): Double {
while (true) {
val y = Math.cos(x)
if (x == y) return y
x = y
}
}

44. tailrec fun cosFixpoint(x: Double = 1.0): Double {
val y = Math.cos(x)
return if (x == y) x else cosFixpoint(y)
}

45. tailrec
fun f(x: Double = 1.0): Double = if (x == cos(x)) x else f(cos(x)))

46. Thank you!

47. Bibliography
1. https://drboolean.gitbooks.io/mostly-adequate-guide
2. https://www.theguardian.com/info/developer-blog/2014/dec/
11/functional-android
3. https://medium.com/@anupcowkur/functional-programming-
for-android-developers-part-3-f9e521e96788
4. https://blog.plan99.net/kotlin-fp-3bf63a17d64a