Functional Programming on Android: is it possible?

@ TDC São Paulo 2017

Transcript

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