Functional Programming on Android: is it possible?

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

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

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

"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

SO I CAN GO FUNCTIONAL WITH JAVA?

Lisp | John McCarthy and Steve Russell | 1958

class Car(var noOfDrivers: Int)

class Car(val noOfDrivers: Int)

"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

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

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

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

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

"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

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

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

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

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

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

enum class Operation { ADD, SUB }

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 } }

applyOperation(Operation.ADD) (1, 3)

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 ]

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

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

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

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

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

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; }

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

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

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

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

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

Functional Programming on Android: is it possible?

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