Introduction to Functional Programing in Kotlin and Arrow

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Welcome

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Introduction to Functional Programing in Kotlin and Arrow Hadi Tok Google Developers Expert (Android) Technical Lead, @CitizenMe https://haditok.com/

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Functional Programming

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Functional Programming In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data.* *https://mauriziostorani.wordpress.com/2008/08/29/functional-programming-examples-methods-and-concepts/

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Functional Programming In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data.

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Functional Programming In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data.

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Mathematical Functions A function is a relation for which each value from the set the first components of the ordered pairs is associated with exactly one value from the set of second components of the ordered pair.

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Function { 1, 2, 3, 4} {A, B, C}

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Function { 1, 2, 3, 4} {A, B, C}

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Function { 1, 2, 3, 4} {A, B, C}

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Functional Programming In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data.

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State class MyClass{ var list = listOf(1,2,3,4,5) fun filterList() { list = list.filter { it%2 !=0 } } }

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State class MyClass{ val list = listOf(1,2,3,4,5) fun filterList(list:List): List { return list.filter { it!=0 } } }

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Functional Programming In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data.

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Mutable data fun filterList(list:List){ val iterator=list.iterator() for(i in iterator){ if(i%2==0){ iterator.remove() } } }

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Mutable data val filteredList = list.filter { it % 2 != 0 }

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Changes in state and data ● Who is the owner of the data? ● What would happen when data changes? ● Should other constructs using the data needs to be notified? ● How other constructs will be notified? ● What happens when there are multiple changes at the same time?

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Pure functions ● Its return value is the same for the same arguments ● Has no side effects

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Side effects ● Mutation of local static variables,non-local variables, mutable reference arguments ● Throwing an exception ● I/O operations

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Kotlin and Functional Programming

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Package Level Functions class Logger{ fun log(message: String){ println(message) } }

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Package Level Functions fun log(message: String){ println(message) }

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interface Callback{ fun onComplete() } fun operationWithACallBack(callback: Callback){ Thread.sleep(1000L) callback.onComplete() } Higher order Functions

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operationWithACallBack(object : Callback{ override fun onComplete() { println("completed") } }) Higher order Functions

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fun operationWithACallBack(callback: () -> Unit) { Thread.sleep(1000L) callback() } Higher order Functions

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operationWithACallBack ({ println("completed") }) Higher order Functions

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operationWithACallBack(){ println("completed") } Higher order Functions

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operationWithACallBack{ println("completed") } Higher order Functions

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data class Customer( val name: String, val city: String ) Higher order Functions

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fun printCustomerByCity(city: String) { val filteredCustomers = customers.filter { it.city == city } filteredCustomers.forEach { println(it) } } fun printCustomerByName(name: String) { val filteredCustomers = customers.filter { it.name == name } filteredCustomers.forEach { println(it) } } Higher order Functions

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fun printCustomers(predicate: (Customer) -> Boolean) { val filteredCustomer = customers.filter(predicate) filteredCustomers.forEach { println(it) } } fun main(args: Array) { printCustomers { it.city == "Istanbul" } printCustomers { it.name == "Hadi" } } Higher order Functions

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object StringUtils { fun getThirdCharacter(string: String): Char { if(string.length>2){ return string[2] } throw IllegalArgumentException() } } Extension Functions

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val thirdLetter = StringUtils.getThirdCharacter("Kotlin") Extension Functions

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fun String.getThirdCharacter(): Char { if(length>2){ return get(2) } throw IllegalArgumentException() } Extension Functions

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val thirdCharacter = "Kotlin".getThirdCharacter() Extension Functions

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list.forEach { println(it) } Extension Functions

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public inline fun Iterable.forEach(action: (T) -> Unit): Unit { for (element in this) action(element) } Extension Functions

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Arrow Λrrow is a library for Typed Functional Programming in Kotlin. https://arrow-kt.io

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Arrow Arrow aims to provide a lingua franca of interfaces and abstractions across Kotlin libraries. For this, it includes the most popular data types, type classes and abstractions such as Option, Either, IO, Functor, Applicative, Monad to empower users to write pure FP apps and libraries built atop higher order abstractions. https://arrow-kt.io

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fun tryGetThirdCharacter(string:String): Try { return Try { if(string.length<2){ throw IllegalArgumentException("string is short") }else{ string[3] } } } Try

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tryGetThirdCharacer("Kotlin") .getOrElse { println(it.message) } Try

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val thirdCharacter = tryGetThirdLetter("Go") .getOrDefault { "a" } Try

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tryGetThirdLetter("Kotlin") .fold( { println(it.message) }, { println("third char is $it") } ) Try

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fun getMessage(useResource: Boolean): Either { return if (useResource) { Either.right(R.string.start) } else { Either.left("Start") } } Either

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val textEither = getMessage(useResource) val text: String = when (textEither) { is Either.Left -> textEither.a is Either.Right -> getString(textEither.b) } Either

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fun divide(x: Int, y: Int): Option { return if (y == 0) { Option.empty() } else { Option.just(x / y) } } Option