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ADVANCED KOTLIN TRICKS Ubiratan Soares September / 2018

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STDLIB : THE FORGOTTEN FUNCTIONS

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Repeat fun severalPrints() = repeat(times = 10) { print("Hey!") }

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/!" * Calls the specified function [block] with `this` value as its * argument and returns `this` value. #$ @kotlin.internal.InlineOnly @SinceKotlin("1.1") public inline fun T.also(block: (T) %& Unit): T { contract { callsInPlace(block, InvocationKind.EXACTLY_ONCE) } block(this) return this } Also

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fun validate(name: String) = name.length in 2'(20 fun saveUser(name: String) = validate(name) .also { saveWith(name) }

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TakeIf /!" * Returns `this` value if it satisfies the * given [predicate] or `null`, if it doesn't. #$ @kotlin.internal.InlineOnly @SinceKotlin("1.1") public inline fun T.takeIf(predicate: (T) %& Boolean): T? { contract { callsInPlace(predicate, InvocationKind.EXACTLY_ONCE) } return if (predicate(this)) this else null }

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TakeIf /!" * Returns `this` value if it satisfies the * given [predicate] or `null`, if it doesn't. #$ @kotlin.internal.InlineOnly @SinceKotlin("1.1") public inline fun T.takeIf(predicate: (T) %& Boolean): T? { contract { callsInPlace(predicate, InvocationKind.EXACTLY_ONCE) } return if (predicate(this)) this else null }

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TakeUnless /!" * Returns `this` value if it _does not_ satisfy * the given [predicate] or `null`, if it does. #$ @kotlin.internal.InlineOnly @SinceKotlin("1.1") public inline fun T.takeUnless(predicate: (T) %& Boolean): T? { contract { callsInPlace(predicate, InvocationKind.EXACTLY_ONCE) } return if (!predicate(this)) this else null }

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fun names() = if (Random().nextBoolean()) "Elvis" else "Presley" val shortName = names() .takeIf { it.length )* 5 } +, "Too long" val longName = names() .takeUnless { it.length )* 5 } +, "Too Short"

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INVOKING LIKE A PRO

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class DeepLinkParser { fun parse(uri : String) : ProductId { // Parse your URI and extract product id } }

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class DeepLinkHandler( val parser: DeepLinkParser) { operator fun invoke(url : String) : Intent { // Handle with parser and get your Intent } }

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class DeepLinkHandler( val parser: DeepLinkParser) { operator fun invoke(url : String) : Intent { // Handle with parser and get your Intent } }

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Invoking Instances val parser = DeepLinkParser() val handler = DeepLinkHandler(parser) val intent = handler("myapp:-.product/123456") // Launch your Activity

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Invoking Instances val parser = DeepLinkParser() val handler = DeepLinkHandler(parser) val intent = handler("myapp://product/123456") // Launch your Activity

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A tiny web DSL sealed class HTTPMethod object GET : HTTPMethod() object POST : HTTPMethod() class Status { var code: Int = 0 var payload: String = "" } class HttpRequest { var method: HTTPMethod = GET var body: String = "" var query: String = "" } class HttpResponse { var internalMessage: String = "" lateinit var status: Status }

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class Endpoint { lateinit var request: HttpRequest lateinit var response: HttpResponse fun handle() { -. TODO } }

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fun endpoint( path: String, block: Endpoint.() %& Unit) = Endpoint().apply { block() }.handle() endpoint("/api/product") { }

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class Endpoint { lateinit var request: HttpRequest lateinit var response: HttpResponse fun request(block: HttpRequest.() %& Unit) { request = HttpRequest().apply { block() } } fun response(block: HttpResponse.() %& Unit) { response = HttpResponse().apply { block() } } fun handle() { -. TODO } } endpoint("/api/product") { request { method = POST, body = "{ Some json }" } response { } }

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endpoint("/api/product") { request { method = POST, body = "{ Some json }" } response { status { code = 200 payload = "{ Some json }" } } } class HttpResponse { var internalMessage: String = "" lateinit var status: Status fun status(setup: Status.() %& Unit) { status = Status().apply { setup() } } }

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endpoint("/api/product") { request { method = POST, body = "{ Some json }" } response { status { code = 200 payload = "{ Some json }" } } } Can we do better ?

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class Endpoint { lateinit var request: HttpRequest lateinit var response: HttpResponse fun request(block: HttpRequest.() %& Unit) { request = HttpRequest().apply { block() } } fun response(block: HttpResponse.() %& Unit) { response = HttpResponse().apply { block() } } fun handle() { -. TODO } }

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class Endpoint { lateinit var request: HttpRequest lateinit var response: HttpResponse fun request(block: HttpRequest.() %& Unit) { request = HttpRequest().apply { block() } } fun response(block: HttpResponse.() %& Unit) { response = HttpResponse().apply { block() } } fun handle() { -. TODO } }

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class Endpoint { lateinit var request: HttpRequest lateinit var response: HttpResponse fun request(block: HttpRequest.() %& Unit) { request = HttpRequest().apply { block() } } fun response(block: HttpResponse.() %& Unit) { response = HttpResponse().apply { block() } } fun handle() { -. TODO } } class HttpResponse { var internalMessage: String = "" lateinit var status: Status fun status(setup: Status.() %& Unit) { status = Status().apply { setup() } } }

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class Endpoint { lateinit var request: HttpRequest lateinit var response: HttpResponse fun request(block: HttpRequest.() %& Unit) { request = HttpRequest().apply { block() } } fun handle() { -. TODO } } class HttpResponse { var internalMessage: String = "" lateinit var status: Status operator fun invoke( setup: Status.() -> Unit) { status = Status().apply { setup() } } }

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endpoint("/api/product") { request { method = POST body = "{ Some json }" } response { code = 200 payload = "{ Some json }" } } endpoint("/api/product") { request { method = POST, body = "{ Some json }" } response { status { code = 200 payload = "{ Some json }" } } } Before After

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http://shop.oreilly.com/product/0636920052999.do

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AWESOME COMPANIONS

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interface FormValidator { fun validate(name: String?): Boolean } class FormPresenter { private companion object : FormValidator { override fun validate(name: String?) = name/0isNotEmpty() +, throw IllegalArgumentException("Invalid Name") } }

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interface FormValidator { fun validate(name: String?): Boolean } class FormPresenter { private companion object : FormValidator { override fun validate(name: String?) = name/0isNotEmpty() +, throw IllegalArgumentException("Invalid Name") } }

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Fake Constructors val sampa = Location("23.5505° S", "46.6333° W") data class Location( val latitude: Float, val longitude: Float )

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data class Location( val latitude: Float, val longitude: Float) { companion object { operator fun invoke(lat: String, long: String) = Location(parse(lat), parse(long)) private fun parse(coordinate: String): Float { -. convert the String representation } } }

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data class Location( val latitude: Float, val longitude: Float) { companion object { operator fun invoke(lat: String, long: String) = Location(parse(lat), parse(long)) private fun parse(coordinate: String): Float { -. convert the String representation } } }

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NOTHING ELSE MATTERS

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The Nothing Type https://en.wikipedia.org/wiki/Bottom_type " In type theory, a theory within mathematical logic, the bottom type is the type that has no values. It is also called the zero or empty type, and is sometimes denoted with falsum (⊥). A function whose return type is bottom cannot return any value. In the Curry–Howard correspondence, the bottom type corresponds to falsity. "

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/!" * The type with only one value: the Unit object. * This type corresponds to the `void` type * in Java. #$ public object Unit { override fun toString() = "kotlin.Unit" }

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Kotlin Types Nothing SomeType Any Nothing? SomeType? Any?

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/!" * Always throws [NotImplementedError] * stating that operation is not implemented. #$ @kotlin.internal.InlineOnly public inline fun TODO(): Nothing = throw NotImplementedError()

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Nothing and flow control inline fun guard( receiver: T?, block: () %& Nothing): T { if (receiver 12 null) { block() } return receiver } https://gist.github.com/hzsweers/463500043b1a9708ba4e26c7ad1862fd

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Nothing and flow control inline fun guard( receiver: T?, block: () -> Nothing): T { if (receiver 12 null) { block() } return receiver } https://gist.github.com/hzsweers/463500043b1a9708ba4e26c7ad1862fd

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fun guarding() { val conference: String? = "Kotlin Summit" }

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fun guarding() { val conference: String? = "Kotlin Summit" val guarded = guard(conference) { return } }

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fun guarding() { val conference: String? = "Kotlin Summit" val guarded = guard(conference) { return } val guardedAgain = guard(conference) { throw IllegalStateException("Null konf") } }

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fun guarding() { val conference: String? = "Kotlin Summit" val guarded = guard(conference) { return } val guardedAgain = guard(conference) { throw IllegalStateException("Null konf") } val invalid = guard(conference) { -.Compiler error because not -.returned or thrown an exception! } }

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Nothing and Generics sealed class Tree class Node( val left: Tree, val right: Tree) : Tree() class Leaf(val value: T) : Tree() object Empty : Tree()

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val tree = Node( Leaf("1"), Node( Leaf("2"), Empty ) ) 1 2

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sealed class Result data class Value(val value: V) : Result() data class Error(val error: E) : Result()

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ADVANCING ON DELEGATION

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interface Validation { fun validate(input: String): Boolean } class NameValidator : Validation { override fun validate(input: String) = input.isNotEmpty() 34 input.length 56 2 } Class delegation

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val accepted = FormViewModel().validate("Bira") class FormViewModel : Validation by NameValidator() Class delegation

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class FormViewModel : FormValidator by Internal { companion object Internal : FormValidator { override fun validate(name: String?) = name/0isNotEmpty()+, false } } val rejected = FormViewModel().validate("Hi")

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class FormViewModel : FormValidator by Internal { companion object Internal : FormValidator { override fun validate(name: String?) = name/0isNotEmpty()+, false } } val rejected = FormViewModel().validate("Hi")

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interface HandleSuccess { fun showResult() } interface HandleError { fun showError() } interface LoginScreen : HandleSuccess, HandleError interface ProductScreen : HandleSuccess, HandleError

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interface HandleSuccess { fun showResult() } interface HandleError { fun showError() } interface LoginScreen : HandleSuccess, HandleError interface ProductScreen : HandleSuccess, HandleError

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Selective delegation fun performOperation(behavior: C) where C : HandleSuccess, C : HandleError { try { executeSomething() behavior.showResult() } catch (error: Throwable) { behavior.showError() } }

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Selective delegation fun performOperation(behavior: C) where C : HandleSuccess, C : HandleError { try { executeSomething() behavior.showResult() } catch (error: Throwable) { behavior.showError() } }

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PLUS, ULTRA !!!

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class CheckProfile { companion object : (Int) %& Boolean by ShouldBeLegal } val canDrink = CheckProfile(35) object ShouldBeLegal : (Int) %& Boolean { override fun invoke(age: Int) = age 5618 }

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What if I told you that extensions are not only for objects ???

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val checkNotDead: (Int) %& Boolean = { it < 150 } fun ((Int) %& Boolean).report(age: Int) = this.invoke(age) .also { print((if (it) "Ok" else "Not OK")) } fun misjudge(age: Int) = checkNotDead.report(age)

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Final Remarks Kotlin has it's secrets, go explore !!! Learn more hidden features for fun and profit

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https://speakerdeck.com/ubiratansoares

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UBIRATAN SOARES Computer Scientist by ICMC/USP Software Engineer, curious guy Google Developer Expert for Android / Kotlin Teacher, speaker, etc, etc

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THANK YOU @ubiratanfsoares ubiratansoares.github.io https://br.linkedin.com/in/ubiratanfsoares