Kotlin’s Mind Blowers

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Kotlin’s Mind Blowers Eugenio Marletti @workingkills

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@Deprecated("Just don't.") fun deprecatedFun(age: Int, name: String) { !" nasty stuff } data class Name(val name: String) data class Age(val age: Int) fun niceFun(name: Name, age: Age) { !" good stuff } fun usage(myName: String, myAge: Int) { !" ⚠ 'deprecatedFun(Int, String): Unit' is deprecated. Just Don't. deprecatedFun(myAge, myName) }

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@Deprecated("Just don't.", level = DeprecationLevel.WARNING) fun deprecatedFun(age: Int, name: String) { !" nasty stuff } data class Name(val name: String) data class Age(val age: Int) fun niceFun(name: Name, age: Age) { !" good stuff } fun usage(myName: String, myAge: Int) { !" ⚠ 'deprecatedFun(Int, String): Unit' is deprecated. Just Don't. deprecatedFun(myAge, myName) }

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@Deprecated("Just don't.", level = DeprecationLevel.ERROR) fun deprecatedFun(age: Int, name: String) { !" nasty stuff } data class Name(val name: String) data class Age(val age: Int) fun niceFun(name: Name, age: Age) { !" good stuff } fun usage(myName: String, myAge: Int) { !" ❌ Using 'deprecatedFun(Int, String): Unit' is an error. Just Don't. deprecatedFun(myAge, myName) }

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@Deprecated("Just don't.", level = DeprecationLevel.HIDDEN) fun deprecatedFun(age: Int, name: String) { !" nasty stuff } data class Name(val name: String) data class Age(val age: Int) fun niceFun(name: Name, age: Age) { !" good stuff } fun usage(myName: String, myAge: Int) { !" ❌ Unresolved reference: deprecatedFun deprecatedFun(myAge, myName) }

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@Deprecated("Just don't.", ReplaceWith("niceFun(Name(name), Age(age))")) fun deprecatedFun(age: Int, name: String) { !" nasty stuff } data class Name(val name: String) data class Age(val age: Int) fun niceFun(name: Name, age: Age) { !" good stuff } fun usage(myName: String, myAge: Int) { !" # deprecatedFun(myAge, myName) }

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typealias TotallyNotThatClass = ThatClass

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typealias AbsSubject = Subject

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class BoringClass { fun washDishes() = "maybe later" } fun usage() { BoringClass().washDishes() }

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class ExcitingClass { fun washDishes() = "maybe later" } fun usage() { BoringClass().washDishes() }

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class ExcitingClass { fun washDishes() = "maybe later" } fun usage() { !" ❌ Unresolved reference: BoringClass BoringClass().washDishes() }

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@Deprecated("Stop being boring.") typealias BoringClass = ExcitingClass class ExcitingClass { fun washDishes() = "maybe later" } fun usage() { !" ⚠ 'typealias BoringClass = ExcitingClass' is deprecated. !" Stop being boring. BoringClass().washDishes() }

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@Deprecated("Stop being boring.", ReplaceWith("ExcitingClass")) typealias BoringClass = ExcitingClass class ExcitingClass { fun washDishes() = "maybe later" } fun usage() { !" ⚠ 'typealias BoringClass = ExcitingClass' is deprecated. !" Stop being boring. BoringClass().washDishes() }

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package first fun topLevelFunction(i: Int) = i fun topLevelFunction(s: String) = s

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package first fun topLevelFunction(i: Int) = i fun topLevelFunction(s: String) = s ----------------------------------- package second import first.topLevelFunction

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package my.package data class ColorInt(val color: Int)

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package my.package import android.support.annotation.ColorInt data class ColorInt(@ColorInt val color: Int)

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package my.package import android.support.annotation.ColorInt data class ColorInt(@ColorInt val color: Int) --------------------------------------------- package not.my.package !" ❌ Conflicting import, imported name 'ColorInt' is ambiguous import android.support.annotation.ColorInt import my.package.ColorInt

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package my.package import android.support.annotation.ColorInt data class ColorInt(@ColorInt val color: Int) --------------------------------------------- package not.my.package !" ✅ import android.support.annotation.ColorInt as ColorIntAnnotation import my.package.ColorInt

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package my.package import android.support.annotation.ColorInt data class ColorInt(@ColorInt val color: Int) --------------------------------------------- package not.my.package !" ✅ import android.support.annotation.ColorInt as ColorIntAnnotationDuplicate import android.support.annotation.ColorInt as ColorIntAnnotation import my.package.ColorInt

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fun oppaFunctionalStyle(list: List) = list.map { it.toCharArray().map { if (it.isDigit()) return@map CharArray(0) !" which map? % else it.toUpperCase() } }

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fun oppaFunctionalStyle(list: List) = list.map listMap@ { it.toCharArray().map charMap@ { if (it.isDigit()) return@listMap CharArray(0) !" that map! & else it.toUpperCase() } }

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fun stringLengthOrNull(maybeString: Any?) = if (maybeString is String) maybeString.length else null

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fun stringLengthOrNull(maybeString: Any?) = (maybeString as? String)!#length

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fun printEverything(vararg args: String) { println(args) } fun usage(stringArray: Array) { printEverything(*stringArray) }

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fun printEverything(vararg args: String) { println(args) } fun usage( singleString: String, anotherSingleString: String, stringArray: Array, anotherStringArray: Array) { printEverything(*stringArray) }

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fun veryDescriptiveFun( arg1: String, arg2: Int, arg3: Int, arg4: Boolean ) = "meh"

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fun veryDescriptiveFun( arg1: String, arg2: Int = arg1.length, arg3: Int = arg2, arg4: Boolean ) = "cool"

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fun veryDescriptiveFun( arg1: String, arg2: Int = arg1.length, arg3: Int = arg2, arg4: Boolean = loadEntireDbInMemory()!#isEmpty() !& areYouSure() ) = "oh no"

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fun iterate(iterable: Iterable) { iterable .onEach { println(it) } .map { it.length } .forEach { println(it) } } fun sequentiate(sequence: Sequence) { sequence .onEach { println(it) } .map { it.length } .forEach { println(it) } } fun usage() { val strings = listOf("a", "bb", "ccc") iterate(strings.asIterable()) sequentiate(strings.asSequence()) }

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fun iterate(iterable: Iterable) { !" iterable !" .onEach { println(it) } !" .map { it.length } !" .forEach { println(it) } !" } !" fun sequentiate(sequence: Sequence) { !" sequence !" .onEach { println(it) } !" .map { it.length } !" .forEach { println(it) } !" } !" fun usage() { val strings = listOf("a", "bb", "ccc") iterate(strings.asIterable()) sequentiate(strings.asSequence()) }

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fun iterate(iterable: Iterable) { !" a iterable !" bb .onEach { println(it) } !" ccc .map { it.length } !" 1 .forEach { println(it) } !" 2 } !" 3 fun sequentiate(sequence: Sequence) { !" sequence !" .onEach { println(it) } !" .map { it.length } !" .forEach { println(it) } !" } !" fun usage() { val strings = listOf("a", "bb", "ccc") iterate(strings.asIterable()) sequentiate(strings.asSequence()) }

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fun iterate(iterable: Iterable) { !" a iterable !" bb .onEach { println(it) } !" ccc .map { it.length } !" 1 .forEach { println(it) } !" 2 } !" 3 fun sequentiate(sequence: Sequence) { !" a sequence !" 1 .onEach { println(it) } !" bb .map { it.length } !" 2 .forEach { println(it) } !" ccc } !" 3 fun usage() { val strings = listOf("a", "bb", "ccc") iterate(strings.asIterable()) sequentiate(strings.asSequence()) }

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void

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void !" not a type

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void !' Unit

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void !' Unit “In type theory, a unit type is a type that allows only one value.”

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void !' Unit !" single instance

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fun unity() { }

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fun unity(): Unit { }

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fun unity() = Unit

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fun unity() = println("sweet")

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fun unity() = java.lang.System.out.println("sweet")

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Void

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Void !" a type that can’t be instantiated

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Void !' Nothing

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Void !' Nothing “In type theory, the bottom type is the type that has no values.” bottom zero empty

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Void !' Nothing !" a type that can’t be instantiated (same!)

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Void !' Nothing?

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fun foreverNull(): Nothing? = null

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fun inifinite() { while (true) { !" nap } } fun naive() { inifinite() !" always believe in yourself!⚠ println("I've achieved all my hopes and dreams!") }

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fun inifinite(): Nothing { while (true) { !" nap } } fun naive() { inifinite() !" ⚠ Unreachable code println("I've achieved all my hopes and dreams!") }

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fun fiveMoreMinutesPlease(): VeryComplexType

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fun fiveMoreMinutesPlease(): VeryComplexType = TODO()

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fun fiveMoreMinutesPlease(): VeryComplexType = TODO() fun TODO(): Nothing = throw NotImplementedError()

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data class Person( val name: String, val age: Int, val likesDataClasses: Boolean)

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data class Person( val name: String, val age: Int, val likesDataClasses: Boolean) fun parsePerson(map: Map): Person? { val name = map["name"] if (name is String) { val age = map["age"] if (age is Int) { val likesDataClasses = map["likes-data-classes"] if (likesDataClasses is Boolean) { return Person( name, age, likesDataClasses) } } } return null }

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data class Person( val name: String, val age: Int, val likesDataClasses: Boolean) fun parsePerson(map: Map): Person? { val name = map["name"] if (name !is String) return null val age = map["age"] if (age !is Int) return null val likesDataClasses = map["likes-data-classes"] if (likesDataClasses !is Boolean) return null return Person( name, age, likesDataClasses) }

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data class Person( val name: String, val age: Int, val likesDataClasses: Boolean) fun parsePerson(map: Map): Person? { val name = map["name"] as? String !& return null val age = map["age"] as? Int !& return null val likesDataClasses = map["likes-data-classes"] as? Boolean !& return null return Person( name, age, likesDataClasses) }

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data class Person( val name: String, val age: Int, val likesDataClasses: Boolean) fun parsePerson(map: Map): Person? { return Person( map["name"] as? String !& return null, map["age"] as? Int !& return null, map["likes-data-classes"] as? Boolean !& return null) }

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!" Elvis operator !&

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!& "hey"

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a += b a.plusAssign(b) a -= b a.minusAssign(b) a!* a.inc() a!, a.dec() a + b a.plus(b) a - b a.minus(b) a * b a.times(b) a / b a.div(b) a % b a.rem(b) a!-b a.rangeTo(b) +a a.unaryPlus() -a a.unaryMinus() !a a.not()

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a[i] a.get(i) a[i_1, !!., i_n] a.get(i_1, !!., i_n) a += b a.plusAssign(b) a -= b a.minusAssign(b) a *= b a.timesAssign(b) a !/ b a.divAssign(b) a %= b a.remAssign(b) a!* a.inc() a!, a.dec() a + b a.plus(b) a - b a.minus(b) a * b a.times(b) a / b a.div(b) a % b a.rem(b) a!-b a.rangeTo(b) +a a.unaryPlus() -a a.unaryMinus() !a a.not()

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a > b a.compareTo(b) > 0 a < b a.compareTo(b) < 0 a !0 b a.compareTo(b) !0 0 a !1 b a.compareTo(b) !1 0 a[i] a.get(i) a[i_1, !!., i_n] a.get(i_1, !!., i_n) a[i] = b a.set(i, b) a[i_1, !!., i_n] = b a.set(i_1, !!., i_n, b) a += b a.plusAssign(b) a -= b a.minusAssign(b) a *= b a.timesAssign(b) a !/ b a.divAssign(b) a %= b a.remAssign(b) a!* a.inc() a!, a.dec() a + b a.plus(b) a - b a.minus(b) a * b a.times(b) a / b a.div(b) a % b a.rem(b) a!-b a.rangeTo(b) !a a.not()

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a() a.invoke() a(i) a.invoke(i) a in b b.contains(a) a !in b !b.contains(a) a > b a.compareTo(b) > 0 a < b a.compareTo(b) < 0 a !0 b a.compareTo(b) !0 0 a !1 b a.compareTo(b) !1 0 a[i] a.get(i) a[i_1, !!., i_n] a.get(i_1, !!., i_n) a[i] = b a.set(i, b) a[i_1, !!., i_n] = b a.set(i_1, !!., i_n, b) a += b a.plusAssign(b) a -= b a.minusAssign(b) a *= b a.timesAssign(b) a !/ b a.divAssign(b) a %= b a.remAssign(b) a - b a.minus(b) a * b a.times(b) a / b a.div(b) a % b a.rem(b) a!-b a.rangeTo(b)

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a() a.invoke() a(i) a.invoke(i) a(i_1, !!., i_n) a.invoke(i_1, !!., i_n) a in b b.contains(a) a !in b !b.contains(a) a > b a.compareTo(b) > 0 a < b a.compareTo(b) < 0 a !0 b a.compareTo(b) !0 0 a !1 b a.compareTo(b) !1 0 a[i] a.get(i) a[i_1, !!., i_n] a.get(i_1, !!., i_n) a[i] = b a.set(i, b) a[i_1, !!., i_n] = b a.set(i_1, !!., i_n, b) a += b a.plusAssign(b) a -= b a.minusAssign(b) a *= b a.timesAssign(b) a !/ b a.divAssign(b) a %= b a.remAssign(b)

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a() a.invoke() a(i) a.invoke(i) a(i_1, !!., i_n) a.invoke(i_1, !!., i_n) a in b b.contains(a) a !in b !b.contains(a) a > b a.compareTo(b) > 0 a < b a.compareTo(b) < 0 a !0 b a.compareTo(b) !0 0 a !1 b a.compareTo(b) !1 0 a[i_1, !!., i_n] a.get(i_1, !!., i_n) a[i] = b a.set(i, b) a[i_1, !!., i_n] = b a.set(i_1, !!., i_n, b)

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getValue, setValue, provideDelegate val someValue by lazy { "don't judge me" }

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Kotlin tools team lead @ JetBrains co-author of “Kotlin in Action” @intelliyole on Twitter “Dmitry Jemerov” in “real life” yole

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“ ” “ ” Kotlin tools team lead @ JetBrains co-author of “Kotlin in Action” @intelliyole on Twitter “Dmitry Jemerov” in “real life” yole

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bit.ly/kotlin-mind-blowers Eugenio Marletti @workingkills