Kotlin’s Mind Blowers

A curated collection of authentic “ohhhhh”, “can’t believe it works”, and “WAIT YOU CAN DO THAT” experiences, evoked by some of the most disruptive properties of the Kotlin language, slowly refined through an artisanal all-natural process. [ gluten free / may contain traces of nuts ]

Presented at Kotlin Night Berlin 2018
https://www.meetup.com/kotlin-berlin/events/247865835/

--> Second part: https://bit.ly/kotlin-mind-blowers2

1. Kotlin’s Mind Blowers Eugenio Marletti @workingkills
4. @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) }
5. @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) }
6. @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) }
7. @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) }
8. @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) }
9. @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) }
10. @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) }
11. @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) { !" ✅ niceFun(Name(myName), Age(myAge)) }
12. typealias TotallyNotThatClass = ThatClass
13. typealias AbsSubject<T> = Subject<T, T>
14. class BoringClass { fun washDishes() = "maybe later" } fun usage() { BoringClass().washDishes() }
15. class ExcitingClass { fun washDishes() = "maybe later" } fun usage() { BoringClass().washDishes() }
16. class ExcitingClass { fun washDishes() = "maybe later" } fun usage() { !" ❌ Unresolved reference: BoringClass BoringClass().washDishes() }
17. @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() }
18. @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() }
19. package first fun topLevelFunction(i: Int) = i fun topLevelFunction(s: String) = s
20. package first fun topLevelFunction(i: Int) = i fun topLevelFunction(s: String) = s ----------------------------------- package second import first.topLevelFunction
21. package my.package data class ColorInt(val color: Int)
22. package my.package import android.support.annotation.ColorInt data class ColorInt(@ColorInt val color: Int)
23. 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
24. 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
25. 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
26. fun oppaFunctionalStyle(list: List<String>) = list.map { it.toCharArray().map { if (it.isDigit()) return@map CharArray(0) !" which map? % else it.toUpperCase() } }
27. fun oppaFunctionalStyle(list: List<String>) = list.map listMap@ { it.toCharArray().map charMap@ { if (it.isDigit()) return@listMap CharArray(0) !" that map! & else it.toUpperCase() } }
28. fun stringLengthOrNull(maybeString: Any?) = if (maybeString is String) maybeString.length else null
29. fun stringLengthOrNull(maybeString: Any?) = (maybeString as? String)!#length
30. fun printEverything(vararg args: String) { println(args) } fun usage(stringArray: Array<String>) { printEverything(*stringArray) }
31. fun printEverything(vararg args: String) { println(args) } fun usage( singleString: String, anotherSingleString: String, stringArray: Array<String>, anotherStringArray: Array<String>) { printEverything(*stringArray) }
32. fun printEverything(vararg args: String) { println(args) } fun usage( singleString: String, anotherSingleString: String, stringArray: Array<String>, anotherStringArray: Array<String>) { printEverything( singleString, *stringArray, anotherSingleString, *anotherStringArray) }
33. fun veryDescriptiveFun( arg1: String, arg2: Int, arg3: Int, arg4: Boolean ) = "meh"
34. fun veryDescriptiveFun( arg1: String, arg2: Int = arg1.length, arg3: Int = arg2, arg4: Boolean ) = "cool"
35. fun veryDescriptiveFun( arg1: String, arg2: Int = arg1.length, arg3: Int = arg2, arg4: Boolean = loadEntireDbInMemory()!#isEmpty() !& areYouSure() ) = "oh no"
36. fun iterate(iterable: Iterable<String>) { iterable .onEach { println(it) } .map { it.length } .forEach { println(it) } } fun sequentiate(sequence: Sequence<String>) { sequence .onEach { println(it) } .map { it.length } .forEach { println(it) } } fun usage() { val strings = listOf("a", "bb", "ccc") iterate(strings.asIterable()) sequentiate(strings.asSequence()) }
37. fun iterate(iterable: Iterable<String>) { !" iterable !" .onEach { println(it) } !" .map { it.length } !" .forEach { println(it) } !" } !" fun sequentiate(sequence: Sequence<String>) { !" sequence !" .onEach { println(it) } !" .map { it.length } !" .forEach { println(it) } !" } !" fun usage() { val strings = listOf("a", "bb", "ccc") iterate(strings.asIterable()) sequentiate(strings.asSequence()) }
38. fun iterate(iterable: Iterable<String>) { !" a iterable !" bb .onEach { println(it) } !" ccc .map { it.length } !" 1 .forEach { println(it) } !" 2 } !" 3 fun sequentiate(sequence: Sequence<String>) { !" sequence !" .onEach { println(it) } !" .map { it.length } !" .forEach { println(it) } !" } !" fun usage() { val strings = listOf("a", "bb", "ccc") iterate(strings.asIterable()) sequentiate(strings.asSequence()) }
39. fun iterate(iterable: Iterable<String>) { !" a iterable !" bb .onEach { println(it) } !" ccc .map { it.length } !" 1 .forEach { println(it) } !" 2 } !" 3 fun sequentiate(sequence: Sequence<String>) { !" 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()) }
40. void
41. void !" not a type
42. void !' Unit
43. void !' Unit “In type theory, a unit type is a type that allows only one value.”
44. void !' Unit !" single instance
45. fun unity() { }
46. fun unity(): Unit { }
47. fun unity() = Unit
48. fun unity() = println("sweet")
49. fun unity() = java.lang.System.out.println("sweet")
50. Void
51. Void !" a type that can’t be instantiated
52. Void !' Nothing
53. Void !' Nothing “In type theory, the bottom type is the type that has no values.” bottom zero empty
54. Void !' Nothing !" a type that can’t be instantiated (same!)
55. Void !' Nothing?
56. fun foreverNull(): Nothing? = null
57. fun inifinite() { while (true) { !" nap } } fun naive() { inifinite() !" always believe in yourself!⚠ println("I've achieved all my hopes and dreams!") }
58. fun inifinite(): Nothing { while (true) { !" nap } } fun naive() { inifinite() !" ⚠ Unreachable code println("I've achieved all my hopes and dreams!") }
59. fun fiveMoreMinutesPlease(): VeryComplexType
60. fun fiveMoreMinutesPlease(): VeryComplexType = TODO()
61. fun fiveMoreMinutesPlease(): VeryComplexType = TODO() fun TODO(): Nothing = throw NotImplementedError()
62. data class Person( val name: String, val age: Int, val likesDataClasses: Boolean)
63. data class Person( val name: String, val age: Int, val likesDataClasses: Boolean) fun parsePerson(map: Map<Any?, Any?>): 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 }
64. data class Person( val name: String, val age: Int, val likesDataClasses: Boolean) fun parsePerson(map: Map<Any?, Any?>): 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) }
65. data class Person( val name: String, val age: Int, val likesDataClasses: Boolean) fun parsePerson(map: Map<Any?, Any?>): 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) }
66. data class Person( val name: String, val age: Int, val likesDataClasses: Boolean) fun parsePerson(map: Map<Any?, Any?>): Person? { return Person( map["name"] as? String !& return null, map["age"] as? Int !& return null, map["likes-data-classes"] as? Boolean !& return null) }
67. !" Elvis operator !&
68. !& "hey"
69. 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()
70. 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()
71. 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()
72. 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)
73. 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)
74. 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.remAssign(b)
75. 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)
76. getValue, setValue, provideDelegate val someValue by lazy { "don't judge me" }
79. Kotlin tools team lead @ JetBrains co-author of “Kotlin in Action” @intelliyole on Twitter “Dmitry Jemerov” in “real life” yole
80. “ ” “ ” Kotlin tools team lead @ JetBrains co-author of “Kotlin in Action” @intelliyole on Twitter “Dmitry Jemerov” in “real life” yole
81. bit.ly/kotlin-mind-blowers Eugenio Marletti @workingkills