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

Transcript

1. Kotlin’s
   Mind
   Blowers
   Eugenio Marletti
   @workingkills
   

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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)
   }
   

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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)
   }
   

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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)
   }
   

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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)
   }
   

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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)
   }
   

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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)
   }
   

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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)
    }
    

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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))
    }
    

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12. typealias TotallyNotThatClass = ThatClass
    

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13. typealias AbsSubject = Subject
    

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

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

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

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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()
    }
    

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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()
    }
    

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

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

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

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

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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
    

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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
    

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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
    

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26. 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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27. 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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28. fun stringLengthOrNull(maybeString: Any?) =
    if (maybeString is String)
    maybeString.length
    else
    null
    

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

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

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

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

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

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

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

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36. 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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37. 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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38. 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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39. 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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40. void
    

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41. void !" not a type
    

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42. void !' Unit
    

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

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44. void !' Unit !" single instance
    

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45. fun unity() { }
    

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46. fun unity(): Unit { }
    

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47. fun unity() = Unit
    

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48. fun unity() = println("sweet")
    

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

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50. Void
    

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51. Void !" a type that can’t be instantiated
    

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52. Void !' Nothing
    

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

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

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55. Void !' Nothing?
    

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56. fun foreverNull(): Nothing? = null
    

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

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59. fun fiveMoreMinutesPlease(): VeryComplexType
    

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60. fun fiveMoreMinutesPlease(): VeryComplexType = TODO()
    

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

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

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63. 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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64. 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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65. 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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66. 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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67. !" Elvis operator
    !&
    

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68. !&
    "hey"
    

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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()
    

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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()
    

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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()
    

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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)
    

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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)
    

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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)
    

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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)
    

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

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77. View Slide

78. View Slide

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

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80. “ ”
    “ ”
    Kotlin tools team lead @ JetBrains
    co-author of “Kotlin in Action”
    @intelliyole on Twitter
    “Dmitry Jemerov” in “real life”
    yole
    

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

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