Understanding Kotlin Type System

Kotlin looks magical, but most of it is possible because of it's powerful type system. Understand how it works and how to take advantage of it while coding.

Transcript

1. Understanding Kotlin Type System
2. @brunoaybarg Bruno125 Bruno Aybar Senior Android Developer @ Avantica https://brunoaybar.com/talks/kotlin-type-system
3. - Jetbrains “Kotlin is a (…) statically typed programming language”
4. String name = ""; Class
5. String name = ""; int age = 10; Class Primitive
 Type
6. String name = ""; int age = 10; List<String> list = ... Class Primitive
 Type Interface
7. Classes ≠ Types you’ll understand how by the end!
8. Kotlin build-in types Logical Boolean Integral Byte Char Int Long Floating Point Float Double Structured Array String Function types And…? Any Nothing Unit
9. User-Defined Types Classes Interfaces Enums
10. User-Defined Types Classes Interfaces Enums Function
 Types Nullable
 operator Generics New types when 
 combined with
11. Kotlin Type Hierarchy … but let’s look at Java’s hierarchy first
12. Integer String MyClass Object MySubClass Number
13. Int String MyClass Any MySubClass Number Nothing
14. Int String MyClass Any MySubClass Number Nothing
15. Int String MyClass Any MySubClass Number Nothing
16. Int String MyClass Any MySubClass Number Nothing
17. Any Object = ? Int String MyClass MySubClass Number Nothing
18. fun test(param: Any) { } public final static test(Ljava/lang/Object;)V @Lorg/jetbrains/annotations/NotNull;() LDC "param" INVOKESTATIC kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull LINENUMBER 3 L1 RETURN L2 LOCALVARIABLE param Ljava/lang/Object; L0 L2 0 bytecode
19. Object test = null; Java: Kotlin: val test: Any = null ✘
20. Any Any? Subtyping by Nullability
21. Int String MyClass Any MySubClass Number Nothing Any?
22. Int String MyClass Any MySubClass Number Nothing Any? Int? String? MyClass? MySubClass? Number? Nothing?
23. Int String MyClass Any MySubClass Number Nothing Any? Int? String? MyClass? MySubClass? Number? Nothing?
24. When we define a class class MyClass We automatically have 2 types available val a: MyClass val b: MyClass?
25. User-Defined Types Classes Interfaces Enums Function
 Types Generics Nullable
 operator New types when 
 combined with
26. Any Any? Subtyping by Nullability
27. When we define a class class MyClass We automatically have 2 types available val a: MyClass val b: MyClass? … while coding
28. When we define a class class MyClass After compilation, the “non nullable” concept no longer
 exists
29. fun test(param: Any) { } public final static test(Ljava/lang/Object;)V @Lorg/jetbrains/annotations/NotNull;() LDC "param" INVOKESTATIC kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull LINENUMBER 3 L1 RETURN L2 LOCALVARIABLE param Ljava/lang/Object; L0 L2 0 bytecode
30. fun test(param: Any?) { } public final static test(Ljava/lang/Object;)V @Lorg/jetbrains/annotations/Nullable;() // invisible, parameter 0 L0 LINENUMBER 3 L0 RETURN L1 LOCALVARIABLE param Ljava/lang/Object; L0 L1 0 bytecode
31. Any Object = ? Int String MyClass MySubClass Number Nothing
32. Any Object = ? Int String MyClass MySubClass Number Nothing
33. Nothing
34. Int String MyClass Any MySubClass Number Nothing
35. Int String MyClass Any MySubClass Number Nothing Nothing never returns a value It is mainly used for control flow
36. data class Data(val name: String?, val lastName: String?)
37. data class Data(val name: String?, val lastName: String?) fun hasValidData(data: Data?): Boolean { val name = data?.name ?: return false val lastName = data.lastName ?: return false return name.isNotBlank() && lastName.isNotBlank() }
38. data class Data(val name: String?, val lastName: String?) fun hasValidData(data: Data?): Boolean { val name = data?.name ?: return false val lastName = data.lastName ?: return false return name.isNotBlank() && lastName.isNotBlank() } val name = data?.name ?: return false
39. val name = data?.name ?: return false Which is the inferred type?
40. val name: String = data?.name ?: return false
41. Why is it possible to assign 
 return 
 to a variable? val name: String = data?.name ?: return false
42. In Kotlin, mostly everything
 is an expression
43. val name = when(number) -> { 1 -> "one" 2 -> “two" ... } Inferred type?
44. val message = if(number % 2) "This is an even number" else "This is an odd number" Inferred type?
45. val function = fun(param: String) { ... } Inferred type?
46. Why is it possible to assign 
 return 
 to a variable? val name: String = data?.name ?: return false
47. val name: String = data?.name ?: return false returns Nothing
48. val name: String = data?.name ?: return false String Nothing
49. Int String MyClass Any MySubClass Number Nothing
50. val name: String = data?.name ?: return false String Nothing Liskov Substitution
 Principle subtype
51. Liskov Substitution
 Principle if S is a subtype of T, 
 then objects of type T may be replaced with objects of type S 
 without altering any of the desirable properties of that program
52. Liskov Substitution
 Principle Fruit Apple Orange val fruit: Fruit = Fruit()
53. Liskov Substitution
 Principle Fruit Apple Orange val fruit: Fruit = Apple()
54. Liskov Substitution
 Principle Fruit Apple Orange val fruit: Fruit = Apple() fun peel(fruit: Fruit) { } peel(Fruit())
55. Liskov Substitution
 Principle Fruit Apple Orange val fruit: Fruit = Apple() fun peel(fruit: Fruit) { } peel(Orange())
56. val name: String = data?.name ?: return false String Nothing
57. data class Data(val name: String?, val lastName: String?) fun hasValidData(data: Data?): Boolean { val lastName = data.lastName ?: return false return name.isNotBlank() && lastName.isNotBlank() } val name: String = data?.name ?: return false
58. data class Data(val name: String?, val lastName: String?) fun hasValidData(data: Data?): Boolean { val name: String = return false val lastName = data.lastName ?: return false return name.isNotBlank() && lastName.isNotBlank() } Perfectly possible
 (but doesn’t make sense)
59. Why is it possible to assign 
 return 
 to a variable? val name: String = data?.name ?: return false
60. Because Nothing is a subtype of everything val name: String = data?.name ?: return false
61. Throw returns Nothing too
62. fun hasValidData(data: Data?): Boolean { val name = data?.name ?: throw RuntimeException() val lastName = data.lastName ?: return false return name.isNotBlank() && lastName.isNotBlank() } Throw returns Nothing too
63. public inline fun TODO(): Nothing = throw NotImplementedError() Kotlin STD example fun test(): Boolean { TODO() return true } WARNING: 
 Unreachable code
64. The compiler knows that after Nothing is returned, 
 the execution flows stops
65. val name: String = data?.name ?: return false so this will actually never be assigned ... because the function ends with the return statement
66. fun forever(): Nothing { while(true) { } } Or you can define your own fun test(): Boolean { forever() return true } WARNING: 
 Unreachable code
67. fun forever(): Nothing { while(false) { } } What if we change the condition? The compiler knows that!
68. fun forever(): Nothing { while(false) { } } What if we change the condition? A ‘return' expression required in a function with a body (‘{…}’)
69. fun forever(): Nothing { while(false) { } return ????? } What if we return something? It is impossible to 
 create an instance of
 Nothing
70. fun forever(): Nothing? { return null while(true) { } } Nothing? ≠ Nothing fun test(): Boolean { forever() return true } NO WARNING: 
 Reachable code
71. fun print(value: Any?) = println(when(value) { is String -> "text: $value" is Int -> "integer: $value" else -> "unexpected value: $value" }) fun main() { print(“Message!”) }
72. fun print(value: Any?) = println(when(value) { is String -> "text: $value" is Int -> "integer: $value" else -> "unexpected value: $value" }) fun main() { print(“Message!”) } text: Message!
73. fun print(value: Any?) = println(when(value) { is String -> "text: $value" is Int -> "integer: $value" else -> "unexpected value: $value" }) fun main() { print(10) }
74. fun print(value: Any?) = println(when(value) { is String -> "text: $value" is Int -> "integer: $value" else -> "unexpected value: $value" }) fun main() { print(10) } integer: 10
75. fun print(value: Any?) = println(when(value) { is String -> "text: $value" is Int -> "integer: $value" else -> "unexpected value: $value" }) fun main() { print(false) }
76. fun print(value: Any?) = println(when(value) { is String -> "text: $value" is Int -> "integer: $value" else -> "unexpected value: $value" }) fun main() { print(false) } Unexpected value: false
77. fun print(value: Any?) = println(when(value) { is String -> "text: $value" is Int -> "integer: $value" else -> "unexpected value: $value" }) fun main() { print(null) }
78. fun print(value: Any?) = println(when(value) { is String -> "text: $value" is Int -> "integer: $value" else -> "unexpected value: $value" }) fun main() { print(null) } Unexpected value: null
79. fun print(value: Any?) = println(when(value) { is String -> "text: $value" is Int -> "integer: $value" is null -> "null value!" else -> "unexpected value: $value" }) fun main() { print(null) }
80. fun print(value: Any?) = println(when(value) { is String -> "text: $value" is Int -> "integer: $value" is null -> "null value!" else -> "unexpected value: $value" }) fun main() { print(null) } Type Expected
81. fun print(value: Any?) = println(when(value) { is String -> "text: $value" is Int -> "integer: $value" is ???? -> "null value!" else -> "unexpected value: $value" }) fun main() { print(null) }
82. fun print(value: Any?) = println(when(value) { is String -> "text: $value" is Int -> "integer: $value" is Nothing? -> "null value!" else -> "unexpected value: $value" }) fun main() { print(null) }
83. fun print(value: Any?) = println(when(value) { is String -> "text: $value" is Int -> "integer: $value” is Nothing? -> "null value!" else -> "unexpected value: $value" }) fun main() { print(null) } null value!
84. User-Defined Types Classes Interfaces Enums Function
 Types Generics Nullable
 operator New types when 
 combined with ✔
85. User-Defined Types Classes Interfaces Enums Function
 Types Nullable
 operator Generics New types when 
 combined with ✔
86. For every class that supports generics class Response<T> It’s not a type by itself
87. For every class that supports generics class Response<T> But these are val a: Response<Int> val b: Response<Boolean>? ...
88. For every class that supports generics class Response<T> But these are val a: Response<Int> val b: Response<Boolean>? ... … while coding
89. For every class that supports generics class Response<T> After compilation, the generic type is erased Type Erasure
90. class Response<T> { fun evaluate() { print("Type: ${T::class.java}") } }
91. class Response<T> { fun evaluate() { print("Type: ${T::class.java}") } } Cannot use ’T' as a reified parameter.
 Use class instead Type is no 
 longer available!
92. class Response<T> { fun evaluate() { print("Type: ${T::class.java}") } } Cannot use ’T' as a reified parameter.
 Use class instead No work around
93. class Response<T> { } fun evaluate() { print("Type: ${T::class.java}") }
94. fun evaluate() { print("Type: ${T::class.java}") }
95. fun <T> evaluate(r: Response<T>) { print("Type: ${T::class.java}") }
96. fun <T> evaluate(r: Response<T>) { print("Type: ${T::class.java}") } Cannot use ’T' as a reified parameter.
 Use class instead
97. inline fun <reified : T> evaluate(r: Response<T>) { print("Type: ${T::class.java}") }
98. inline fun <reified : T> evaluate(r: Response<T>) { print("Type: ${T::class.java}") } No error! Because value is inlined (homework)
99. Type Hierarchy for Generics
100. Fruit Apple Orange class Box<T> Box<Fruit> Box<Apple> Box<Orange>
101. Fruit Apple Orange class Box<T> Box<Fruit> Box<Apple> Box<Orange> ✘ ✘
102. Fruit Apple Orange class Box<T> Box<Fruit> Box<Apple> Box<Orange>
103. Covariance and Contravariance
104. Fruit Apple Orange class Box<T> Box<Fruit> Box<Apple> Box<Orange>
105. Fruit Apple Orange Box<Fruit> Box<Apple> Box<Orange> class Box<out T>
106. Fruit Apple Orange class Box<out T> Box<Fruit> Box<Apple> Box<Orange>
107. Fruit Apple Orange class Box<in T> Box<Fruit> Box<Apple> Box<Orange>
108. User-Defined Types Classes Interfaces Enums Function
 Types Nullable
 operator Generics New types when 
 combined with ✔ ✔
109. User-Defined Types Classes Interfaces Enums Generics Nullable
 operator Function
 Types New types when 
 combined with ✔ ✔ no time left :(
110. Kotlin build-in types Logical Boolean Integral Byte Char Int Long Floating Point Float Double Structured Array String Function types And…? Any Nothing ✔ ✔ Unit
111. Int String MyClass Any MySubClass Number Nothing Unit
112. Int String MyClass MySubClass Number Nothing Any Unit
113. fun test() { } fun test(): Unit { } public object Unit { override fun toString() = "kotlin.Unit" }
114. fun test() { } fun test(): Unit { } These are not pure functions,
 they only have side effects
115. Pure function 1. Given an input, always emits same output 2. No side effects
 
 3. 100% Unit testable
116. fun sum(a: Int, b: Int): Int { return a + b } Pure ✔
117. fun sum(a: Int, b: Int): Int { return a + b + random() } Not pure ✘
118. Not pure ✘ fun sum(a: Int, b: Int): Int { print("Side effect!") return a + b }
119. Not pure ✘ fun sum(a: Int, b: Int): Int { updateDatabase(a) return a + b }
120. Study material - Exploring the Kotlin Type Hierarchy from Top to Bottom
 https://www.youtube.com/watch?v=juFkdMv4B9s 
 - Programmer dictionary: Class vs Type vs Object
 https://blog.kotlin-academy.com/programmer-dictionary-class-vs-type-vs- object-e6d1f74d1e2e - An Illustrated Guide to Covariance and Contravariance
 in Kotlin https://typealias.com/guides/illustrated-guide-covariance- contravariance/
 - Getting Real with Kotlin's Reified Type Parameters
 https://typealias.com/guides/getting-real-with-reified-type-parameters/
121. @brunoaybarg Bruno125 Bruno Aybar Senior Android Developer @ Avantica Thanks! https://brunoaybar.com/talks/kotlin-type-system