Understanding Kotlin Type System

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@brunoaybarg Bruno125 Bruno Aybar Senior Android Developer @ Avantica https://brunoaybar.com/talks/kotlin-type-system

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- Jetbrains “Kotlin is a (…) statically typed programming language”

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String name = ""; Class

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String name = ""; int age = 10; Class Primitive  Type

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String name = ""; int age = 10; List list = ... Class Primitive  Type Interface

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Classes ≠ Types you’ll understand how by the end!

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Kotlin build-in types Logical Boolean Integral Byte Char Int Long Floating Point Float Double Structured Array String Function types And…? Any Nothing Unit

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User-Defined Types Classes Interfaces Enums

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User-Defined Types Classes Interfaces Enums Function  Types Nullable  operator Generics New types when   combined with

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Kotlin Type Hierarchy … but let’s look at Java’s hierarchy ﬁrst

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Integer String MyClass Object MySubClass Number

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Int String MyClass Any MySubClass Number Nothing

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Int String MyClass Any MySubClass Number Nothing

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Int String MyClass Any MySubClass Number Nothing

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Int String MyClass Any MySubClass Number Nothing

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Any Object = ? Int String MyClass MySubClass Number Nothing

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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

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Object test = null; Java: Kotlin: val test: Any = null ✘

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Any Any? Subtyping by Nullability

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Int String MyClass Any MySubClass Number Nothing Any?

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Int String MyClass Any MySubClass Number Nothing Any? Int? String? MyClass? MySubClass? Number? Nothing?

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Int String MyClass Any MySubClass Number Nothing Any? Int? String? MyClass? MySubClass? Number? Nothing?

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When we define a class class MyClass We automatically have 2 types available val a: MyClass val b: MyClass?

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User-Defined Types Classes Interfaces Enums Function  Types Generics Nullable  operator New types when   combined with

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Any Any? Subtyping by Nullability

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When we define a class class MyClass We automatically have 2 types available val a: MyClass val b: MyClass? … while coding

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When we define a class class MyClass After compilation, the “non nullable” concept no longer  exists

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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

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Any Object = ? Int String MyClass MySubClass Number Nothing

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Any Object = ? Int String MyClass MySubClass Number Nothing

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Nothing

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Int String MyClass Any MySubClass Number Nothing

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Int String MyClass Any MySubClass Number Nothing Nothing never returns a value It is mainly used for control flow

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data class Data(val name: String?, val lastName: String?)

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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() }

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val name = data?.name ?: return false Which is the inferred type?

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val name: String = data?.name ?: return false

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Why is it possible to assign   return   to a variable? val name: String = data?.name ?: return false

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In Kotlin, mostly everything  is an expression

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val name = when(number) -> { 1 -> "one" 2 -> “two" ... } Inferred type?

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val message = if(number % 2) "This is an even number" else "This is an odd number" Inferred type?

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val function = fun(param: String) { ... } Inferred type?

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Why is it possible to assign   return   to a variable? val name: String = data?.name ?: return false

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val name: String = data?.name ?: return false returns Nothing

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val name: String = data?.name ?: return false String Nothing

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Int String MyClass Any MySubClass Number Nothing

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val name: String = data?.name ?: return false String Nothing Liskov Substitution  Principle subtype

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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

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Liskov Substitution  Principle Fruit Apple Orange val fruit: Fruit = Fruit()

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Liskov Substitution  Principle Fruit Apple Orange val fruit: Fruit = Apple()

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Liskov Substitution  Principle Fruit Apple Orange val fruit: Fruit = Apple() fun peel(fruit: Fruit) { } peel(Fruit())

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Liskov Substitution  Principle Fruit Apple Orange val fruit: Fruit = Apple() fun peel(fruit: Fruit) { } peel(Orange())

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val name: String = data?.name ?: return false String Nothing

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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

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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)

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Why is it possible to assign   return   to a variable? val name: String = data?.name ?: return false

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Because Nothing is a subtype of everything val name: String = data?.name ?: return false

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Throw returns Nothing too

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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

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public inline fun TODO(): Nothing = throw NotImplementedError() Kotlin STD example fun test(): Boolean { TODO() return true } WARNING:   Unreachable code

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The compiler knows that after Nothing is returned,   the execution flows stops

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val name: String = data?.name ?: return false so this will actually never be assigned ... because the function ends with the return statement

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fun forever(): Nothing { while(true) { } } Or you can define your own fun test(): Boolean { forever() return true } WARNING:   Unreachable code

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fun forever(): Nothing { while(false) { } } What if we change the condition? The compiler knows that!

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fun forever(): Nothing { while(false) { } } What if we change the condition? A ‘return' expression required in a function with a body (‘{…}’)

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fun forever(): Nothing { while(false) { } return ????? } What if we return something? It is impossible to   create an instance of  Nothing

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fun forever(): Nothing? { return null while(true) { } } Nothing? ≠ Nothing fun test(): Boolean { forever() return true } NO WARNING:   Reachable code

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fun print(value: Any?) = println(when(value) { is String -> "text: $value" is Int -> "integer: $value" else -> "unexpected value: $value" }) fun main() { print(“Message!”) }

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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!

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fun print(value: Any?) = println(when(value) { is String -> "text: $value" is Int -> "integer: $value" else -> "unexpected value: $value" }) fun main() { print(10) }

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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

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fun print(value: Any?) = println(when(value) { is String -> "text: $value" is Int -> "integer: $value" else -> "unexpected value: $value" }) fun main() { print(false) }

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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

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fun print(value: Any?) = println(when(value) { is String -> "text: $value" is Int -> "integer: $value" else -> "unexpected value: $value" }) fun main() { print(null) }

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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

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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) }

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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

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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) }

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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) }

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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!

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User-Defined Types Classes Interfaces Enums Function  Types Generics Nullable  operator New types when   combined with ✔

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User-Defined Types Classes Interfaces Enums Function  Types Nullable  operator Generics New types when   combined with ✔

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For every class that supports generics class Response It’s not a type by itself

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For every class that supports generics class Response But these are val a: Response val b: Response? ...

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For every class that supports generics class Response But these are val a: Response val b: Response? ... … while coding

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For every class that supports generics class Response After compilation, the generic type is erased Type Erasure

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class Response { fun evaluate() { print("Type: ${T::class.java}") } }

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class Response { fun evaluate() { print("Type: ${T::class.java}") } } Cannot use ’T' as a reified parameter.  Use class instead Type is no   longer available!

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class Response { fun evaluate() { print("Type: ${T::class.java}") } } Cannot use ’T' as a reified parameter.  Use class instead No work around

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class Response { } fun evaluate() { print("Type: ${T::class.java}") }

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fun evaluate() { print("Type: ${T::class.java}") }

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fun evaluate(r: Response) { print("Type: ${T::class.java}") }

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fun evaluate(r: Response) { print("Type: ${T::class.java}") } Cannot use ’T' as a reified parameter.  Use class instead

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inline fun evaluate(r: Response) { print("Type: ${T::class.java}") }

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inline fun evaluate(r: Response) { print("Type: ${T::class.java}") } No error! Because value is inlined (homework)

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Type Hierarchy for Generics

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Fruit Apple Orange class Box Box Box Box

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Fruit Apple Orange class Box Box Box Box ✘ ✘

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Fruit Apple Orange class Box Box Box Box

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Covariance and Contravariance

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Fruit Apple Orange class Box Box Box Box

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Fruit Apple Orange Box Box Box class Box

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Fruit Apple Orange class Box Box Box Box

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Fruit Apple Orange class Box Box Box Box

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User-Defined Types Classes Interfaces Enums Function  Types Nullable  operator Generics New types when   combined with ✔ ✔

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User-Defined Types Classes Interfaces Enums Generics Nullable  operator Function  Types New types when   combined with ✔ ✔ no time left :(

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Kotlin build-in types Logical Boolean Integral Byte Char Int Long Floating Point Float Double Structured Array String Function types And…? Any Nothing ✔ ✔ Unit

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Int String MyClass Any MySubClass Number Nothing Unit

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Int String MyClass MySubClass Number Nothing Any Unit

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fun test() { } fun test(): Unit { } public object Unit { override fun toString() = "kotlin.Unit" }

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fun test() { } fun test(): Unit { } These are not pure functions,  they only have side effects

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Pure function 1. Given an input, always emits same output 2. No side effects    3. 100% Unit testable

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fun sum(a: Int, b: Int): Int { return a + b } Pure ✔

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fun sum(a: Int, b: Int): Int { return a + b + random() } Not pure ✘

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Not pure ✘ fun sum(a: Int, b: Int): Int { print("Side effect!") return a + b }

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Not pure ✘ fun sum(a: Int, b: Int): Int { updateDatabase(a) return a + b }

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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/

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@brunoaybarg Bruno125 Bruno Aybar Senior Android Developer @ Avantica Thanks! https://brunoaybar.com/talks/kotlin-type-system