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Idiomatic Kotlin

1bc80e2eee2adeaa8bb577798d92e9d0?s=47 Anton Arhipov
May 06, 2021
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Idiomatic Kotlin

1bc80e2eee2adeaa8bb577798d92e9d0?s=128

Anton Arhipov

May 06, 2021
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  1. @antonarhipov Idiomatic Kotlin from formatting to DSLs

  2. Agenda • Expressions • Examples from standard library • DSL

  3. Anton Arhipov @antonarhipov Developer Advocate @ JetBrains

  4. None
  5. None
  6. None
  7. Idiomatic - using, containing, or denoting expressions that are natural

    to a native speaker
  8. Idiomatic - using, containing, or denoting expressions that are natural

    to a native speaker In case of a programming language: •Conforms to a commonly accepted style •E ff ectively uses features of the programming language
  9. None
  10. Expressions try, if, when

  11. fun adjustSpeed(weather: Weather): Drive { var result: Drive if (weather

    is Rainy) { result = Safe() } else { result = Calm() } return result }
  12. fun adjustSpeed(weather: Weather): Drive { var result: Drive if (weather

    is Rainy) { result = Safe() } else { result = Calm() } return result }
  13. fun adjustSpeed(weather: Weather): Drive { val result: Drive if (weather

    is Rainy) { result = Safe() } else { result = Calm() } return result }
  14. fun adjustSpeed(weather: Weather): Drive { val result: Drive if (weather

    is Rainy) { result = Safe() } else { result = Calm() } return result }
  15. fun adjustSpeed(weather: Weather): Drive { val result: Drive result =

    if (weather is Rainy) { Safe() } else { Calm() } return result }
  16. fun adjustSpeed(weather: Weather): Drive { val result: Drive result =

    if (weather is Rainy) { Safe() } else { Calm() } return result }
  17. fun adjustSpeed(weather: Weather): Drive { val result: Drive = if

    (weather is Rainy) { Safe() } else { Calm() } return result }
  18. fun adjustSpeed(weather: Weather): Drive { val result: Drive = if

    (weather is Rainy) { Safe() } else { Calm() } return result }
  19. fun adjustSpeed(weather: Weather): Drive { return if (weather is Rainy)

    { Safe() } else { Calm() } }
  20. fun adjustSpeed(weather: Weather): Drive { return if (weather is Rainy)

    { Safe() } else { Calm() } }
  21. fun adjustSpeed(weather: Weather): Drive = if (weather is Rainy) {

    Safe() } else { Calm() }
  22. fun adjustSpeed(weather: Weather): Drive = if (weather is Rainy) {

    Safe() } else { Calm() }
  23. fun adjustSpeed(weather: Weather) = if (weather is Rainy) { Safe()

    } else { Calm() }
  24. fun adjustSpeed(weather: Weather) = if (weather is Rainy) { Safe()

    } else { Calm() }
  25. fun adjustSpeed(weather: Weather) = if (weather is Rainy) Safe() else

    Calm()
  26. fun adjustSpeed(weather: Weather) = if (weather is Rainy) Safe() else

    Calm() fun adjustSpeed(weather: Weather): Drive { var result: Drive if (weather is Rainy) { result = Safe() } else { result = Calm() } return result }
  27. fun adjustSpeed(weather: Weather) = if (weather is Rainy) Safe() else

    Calm()
  28. fun adjustSpeed(weather: Weather) = if (weather is Rainy) Safe() else

    Calm()
  29. abstract class Weather class Sunny : Weather() class Rainy :

    Weather() fun adjustSpeed(weather: Weather) = when (weather) { is Rainy -> Safe() else -> Calm() }
  30. sealed class Weather class Sunny : Weather() class Rainy :

    Weather() fun adjustSpeed(weather: Weather) = when (weather) { is Rainy -> Safe() / / else -> Calm() }
  31. sealed class Weather class Sunny : Weather() class Rainy :

    Weather() fun adjustSpeed(weather: Weather) = when (weather) { is Rainy -> Safe() / / else -> Calm() }
  32. sealed class Weather class Sunny : Weather() class Rainy :

    Weather() fun adjustSpeed(weather: Weather) = when (weather) { is Rainy -> Safe() is Sunny -> TODO() }
  33. sealed class Weather class Sunny : Weather() class Rainy :

    Weather() fun adjustSpeed(weather: Weather) = when (weather) { is Rainy -> Safe() is Sunny -> TODO() } Use expressions! Use when as expression body Use sealed classes with when
  34. Use try as expression body fun tryParse(number: String) : Int?

    { try { return Integer.parseInt(number) } catch (e: NumberFormatException) { return null } }
  35. Use try as expression body fun tryParse(number: String) = try

    { Integer.parseInt(number) } catch (e: NumberFormatException) { null }
  36. Use try as expression fun tryParse(number: String) : Int? {

    val n = try { Integer.parseInt(number) } catch (e: NumberFormatException) { null } println(n) return n }
  37. Use elvis operator class Person(val name: String?, val age: Int?)

    val p = retrievePerson() ?: Person()
  38. Use elvis operator as return and throw class Person(val name:

    String?, val age: Int?) fun processPerson(person: Person) { val name = person.name if (name = = null) throw IllegalArgumentException("Named required") val age = person.age if (age == null) return println("$name: $age") }
  39. Use elvis operator as return and throw class Person(val name:

    String?, val age: Int?) fun processPerson(person: Person) { val name = person.name if (name = = null) throw IllegalArgumentException("Named required") val age = person.age if (age == null) return println("$name: $age") }
  40. Use elvis operator as return and throw class Person(val name:

    String?, val age: Int?) fun processPerson(person: Person) { val name = person.name if (name = = null) throw IllegalArgumentException("Named required") val age = person.age if (age == null) return println("$name: $age") }
  41. Use elvis operator as return and throw class Person(val name:

    String?, val age: Int?) fun processPerson(person: Person) { val name = person.name ? : throw IllegalArgumentException("Named required") val age = person.age ?: return println("$name: $age") }
  42. Nullability

  43. Consider using null-safe call val order = retrieveOrder() if (order

    == null || order.customer = = null || order.customer.address == null){ throw IllegalArgumentException("Invalid Order") } val city = order.customer.address.city
  44. Consider using null-safe call val order = retrieveOrder() val city

    = order ?. customer ? . address ?. city
  45. Consider using null-safe call val order = retrieveOrder() val city

    = order ?. customer ? . address ?. city ?: throw IllegalArgumentException("Invalid Order")
  46. Avoid not-null assertions !! val order = retrieveOrder() val city

    = order !! .customer !! .address !! .city “You may notice that the double exclamation mark looks a bit rude: it’s almost like you’re yelling at the compiler. This is intentional.” - Kotlin in Action
  47. Avoid not-null assertions !! class MyTest { class State(val data:

    String) private var state: State? = null @BeforeEach fun setup() { state = State("abc") } @Test fun foo() { assertEquals("abc", state !! .data) } }
  48. Avoid not-null assertions !! class MyTest { class State(val data:

    String) private var state: State? = null @BeforeEach fun setup() { state = State("abc") } @Test fun foo() { assertEquals("abc", state !! .data) } } class MyTest { class State(val data: String) private lateinit var state: State @BeforeEach fun setup() { state = State("abc") } @Test fun foo() { assertEquals("abc", state.data) } } - use lateinit
  49. Consider using ?.let for null-checks val order = retrieveOrder() if

    (order != null){ processCustomer(order.customer) }
  50. Consider using ?.let for null-checks val order = retrieveOrder() if

    (order != null){ processCustomer(order.customer) } retrieveOrder() ?. let { processCustomer(it.customer) } retrieveOrder() ?. customer ?. let { :: processCustomer } or
  51. Consider using ?.let for null-checks val order = retrieveOrder() if

    (order != null){ processCustomer(order.customer) } retrieveOrder() ?. let { processCustomer(it.customer) } No need for an extra variable retrieveOrder() ?. let { processCustomer(it.customer) } retrieveOrder() ?. customer ?. let { :: processCustomer } or
  52. Consider using ?.let for null-checks val order = retrieveOrder() if

    (order != null){ processCustomer(order.customer) } retrieveOrder() ?. let { processCustomer(it.customer) } retrieveOrder() ?. let { processCustomer(it.customer) } retrieveOrder() ?. customer ?. let { :: processCustomer } or
  53. Consider using safe cast for type checking override fun equals(other:

    Any?) : Boolean { val command = other as Command return command.id == id }
  54. Consider using safe cast for type checking override fun equals(other:

    Any?) : Boolean { val command = other as Command return command.id == id } override fun equals(other: Any?) : Boolean { return (other as? Command) ?. id == id }
  55. Use range checks instead of comparison pairs fun isLatinUppercase(c: Char)

    = c > = 'A' && c < = 'Z'
  56. Use range checks instead of comparison pairs fun isLatinUppercase(c: Char)

    = c > = 'A' && c < = 'Z'
  57. Use range checks instead of comparison pairs fun isLatinUppercase(c: Char)

    = c in 'A' . . 'Z'
  58. Use range checks instead of comparison pairs fun isLatinUppercase(c: Char)

    = c in 'A' . . 'Z'
  59. Ranges in loops fun main(args: Array<String>) { for (i in

    0 .. args.size - 1) { println("$i: ${args[i]}") } }
  60. Ranges in loops fun main(args: Array<String>) { for (i in

    0 .. args.size - 1) { println("$i: ${args[i]}") } }
  61. Ranges in loops fun main(args: Array<String>) { for (i in

    0 .. args.size - 1) { println("$i: ${args[i]}") } } for (i in 0 until args.size) { println("$i: ${args[i]}") }
  62. Ranges in loops fun main(args: Array<String>) { for (i in

    0 .. args.size - 1) { println("$i: ${args[i]}") } } for (i in 0 until args.size) { println("$i: ${args[i]}") } for (i in args.indices) { println("$i: ${args[i]}") }
  63. Ranges in loops fun main(args: Array<String>) { for (i in

    0 .. args.size - 1) { println("$i: ${args[i]}") } } for (i in 0 until args.size) { println("$i: ${args[i]}") } for (i in args.indices) { println("$i: ${args[i]}") } for ((i, arg) in args.withIndex()) { println("$i: $arg") }
  64. Classes and Functions

  65. Don’t create classes just to hold functions class StringUtils {

    companion object { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } } }
  66. Don’t create classes just to hold functions class StringUtils {

    companion object { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } } } object StringUtils { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } }
  67. Don’t create classes just to hold functions class StringUtils {

    companion object { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } } } object StringUtils { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } } fun isPhoneNumber(s: String) = s.length == 7 && s.all { it.isDigit() }
  68. Use extension functions class StringUtils { companion object { fun

    isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } } } object StringUtils { fun isPhoneNumber(s: String) = s.length = = 7 & & s.all { it.isDigit() } } fun isPhoneNumber(s: String) = s.length == 7 && s.all { it.isDigit() } fun String.isPhoneNumber() = length == 7 && all { it.isDigit() }
  69. Extension or a member? https://kotlinlang.org/docs/coding-conventions.html#extension-functions •Use extension functions liberally. •If

    a function works primarily on an object, consider making it an extension with that object as a receiver. •Minimize API pollution, restrict the visibility. •As necessary, use local extension functions, member extension functions, or top-level extension functions with private visibility.
  70. Use default values instead of overloading class Phonebook { fun

    print() { print(",") } fun print(columnSeparator: String) {} } fun main(args: Array<String>) { Phonebook().print("|") }
  71. Use default values instead of overloading class Phonebook { fun

    print() { print(",") } fun print(columnSeparator: String) {} } fun main(args: Array<String>) { Phonebook().print("|") } class Phonebook { fun print(separator: String = ",") {} fun someFun(x: Int) {} } fun main(args: Array<String>) { Phonebook().print(separator = "|") }
  72. Return multiple values using data classes fun namedNum(): Pair<Int, String>

    = 1 to "one" // same but shorter fun namedNum2() = 1 to "one" fun main(args: Array<String>) { val pair = namedNum() val number = pair.first val name = pair.second }
  73. Return multiple values using data classes fun namedNum(): Pair<Int, String>

    = 1 to "one" // same but shorter fun namedNum2() = 1 to "one" fun main(args: Array<String>) { val pair = namedNum() val number = pair.first val name = pair.second } data class GameResult( val rank: Int, val name: String ) fun namedNum() = GameResult(1, "Player 1") fun main(args: Array<String>) { val (rank, name) = namedNum() println("$name, rank $rank") }
  74. Return multiple values using data classes data class GameResult( val

    rank: Int, val name: String ) fun namedNum() = GameResult(1, "Player 1") fun main(args: Array<String>) { val (rank, name) = namedNum() println("$name, rank $rank") } GameResult var1 = namedNum(); int var2 = var1.component1(); String var3 = var1.component2();
  75. Destructuring in loops fun printMap(map: Map<String, String>) { for (item

    in map.entries) { println("${item.key} -> ${item.value}") } }
  76. Destructuring in loops fun printMap(map: Map<String, String>) { for (item

    in map.entries) { println("${item.key} -> ${item.value}") } } fun printMap(map: Map<String, String>) { for ((key, value) in map) { println("$key -> $value") } }
  77. Destructuring in lists data class NameExt( val name: String, val

    ext: String? ) fun splitNameExt(filename: String): NameExt { if ('.' in filename) { val parts = filename.split('.', limit = 2) return NameExt(parts[0], parts[1]) } return NameExt(filename, null) } fun splitNameAndExtension(filename: String): NameExt { if ('.' in filename) { val (name, ext) = filename.split('.', limit = 2) return NameExt(name, ext) } return NameExt(filename, null) }
  78. None
  79. Use type aliases for functional types class Event class EventDispatcher

    { fun addClickHandler(handler: (Event) -> Unit) {} fun removeClickHandler(handler: (Event) -> Unit) {} }
  80. Use type aliases for functional types class Event class EventDispatcher

    { fun addClickHandler(handler: (Event) -> Unit) {} fun removeClickHandler(handler: (Event) -> Unit) {} } typealias ClickHandler = (Event) -> Unit class EventDispatcher { fun addClickHandler(handler: ClickHandler) { } fun removeClickHandler(handler: ClickHandler) { } }
  81. Standard Library

  82. Verify parameters using require() class Person( val name: String?, val

    age: Int ) fun processPerson(person: Person) { if (person.age < 18) { throw IllegalArgumentException("Adult required") } }
  83. Verify parameters using require() class Person( val name: String?, val

    age: Int ) fun processPerson(person: Person) { if (person.age < 18) { throw IllegalArgumentException("Adult required") } } fun processPerson(person: Person) { require(person.age > = 18) { "Adult required" } }
  84. Select objects by type with filterIsInstance fun findAllStrings(objects: List<Any>) =

    objects.filter { it is String }
  85. Select objects by type with filterIsInstance fun findAllStrings(objects: List<Any>) =

    objects.filter { it is String } fun findAllStrings(objects: List<Any>) = objects.filterIsInstance<String>()
  86. Select objects by type with filterIsInstance fun findAllStrings(objects: List<Any>) :

    List<Any> = objects.filter { it is String } fun findAllStrings(objects: List<Any>) : List<String> = objects.filterIsInstance<String>()
  87. Apply operation to non-null elements mapNotNull data class Result( val

    data: Any?, val error: String? ) fun listErrors(results: List<Result>): List<String> = results.map { it.error }.filterNotNull() fun listErrors(results: List<Result>): List<String> = results.mapNotNull { it.errorMessage }
  88. compareBy compares by multiple keys class Person( val name: String,

    val age: Int ) fun sortPersons(persons: List<Person>) = persons.sortedWith(Comparator<Person> { person1, person2 -> val rc = person1.name.compareTo(person2.name) if (rc != 0) rc else person1.age - person2.age })
  89. compareBy compares by multiple keys class Person( val name: String,

    val age: Int ) fun sortPersons(persons: List<Person>) = persons.sortedWith(Comparator<Person> { person1, person2 -> val rc = person1.name.compareTo(person2.name) if (rc != 0) rc else person1.age - person2.age }) fun sortPersons(persons: List<Person>) = persons.sortedWith(compareBy(Person :: name, Person : : age))
  90. groupBy to group elements class Request( val url: String, val

    remoteIP: String, val timestamp: Long ) fun analyzeLog(log: List<Request>) { val map = mutableMapOf<String, MutableList<Request >> () for (request in log) { map.getOrPut(request.url) { mutableListOf() } .add(request) } }
  91. groupBy to group elements class Request( val url: String, val

    remoteIP: String, val timestamp: Long ) fun analyzeLog(log: List<Request>) { val map = mutableMapOf<String, MutableList<Request >> () for (request in log) { map.getOrPut(request.url) { mutableListOf() } .add(request) } } fun analyzeLog(log: List<Request>) { val map = log.groupBy(Request :: url) }
  92. Use coerceIn to ensure numbers in range fun updateProgress(value: Int)

    { val actualValue = when { value < 0 - > 0 value > 100 -> 100 else - > value } } fun updateProgress(value: Int) { val actualValue = value.coerceIn(0, 100) }
  93. Initializing objects with apply val dataSource = BasicDataSource( ) dataSource.driverClassName

    = "com.mysql.jdbc.Driver" dataSource.url = "jdbc:mysql://domain:3309/db" dataSource.username = "username" dataSource.password = "password" dataSource.maxTotal = 40 dataSource.maxIdle = 40 dataSource.minIdle = 4 val dataSource = BasicDataSource().apply { driverClassName = "com.mysql.jdbc.Driver" url = "jdbc:mysql://domain:3309/db" username = "username" password = "password" maxTotal = 40 maxIdle = 40 minIdle = 4 }
  94. Initializing objects with apply final ClientBuilder builder = new ClientBuilder();

    builder.setFirstName("Anton"); builder.setLastName("Arhipov"); final TwitterBuilder twitterBuilder = new TwitterBuilder(); twitterBuilder.setHandle("@antonarhipov"); builder.setTwitter(twitterBuilder.build()); final CompanyBuilder companyBuilder = new CompanyBuilder(); companyBuilder.setName("JetBrains"); companyBuilder.setCity("Tallinn"); builder.setCompany(companyBuilder.build()); final Client client = builder.build(); System.out.println("Created client is: " + client);
  95. Initializing objects with apply val builder = ClientBuilder() builder.firstName =

    "Anton" builder.lastName = "Arhipov" val twitterBuilder = TwitterBuilder() twitterBuilder.handle = "@antonarhipov" builder.twitter = twitterBuilder.build() val companyBuilder = CompanyBuilder() companyBuilder.name = "JetBrains" companyBuilder.city = "Tallinn" builder.company = companyBuilder.build() val client = builder.build() println("Created client is: $client")
  96. Initializing objects with apply val builder = ClientBuilder() builder.firstName =

    "Anton" builder.lastName = "Arhipov" val twitterBuilder = TwitterBuilder() twitterBuilder.handle = "@antonarhipov" builder.twitter = twitterBuilder.build() val companyBuilder = CompanyBuilder() companyBuilder.name = "JetBrains" companyBuilder.city = "Tallinn" builder.company = companyBuilder.build() val client = builder.build() println("Created client is: $client") val client = ClientBuilder().apply { firstName = "Anton" lastName = "Arhipov" twitter = TwitterBuilder().apply { handle = "@antonarhipov" }.build() company = CompanyBuilder().apply { name = "JetBrains" city = "Tallinn" }.build() }.build() println("Created client is: $client")
  97. Domain Specific Languages

  98. “Domain Speci fi c”, i.e. tailored for a speci fi

    c task
  99. “Domain Speci fi c”, i.e. tailored for a speci fi

    c task Examples: •Compose strings - stringBuilder •Create HTML documents - kotlinx.html •Con fi gure routing logic for a web app - ktor •Generally, build any object graphs. See “type-safe builders”
  100. buildString //Java String name = "Joe"; StringBuilder sb = new

    StringBuilder(); for (int i = 0; i < 5; i++) { sb.append("Hello, "); sb.append(name); sb.append("!\n"); } System.out.println(sb); //Kotlin val name = "Joe" val s = buildString { repeat(5) { append("Hello, ") append(name) appendLine("!") } } println(s)
  101. kotlinx.html System.out.appendHTML().html { body { div { a("http: // kotlinlang.org")

    { target = ATarget.blank +"Main site" } } } }
  102. Ktor fun main() { embeddedServer(Netty, port = 8080, host =

    "0.0.0.0") { routing { get("/html-dsl") { call.respondHtml { body { h1 { +"HTML" } ul { for (n in 1 .. 10) { li { +"$n" } } } } } } } }.start(wait = true) }
  103. Ktor fun main() { embeddedServer(Netty, port = 8080, host =

    "0.0.0.0") { routing { get("/html-dsl") { call.respondHtml { body { h1 { +"HTML" } ul { for (n in 1 .. 10) { li { +"$n" } } } } } } } }.start(wait = true) } Ktor’s routing
  104. Ktor fun main() { embeddedServer(Netty, port = 8080, host =

    "0.0.0.0") { routing { get("/html-dsl") { call.respondHtml { body { h1 { +"HTML" } ul { for (n in 1 .. 10) { li { +"$n" } } } } } } } }.start(wait = true) } kotlinx.html Ktor’s routing
  105. Lambda with receiver T.() -> Unit

  106. Build your vocabulary to abstract from scope functions val client

    = ClientBuilder().apply { firstName = "Anton" lastName = "Arhipov" twitter = TwitterBuilder().apply { handle = "@antonarhipov" }.build() company = CompanyBuilder().apply { name = "JetBrains" city = "Tallinn" }.build() }.build() println("Created client is: $client")
  107. Build your vocabulary to abstract from scope functions fun client(c:

    ClientBuilder.() - > Unit): Client { val builder = ClientBuilder() c(builder) return builder.build() } fun ClientBuilder.company(block: CompanyBuilder.() - > Unit) { company = CompanyBuilder().apply(block).build() } fun ClientBuilder.twitter(block: TwitterBuilder.() - > Unit) { twitter = TwitterBuilder().apply(block).build() } val client = ClientBuilder().apply { firstName = "Anton" lastName = "Arhipov" twitter = TwitterBuilder().apply { handle = "@antonarhipov" }.build() company = CompanyBuilder().apply { name = "JetBrains" city = "Tallinn" }.build() }.build() println("Created client is: $client")
  108. val client = client { firstName = "Anton" lastName =

    "Arhipov" twitter { handle = "@antonarhipov" } company { name = "JetBrains" city = "Tallinn" } } println("Created client is: $client") Build your vocabulary to abstract from scope functions val client = ClientBuilder().apply { firstName = "Anton" lastName = "Arhipov" twitter = TwitterBuilder().apply { handle = "@antonarhipov" }.build() company = CompanyBuilder().apply { name = "JetBrains" city = "Tallinn" }.build() }.build() println("Created client is: $client")
  109. https://speakerdeck.com/antonarhipov https://github.com/antonarhipov/idiomatic-kotlin @antonarhipov