Kotlin Fest 2019: Kotlin型実践入門

Kotlin Fest 2019 ࠤ౻ ൏ / Sato Shun Twitter: @stsn_jp
GitHub: satoshun Kotlinܕ࣮ફೖ໳

֓ཁ • KotlinͰ͸Javaʹ͸ͳ͍༷ʑͳܕʹؔ͢Δػೳ͕௥Ճ͞Εͨ ͷͰɺͦΕΒͷղઆ • ॳֶऀ޲͚ͷ಺༰ʹͳ͓ͬͯΓ·͢

໨࣍ • Smart Casts • AnyɺUnitɺNothing • δΣωϦΫε • Sealed
class • ؔ਺ܕ

•Smart Casts • AnyɺUnitɺNothing • δΣωϦΫε • Sealed class •
ؔ਺ܕ

Smart Casts • ifࣜɺwhenࣜɺisɺasͳͲͷ࣮ߦޙʹɺ໌ࣔతʹܕΩϟετ ͠ͳͯ͘΋ɺܕΛਪ࿦ͯ͘͠ΕΔ • ﬂow-sensitive typing (ﬂow typing)ͱݺ͹ΕΔܕγεςϜ

Smart Casts • ifࣜɺwhenࣜɺisɺasͳͲͷ࣮ߦޙʹɺ໌ࣔతʹܕΩϟετ ͠ͳͯ͘΋ɺܕΛਪ࿦ͯ͘͠ΕΔ • ﬂow-sensitive typing (ﬂow typing)ͱݺ͹ΕΔܕγεςϜ
• ৑௕ͳܕΩϟετΛܰݮ͢Δ • ಈతܕ෇͚ݴޠͷμοΫλΠϐϯάͷΑ͏ͳॻ͖৺஍ • https://en.wikipedia.org/wiki/Flow-sensitive_typing

if (obj is String) { a print((obj as String).length) }
a if (obj !is String) return print((obj as String).length) when (obj) { aa is Int -> print((obj as Int) + 1) is String -> print((obj as String).length + 1) is IntArray -> print((obj as IntArray).sum() + 1) } a

if (obj is String) { a print(obj.length) } a if
(obj !is String) return print((obj as String).length) when (obj) { aa is Int -> print((obj as Int) + 1) is String -> print((obj as String).length + 1) is IntArray -> print((obj as IntArray).sum() + 1) } a

(obj !is String) return print(obj.length) when (obj) { aa is Int -> print((obj as Int) + 1) is String -> print((obj as String).length + 1) is IntArray -> print((obj as IntArray).sum() + 1) } a

(obj !is String) return print(obj.length) when (obj) { a is Int -> print(obj + 1) is String -> print(obj.length + 1) is IntArray -> print(obj.sum() + 1) } a

if (hoge() && obj is Int) { a (obj as
Int).toLong() } a if (obj is String && obj is Int) { a (obj as String).length (obj as Int).toLong() } a

if (hoge() && obj is Int) { a obj.toLong() }
a if (obj is String && obj is Int) { a (obj as String).length (obj as Int).toLong() } a

if (hoge() && obj is Int) { a obj.toLong() }
a if (obj is String && obj is Int) { a obj.length obj.toLong() } a

class Hoge { a private var obj: Any = "a"
fun test() { a if (obj is String) { a print((obj as String).length) } a } a } a

fun test() { a if (obj is String) { a print(obj.length) } a } a } a

fun test() { a if (obj is String) { a print(obj.length) } a } a } a ❌

fun test() { a val obj = obj if (obj is String) { a print(obj.length) } a } a } a

fun test() { a obj.let { a if (it is String) { a print(it.length) } a } a } a } a

val strs: List<String>? = … a if (!strs.isNullOrEmpty()) { a
println(strs!!.size) } a

println(strs.size) } a

Contracts • Smart CastsΛ֦ு͢Δػೳ • ؔ਺ݺͼग़͠ͷ݁ՌΛ΋ͱʹɺSmart Casts͢Δ͜ͱ͕ग़དྷ Δ

public inline fun <T> Collection<T>?.isNullOrEmpty(): Boolean { a contract {
a returns(false) implies (this@isNullOrEmpty != null) } a return this == null || this.isEmpty() } a

println(strs!!.size) } a

println(strs.size) } a

fun assertTrue(actual: Boolean, message: String? = null) { a contract
{ returns() implies actual } a return asserter.assertTrue(message ?: "Expected value to be true.", actual) } a

{ returns() implies actual } a return asserter.assertTrue(message ?: "Expected value to be true.", actual) } a assertTrue(obj != null) print(obj!!.length)

{ returns() implies actual } a return asserter.assertTrue(message ?: "Expected value to be true.", actual) } a assertTrue(obj != null) print(obj.length)

fun assertFalse(actual: Boolean, message: String? = null) { contract {
returns() implies (!actual) } return asserter.assertTrue(message ?: "Expected value to be false.", !actual) } assertFalse(obj == null) print(obj.length)

public inline fun <R> run(block: () -> R): R {
a contract { a callsInPlace(block, InvocationKind.EXACTLY_ONCE) } a return block() } a

a contract { a callsInPlace(block, InvocationKind.EXACTLY_ONCE) } a return block() } a var s: String? = null a run { a s = "" a } a println(s!!) a

a contract { a callsInPlace(block, InvocationKind.EXACTLY_ONCE) } a return block() } a val s: String a run { a s = "" a } a println(s) a

fun checkObj(obj: Any?): Boolean { a return obj != null
} a if (checkObj(obj)) { a obj!!.javaClass } a

fun checkObj(obj: Any?): Boolean { a contract { a returns(true)
implies (obj != null) } a return obj != null } a if (checkObj(obj)) { a obj!!.javaClass } a

fun checkObj(obj: Any?): Boolean { a contract { a returns(true)
implies (obj != null) } a return obj != null } a if (checkObj(obj)) { a obj.javaClass } a

@UseExperimental(ExperimentalContracts::class) fun checkObj(obj: Any?): Boolean { a contract { a
returns(true) implies (obj != null) } a return obj != null } a if (checkObj(obj)) { a obj.javaClass } a

• Smart Casts •AnyɺUnitɺNothing • δΣωϦΫε • Sealed class •
ؔ਺ܕ

Anyܕ • ͢΂ͯͷKotlinͷΫϥεͷεʔύʔΫϥε • ϧʔτΫϥε • Javaͷ৔߹ɺjava.lang.Object͕ϧʔτΫϥε

Anyܕ • JavaͷObjectͱҧ͏఺͸Ͳ͔͜ʁ • waitɺnotifyϝιου͕࢖͑ͳ͍ • ໌ࣔతʹjava.lang.ObjectܕʹΩϟετ͢Ε͹ݺͼग़ͤΔ • getClassϝιου͕֦ுؔ਺ʹ •
public inline val <T : Any> T.javaClass: Class<T> • cloneɺﬁnalizeϝιου

val s = "" s.notify()

val s = "" (s as Object).notify()

Unitܕ • JavaͰ͍͏ͱ͜ΖͷvoidΛදݱ͢ΔΫϥε • ฦΓ஋͕ۭͰ͋Δ͜ͱΛද͢

fun empty() { a } a empty()

fun empty(): Unit { a } a empty()

fun empty(): Unit { a } a val e =
empty() println(e)

fun empty() { a } a val e = empty()
println(e)

Nothingܕ • ͢΂ͯͷKotlinͷΫϥεͷαϒΫϥε • ஋͕ଘࡏ͠ͳ͍͜ͱΛࣔ͢ • ਖ਼ৗऴྃ͢Δ͜ͱ͕ͳ͍ؔ਺ͷฦΓ஋ʹ࢖͑Δ

fun getName(): String { a return "name" } a fun
fail() { a throw RuntimeException() } a

fail() { a throw RuntimeException() } a val name = if (isFriend()) { a getName() } a else { a fail() } a

fail() { a throw RuntimeException() } a val name: Any = if (isFriend()) { a getName() } a else { a fail() } a

fail() { a throw RuntimeException() } a val name: Any = if (isFriend()) { a getName() } a else { a fail() } a println((name as String).length)

fail(): Nothing { a throw RuntimeException() } a val name: Any = if (isFriend()) { a getName() } a else { a fail() } a println((name as String).length)

fail(): Nothing { a throw RuntimeException() } a val name: String = if (isFriend()) { a getName() } a else { a fail() } a println((name as String).length)

fail(): Nothing { a throw RuntimeException() } a val name: String = if (isFriend()) { a getName() } a else { a fail() } a println(name.length)

fail(): Nothing { a throw RuntimeException() } a val name = if (isFriend()) { a getName() } a else { a fail() } a println(name.length)

fun test(any: Any): String { a when (any) { a
is Int -> return "int" else -> fail() } a return "error" } a

is Int -> return "int" else -> fail() } a return "error" } a NothingܕͳͷͰɺ͜ͷݺͼग़͠͸ਖ਼ৗʹऴྃ͠ͳ͍ ͜ͷelse͸ඞࣦͣഊ͢Δ

is Int -> return "int" else -> fail() } a } a

// ඪ४ؔ਺ public inline fun TODO(): Nothing = throw NotImplementedError()
// KotlinTest fun fail(msg: String): Nothing = throw Failures.failure(msg)

// Kotlin/kotlinx.collections.immutable internal companion object { internal val EMPTY =
TrieNode<Nothing>(0, emptyArray()) } // airbnb/MvRx object Uninitialized : Async<Nothing>(complete = false, shouldLoad = true), Incomplete

fun test2() { a while (true) { a println(".") }
a } a

fun test2(): Nothing { a while (true) { a println(".")
} a } a

Nothing?ܕ • Nothing?ܕ΋ଘࡏ͢Δ • Nothing͸஋ͱͯ͠ଘࡏ͠ͳ͍ͷͰɺnullͷΈΛڐՄ͢Δ

// kotlintest/kotlintest interface Show<in A> { a fun supports(a: Any?):
Boolean fun show(a: A): String } a object NullShow : Show<Nothing?> { a override fun supports(a: Any?): Boolean = a == null override fun show(a: Nothing?): String = "<null>" } a

• Smart Casts • AnyɺUnitɺNothing •δΣωϦΫε • Sealed class •
ؔ਺ܕ

δΣωϦΫε • ෼ࢄΞϊςʔγϣϯʢvariance annotationʣ • outɺinम০ࢠ • એݴଆมੑ: declaration-site variance
• where • reiﬁed • ֦ுؔ਺ + δΣωϦΫε

෼ࢄΞϊςʔγϣϯ • out - ڞม: covariance • in - ൓ม:
contravariance • ࢦఆͳ͠: ෆม: invariance

ڞมɺ൓มɺෆม • Number <|- Intͷܧঝؔ܎͕͋Δ

ڞมɺ൓มɺෆม • Number <|- Intͷܧঝؔ܎͕͋Δ • A<Number> <|- A<Int>ͷ৔߹ɺA͸ڞมͰ͋Δ

ڞมɺ൓มɺෆม • Number <|- Intͷܧঝؔ܎͕͋Δ • A<Number> <|- A<Int>ͷ৔߹ɺA͸ڞมͰ͋Δ •
A<Number> -|> A<Int>ͷ৔߹ɺA͸൓มͰ͋Δ

ڞมɺ൓มɺෆม • Number <|- Intͷܧঝؔ܎͕͋Δ • A<Number> <|- A<Int>ͷ৔߹ɺA͸ڞมͰ͋Δ •
A<Number> -|> A<Int>ͷ৔߹ɺA͸൓มͰ͋Δ • A<Number>ɺ A<Int>͕ؔ܎ͳ͍৔߹ɺA͸ෆมͰ͋Δ

// ڞม val a: A<Int> = A<Int>() a val b:
A<Number> = a

A<Number> = a // ൓ม val a: A<Number> = A<Number>() a val b: A<Int> = a

A<Number> = a // ൓ม val a: A<Number> = A<Number>() a val b: A<Int> = a // ෆม val a: A<Int> = A<Int>() a

interface Mapper<T> { a fun map(s: String): T } a
class IntMapper : Mapper<Int> { a override fun map(s: String): Int = s.toInt() } a

class IntMapper : Mapper<Int> { a override fun map(s: String): Int = s.toInt() } a fun hoge(mapper: Mapper<Number>) { a mapper.map("10") } a

class IntMapper : Mapper<Int> { a override fun map(s: String): Int = s.toInt() } a fun hoge(mapper: Mapper<Number>) { a mapper.map("10") } a val mapper = IntMapper() hoge(mapper)

class IntMapper : Mapper<Int> { a override fun map(s: String): Int = s.toInt() } a fun hoge(mapper: Mapper<Number>) { a mapper.map("10") } a val mapper = IntMapper() hoge(mapper) ❌

interface Mapper<out T> { a fun map(s: String): T }
a class IntMapper : Mapper<Int> { a override fun map(s: String): Int = s.toInt() } a fun hoge(mapper: Mapper<Number>) { a mapper.map("10") } a val mapper = IntMapper() hoge(mapper) ❌

interface Mapper<out T> { a fun map(s: String): T }
a class IntMapper : Mapper<Int> { a override fun map(s: String): Int = s.toInt() } a fun hoge(mapper: Mapper<Number>) { a mapper.map("10") } a val mapper = IntMapper() hoge(mapper)

ListͱMutableList public interface List<out E> : Collection<E> public interface MutableList<E>
: List<E>, …

ListͱMutableList public interface List<out E> : Collection<E> public interface MutableList<E>
: List<E>, … • List͸outम০ࢠ͕෇͍͍ͯΔͷͰɺڞม • MutableList͸Կ΋෇͍͍ͯͳ͍ͷͰɺෆม

val strs: List<String> = listOf("a", “b")

val strs: List<String> = listOf("a", “b") val anys: List<Any> =
strs

strs val a: Any = anys[0]

strs val a: Any = anys[0] val a2: Any? = anys2.getOrNull(1)

val strs: MutableList<String> = mutableListOf("a", "b")

val strs: MutableList<String> = mutableListOf("a", "b") val anys: MutableList<Any> =
strs

strs anys.add(10)

strs anys.add(10) val str: String = strs.last()

strs anys.add(10) val str: String = strs.last() ❌

strs anys.add(10) val str: String = strs.last()

PECS • Producer Extends Consumer Super • Producer͸readͷੑ࣭Λ࣋ͭ • Consumer͸writeͷੑ࣭Λ࣋ͭ
• List͸Producerͷੑ࣭ͷΈΛ࣋ͭͷͰɺExtends = out ڞมʹ ग़དྷΔ • MutableList͸ProducerͱConsumerͷੑ࣭Λ࣋ͭͷͰɺෆม

2ͭҎ্ͷ্քΛ࣋ͭܕม਺ • 1ͭͷ্քʢupper boundʣΛ࣋ͭͱ͖͸ɺׅހ಺ʹॻ͚͹ྑ ͍

2ͭҎ্ͷ্քΛ࣋ͭܕม਺ • 1ͭͷ্քʢupper boundʣΛ࣋ͭͱ͖͸ɺׅހ಺ʹॻ͚͹ྑ ͍ • fun <T : CharSequence>
hoge(t: T) {…}

2ͭҎ্ͷ্քΛ࣋ͭܕม਺ • 1ͭͷ্քʢupper boundʣΛ࣋ͭͱ͖͸ɺׅހ಺ʹॻ͚͹ྑ ͍ • fun <T : CharSequence>
hoge(t: T) {…} • 2ͭҎ্ͷ্քΛ࣋ͭ৔߹͸Ͳ͏͢Ε͹Α͍͔ʁ • ྫ͑͹ɺActivity͔ͭOnClickListenerΛ࣋ͭܕΛड͚ೖΕΔܕ ม਺Λఆ͍ٛͨ͠

class OnClickActivity() : Activity(), OnClickListener

class OnClickActivity() : Activity(), OnClickListener fun setClick(activity: OnClickActivity) { a
} a

} a val activity = OnClickActivity() setClick(activity)

} a val activity = OnClickActivity() setClick(activity) class OnClickActivity2() : Activity(), OnClickListener

} a val activity = OnClickActivity() setClick(activity) class OnClickActivity2() : Activity(), OnClickListener val activity2 = OnClickActivity2() setClick(activity2)

} a val activity = OnClickActivity() setClick(activity) class OnClickActivity2() : Activity(), OnClickListener val activity2 = OnClickActivity2() setClick(activity2) ❌

fun setClick(activity: OnClickActivity) { a } a

fun <T> setClick(activity: T)

fun <T> setClick(activity: T) where T : Activity, T :
OnClickListener { a } a

OnClickListener { a } a val activity = OnClickActivity() setClick(activity)

OnClickListener { } val activity = OnClickActivity() setClick(activity) val activity2 = OnClickActivity2() setClick(activity2)

internal abstract class NIOSocketImpl<out S>( override val channel: S, val
selector: SelectorManager, val pool: ObjectPool<ByteBuffer>? ) : ReadWriteSocket, SelectableBase(channel), CoroutineScope where S : java.nio.channels.ByteChannel, S : java.nio.channels.SelectableChannel Ktor

selector: SelectorManager, val pool: ObjectPool<ByteBuffer>? ) : ReadWriteSocket, SelectableBase(channel), CoroutineScope where S : java.nio.channels.ByteChannel, S : java.nio.channels.SelectableChannel internal class DatagramSocketImpl(override val channel: DatagramChannel, selector: SelectorManager) : BoundDatagramSocket, ConnectedDatagramSocket, NIOSocketImpl<DatagramChannel>(channel, selector, DefaultDatagramByteBufferPool) Ktor

selector: SelectorManager, val pool: ObjectPool<ByteBuffer>? ) : ReadWriteSocket, SelectableBase(channel), CoroutineScope where S : java.nio.channels.ByteChannel, S : java.nio.channels.SelectableChannel internal class DatagramSocketImpl(override val channel: DatagramChannel, selector: SelectorManager) : BoundDatagramSocket, ConnectedDatagramSocket, NIOSocketImpl<DatagramChannel>(channel, selector, DefaultDatagramByteBufferPool) internal class SocketImpl<out S : SocketChannel>( override val channel: S, private val socket: java.net.Socket, selector: SelectorManager ) : NIOSocketImpl<S>(channel, selector, pool = null), Socket Ktor

reiﬁed • ؔ਺಺͔Βܕม਺ʹΞΫηεग़དྷΔ • JavaͰ͸ܕม਺ʹϥϯλΠϜ࣌ʹΞΫηεͰ͖ͳ͍ • Type Erasureͱݺ͹ΕΔݴޠ࢓༷ͷͨΊɺίϯύΠϧޙʹܕม਺͕ ࡟আ͞ΕΔͨΊ

inline fun <reified T> hoge(obj: Any) { a println(T::class.java) a
if (obj is T) { a println("obj is T") a } a } a

֦ுؔ਺ + δΣωϦΫε • Kotlinͷ֦ுؔ਺ͱδΣωϦΫεΛ૊Έ߹ΘͤΔͱΑΓॊೈʹ ܕม਺Λίϯτϩʔϧ͢Δ͜ͱ͕ग़དྷΔ

class A<T>(val value: T) { a fun isNull(): Boolean {
a return value != null } a } a

a return value != null } a } a val a1: A<Int?> = A(null) a a1.isNull() a

a return value != null } a } a val a1: A<Int?> = A(null) a a1.isNull() a val a2: A<Int> = A(10) a a2.isNull() a

class A<T>(val value: T) a fun <T : Any> A<T?>.isNull():
Boolean { a return value != null a } a val a1: A<Int?> = A(null) a a1.isNull() a val a2: A<Int> = A(10) a a2.isNull() a

class A<T>(val value: T) a fun <T : Any> A<T?>.isNull():
Boolean { a return value != null a } a val a1: A<Int?> = A(null) a a1.isNull() a val a2: A<Int> = A(10) a a2.isNull() a ❌

• Smart Casts • AnyɺUnitɺNothing • δΣωϦΫε •Sealed class •
ؔ਺ܕ

Sealed class • ੍ݶ͞ΕͨαϒΫϥε֊૚ߏ଄Λ࡞Δ͜ͱ͕ग़དྷΔ • ಛघͳ༻్ͷabstract class

abstract class Either object Left : Either() object Right :
Either() when (obj) { a Left -> println("is Left") Right -> println("is Right") else -> println("else") } a

sealed class Either object Left : Either() object Right :
Either() when (obj) { a Left -> println("is Left") Right -> println("is Right") else -> println("else") } a

sealed class Either object Left : Either() object Right :
Either() when (obj) { a Left -> println("is Left") Right -> println("is Right") } a

sealed͸classͰͷΈ༗ޮ • interfaceͩͱɺ༰қʹܧঝग़དྷͯ͠·͏ͨΊɺ੍໿ΛഁΕͯ ͠·͏

public abstract class Either { private Either() { } //
$FF: synthetic method public Either(DefaultConstructorMarker $constructor_marker) { this(); } }

• Smart Casts • AnyɺUnitɺNothing • δΣωϦΫε • Sealed class
•ؔ਺ܕ

ؔ਺ܕ • KotlinͰ͸ؔ਺ʢFunctionʣ͕ϑΝʔετΫϥεΦϒδΣΫτ • ม਺ɺؔ਺ͷҾ਺ɺ໭Γ஋ͳͲʹؔ਺ܕΛ࢖͏͜ͱ͕ग़དྷΔ

ؔ਺ܕ • Function0 ~ Function22ͷ22Ҿ਺·Ͱͷؔ਺ΠϯλʔϑΣʔε ͱɺͦΕҎ্ͷҾ਺༻ͷɺFunctionNΠϯλʔϑΣʔε͕ఆٛ ͞Ε͍ͯΔ • ্هͷؔ਺ΠϯλʔϑΣʔεʹίϯύΠϧ࣌ʹม׵͞ΕΔ

fun hoge(body: Function1<Int, String>) { a body(10) } a

fun hoge(body: (Int) -> String) { a body(10) } a

fun hoge(body: (Int) -> String) { a body(10) } a
hoge { it.toString() } a

fun add(a1: Int, a2: Int): Int { a return a1
+ a2 } a add add(10, 20)

+ a2 } a add add(10, 20) val a: (Int, Int) -> Int = ::add

+ a2 } a add add(10, 20) val a: (Int, Int) -> Int = ::add a(10, 20)

} a add add(10, 20) val a: (Int, Int) ->
Int = ::add a(10, 20) fun <P1, P2, R> ((P1, P2) -> R).curried(): (P1) -> (P2) -> R = { p1: P1 -> { p2: P2 -> this(p1, p2) } } a

Int = ::add a(10, 20) fun <P1, P2, R> ((P1, P2) -> R).curried(): (P1) -> (P2) -> R = { p1: P1 -> { p2: P2 -> this(p1, p2) } } a val curried = a.curried()

Int = ::add a(10, 20) fun <P1, P2, R> ((P1, P2) -> R).curried(): (P1) -> (P2) -> R = { p1: P1 -> { p2: P2 -> this(p1, p2) } } a val curried = a.curried() curried(10)(20)

class A : (Int) -> Unit { a override fun
invoke(p1: Int) { a println("call $p1") } a } a val a = A() a.invoke(10)

class A : (Int) -> Unit { a override fun
invoke(p1: Int) { a println("call $p1") } a } a val a = A() a(10)

public interface Function1<in P1, out R> : Function<R> { public
operator fun invoke(p1: P1): R } a

operator fun invoke(p1: P1): R } a val a: (Number) -> Number = { it } a

operator fun invoke(p1: P1): R } a val a: (Number) -> Number = { it } a val a1: (Int) -> Number = a

operator fun invoke(p1: P1): R } a val a: (Number) -> Number = { it } a val a1: (Int) -> Number = a val a2: (Number) -> Any = a

operator fun invoke(p1: P1): R } a val a: (Number) -> Number = { it } as val a1: (Int) -> Number = a val a2: (Number) -> Any = a

SAMม׵ • SAM? • Single Abstract Method • 1ͭͷந৅ϝιουΛ࣋ͭ •
Kotlin͔ΒJavaఆٛͷSAMΛ࢖͏ͱ͖ʹɺϥϜμࣜͰ؆ܿʹ ॻ͘͜ͱ͕ग़དྷΔ

public class SamTest { a public static void foo(Runnable a,
Runnable b) { a } a } a fun test() { a SamTest.foo({}) { a } a } a

Runnable b) { a } a } a fun test() { a SamTest.foo({}) { a } a } a public interface Runnable { public abstract void run(); }

Runnable b) { a } a } a fun test(r: Runnable) { a SamTest.foo(r) { } } a

Runnable b) { a } a } a fun test(r: Runnable) { a SamTest.foo(r) { } } a ❌

KotlinͷSAMม׵ͷ՝୊ • ෳ਺ͷSAMΛҾ਺ʹऔΔ৔߹ɺ1ͭͰ΋࣮ମΛ࢖ͬͯ͠·͏ ͱɺҙਤͨ͠ม׵͕ߦΘΕͳ͍

// RxJava val o1 = Observable.just(1) val o2 = Observable.just(2)
val combination = Observable .combineLatest(o1, o2, { n1, n2 -> n1 to n2 }) a

val combination = Observable .combineLatest(o1, o2, { n1, n2 -> n1 to n2 }) public static <T1, T2, R> Observable<R> combineLatest( ObservableSource<? extends T1> source1, ObservableSource<? extends T2> source2, BiFunction<? super T1, ? super T2, ? extends R> combiner ) a

val combination = Observable .combineLatest(o1, o2, { n1, n2 -> n1 to n2 }) public static <T1, T2, R> Observable<R> combineLatest( ObservableSource<? extends T1> source1, ObservableSource<? extends T2> source2, BiFunction<? super T1, ? super T2, ? extends R> combiner ) a public interface ObservableSource<T> { a void subscribe(@NonNull Observer<? super T> observer); } ˇ

val combination = Observable .combineLatest(o1, o2, { n1, n2 -> n1 to n2 })

val combination = Observable .combineLatest(o1, o2, { n1, n2 -> n1 to n2 }) ❌

val combination = Observable .combineLatest(o1, o2, BiFunction { n1, n2 -> n1 to n2 })

public static <T1, T2, R> Observable<R> combineLatest( ObservableSource<? extends T1>
source1, ObservableSource<? extends T2> source2, BiFunction<? super T1, ? super T2, ? extends R> combiner ) a

public static <T1, T2, R> Observable<R> combineLatest( Observable<? extends T1>
source1, Observable<? extends T2> source2, BiFunction<? super T1, ? super T2, ? extends R> combiner ) a

source1, Observable<? extends T2> source2, BiFunction<? super T1, ? super T2, ? extends R> combiner ) a val o1 = Observable.just(1) val o2 = Observable.just(2) val combination = Observable .combineLatest(o1, o2, BiFunction { n1, n2 -> n1 to n2 })

source1, Observable<? extends T2> source2, BiFunction<? super T1, ? super T2, ? extends R> combiner ) a val o1 = Observable.just(1) val o2 = Observable.just(2) val combination = Observable .combineLatest(o1, o2, { n1, n2 -> n1 to n2 })

NewInference • kotlinͰ͸SAMม׵ͷ໰୊ʹରͯ͠ɺ৽͍͠ܕਪ࿦Λ։ൃͯ͠ ͍Δ • ݱࡏɺexperimentalͷཱͪҐஔ͕ͩɺઃఆͰenableʹ͢Δ͜ͱ ͕Մೳ

·ͱΊ • KotlinͷSmart Castsͱܕਪ࿦Ͱɺ৑௕ͳίʔυΛݮΒ͠ɺҰ ෦μοΫλΠϐϯάͷΑ͏ͳॻ͖৺஍ΛಘΔ͜ͱ͕Մೳʂ • ॊೈͳδΣωϦΫεAPIΛ࢖͏͜ͱͰɺॊೈͳAPI͕ߏஙग़དྷ Δʂ • NothingܕΛ࢖͏͜ͱͰɺॊೈͳAPIΛߏஙͰ͖Δʂ
• Sealed classΛ࢖͏͜ͱͰɺॊೈͳAPIΛߏஙͰ͖Δʂ

Kotlin Fest 2019 ࠤ౻ ൏ / Sato Shun Twitter: @stsn_jp
GitHub: satoshun Kotlinܕ࣮ફೖ໳

Kotlin Fest 2019: Kotlin型実践入門

Kotlin Fest 2019: Kotlin型実践入門

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