関数にLambdaを渡す

ラムダを受け取る関数

高階関数の宣言

fun applyTwice(n: Int, op: (Int) -> Int): Int {
    return op(op(n))
}
println(applyTwice(3, { it * 2 }))  // 12

末尾ラムダ構文

fun applyTwice(n: Int, op: (Int) -> Int) = op(op(n))

// Standard:
applyTwice(3, { it * 2 })
// Trailing lambda:
applyTwice(3) { it * 2 }

かっこなしの末尾ラムダ

fun execute(block: () -> Unit) = block()

// With parens:
execute({ println("Hello") })
// Without parens (idiomatic):
execute { println("Hello") }

ラムダ変数を渡す

val double = { n: Int -> n * 2 }
fun applyTwice(n: Int, op: (Int) -> Int) = op(op(n))
println(applyTwice(5, double))  // 20

::による関数参照

fun triple(n: Int) = n * 3
fun applyTwice(n: Int, op: (Int) -> Int) = op(op(n))
println(applyTwice(2, ::triple))  // 18

複数のラムダパラメーター

fun transform(
    value: Int,
    before: (Int) -> Int,
    after: (Int) -> Int
) = after(before(value))

println(transform(5, { it * 2 }, { it + 1 }))  // 11

filter にラムダを渡す

val numbers = listOf(1, 2, 3, 4, 5, 6)
val evens = numbers.filter { it % 2 == 0 }
println(evens)  // [2, 4, 6]

高階関数の連結

val result = (1..10)
    .filter { it % 2 != 0 }  // odds
    .map { it * it }          // square
    .take(3)                  // first 3
println(result)  // [1, 9, 25]

ラムダによる変数のキャプチャ

var total = 0
listOf(1, 2, 3, 4).forEach { total += it }
println(total)  // 10

関数からラムダを返す

fun multiplierOf(factor: Int): (Int) -> Int {
    return { n -> n * factor }
}
val double = multiplierOf(2)
val triple = multiplierOf(3)
println(double(5))  // 10
println(triple(5))  // 15

簡単な確認

まとめ