Merangkai Pipeline dan Menghindari List Perantara dengan Sequence

Masalah: Daftar Perantara

Evaluasi Langsung vs Malas

Mengonversi ke Urutan

val result = (1..1_000_000)
    .asSequence()
    .filter { it % 2 == 0 }
    .map { it * it }
    .take(5)
    .toList()
println(result)  // [4, 16, 36, 64, 100]

Urutan Bersifat Malas: Tidak Ada Pekerjaan hingga Operasi Terminal

val seq = generateSequence(1) { it + 1 }  // infinite!
    .filter { it % 3 == 0 }
    .map { it * it }
val first5 = seq.take(5).toList()
println(first5)  // [9, 36, 81, 144, 225]

generateSequence: Urutan Tak Terbatas

val fibonacci = generateSequence(Pair(0, 1)) { (a, b) -> Pair(b, a + b) }
    .map { it.first }
val first10 = fibonacci.take(10).toList()
println(first10)  // [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

Pembuat Urutan

val evens = sequence {
    var n = 0
    while (true) {
        yield(n)
        n += 2
    }
}
println(evens.take(5).toList())  // [0, 2, 4, 6, 8]

Kapan Menggunakan Urutan

Kapan NOT Menggunakan Urutan

// Overkill for small list:
listOf(1, 2, 3).asSequence().map { it * 2 }.toList()
// Just use:
listOf(1, 2, 3).map { it * 2 }

Operasi Terminal Urutan

val s = (1..100).asSequence().filter { it % 7 == 0 }
println(s.first())          // 7
println(s.count())          // 14
println(s.sum())            // 728

Operasi Berstatus vs Tanpa Status

val s = (1..10).asSequence()
    .filter { it > 3 }   // stateless: lazy
    .sorted()            // stateful: must see all elements
    .take(3).toList()
println(s)  // [4, 5, 6]

constrainOnce: Keamanan Iterasi Tunggal

val seq = generateSequence(1) { if (it < 5) it + 1 else null }
    .constrainOnce()
val first = seq.toList()   // OK
// val second = seq.toList()  // IllegalStateException!

Pemeriksaan Cepat

Ringkasan