kotlin-stdlib/kotlin.sequences/sequence

sequence

fun <T> sequence(block: suspend SequenceScope<T>.() -> Unit): Sequence<T>(source)

Builds a Sequence lazily yielding values one by one.

If the consuming code stops iterating the sequence before it's completed, the remainder of the computation will not run at all. In particular, it means that finally blocks may be left unexecuted:

val sequenceOfOne = sequence {
    try {
        yield(1)
        // no code after the `yield(1)` line will be executed!
    } finally {
        println("This line will not run")
    }
}
// only get the first element, do not attempt evaluating the rest of them
println(sequenceOfOne.first())
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Since Kotlin

1.3

See also

Samples

import kotlin.test.*

fun main() { 
   //sampleStart 
   val sequence = sequence {
    val start = 0
    // yielding a single value
    yield(start)
    // yielding an iterable
    yieldAll(1..5 step 2)
    // yielding an infinite sequence
    yieldAll(generateSequence(8) { it * 3 })
}

println(sequence.take(7).toList()) // [0, 1, 3, 5, 8, 24, 72] 
   //sampleEnd
}
import kotlin.test.*

fun main() { 
   //sampleStart 
   fun fibonacci() = sequence {
    var terms = Pair(0, 1)

    // this sequence is infinite
    while (true) {
        yield(terms.first)
        terms = Pair(terms.second, terms.first + terms.second)
    }
}

println(fibonacci().take(10).toList()) // [0, 1, 1, 2, 3, 5, 8, 13, 21, 34] 
   //sampleEnd
}