Is it possible to pass an argument into a sequence function? - kotlin

Question

I'm looking for a way to pass an argument into a Kotlin sequence function similar to how it works in JS:
function *gen () {
console.log(yield) // prints 1
console.log(yield) // prints 2
}
const it = gen()
it.next() // first iteration will execute the first yield and pause
it.next(1) // we pass 1 to the first yield which will be printed
it.next(2) // we pass 2 to the second yield which will be printed
Something like this in Kotlin:
fun main() {
val it = gen().iterator()
// Iterator#next() doesn't expect an argument
it.next(1)
it.next(2)
}
fun gen() = sequence {
println(yield(null)) // Would print 1
println(yield(null)) // Would print 2
}

Answer 1

Kotlin Sequences do not support passing arguments to each yield, but you have at least 2 ways to implement needed behaviour:
Using actors:
class NextQuery<A, T>(val arg: A, val next: CompletableDeferred<T> = CompletableDeferred())
fun test() = runBlocking {
val actor = GlobalScope.actor<NextQuery<String, Int>> {
for (nextQuery in channel) {
nextQuery.next.complete(nextQuery.arg.length)
}
}
val query1 = NextQuery<String, Int>("12345")
actor.send(query1)
println(query1.next.await())
val query2 = NextQuery<String, Int>("1234")
actor.send(query2)
println(query2.next.await())
}
Using channels:
class ArgSequenceScope<out A, in T>(
private val argChannel: ReceiveChannel<A>,
private val nextChannel: SendChannel<T>
) {
suspend fun yield(next: T) {
nextChannel.send(next)
}
suspend fun arg(): A = argChannel.receive()
}
class ArgSequence<in A, out T>(
private val argChannel: SendChannel<A>,
private val nextChannel: ReceiveChannel<T>
) {
suspend fun next(arg: A): T {
argChannel.send(arg)
return nextChannel.receive()
}
}
fun <A, T> sequenceWithArg(block: suspend ArgSequenceScope<A, T>.() -> Unit): ArgSequence<A, T> {
val argChannel = Channel<A>()
val nextChannel = Channel<T>()
val argSequenceScope = ArgSequenceScope(argChannel, nextChannel)
GlobalScope.launch {
argSequenceScope.block()
argChannel.close()
nextChannel.close()
}
return ArgSequence(argChannel, nextChannel)
}
fun test() {
val sequence = sequenceWithArg<String, Int> {
yield(arg().length)
yield(arg().length)
}
runBlocking {
println(sequence.next("12345"))
println(sequence.next("1234"))
}
}