Is there a way to poll a single element from Flow,...

# coroutines

[tateisu]

Is there a way to poll a single element from Flow, like Channel.receive ()? I want to poll the data inside a recursive parser function instead of always receiving the data in the same lambda.

[gildor]

No, there is no way, Flow is cold stream, and even hot version of it doesn’t work like that

[gildor]

What is your use case?

[gildor]

Channel may be right abstraction if this what you need

[tateisu]

// parser with recursive call
suspend fun simpleParser(ch: ReceiveChannel<Char>, nest: Int = 0) {
   while (true) {
      when (val a = ch.receiveOrNull()) {
         null, ']' -> break
         '[' -> simpleParser(ch, nest + 1)
         else -> println("$nest $a")
      }
   }
}

runBlocking {
   simpleParser(
      flowOf('a', '[', 'b', ']', 'c')
         .produceIn(this)
   )
}

[tateisu]

I want to do the same thing with keeping the cold stream

[tateisu]

maybe I want Flow.iterator()

[tateisu]

it create reading context similar to collect(), but also provide reader object. like flow.collectIterator{ it -> while(it.hasNext() ) it.read() }

[gildor]

There is no such thing as hasNext() in Flow, that the whole point of cold streams, they are push based, not pull based

[gildor]

I don't see why in example above you need while (true), why not simple collect? Maybe there is some more wider context, because now I don't see what converting to channel gives here. Just throw cancellation exception instead of break to stop collecting

[gildor]

You also can use takeUntil operator to stop collecting

[tateisu]

In the above example, when the parentheses appear, it make a nested recursive call and read the continuation of the flow.
but in case of your "stop collecting", how can I write the such control ?