I want to generate a finite sequence of elements from a collection, is there a way to signal that th...

# getting-started

[Jens Lindgren]

I want to generate a finite sequence of elements from a collection, is there a way to signal that the sequence has ended using the sequence {} syntax? consider:

val mySequence = sequence { 
  yield (
      when (takeSubListOf(foo, 0, n--)) { subListOfFoo ->
      hasMultipleElements(subListOfFoo) -> subListOfFoo.reduce { ... }
      hasOneElement(subListOfFoo) -> subListOfFoo.first()
      else -> //sequence ended
    }
  )
}

[Jens Lindgren]

For the curious, here is a possible solution:

val mySequence = sequence { 
    when (takeSubListOf(foo, 0, n--)) { subListOfFoo ->
    hasMultipleElements(subListOfFoo) -> yield(subListOfFoo.reduce { ... })
    hasOneElement(subListOfFoo) -> yield(subListOfFoo.first())
    else -> yieldAll(emptySequence())
  }
}

[Klitos Kyriacou]

Or you could just not use yield.

[Jens Lindgren]

Thank you, excellent! It could be useful to showcase an example sequence chunk without yield calls at https://kotlinlang.org/docs/sequences.html for the less initiated.

[Sam]

Sequence builders are a lot like functions—they finish automatically when they reach the end of their code. So in this example, just doing nothing is fine. But in cases where you want to end the sequence early, before reaching the end of the builder code, you can also use return@sequence.

[Klitos Kyriacou]

Also, it seems that you are creating either an empty sequence or a single-element sequence. You might prefer to use sequenceOf for that:

val subListOfFoo = takeSubListOf(foo, 0, n--)
val mySequence = when {
    hasMultipleElements(subListOfFoo) -> sequenceOf(subListOfFoo.reduce { ... })
    hasOneElement(subListOfFoo) -> sequenceOf(subListOfFoo.first())
    else -> emptySequence()
}

[Klitos Kyriacou]

Also, in your case, subListOfFoo.reduce { ... } will return subListOfFoo.first() when it only has one element, so you can get rid of that when branch and rely on reduce instead.

[Jens Lindgren]

My thanks to everyone for all the great replies and insights!