Understanding Kotlin's yield function

Question

I don't see a very clear definition of the yield function in Kotlin.

Example in the link above doesn't mention much but the following,

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] 

But the above example doesn't point out the significance of yield.

• What does it mean that it is a suspending function?
• In what scenarios can it be advantageous?

Answer 1

You can think of yield() as "return, and next time start from where you stopped":

val sequence = sequence {     val start = 0     yield(start) // first return     yieldAll(1..5 step 2) // if called again, will start from here     yieldAll(generateSequence(8) { it * 3 }) // if called more that six times, start from here } 

It creates state machine and stuff, but you can translate it to something like following in Java:

class Seq {     private AtomicInteger count = new AtomicInteger(0);     private int state = 0;          public int nextValue() {         if (count.get() == 0) {            return state;         }         else if (count.get() >= 1 && count.get() <= 5) {            state += 2;            return state;         }         else {            state *= 3;            return state;         }     } } 

In Java class we maintain explicit state by having two variables: count and state. Combination of sequence and yield allow this state to be maintained implicitly.

Note that yield() is a suspending function, so it may be invoked only from another suspend function or from within a coroutine.