What are differents between foo().collect{...} and launch { foo().collect{...} }? - kotlin

Question

The Code A get the Result A, and the Code B get the Result B .
1: Is the fun foo() coroutines ? why doesn't it add keyword suspend before if it's a coroutines function?
2: In my mind, a coroutines function is fired using launch {...} or aync {...} if fun foo() is a coroutines function, why can the main() fun in Code B invoke it directly?
3: What are differents between Code A and Code B ?
Code A
fun main() = runBlocking<Unit> {
launch {
for (k in 1..3) {
println("I'm not blocked $k")
delay(100)
}
}
launch {
foo().collect { value -> println(value) }
}
}
fun foo(): Flow<Int> = flow {
for (i in 1..3) {
delay(100)
emit(i+5)
}
}
Result A
I'm not blocked 1
I'm not blocked 2
6
I'm not blocked 3
7
8
Code B
fun main() = runBlocking<Unit> {
launch {
for (k in 1..3) {
println("I'm not blocked $k")
delay(100)
}
}
foo().collect { value -> println(value) }
}
fun foo(): Flow<Int> = flow {
for (i in 1..3) {
delay(100)
emit(i+5)
}
}
Result B
I'm not blocked 1
6
I'm not blocked 2
7
I'm not blocked 3
8

Answer 1

The foo function is not a suspending function, so you can call it outside a coroutine scope. This function builds a flow and returns the flow, but the flow does not start.
The flow only starts when you call collect on it, and that is a suspending function, so calling collect must be done in a coroutine scope.
flows are cold, they only start emitting items when you call collect on them, so building the flow is not a suspending function because it's a fast operation.

Answer 2

1: Is the fun foo() coroutines ? why doesn't it add keyword suspend before if it's a coroutines function?
Actually foo() is a regular function that returns a Flow.
2: In my mind, a coroutines function is fired using launch {...} or aync {...} if fun foo() is a coroutines function, why can the main() fun in Code B invoke it directly?
Also using runBlocking{...}, runBlockingTest{...}, produce{...} and actor{...}.
It doesn't matter where you build/create/declare a Flow, what matters is where you call terminal operators (like collect) which are suspending functions and must be called inside a coroutine or inside another suspending function. Btw, you're running all the main method inside a coroutine since you're wrapping it inside a runBlocking block.
3: What are differents between Code A and Code B ?
None. You're just creating another coroutine on Code A which will also run on the main thread.