Given two streams A and B, I want to merge them such that the merge emits items from A only until B emits. At that point it emits B only. How can I do this? Thanks!

could be something like this:

var emittedB = false
merge(a.takeWhile { !emittedB }, b.onEach { emittedB = true })

so, takeWhile has a stream overload - playing around with that

Do not use external states and side effects

it's fine if it's encapsulated. if the resulting flow may be subscribed to multiple times,

flow {
    var emittedB = false
    emitAll(merge(a.takeWhile { !emittedB }, b.onEach { emittedB = true }))
}

would do