Stream.repeatedly doesn’t take an arg, so it’d look more like:
Stream.repeatedly(&start_workers/1)
|> Enum.find(fn _ -> worker_online() == target end)
Regardless though, the root issue in this case is less about the chosen method of iteration and more about polling workers_online to determine if workers are in fact online. Message passing would be a lot more idiomatic here.
I had been working with ruby for 10+ years and there was not a single occasion of me resorting to while loop. I even went to docs now to check if ruby indeed has while in a core.
Imperative loops are extremely counter-idiomatic even in ruby, in Elixir they are plain alien. Besides that Stream module provides several ways to accomplish the task, the plain recursion would perfectly suit here.
Or, alternatively, GenServer.handle_continue/2 that blocks (returning {:noreply, _state, {:continue, :wait_for_workers}}) unless there are N workers. One might put this watchdog into a dedicated start_phase to precisely tune up the starting process.
To make this happen, you could do something like:
Stream.repeatedly(&start_new_worker/0)
|> Enum.take(max(target - workers_online(), 0))