r1 = Ractor.new do
  1.upto(10) do |i|
    sleep(0.2)
    Ractor.yield i
  end
end

r2 = Ractor.new do
  11.upto(15) do |i|
    sleep(0.2)
    Ractor.yield i
  end
end

loop do
  _, obj = Ractor.select(r1, r2)
  puts obj
end


Here is output:
> ruby ractor_play.rb
ractor_play.rb:1: warning: Ractor is experimental, and the behavior may change in future versions of Ruby! Also there are many implementation issues.
11
1
12
2
13
3
14
4
5
15
11