class Concurrent::Promise

def flat_map(&block)
  child = Promise.new(
    parent: self,
    executor: ImmediateExecutor.new,
  )
  on_error { |e| child.on_reject(e) }
  on_success do |result1|
    begin
      inner = block.call(result1)
      inner.execute
      inner.on_success { |result2| child.on_fulfill(result2) }
      inner.on_error { |e| child.on_reject(e) }
    rescue => e
      child.on_reject(e)
    end
  end
  child
end