concurrency.rb

def call(agent:, pages: 10, filter: nil)
  jobs = 1.upto(pages).map do |n|
    Concurrent::Promise.execute do
      beers = fetch_page(n)
      new_elements = filter.is_a?(Proc) ? beers.select(&filter) : beers

      agent.send { |set| set | new_elements }
    end
  end

  jobs.map(&:value)
  agent.await.value
end