config.ru

class ReplaceWords
  attr_reader :app

  def initialize(app)
    @app = app
  end

  def call(env)
    binding.irb
    http_status_code, headers, body = @app.call(env)
    return [http_status_code, headers, body.map { _1.gsub!(/ruby/i, 'rack') }]
  end
end

class UpperWords
  attr_reader :app

  def initialize(app)
    @app = app
  end

  def call(env)
    binding.irb
    http_status_code, headers, body = @app.call(env)
    return [http_status_code, headers, body.map { _1.upcase }]
  end
end

class App
  attr_reader :app

  def call(env)
    binding.irb
    [200, {'Content-Type' => 'text/html'}, ['Hello, Ruby world!']]
  end
end

use UpperWords
use ReplaceWords

binding.irb
run App.new