config.ru

class App
  def call(env)
    [200, {"Content-Type" => "text/html"}, ["Hello!"]]
  end
end

class Upper
  def initialize(app)
    @app = app
  end

  def call(env)
    status, headers, body = @app.call(env)
    body.map {|e| e.upcase!}
    [status, headers, body]
  end
end

class Headify
  def initialize(app)
    @app = app
  end
  
  def call(env)
    status, headers, body = @app.call(env)
    body.unshift "<h1>A header</h1>"
    [status, headers, body]
  end
end

use Upper
use Headify

run App.new