Gemfile

source "https://rubygems.org"
gem "rack"
gem "sinatra"
# gem "pry"

racky-app.rb

require "sinatra"
require "rack"

module RackyRack
  class Middleware
    def initialize(app)
      @app = app
    end

    def call(env)
      response = Rack::Response[*@app.call(env)]

      # response.body は配列です
      # ここでいろいろ加工したり、代入しなおして試してみよう
      response.body = response.body.map(&:reverse)

      response.finish
    end
  end
end

get "/" do
  "This is sample application!!"
end

use RackyRack::Middleware

# Usage: bundle exec ruby racky-app.rb

zz-Usage.console

$ bundle exec ruby racky-app.rb
Ignoring curses-1.3.2 because its extensions are not built. Try: gem pristine curses --version 1.3.2
[2020-10-22 18:14:50] INFO  WEBrick 1.6.0
[2020-10-22 18:14:50] INFO  ruby 2.7.1 (2020-03-31) [x86_64-darwin19]
== Sinatra (v2.1.0) has taken the stage on 4567 for development with backup from WEBrick
[2020-10-22 18:14:50] INFO  WEBrick::HTTPServer#start: pid=33267 port=4567
127.0.0.1 - - [22/Oct/2020:18:14:54 +0900] "GET / HTTP/1.1" 200 28 0.0066
127.0.0.1 - - [22/Oct/2020:18:14:54 JST] "GET / HTTP/1.1" 200 28
- -> /
^C== Sinatra has ended his set (crowd applauds)
[2020-10-22 18:14:57] INFO  going to shutdown ...
[2020-10-22 18:14:57] INFO  WEBrick::HTTPServer#start done.

----

$ curl localhost:4567
!!noitacilppa elpmas si sihT
#=> ???