RackImitation.rb

#coding: utf-8                                                                                                                             

class M1
    def initialize(app)
        @app = app 
    end 

    def call(env)
        env = env + '1' 
        res = @app.call(env)
        res = res + '6' 
    end 
end

class M2
    def initialize(app)
        @app = app 
    end 

    def call(env)
        env = env + '2' 
        res = @app.call(env)
        res = res + '5' 
    end 
end

class App
    def call(env)
        env = env + '3' 
        res = env + '4' 
    end 
end

p M1.new(M2.new(App.new)).call 'request'

# 以下と同義
#app = App.new
#m2 = M2.new app
#m1 = M1.new m2
#p m1.call 'request'