command.rb

class Command
  attr_reader :result

  def initialize(result)
    @result = result
  end

  def execute
  end
end

class CompositeCommand < Command
  def initialize(result)
    @commands = []
    @result = result
  end

  def add_commmand command
    @commands.push command
  end
  
  def execute
    @commands.each {|command| @result = command.execute(@result) }
  end
end

class MultiplyRule < Command
  def initialize(multiplier)
    @multiplier = multiplier
    super
  end

  def execute(result)
    result * @multiplier
  end
end

class SumRule < Command
  def initialize(sum)
    @sum = sum
    super
  end

  def execute(result)
    result + @sum
  end
end

cmd = CompositeCommand.new 1
cmd.add_commmand MultiplyRule.new 1 # 1*1=1
cmd.add_commmand SumRule.new 2 # 1+2=3
cmd.add_commmand MultiplyRule.new 3 # 3*3=9
cmd.add_commmand SumRule.new 2 # 9+2=11
cmd.execute
p cmd.result # ~> 11