composite.rb

#!/usr/bin/env ruby

class Node
  def value
    raise "Missing implementation for #{self.class.name}#value"
  end
end

class ValueNode < Node
  attr_reader :value

  def initialize(value)
    @value = value
  end
end

class BinaryNode < Node
  attr_reader :left, :right

  def initialize(left, right)
    @left = left
    @right = right
  end
end

class Sum < BinaryNode
  def value
    left.value + right.value
  end
end

class Multiply < BinaryNode
  def value
    left.value * right.value
  end
end

puts Sum.new(ValueNode.new(3), Multiply.new(ValueNode.new(4), ValueNode.new(5))).value # 3 + (4 * 5) = 23