rev_polish.rb

class RevPolish
  def initialize(expression)
    @expression = expression.split
    @stack = []
  end

  def result
    @expression.each{|n| add n }
    raise SyntaxError, @expression unless @stack.size == 1
    @stack.last
  end

  private

  def add n
    case n
    when /\A[+-\/*]\z/ then calcurate n.to_sym
    when /\A[0-9]+\z/  then stack n.to_i
    end
    p @stack
    self
  end

  def stack operand
    @stack.push operand
  end

  def calcurate operator
    @stack.push( -> n { @stack.pop.send operator, n }.call @stack.pop )
  end
end

p RevPolish.new(ARGV.shift).result