rpn.rb

#!/usr/bin/env ruby

until (words = gets.chomp.split).empty?
  stack = []
  words.each do |w|
    begin
      stack.push Float(w)
    rescue ArgumentError
      args = stack.pop(Float.instance_method(w).arity + 1)
      stack.push args.first.send(w, *args[1..-1])
    end
  end
  puts stack.join(" ")
end