RPN i Ruby

rpn.rb

def rpn(expr)
  head, tail = expr.first, expr[1..-1]
  case head
    when nil
      return lambda { |n| n }
    when /\d+/
      rpn(tail).call(Integer(head))
    else
      rest = rpn(tail)
      lambda { |n|
        lambda { |m|
          rest.call(m.send(expr[0].to_sym, n))
        }
      }
  end
end

input = ARGV[0].split
puts rpn(input)