rpn-spec.rb

require "./rpn"

describe "rpn" do
  it "" do
    expect(expr(4).("+").(3).("*").(2).("-").(1).("=")).
      to eq(->(){
              comp1 = 3 * 2;
              comp2 = 3 + comp1;
              comp3 = comp2 - 1;
              return comp3 }.call)
  end
end

rpn.rb

def expr(e, stack=[], rpn=[])
  case e
  when "="
    rpn += stack.reverse
    return rpn.inject([]) do |arr, token|
        case token.to_s
        when /[-+*\/]/ then arr << arr.pop(2).inject($&)
        when /\d+/ then arr << $&.to_f
        end
    end.shift
  when /[\-+*\/]/
    case e
    when /[*\/]/ then stack << e
    else rpn += stack.reverse && stack = [e]
    end
  else rpn << e
  end
  ->(e){expr(e, stack, rpn)}
end

puts expr(4).("+").(3).("*").(2).("-").(1).("=")
puts expr(1).("*").(2).("+").(6).("/").(3).("=")
puts expr(1).("+").(2).("+").(6).("/").(3).("=")