expr2rpn.rb

#! /usr/local/bin/ruby27

module Expr2RPN
  class Node
    def method_missing mname, *args
      unless [:+, :*].include? mname then
        super
      end
      args_length = args.length
      if args_length != 1 then
        raise ArgumentError, "#{mname}: wrong number of arguments (given #{args_length}, expected 1)"
      end
      NodeRPN.new "#{self.to_rpn} #{args[0].to_rpn} #{mname}"
    end
  end

  class NodeRPN < Node
    attr :val

    def initialize arg
      @val = arg
    end

    def to_rpn
      @val
    end
  end

  class Term < Node
    attr :val

    def initialize arg
      @val = arg
    end

    def to_rpn
      "#{@val}"
    end
  end

  class Runner < BasicObject
    def term arg
      Term.new arg
    end
  end

  def self.run &c
    Runner.new.instance_eval &c
  end
end

print(Expr2RPN.run{
  a = term 3
  b = term 4
  c = term 5
  d = term 6

  "#{(a*b + c*d).to_rpn}\n"
})