purelisp.rb

module MS
  def print(token)
    case token
    when MAtom
      token.token
    when Array
      "( #{token.map{ |t| print(t) }.join(' ')} )"
    else
      raise 'Syntax Error!!'
    end
  end

  module_function :print
end

class MAtom
  attr_reader :token

  def initialize(token)
    @token = token
  end
end

class MNil
  attr_reader :nil

  def initialize
    @nil = nil
  end
end

class MPaer
  attr_reader :car, :cdr

  def initialize(car, cdr)
    @car = car
    @cdr = cdr
  end
end

class MLexer
  def initialize(str)
    @chars = str.split('')
    @index = 0
    @depth = 0
    @ret = []
  end

  def run
    if current_token == '('
      @depth = 1
      next!
      while @chars.size > @index
        case current_token
        when '('
          @ret.push( [] )
          @depth += 1
          next!
        when %r([a-zA-Z0-9])
          tmp = ""

          loop do
            tmp += current_token
            next!

            break unless current_token =~ %r([a-zA-Z0-9])
          end

          if @depth == 1
            @ret.push( MAtom.new(tmp) )
          else
            @ret[@depth-1].push( MAtom.new(tmp) )
          end
        when ')'
          @depth -= 1
          next!
        when ' '
          next!
        else
          raise 'Parse Error!!'
        end
      end
    else
      raise 'Parse Error!!'
    end

    raise "Syntax Error!!" if @depth != 0

    @ret
  end

  private

  def current_token
    @chars[@index]
  end

  def next!
    @index += 1
  end
end

a = MLexer.new('( atom ( 2 3 ) )').run

puts MS.print(a)