require 'treetop'

class MyPP
  def initialize(ast)
    @ast = ast
    @indentation = ''
  end

  def inspect
    @inspected ||= pretty_print @ast
  end

  private

  def pretty_print(ast)
    if !ast.terminal?
      ast.elements.inject('') { |printed, child| printed << pretty_print(child) }
    else
      case val = ast.text_value
      when /\A\d+\Z/ then val
      when '('       then indent!;   "("
      when ')'       then outdent!;  ")"
      when '+'       then "\n#{@indentation}#{val} "
      end
    end
  end

  def indent!
    @indentation << '  '
  end

  def outdent!
    @indentation.chomp! '  '
  end
end


Treetop.load_from_string <<GRAMMAR
grammar A
  rule operator_sequence
    lhs:single_element rest:(operator operator_sequence)*
  end

  rule single_element
    '(' operator_sequence ')' / num
  end

  rule num
    [0-9]+
  end

  rule operator
    "+"
  end
end
GRAMMAR

ast = AParser.new.parse '1+(1)+(2+2)+3+(1+2+3+((4+5)+(6+7)))+8'
p MyPP.new ast

# >> 1
# >> + (1)
# >> + (2
# >>   + 2)
# >> + 3
# >> + (1
# >>   + 2
# >>   + 3
# >>   + ((4
# >>       + 5)
# >>     + (6
# >>       + 7)))
# >> + 8