module Sass
  module Tree
    class Node
      attr_accessor :children
      attr_accessor :line
      attr_accessor :filename

      def initialize(style)
        @style = style
        @children = []
      end

      def <<(child)
        if msg = invalid_child?(child)
          raise Sass::SyntaxError.new(msg, child.line)
        end
        @children << child
      end
      
      def to_s
        result = String.new
        children.each do |child|
          if child.is_a? AttrNode
            raise SyntaxError.new('Attributes aren\'t allowed at the root of a document.', child.line)
          end

          begin
            result += "#{child.to_s(1)}\n"
          rescue SyntaxError => e
            raise e
          end
        end
        result[0...-1]
      end

      private

      # This method should be overridden by subclasses to return an error message
      # if the given child node is invalid,
      # and false or nil otherwise.
      def invalid_child?(child)
        false
      end
    end
  end
end