gosukiwi

class BoldVisitor
  def visit(node)
    "<strong>#{node.value}</strong>"
  end
end

class EmphasisVisitor
  def visit(node)
    "<em>#{node.value}</em>"
  end

gosukiwi

class SentenceVisitor
  SENTENCE_VISITORS = {
    "BOLD"     => BoldVisitor,
    "EMPHASIS" => EmphasisVisitor,
    "TEXT"     => TextVisitor
  }.freeze

  def visit(node)
    visitor_for(node).visit(node)
  end

gosukiwi

class ParagraphVisitor
  def visit(paragraph_node)
    "<p>#{sentences_for(paragraph_node)}</p>"
  end

  private

  def sentences_for(paragraph_node)
    paragraph_node.sentences.map do |sentences|
      sentence_visitor.visit(sentences)

gosukiwi

class BodyVisitor
  def visit(body_node)
    body_node.paragraphs.map do |paragraph|
      paragraph_visitor.visit(paragraph)
    end.join
  end

  private

  def paragraph_visitor

gosukiwi

class Generator
  def generate(ast)
    body_visitor.visit(ast)
  end

  private

  def body_visitor
    BodyVisitor.new
  end

gosukiwi

Body               := Paragraph*

Paragraph          := SentenceAndNewline
                    | SentenceAndEOF
                    
SentenceAndNewline := Sentence+ NEWLINE NEWLINE
SentencesAndEOF    := Sentence+ NEWLINE EOF
                    | Sentence+ EOF
                    
Sentence           := EmphasizedText

gosukiwi

Start          := Binop
Binop          := Subtraction
Subtraction    := Adition "-" Binop
                | Adition
Adition        := Division "+" Binop
                | Division
Division       := Multiplication "/" Binop
                | Multiplication
Multiplication := Number "*" Binop
                | Number

gosukiwi

class BaseParser
  private

  # We use some reflection to prettify the parser depedencies. Basically, from
  # calling a `foo_parser` method is the same as doing `ParserFactory.build('foo_parser')`.
  #
  def method_missing(name, *args, &block)
    raise ArgumentError.new("Method #{name} does not exist.") unless name.to_s.end_with?('_parser')
    ParserFactory.build(name, *args, &block)
  end

gosukiwi

Start    := Binop
Binop    := Binop Operator Binop
          | Number
Operator := + | - | * | /
Number   := 0 | 1 | 2 | ... | 9

gosukiwi

   +------[START]----------+
   |          |            |
   v          v            v
[NUMBER=2] [OPERATOR=+] [BINOP]
                           |
              +------------+----------+
              |            |          |
              v            v          v
           [NUMBER=2] [OPERATOR=-] [NUMBER=4]