creating_rules.md

title
Creating Rules

Rules are written in ruby, using a rule DSL for defining rules. A rule looks like:

rule "MD000", "Rule description" do
  tags :foo, :bar
  aliases 'rule-name'
  params :style => :foo
  check do |doc|
    # check code goes here
    # return a list of line numbers that break the rule, or an empty list
    # (or nil) if there are no problems.
  end
end

The first line specifies the rule name and description. By convention, built in markdownlint rules use the prefix 'MD' followed by a number to identify rules. Any custom rules should use an alternate prefix to avoid conflicting with current or future rules. The description is simply a short description explaining what the rule is checking, which will be printed alongside the rule name when rules are triggered.

Next, the rule's tags are specified. These are simply ruby symbols, and can be used by a user to limit which rules are checks. For example, if your rule checks whitespace usage in a document, you can add the :whitespace tag, and users who don't care about whitespace can exclude that tag on the command line or in style files.

You can also specify aliases for the rule, which can be used to refer to the rule with a human-readable name rather than MD000. To do this, add then with the 'aliases' directive. Whenever you refer to a rule, such as for including/excluding in the configuration or in style files, you can use an alias for the rule instead of its ID.

After that, any parameters the rule takes are specified. If your rule checks for a specific number of things, or if you can envision multiple variants of the same rule, then you should add parameters to allow your rule to be customized in a style file. Any parameters specified here are accessible inside the check itself using params[:foo].

Finally, the check itself is specified. This is simply a ruby block that should return a list of line numbers for any issues found. If no line numbers are found, you can either return an empty list, or nil, whichever is easiest for your check.

Document objects

The check takes a single parameter 'doc', which is an object containing a representation of the markdown document along with several helper functions used for making rules. The doc.rb file is documented using rdoc, and you will want to take a look there to see all the methods you can use, as well as look at some of the existing rules, but a quick summary is as follows:

• doc - Object containing a representation of the markdown document
• doc.lines - The raw markdown file as an array of lines
  • You can also look up a line given an element with doc.element_line(element)
• doc.parsed - The kramdown internal representation of the doc. Most of the time you will want to interact with the parsed version of the document rather than looking at doc.lines.
• doc.find_type_elements - A method to find all elements of a given type. You pass the type as a symbol, such as :ul or :p. Most element types match the name of the element in HTML output. This method returns a list of the matching elements.
• doc.find_type - This is like doc.find_type_elements, but returns just the options hashes (see below) for each element. This is useful if you don't need all the element information, but you do need the line numbers.
• doc.element_line_number - Pass in an element (or an options hash), and this will return the line number for the element. You need to return the line number in the list of errors.

Element objects

The document contains an internal representation of the markdown document as parsed by kramdown. Kramdown's representation of the document is as a tree of 'element' objects. The following is a quick summary of those objects:

• element.type - a symbol denoting the type of the element, such as :li, :p, :text
• element.value - the value of the element. Note that most block level elements such as paragraphs don't have any value themselves, but have child text elements containing their contents instead.
• element.children - A list of the element's child elements.
• element.options - A hash containing:
  • :location - line number of element
  • :element_level - A value filled in by markdownlint to denote the nesting level of the element, i.e. how deep in the tree is it.
  • Other options that are element type specific.