markdown_handler.rb

require 'redcarpet'

module MarkdownHandler
  def self.erb
    @erb ||= ActionView::Template.registered_template_handler(:erb)
  end

  def self.call(template, source)
    _frontmatter, markdown = Page.parse_frontmatter(source, raw: true)
    compiled_source = ActionView::OutputBuffer.new(erb.call(template, markdown)).to_s
    # Stringify the output until RedCarpet can accept OutputBuffer.
    # https://github.com/vmg/redcarpet/issues/783
    # https://gist.github.com/KeithP/5f959ca70f0b09a1ce5b5cf4831829f6
    compiled_source << '.to_s'
    "Redcarpet::Markdown.new(Redcarpet::Render::HTML, autolink: true, fenced_code_blocks: true).render(begin;#{compiled_source};end).html_safe"
  end
end

ActionView::Template.register_template_handler :md, MarkdownHandler