ERM: Embedded Ruby Macro, code to write code

erm.rb

#!/usr/bin/ruby

require "fileutils"
require "erubis"
require "yaml"

def erm(source)
  content, data = "", {}
  loop do
    properties, fragment = File.read(source).split("---\n", 2)
    properties, fragment = "", (properties || "") if fragment.nil?
    content = fragment.gsub(/<%=\s*yield\s*%>/, content)
    data = (YAML.load(properties) || {}).merge(data)
    if data.has_key? "layout"
      name = data.delete "layout"
      source = "layout/#{name}.erm"
    else
      break
    end
  end
  Erubis::Eruby.new(content).evaluate(data)
end

Dir["src/**/*.erm"].each do |source|
  target = source.sub(/^src/, "target").sub(/\.erm$/, "")
  FileUtils.mkdir_p File.dirname target
  File.write target, erm(source)
end