build do
  Environment.new do |env|
    require 'json'
    require 'yaml'
    env.add_builder(:JsonToYaml) do |params|
      unless @cache.up_to_date?(@target, :JsonToYaml, @sources, @env)
        print_run_message("JsonToYaml #{@target}", nil)
        @cache.mkdir_p(File.dirname(@target))
        File.open(@target, 'w') do |f|
          f.write(YAML.dump(JSON.load(IO.read(@sources.first))))
        end
        @cache.register_build(@target, :JsonToYaml, @sources, @env)
      end
      true
    end
    env.JsonToYaml('foo.yml', 'foo.json')
  end
end