Rscons::Environment.new do |env|
  require 'json'
  require 'yaml'
  env.add_builder(:JsonToYaml) do |target, sources, cache, env, vars|
    unless cache.up_to_date?(target, :JsonToYaml, sources, env)
      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
    target
  end
  env.JsonToYaml('foo.yml', 'foo.json')
end