Rack rack

I want to apply certain middlewares only to some paths. I asked a question on StackOverflow, but haven't come up with any good solutions.

So I extended Rack::Builder to allow it to act like middleware. Thus, in a rackup file (or within another Builder):

use SomeMiddleware      # for all requests
use SomeOtherMiddleware # ditto
use Rack::Builder do
  map '/secret' do
    use Rack::Auth::Basic # Basic auth only for protected stuff
  end
end
run lambda { |env| ... }

This is particularly useful for Rails apps, which can add middlewares, but not endpoints, in their environment.rb.

There is a branch on GitHub, along with the relevant commit. I have also attached a patch file.