can I use Rack::Directory inside Rack::Builder block?

[Stephen Bannasch <stephen.bannasch@deanbrook.org>]

I want to emulate a static web server with directory indexes using rack.

Is there a way to use Rack::Directory inside a Rack::Builder block?

This works fine serving static files:

   myapp = Rack::Builder.new do
     map "/" do
       use Rack::Static, :urls => ["/"], :root => 'cache'
       run lambda {|env| [404, {}, '']}
     end
   end

   Rack::Handler::WEBrick.run(myapp, :Port => 4321)

But when I add Rack::Directory

   myapp = Rack::Builder.new do
     map "/" do
       use Rack::Static, :urls => ["/"], :root => 'cache'
       use Rack::Directory, 'cache'
       run lambda {|env| [404, {}, '']}
     end
   end

   Rack::Handler::WEBrick.run(myapp, :Port => 4321)

I'm getting this error:

   rack-1.1.0/lib/rack/directory.rb:46:in `initialize': can't convert Proc into String (TypeError)
           from ./rack-1.1.0/lib/rack/builder.rb:54:in `new'
           from ./rack-1.1.0/lib/rack/builder.rb:54:in `use'
           from ./rack-1.1.0/lib/rack/builder.rb:73:in `call'
           from ./rack-1.1.0/lib/rack/builder.rb:73:in `to_app'
           from ./rack-1.1.0/lib/rack/builder.rb:73:in `each'
           from ./rack-1.1.0/lib/rack/builder.rb:73:in `inject'
           from ./rack-1.1.0/lib/rack/builder.rb:73:in `to_app'
           from ./rack-1.1.0/lib/rack/builder.rb:63:in `map'
           from ./rack-1.1.0/lib/rack/builder.rb:66:in `map'