application.rb

class Application < Cuba
  plugin Cuba::Render
  settings[:render][:template_engine] = 'slim'

  define do
    on get do
      on ':path' do |path|
        # ...
        res.write view('directory')
      end
    end
  end
end

directory.slim

foobar

layout.slim

doctype html
html
  body
    == yield

Output

LocalJumpError: no block given (yield)