app.rb

  extend Controllers

  plugin :flow
  plugin :container

  route do |r|
    r.on("users") do
      r.is do
        r.get to: 'users#index'
      end

      r.on(:id) do |user_id|
        r.is do
          r.get to: 'users#show', call_with: [user_id]
        end
      end
    end
  end

  register_controllers

controllers.rb

module Controllers
  def register_controllers
    register('users') { UsersController.new }
  end
end

Dir["controllers/*.rb"].each { |file| require_relative file }

controllers_users.rb

class UsersController
  def index
    "users#index"
  end

  def show(user_id)
    if user_id == "666"
      "NOT FOUND"
    else
      "user show ##{user_id}"
    end
  end
end