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A Controller DSL to Complement MVC

July 3, 2007

Bruce Williams has been working on the view pattern over the past years; Notably the problems it raises: heaviness, conciseness, complexity, depending on the solution you choose. The presentation he gave at RailsConf, When “V” is for “Vexing”, introduced a Controller DSL: folder_for. Bruce gave an exhaustive use case of it on his blog.
Starting from a simple example that will need Folder interfaces, he naturally chose to use a DSL: folder_for.

 class CarsController < ApplicationController

    # ...
    folder_for :show do
      tab "General Information" do
        @score = current_user.score_for_car(@car)
      end
      tab "History"
      tab "Photos"
    end
    # ...

 end

Bruce uses convention over the use of one view template to render each selected tab partial (cars/show.html.erb). His views will be structured this way then:

 cars/
    show.html.erb
    show/
      _general_information.html.erb
      _history.html.erb
      _photos.html.erb

Bruce doesn’t only bring a new plugin which is more like a sample code; But he showed that a hundred lines of code was enough to accomplish his task demonstrating that a DSL can be deployed effectively with Ruby. The code is cleaner even if breaking some rules:

There are likely several of you reading this that are disturbed to see a folder reference within a controller, as this amounts to some level of MVC “separation of concerns” blasphemy in your very strict, very well-worn code rulebook.

I’m here to tell you it’s okay, and you’ll recover in time.

Let’s keep in mind here that the MVC separation of concerns, while a great rule of thumb, is just that … it is not an ivory tower to be left unassailed in times of dire need. At times, it makes sense to allow abstractions to cross these boundaries for the sake of reducing our own overhead, and in the cause of developing your own app-wide domain specific language—something, that in my book, is the principal sign of a good Rails developer (for what it’s worth).

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