Does anybody have any exercises for practicing Law of Demeter?

aaronmcadam · March 18, 2015, 5:25pm

Hi there

I’ve been trying to get into the habit of not flagging LoD violations in code reviews based on the number of methods called on an object, the old “2 dots” rule (a.method.another_method).

So I’m looking for some code exercises that will help me learn the spirit of the law and teach the rest of my team.

Any ideas? Maybe it could be an exercise @benorenstein?

Thanks

r00k · March 20, 2015, 2:38pm

Good idea!

aaronmcadam · March 20, 2015, 2:46pm

It might be a little difficult to come up with suitably-complex examples though. I think contrived examples are one of the reasons why Law of Demeter gets seen as the “2 dots rule” in the first place. Avdi’s old blog post on the principle is still a great resource.

cored · May 8, 2015, 3:24pm

@aaronmcadam I think just thinking about having ‘2 dots’ could be misleading just because if you are calling a method that return the same data type like String.gsub.upcase that’s not actually a violation of the law of demeter.

aaronmcadam · May 8, 2015, 3:36pm

That’s exactly the kind of thing I think having lots of exercises about it would mitigate.

nozpheratu · May 8, 2015, 6:20pm

I wish I could share all the crappy Rails code I wrote over the past three years of being self-taught. I have production code that I’m stilling maintaining to this day no doubt rife with LoD violations that I wish I could share, but sadly it’s proprietary . I bet the upcase community would have a field day picking it apart.

I’ve heard that once you get really really good at writing solid code it’s kind of difficult to reproduce bad design for the sake of example. I can’t attest to this as I’m still a pretty terrible programmer.

andyw8 · May 8, 2015, 7:24pm

Not sure if it helps, but here’s what rails_best_practices detects as LOD violations on the Upcase repo. All very minor:

github.com

thoughtbot/upcase/blob/master/app/controllers/videos_controller.rb#L33


      
          class VideosController < ApplicationController
            def show
              @video = Video.find(params[:id])
              @watchable = @video.watchable
          
          
    respond_to do |format|
                format.html { render_or_redirect }
              end
            end
          
          
  private
          
          
  def render_or_redirect
              if params[:id] != @video.slug
                redirect_to video_path(@video.slug), status: :moved_permanently
              else
                render
              end
            end
          end

https://github.com/thoughtbot/upcase/blob/master/app/views/subscriptions/_subscription.html.erb#L3

github.com

thoughtbot/upcase/blob/master/app/views/teams/edit.html.erb#L15


      
          
          
<div class="text-box-wrapper solo">
            <div class="text-box">
              <h4>
                Members
              </h4>
          
          
    <ul class="members">
                <% @team.users.each do |user| %>
                  <li class="<%= "owner" if @team.owner?(user) %>">
                    <span class="username"><%= user.name %></span>
                    <%= link_to(
                      "✖",
                      membership_path(user),
                      method: :delete,
                      id: dom_id(user, "remove"),
                      data: {
                        confirm: "Are you sure you want to remove this user from your team? They will lose access to all the content they have as part of the team."
                      }
                    ) %>
                  </li>

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Does anybody have any exercises for practicing Law of Demeter?

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