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Faithful Elm and the Amazing Router
By Amitai Burstein (@amitaibu) // 16 Jun 2016

I gave an Elm session in YGLF conf. This was a great excuse to free up some hours to work on a new v0.17 SPA (Single Page Application). You won’t believe what happened next…

Well, actually, you would: it was an awesome experience. :)
In fact, I’ve reached the point that the backend me is becoming jealous of the frontend me.

Fetch GitHub user's info on this fake login.

My goal with building this demo app, was to give a small, yet realistic, look into how Elm allows us to accomplish daily tasks such as HTTP requests, routing, access, and more. It was important for me to structure it in the same way that we structure larger apps built for production, so that it could demonstrate more effectively how Elm can be used in a project.

If you are interested in Elm, and want to get a feeling of how it could be built for your apps, this might be a good starting point. I even wanted to add a single test to show how it could be done. But Elm being such a fun, predictable, opinionated, and fun (no mistake here, it deserves the double fun) to work with, I ended up adding more and more tests.
Isn’t that yet another great sign for Elm? I was adding unit tests for a demo app, while we hardly added any unit tests for our Angular apps in production!

I held myself back from adding too many features, but I couldn’t resist polishing the existing ones, and adding lots of comments. With the compiler’s tough love and ever growing unit tests, changes were so easy it almost felt like cheating (and note that I rarely write “easy” or “trivial” about development issues).

In general, at least for me, the great thing that Elm brings to the table is the reduced cognitive load. We don’t need to remember too much about our application at every single moment. This means we can concentrate on our current component and make sure it’s being wired in correctly to the entire application.

With Elm-format we don’t even need to bother ourselves with the file format; and along with the Atom integration on every save, we get consistent looking files.

Wiring Multiple Components

One of the questions I see people struggle with is how should the different components communicate with each other. I won’t get into depth here, but rather give some pointers to the code base. The gist of things:

  1. Main module is mostly boilerplate, and it cares only about the App module.
  2. The App module is conveniently split into Model, Update, and View files. This is the heart of our web app, and it is responsible for calling and delegating the work to the right modules (a.k.a components) via its update function.
  3. Same for the view: it’s calling the child modules.
  4. Any module, like the App for example, is always communicating downwards to its child modules. It needs to know about the child module, but the child doesn’t need to know about it. Nor does the parent module need to know about the grandchildren, if they exist.
  5. Child modules can pass information back to the parent module. Really! It’s done in the update function by simply returning more info. While the typical signature of an update function is (Model, Cmd Msg) nothing stops the child’s update function from being (Model, Cmd Msg, Int), where Int will be some numeric value the parent can act on.

Elm Route Url

With the new official Navigation module in place, the need for SPA routers has been met. However, Navigation isn’t just a router module, it’s providing the building blocks for other routing solutions. It merely has a certain solution as a “serving suggestion”.

@rgrempel’s elm-route-url module indeed builds on the foundations of Navigation, however it takes a different approach, which I personally prefer. As you may have read in one of my previous posts under the Url change is not routing paragraph, elm-route-url allows us to add routing (i.e. actually changing the URL) after the fact, with very little change to the existing code base. For example, when I wrote the demo app, I started step by step, wiring different components into my app.

The App.Model has a property called activePage which is, as the name implies, responsible for knowing which page is active. In fact, I have written the entire app without having a URL change. Just to emphasize, there were “page” changes, they were simply not reflected in the URL.

So once I wanted to wire-in the Navigation, I realized I needed to change my existing app. For example, in my update function I should have changed the return value from (model, Cmd.none) to (model, Navigation.newUrl (toUrl newModel))

Same with the init function: I had to change the signature from init : ( Model, Cmd Msg ) to init : Result String Int -> (Model, Cmd Msg). Nothing too dramatic, but I preferred avoiding it.

This is where I believe Elm route URL’s approach shines. The init and update function can remain exactly as they were, while we can add our routing needs via a completely separate set of functions. This makes the addition of routing consistent with my experience of building the app one step at a time, and knowing that what went into it is stable, solid, and most likely not to change if there are no new requirements.