Tour of an Open-Source Elm SPA
Richard Feldman May 08, 2017
People often ask me if I can point them to an open-source Elm Single Page Application so they can peruse its code.
Ilias van Peer linked me to the Realworld project, which seemed perfect for this. They provide a back-end API, static markup, styles, and a spec, and you build a SPA front-end for it using your technology of choice.
Here's the result. I had a ton of fun building it!
Fair warning: This is not a gentle introduction to Elm. I built this to be something I'd like to maintain, and did not hold back. This is how I'd build this application with the full power of Elm at my fingertips.
Routing for User Experience
I went with a routing design that optimizes for user experience. I considered three use cases, illustrated in this gif:
The use cases:
- The user has a fast connection
- The user has a slow connection
- The user is offline
On fast connections, I want users to transition from one page to another seamlessly, without seeing a flash of a partially-loaded page in between.
To accomplish this, I had each page expose
init : Task PageLoadError Model. When the router receives a request to transition to a new page, it doesn't transition immediately; instead, it first calls
Task.attempt on this
init task to fetch the data the new page needs.
If the task fails, the resulting
PageLoadError tells the router what error message to show the user. If the task succeeds, the resulting
Model serves as the initial model necessary to render 100% of the new page right away.
No flash of partially-loaded page necessary!
On slow connections, I want users to see a loading spinner, to reassure them that there's something happening even though it's taking a bit.
To do this, I'm rendering a loading spinner in the header as soon as the user attempts to transition to a new page. It stays there while the
Task is in-flight, and then as soon as it resolves (either to the new page or to an error page), the spinner goes away.
For a bit of polish, I prevented the spinner from flashing into view on fast connections by adding a CSS
animation-delay to the spinner's animation. This meant I could add it to the DOM as soon as the user clicked the link to transition (and remove it again once the destination page rendered), but the spinner would not become visible to the user unless a few hundred milliseconds of delay had elapsed in between.
I'd like at least some things to work while the user is offline.
I didn't go as far as to use Service Worker (or for that matter App Cache, for those of us who went down that bumpy road), but I did want users to be able to visit pages like New Post which could be loaded without fetching data from the network.
init returned a
Model instead of a
Task PageLoadError Model. That was all it took.
We have over 100,000 lines of Elm code in production at NoRedInk, and we've learned a lot along the way! (We don't have a SPA, so our routing logic lives on the server, but the rest is the same.) Naturally every application is different, but I've been really happy with how well our code base has scaled, so I drew on our organizational scheme when building this app.
Keep in mind that although using
exposing to create guarantees by restricting what modules expose is an important technique (which I used often here), the actual file structure is a lot less important. Remember, if you change your mind and want to rename some files or shift directories around, Elm's compiler will have your back. It'll be okay!
Here's how I organized this application's modules.
These modules hold the logic for the individual pages in the app.
Pages that require data from the server expose an
init function, which returns a
Task responsible for loading that data. This lets the routing system wait for a page's data to finish loading before switching to it.
These modules hold reusable views which multiple
Page modules import.
Views.User, are very simple. Others, like
Views.Article.Feed, are very complex. Each exposes an appropriate API for its particular requirements.
Views.Page module exposes a
frame function which wraps each page in a header and footer.
These modules describe common data structures, and expose ways to translate them into other data structures.
Data.User describes a
User, as well as the encoders and decoders that serialize and deserialize a
User to and from JSON.
Identifiers such as
Slug - which are used to uniquely identify comments, users, and articles, respectively - are implemented as union types. If we used e.g.
type alias Username = String, we could mistakenly pass a
Username to an API call expecting a
Slug, and it would still compile. We can rule bugs like that out by implementing identifiers as union types.
These modules expose functions to make HTTP requests to the app server. They expose
Http.Request values so that callers can combine them together, for example on pages which need to hit multiple endpoints to load all their data.
I don't use raw API endpoint URL strings anywhere outside these modules. Only
Request.* modules should know about actual endpoint URLs.
This exposes functions to translate URLs in the browser's Location bar to logical "pages" in the application, as well as functions to effect Location bar changes.
Similarly to how
Request modules never expose raw API URL strings, this module never exposes raw Location bar URL strings either. Instead it exposes a union type called
Route which callers use to specify which page they want.
Centralizing all the ports in one
port module makes it easier to keep track of them. Most large applications end up with more than just two ports, but in this application I only wanted two. See
At NoRedInk our policy for both ports and flags is to use
This kicks everything off, and calls
Html.map on the various
Page modules to switch between them.
Based on discussions around how asset management features like code splitting and lazy loading have been shaping up, I expect most of this file to become unnecessary in a future release of Elm.
These are miscellaneous helpers that are used in several other modules.
It might be more honest to call this
- With server-side rendering, it's possible to offer a better user experience on first page load by using cookie-based authentication. There are security risks on that path, though!
- If I were making this from scratch, I'd use
elm-cssto style it. However, since Realworld provided so much markup, I ended up using
html-to-elmto save myself a bunch of time instead.
- There's a beta of
elm-testin progress, and I'd like to use the latest and greatest for tests. I debated waiting until the new
elm-testlanded to publish this, but decided that even in its untested form it would be a useful resource.
I hope this has been useful to you!
And now, back to writing another chapter of Elm in Action. 😉