Update on Module Unification

– By Tom Dale

Ember's conventions for project layout and file naming are central to the developer experience. It's crucial that we get both the technical and ergonomic details right. I wanted to provide an update about Module Unification and our plans for the file structure in Ember Octane.

In short, we do not plan to ship Module Unification in Octane. Instead, Octane will ship with today's file system layout, with one change: support for nested components in <AngleBracket /> invocation syntax

Because Octane apps will continue with today's file system layout, we want to address the largest barrier to <AngleBracket /> adoption today: components nested inside other directories.

For example, if you have a component located at app/components/icons/download-icon.js (i.e., nested inside an icons directory), you can invoke it with curly invocation syntax like this:


However, it's not possible to invoke the same nested component with angle bracket syntax without resorting to clunky workarounds.

As proposed in the Nested Invocations in Angle Bracket Syntax RFC, we plan to address this by adding support for nested components via the :: separator.

With this proposal, the component described above could be invoked with angle bracket syntax like this:

<Icons::DownloadIcon />

Because this is a small change, it can be implemented quickly without requiring us to delay the Ember Octane release.

Status of Module Unification

Given that the Module Unification RFC was merged in late 2016, and we talked about shipping Module Unification in the 2018 Roadmap RFC, it's fair to ask what happened and why we're making this decision now.

Heads up: this post gets long and detailed, so if you only care about the plan going forward, you can safely skip the rest.

In the spirit of transparency and overcommunication, though, I wanted to share a little bit of the history and evolution of MU from my perspective.

Let's call the file system layout that Ember apps use today the "classic" layout. While this structure has served us well, it also has several shortcomings.

In the classic layout, files are grouped by type, not by name. Sometimes, this means that closely related files (like a component and its template) are separated from each other, and navigating between them can be frustrating.

Ideally, related files would be close to each other in the file system. For example, you may want an Ember Data model and its associated serializer to be co-located in the same directory.

Early on, Ember CLI implemented an experimental "pods" layout that grouped files by name rather than by kind. For example, a User model and its serializer would be grouped together in app/user/, as model.js and serializer.js respectively.

Feedback from the community was that pods felt more productive than the classic layout. However, pods had several problems that needed to be addressed before it could be enabled by default.

The effort to address the design flaws of pods led to the Module Unification RFC, a ground-up rethink of how the file system should work in Ember apps. Importantly, this design grappled with overhauling Ember's resolver and dependency injection systems as well, which was just as important as shuffling around where particular files went on disk.

The MU RFC was merged at the end of 2016, but new Ember apps are still generated with the classic layout. So, why can't we flip the switch on enabling Module Unification-style layout by default today?

Remember that MU described not just a new file system but a significant overhaul of Ember's resolver. Implementing MU was a huge initiative that, while often hidden, touched nearly every part of the framework. Despite this large scope, and thanks to the incredible amount of time and energy our community devoted to the work, MU implementation has made great progress, and nearly everything needed to make Module Unification work has landed in Ember.

Nearly everything. When we merged the Roadmap RFC last year, there was one last major piece of MU that still needed to be designed: component namespacing, or how to refer to components that come from other addons in templates.

While we had a plan, a shift in the JavaScript ecosystem sent us back to the drawing board. And the harder we worked on solving this last piece, the more we realized that fundamental aspects of the overall design needed to be rethought.

Namespaces and Scoped Packages

Besides files being spread out, another problem with the classic layout is that everything goes into one big global pool of names.

So, for example, Ember Data defines a service called store that you can inject into components and services. If you install another addon that also has a service called store, there's no easy way to use both.

Similarly, if your app has a component called small-button and you install an addon that also has a component called small-button, you have no way to tell Ember which one you mean when you type {{small-button}} in a template.

One of the key benefits of the MU design is that names are no longer global, but namespaced to the package where they come from. So, for example, if an npm package called ember-ui-library contains a component called small-button, the MU RFC proposed referring to it in your template like this:


Around the same time we were designing MU, however, npm announced support for scoped package names. Prior to this change, npm package names were limited to alphanumeric characters and dashes. Now, though, packages could start with an @ and contain a /, like @glimmer/component.

Component invocations with scoped packages blew past the verbosity limit. We simply could not bring ourselves to accept a syntax that commonly produced code like this:


We also had concerns that this was confusing to scan visually, considering that @ already means a component argument and / already means a closing tag.

While Ember had been using it for years, around this time JavaScript module syntax (import Post from './post') started gaining significant traction in the wider ecosystem, along with tools like webpack that could use these modules to perform code splitting.

After scoped npm packages scuttled our original plan for namespaced components, we went back to the drawing board, and something similar to JavaScript's import seemed like a promising solution. However, we immediately hit some challenges while exploring this idea.

The concept of a "component" in Ember isn't a singular thing, but the union of a template and a component class. With the component and template in separate files, it isn't clear which one you're supposed to import. Because of this, we wanted something where you would import a component by name (my-component), not a specific file like in JavaScript.

But despite the differences, the overall concept of importing modules was similar. We wanted to find a syntax that would give users who already knew JavaScript an intuition about the similarities, while not making it look so similar that people would be misled into thinking it was literally the same thing.

We tried to find a balance between these two constraints with a syntax that looked like this:

{{use component-name from 'package-name'}}

We hoped that it looked similar enough to JavaScript's import syntax to give you a clue about what it was doing, but by adopting the use keyword instead of import, signal that this was not exactly the same as JavaScript modules.

Matthew Beale poured significant time into capturing all of these conflicting constraints in the Module Unification Packages RFC, but, even after months of discussion, we couldn't come to consensus and the RFC was never merged.

Despite everyone agreeing this was an urgent problem, we couldn't convince ourselves that having different module systems for JavaScript and templates was a viable solution. Unfortunately, there wasn't an obvious alternative plan, and not having an answer meant MU was indefinitely blocked until we could figure out this last piece of the puzzle.

We felt stuck.

Real-World Feedback

In the meantime, enough of Module Unification was shipping behind feature flags (and in Glimmer.js) that we were able to get feedback from early adopters. While overall people really liked the new file system and really appreciated not dealing with frustrating name collisions, something felt off.

One common theme in the feedback was that MU felt too rigid and frequently got in the way of simple tasks. To understand why, it's important to understand that MU is about more than a file system. MU is really a system for controlling scope.

For example, a feature of MU is the ability to have private components that don't leak into the rest of the app. If we have a component called list-paginator and it has a child component called paginator-control, MU allows us to organize them like this:

├── ui
│   ├── components
│   │   └── list-paginator
│   │       ├── paginator-control
│   │       │   ├── component.js
│   │       │   └── template.hbs
│   │       ├── component.js
│   │       └── template.js

In this example, the list-paginator template can invoke {{paginator-control}} to render its child component. However, if you try to invoke {{paginator-control}} from any template outside the list-paginator directory, you'll get an error. In other words, paginator-control is local to list-paginator.

MU, then, is about scope, and controlling who has access to what. Where a module lives in the MU file system determines what it can see to and who else can see it.

This is a clever idea that eliminates a lot of boilerplate. If you have to organize your files anyway, and if you want to group related things together anyway, it makes sense to try to create a single system that solves organization and scoping at the same time.

In practice, though, we ran into a few challenges:

  1. This idea is not common in the JavaScript ecosystem, so the file system controlling scope isn't intuitive for new learners. They also have to memorize these naming rules, which are implicit and get quite complex.
  2. Similarly, ecosystem tools don't understand MU. We have to build custom integrations to get things like "Go to Definition" to work in IDEs or code splitting to work in webpack, that other libraries get for free.
  3. JavaScript files in Ember use module syntax, which doesn't go through the MU system, adding to the confusion. Having one system in a component's JavaScript and another in its template is not ideal.
  4. When file names and locations are so meaningful, it can be frustrating if you want to create a file that isn't part of the MU world. Tasks that are normally trivial, such as extracting utility functions into a separate file or grouping related files together in a directory, can easily turn into a battle where your build starts erroring because you're not playing by the MU rules.

A Personal Anecdote

Personally, this last one was what really caused me to step back and re-evaluate our plan for MU. It happened during a project at work where we were using both Glimmer.js (with Module Unification) and Preact.

As the number of components grew, a co-worker created a directory called icons in the Preact app to hold all of the components for rendering different SVG icons. I'm sure it didn't take more than a few minutes to create the directory, drag the appropriate component files in, and update the paths everywhere those components were imported. (In fact, VS Code probably updated the import paths automatically.)

When we tried to do the same thing in the Glimmer app, it was a much different experience that turned into an hours-long slog. And despite all the great things it does do, MU doesn't really have a way to do this kind of lightweight grouping.

We could have found a workaround. We could have extracted the SVG icon components into a separate package, or created a higher-order component that wrapped all of the child icon components. But it seemed ridiculous to burn so much time looking for a "workaround" to perform a task that felt like it should have been (and was, in Preact) trivial.

I knew, intellectually, the benefits of MU. I knew how carefully it was designed to enforce structure and consistency as your application grew and had different engineers of different experience levels working on it. (Indeed, by the end of the project, I found the Glimmer app much easier to navigate, while the Preact app had several inconsistently-followed conventions.)

But I never forgot how viscerally bad it felt to have my co-workers fight so hard to do something that felt like it should be so easy.

So this was the status quo last year. We were all incredibly frustrated that we couldn't make progress on the scoped package problem, but I was even more overwhelmed trying to figure out what, if anything, to do about the negative experiences my co-workers had had when using MU.

Discovering problems in a design you've been working on for so long is painful, especially in cases like this where the majority of the work was already complete, and we thought we were so close to the finish line.

Programming language and API design is really hard. Sometimes I read old RFCs and marvel at how obvious the solution seems now, in contrast to the weeks, months, or years I know it took to tease it out from the millions of possible alternatives.

When you're trying to balance so many competing constraints, sometimes a small change is all it takes to get you out of a design conundrum you've struggled with for months. In this case, that change was angle bracket component invocation.

One thing making JavaScript imports difficult to use with templates was the constraint that components had to have a dash in their name. Unfortunately, dashes aren't valid in JavaScript identifiers, so something like import some-component from './some-component' produces a syntax error.

Angle bracket components, on the other hand, start with a capital letter to disambiguate themselves from normal HTML tags: <MyComponent /> instead of {{my-component}}. Most importantly, there's no dash.

As the Ember community started using angle bracket syntax, early feedback was very positive. All of a sudden, JavaScript import syntax was back on the table.

The Path Forward: Template Imports

Speaking for the Ember.js core team, we are trying to get better at updating the community when plans have changed but the new plan isn't fully locked in yet. So, consider this one of those situations.

We know that the exact plan for Module Unification (MU), as described in the original RFC, will need to change. How it changes is not yet certain, but we believe that some of the problems we wanted to solve with MU are better solved with template imports.

With template imports, we intend to make templates play nicely with JavaScript, so you can use the import feature you already know and love. By having components and helpers be modules you can import, we can eliminate the most complex parts of Module Unification so it's easier to learn and use.

We recently posted the SFC & Template Imports RFC, which describes some of the low-level APIs needed in Ember to make template imports possible.

Learning from past mistakes, this RFC focuses on the primitives so we can quickly experiment, get feedback, and iterate on template import proposals in addons, before stabilizing them in the core framework.

While the Ember.js core team has reached consensus that template imports are the intended path forward, please note that the current RFC only covers low-level primitives, not the API that would be used by Ember developers to author templates.

Here is one example of a very hypothetical template imports syntax:

import UserProfile from './user-profile';
import UserIcon from './icons/user';
<UserIcon />
<UserProfile @userId={{this.blog.authorId}} />

The syntax is up in the air and will almost certainly be different from this example. Regardless, it shows the benefit of template imports clearly: we've imported two components—UserProfile and UserIcon—just like how we would refer to any other JavaScript module. This makes it very easy for everyone—from developer who are new to Ember, to IDEs, and other tooling in the JavaScript ecosystem—to understand what came from where.

You can even imagine an (again, very hypothetical) single-file component format that places the template right within the component's class. Here, a unified imports solution feels especially natural:

// src/ui/components/blog-post.gbs

import Component from '@glimmer/component';
import UserIcon from './icons/user';

export class BlogPost extends Component {
  blog = { title: 'Coming soon in Octane', authorId: '1234' };

    <UserIcon />
    <UserProfile @userId={{this.blog.authorId}} />

Again, the exact syntax is up in the air and will almost certainly be different from this example. The benefit of exposing the low-level primitives first is that we can try out multiple competing designs relatively easily before comitting. And if you don't like what we eventually decide on, you can build an alternative that is just as stable as the default implementation.

But note that template imports are not a replacement for MU. They don't help with things like better isolation of an addon's services, or a more intuitive file system layout. Instead, we hope that template imports will better solve one aspect of MU, so the overall design can be simplified and its benefits can shine through more clearly.

Template imports also give us a good opportunity to try to address the ergonomic problems people reported when trying MU.

Without template imports, we had to rely on MU to resolve component names, meaning the files in the src/ui/components directory had to follow strict rules. But with template imports, users can just tell us which module on disk they want. We can skip resolving components through MU altogether, and let Ember users organize their component files however they want.

As frustrating as it was at the time, the inability to make progress on MU may have been a blessing in disguise. It gave us time to implement angle bracket syntax for components, which allowed template imports to seem feasible again—which means we now have a solution that seems to address both the scoped package problem and the ergonomics problem. Even better, template imports make things like treeshaking and code-splitting in Embroider much easier.

I believe the dead-end we found ourselves in was a sign from the universe that something better was just around the corner. Time will tell, but my hunch is that template imports solve these important problems much more elegantly than what we had before. This process also pushed us to explore single-file components, which I think will be a surprisingly big productivity improvement for Ember developers.

While we're excited about template imports, we want to keep our promise to finish what we started. We are prioritizing Ember Octane and making sure that our first edition is a polished, cohesive experience. Only once Octane is out the door will we turn our attention to new initiatives like template imports.

Hopefully, this post helps provide some context about the state of MU. Of course, what I've written here is my personal, imperfect recollection of events, simplified for clarity. The reality of technical design is messy and feels a lot more like going around in circles than the tidy sequence I've presented here.

I will also mention that, as a project, I think we've dramatically improved how we design, implement, validate, and iterate on features since we originally started the Module Unification RFC. The MU RFC is the last of the proposals from the "mega-RFC" era, where we had a tendency to do a ton of upfront design and implementation before we had any feedback from real users.

Nowadays, I think we're a lot better about making sure RFCs are relatively small and focused on doing one thing. We also tend to prioritize exposing hooks or other primitives that let us test out new ideas in addons. This allows us to improve designs based on early feedback, without the strict stability requirements that come with landing something in the framework proper.

This has worked well for things like ember-decorators and Glimmer components, where real world feedback and the ability to make breaking changes based on that feedback was critical. I'm hopeful that a similar strategy for template imports will be just as successful.

My sincere thanks to everyone who has worked so hard on MU and related efforts. I'm proud to be part of a community that refuses to charge ahead with something we know isn't right. Ember's longevity is, at least in part, explained by our willingness to make these kinds of hard decisions.

I'm so excited about Ember, our roadmap, and the newfound energy in our community. In 2019, a thriving Ember is more important for the web than ever. Thank you for being a part of our community, and I hope to see you at EmberConf in a few weeks. It's gonna be a good one.