Erlang Tooling in 2020

By Iñaki Garay

Iñaki Garay
The Henry Studley carpenter’s tool chest — 1838

The vox populi is that Erlang tooling has always been lagging behind other languages’, and that when Elixir came along, it somewhat forced the community into action. Whatever the exact causes, it’s true today that Erlang has been benefiting from cross-pollination with the new communities that are building systems around the BEAM platform.

One of these fruits is the new rebar3 plugin adding a formatter to Erlang. The project’s initiator, Brujo, wrote a blog post when it was presented. We asked him some follow-up questions.

Another sorely needed piece of tooling was a working implementation of a Language Server Protocol server for Erlang. Several efforts have been underway, but in particular erlang-ls (Github) headed by Roberto Aloi has reached 1.0 recently.

Given this serendipitous situation where two important developments for Erlang were launched at the same time, we decided to do a joint interview with both Brujo and Roberto.


What led you to the decision to start working on an Erlang code formatter?

Almost since I started working as a developer, I’ve always cared a lot about software maintainability.

That’s why I generally advocate for the usage of tools like xref, dialyzer, etc.

A while back, while working at Inaka I learned about Hound for Ruby. I wanted something like that for Erlang and found nothing similar. Therefore I worked with Juan Facorro and other inakos to create Elvis to fill that void.

In the same way, since I learned about mix format I started looking for a similar tool for Erlang. I knew that I could not create such a tool on my own. So I presented it as a project idea for Spawnfest (twice) with no luck. But then, the opportunity presented itself: HackWeek at NextRoll. It’s a week-long hackathon that happens twice a year. I presented my project and Juan Bono and Diego Calero were thrilled to help, so we built a team and in a week we had our first MVP, which was even published in and all.

Do you have any thoughts on why Erlang is a latecomer to the code formatting game?

I believe that, except for languages that are generally ahead of its time (like Smalltalk), having a code formatter that everybody uses is a relatively new thing. It’s something that’s expected from a modern language now, but that was not the case 10 or even 5 years ago. On the other hand, getting traction for an official formatter for a new language (like Elixir or Go) is not hard, since there are not many huge codebases around and people didn’t have enough time to get attached to their own formatting style yet.

For older ecosystems, like Erlang, where we have impressively huge repositories with widely different code styles (even within themselves), convincing developers to start using a single universal code format is not easy.

And the fact that different editors (like Intellij, emacs, etc.) provide their own auto-formatting tools, each one producing code in their very very distinct style that shares nothing with the others makes it even worse.

What did the design process for the tool look like?

Well… It was a hack week, right? It was 90% trial & error. Or, more accurately: digging, trial & error.

Jokes aside, we knew what we wanted from the very first day: We were going to create a tool capable of parsing, formatting and not breaking Erlang code. We didn’t want the formatted code to be pretty, we just wanted it to be consistently ugly. And we achieved that: as I said before… you can check that version (0.0.1) on

To achieve that goal, we wanted to use as many existing tools as we could find (we didn’t want to reinvent the wheel). That’s when Juan Bono came in. In pure hackathon-style, we were working on different time-zones. I was in Barcelona and Juan was in Buenos Aires. By the time I was done for the day, his day had just started. So I told him, for instance: “We need a tool to parse Erlang terms and another one to print them out”. My next day started with a message from him like “I found epp to parse and erl_prettypr to produce the formatted output”. I implemented our solution with them and came back with “But epp doesn’t handle macros well” and “erl_prettypr is very limited”, so he dug up stuff again and told me about epp_dodger and the many different reimplementations of erl_prettypr in different projects… trial & error again… and the process repeated.

At the same time, Diego was constantly trying to find strange edge cases to break our formatter and he found some very very strange stuff, too. Check the test app on the rebar3_format repo and I promise you, you will say “but… you can actually do that?” more than once.

How does the Erlang formatter compare to other well-known formatting tools (e.g. go fmt, mix format, or rustfmt)?

On one hand, rebar3 format is still much younger and less battle-tested than those tools.

It’s also not the official formatter… yet? Hey! OTP team… if you read this maybe it would be a good idea to have a conversation ;) — I’ll visit Stockholm soon, just saying…

On the other hand, the tools that you mention (particularly mix format) are much more strongly opinionated than rebar3 format. There is practically just one way to format code with them.

Knowing that it will be very very hard to convince every Erlang developer in the world to format their code in the same way, we aimed at a more humble goal: To convince every Erlang developer in the world to format their code with the same tool. You can keep your style, any style, but you use rebar3 format to apply it to your whole project at once. That way, reformatting your code in my style is just a matter of running a single command on the console: rebar3 as brujo format. I’m a ROK-style comma-first fan after all… and that’s certainly not the default formatting style ;)

Which were the main difficulties you found on the way to developing it?

I think the main issue we had was the lack of clear documentation and extensibility around existing tools. The work Juan did finding very obscure OTP modules with names like epp_dodger that actually had functions that resembled what we actually needed or were close enough was invaluable. Much of the code was there, but it was invisible for the unprepared eye.

If all those tools, like epp_dodger, erl_prettypr, etc were more visible and extensible, this formatter would’ve been a reality many years ago.

Handling comments along with valid code is a common difficulty that code analysing tools must deal with. How does the Erlang formatter handle this?

Luckily for us, the tools provided by OTP already did most of the work. We need a few patches here and there, but it was mostly trivial stuff. On the other hand, there are still some comments that rebar3 format can’t handle in the most beautiful manner. Rebar3 format won’t lose any comment in your files, but it might misplace them a bit, particularly if you want to add comments in things like type fields. Nothing that can’t be solved by manually moving the comments to a better place in the file.

Likewise, how does the code formatter handle incorrect code?

Since we use the same tools that the compiler uses to parse code, if the formatter finds syntactically incorrect code, it fails just like the compiler would. On the other hand, since the formatter doesn’t process macros, if your code is invalid because of a macro then the formatter will format it anyway. Even further, syntactically correct but semantically incorrect code (e.g. functions that are unused and not exported, variables that are used before being bound, etc.) is just formatted anyway.

What the formatter actually does, and we learned that from @old-reliable’s steamroller, is to verify that the AST generated from the code is not affected by the formatter itself (i.e. that the AST generated from the code before formatting it is exactly the same -except for the line numbers- as the AST generated from the code after running the formatter on it).

How configurable is the code formatter?

A lot. In basically two levels:

  • First, you need to choose what formatter you’ll use (i.e. what implementation of rebar3_formatter behavior will format your code). Rebar3 format comes with two by default: otp_formatter (basically, erl_prettypr) and default_formatter (NextRoll’s preferred way of formatting code).
  • Then each formatter has its own list of options to use, the default formatter has approximately 10 different options you can tweak, but the OTP formatter only allows you to change paper and ribbon.

Of course, you can create your own formatter, too. You just need to implement the rebar3_formatter behavior. We accept contributions! If you create a ROK-style formatter, please send a pull request with it!!

Which obstacles to adoption do you perceive? Or, What kind of adoption do you expect?

Oddly enough, the biggest obstacle now will likely be the competition. As Daniel (@old-reliable) puts it: ‘Do you think that this time will be remembered as “The Great Formatter Wars”?’ For ages, there were no formatters for Erlang and many people complained about that. But now, there is not one, not two, but soon there will be three of them: rebar3 format, steamroller and one that Michał Muskała will soon present.

In any case, we’re not sad, we believe that having three different formatters is far better than having none of them ;)

What do you think is in the future for the Erlang formatter?

It’ll become the de facto formatter for all Erlang code in the whole wide world, of course! You have to dream big, right?

Seriously, my greatest dream for the formatter would be for it to be considered the default Erlang formatter. Maybe the OTP team adopts it and starts requiring all contributions to Erlang/OTP code to be formatted with it, etc. etc. …

More realistically, though… I have to say that we plan to use it for all of our Erlang projects within NextRoll. We are already using it for all the open-source ones and we’re progressively adding it to our private ones that are much more massive. Keeping that in mind, we (as a company) are committed to providing support for it actively. And we expect some help from the community in the form of bug reports, configuration and formatter ideas and… of course… pull requests!

Now that the Erlang formatter is usable, what else are you up to?

I think that my main focus these days is the Education Working Group in the Erlang Ecosystem Foundation. As an active member of it, I’m now working on an initiative called Meetup Interventions. It’s in its early stages, but the idea is to help meetup organizers around the world to provide workshop sessions with training in different subjects, some introductory, others more advanced. The first round of workshops is about to start very soon with Miriam Pena, Simon Thompson, Johnny Winn and me as trainers. We’ll see how that goes and then we’ll decide how to properly scale up the idea. I think it will be a great success.

Besides that, I recently moved to Sant Cugat del Vallès, a beautiful town near Barcelona, Spain which gives me access to many European things. I’ll be giving a talk at CodeBEAM STO in May and before that, I’ll be attending ElixirConf EU. I’m also helping some new friends revitalize the Elixir meetups at Madrid and hopefully visiting A Coruña’s university soon to give a guest lecture, too.

I’m very excited about all the opportunities that are now much closer to me. I’m still just settling in my new home, but I already love it!

Finally, I would like to mention that even when I was not particularly active in Erlang Battleground lately, I’m still looking for writers… If you happen to have an idea of something BEAM-related you want to write about, contact me… I’ll help you get it published and shared with a lot of people!

Roberto Aloi

How and why did the project start?

My original plan in 2018 was to simply start contributing to the [EDTS]( project, the Erlang Development Tool Suite for the Emacs editor. The project was not actively maintained any longer and I was experiencing a few issues after upgrading to the latest version of Erlang/OTP.

I started diving into the EDTS code-base and discovered that its architecture was particularly interesting. The Emacs client, implemented in Emacs Lisp, was communicating via HTTP to a server, implemented in Erlang, which was responsible for the heavy-lifting. I played with the idea that, with a bit of effort, one could grow the HTTP API that was used for the client-server communication in EDTS into something more solid and generic, allowing other editors to re-use the logic implemented by the EDTS server.

Some weeks after I was at the Code Beam Stockholm conference. I attended a talk from the author of the Sourcer project, who gave an introduction to the Language Server Protocol. Apparently, Microsoft had already thought about this. To me it meant that I didn’t have to go through the process of designing a brand-new protocol, but that I could simply focus on implementing a server, maybe based on the EDTS one, which would comply to the protocol. I started looking at existing Language Server implementations in Erlang and I guess that leads to your second question…

How does it compare to other similar efforts such as sourcer or other LSP implementations?

There are currently three major implementations of the Language Server I am aware of:

Sourcer is a 7 years old code-base (the Erlang plugin for Eclipse) converted to a Language Server. This makes it hard to contribute, if you are not familiar with the original project. Sourcer makes heavy use of custom libraries, such as scanners and parsers, while Erlang LS tries to leverage OTP libraries as much as possible. The project was last updated six months ago, so I don’t know the current status of its development. You can check-out this blog post for a more accurate comparison between Erlang LS and Sourcer.

VSCode Erlang is a Visual Studio extension which includes an Erlang Language server. The project is tightly coupled to the VSCode editor, though, and to me that defeats the purpose of a Language Server in the first place.

Erlang LS is built from scratch to be a Language Server and it aims at being truly IDE-agnostic. Lot of effort is being put in documenting the installation process with different editors and in providing clear troubleshooting procedures for the end users.

How does erlang_ls integrate with existing Erlang tooling? (e.g. rebar3 or elvis)

The way I see Erlang LS is that it should act as a platform that allows third-party tools such as elvis to smoothly integrate with your own development workflows.

In the Erlang Community many tools exist which have lot of potential, but they are used in a very limited way by developers. The refactoring tools Wrangler and RefactorErl are good examples. In some cases this is because the installation process for a tool is not straightforward, in other cases tools are tightly coupled with a specific editor, which unavoidably limits their adoption. These tools could live a reinassance period if we get things right with Erlang LS. My hope is that, once a cross-editor platform is available, Erlang tools can find the attention they deserve.

Another interesting situation is when competing tools exist for solving the same problem: code formatters, for example. The Erlang scene is currently experiencing a little explosion in terms of formatting tools: AdRoll just released rebar3_format, and WhatsApp is presenting a new formatter at the Code Beam San Francisco as we speak. Dialyzer and Gradualizer are another example: two different strategies, both with pros and cons, to achieve the very same goal. In these cases, the idea is to provide integrations for these tools via the Erlang LS platform. The tools should work out-of-the-box and it should always possible for the end user to opt-in or opt-out a specific integration. The major challenge with this approach is not to end up with a Language Server bloated with dependencies.

I’m also glad you mentioned `rebar3` since it falls into a very different category of tools which deserve a separate discussion. `rebar3` is a building tool. Erlang LS accepts low-level configuration parameters (dependency paths, include directories, custom macro definitions, etc), which can be set via a `erlang_ls.config` for a specific project and are used to instruct the server on how to build Erlang modules. We are also going to introduce the concept of project type, the first one being `rebar3`. Once a user marks a project as a `rebar3` one (we can of course automate this process by looking for presence of a `rebar.config` file), all the low-level configuration parameters required by the Language Server will be automatically inferred by the `rebar.config` file, minimizing the required configuration. This approach will make `rebar3` projects work out-of-the-box with Erlang LS, while ensuring that Erlang LS can still be used by all those projects which do not use `rebar3` as their building tool of choice. We are also keeping an eye on the Build Server Protocol, but that will require some more investigation.

Did you encounter any challenges on the way to implementing the LSP protocol? any specific to Erlang?

The LSP protocol is fairly new and it still has to go through its own maturity process. Some parts of the protocol are over-engineered, such as the handshaking and the shutdown sequence. The fact that the protocol supports two different transports (TCP and STDIO) feels unnecessary. Some of the concepts mentioned in the protocol are either not well explained (e.g. document links) or coupled to specific languages needs (e.g. the distinction between a `go-to-definion` vs a `go-to-declaration` vs a `go-to-implementation`). There is an interesting Reddit post by the author of one of the Vim LSP clients which highlights some of the LSP protocol imperfections.

Even if most of the IDE nowadays have decent LSP clients, some of them don’t. The IntelliJ integration has been particularly challenging for us because of this. Testing a Language Server is also not a trivial task. I actually have in mind to create of a testing framework for Language Servers combining property-based testing and the Language Server Index Format, but that deserves a blog post on its own. Windows support has also revealed particularly challenging due to the fact of Windows/Erlang stacks in Travis CI or GitHub Actions.

In terms of Erlang specific challenges, the major one has probably been to decide the database to use to persist indexed information. We currently settled on Mnesia, used as a key-value store, for two main reason: minimize the setup for the end user (Mnesia comes with Erlang/OTP, so it does not require any additional installation step) and it allows us to store Erlang terms without the need of encoding/decoding them. Mnesia comes with its own challenges, though. To get the performance we needed for big projects (up to 2 million lines of code) we had to dig into Mnesia internals and tweak a few settings. Mnesia is also a distributed database and it feels a bit like an overkill for a single-node project like Erlang LS. Will see if we decide to keep it or move to a different solution.

Finally, the Erlang standard libraries still lack what, in my opinion, are very basic functionalities that one would expect from `stdlib`. We had to implement our own functions to delete a non-empty directory or deal with relative and absolute paths. Even if these functions are not hard to implement, they require time and we (as in members of the Erlang community) should all be a bit more pro-active in ensuring to contribute this type of functionalities to Erlang/OTP.

Did you find any advantages within the Erlang/BEAM tech stack when designing and implementing erlang_ls?

Plenty of them. The experience of using Erlang to implement a Language Server has been awesome so far.

Pattern matching and message passing, combined with the flexibility of maps to encode/decode JSON entities, make the implementation of the protocol straightforward. Lightweight processes allow us to implement features such as cancellable requests and background jobs — sometimes hard to implement using other technology stacks — in a trivial and idiomatic way. Behaviours give a wonderful way to separate generic and implementation-specific parts of the code (we use them heavily for implementing providers and transports). The syntax_tools libraries contain all the building blocks which are necessary to play with the Abstract Syntax Tree of your Erlang programs (even if they have their own quirks).

The ability to attach to a running Language Server using a remote shell and to trace individual requests from a client plays a crucial role when you have to troubleshoot a bug, which would have required plenty of step-by-step debugging or io-formatting otherwise. Common Test groups are precious to parameterize your tests (we run them for both `stdout` and `TCP` transports).

I could continue for a long time, but I will stop here. I really hope that Erlang LS will make it easier for newcomers to the Erlang Community to get a chance to get started with Erlang with minimal friction and to experience all of these wonderful advantages that a BEAM tech stack can provide you when implementing their own Erlang project.