This is a very early release. It only works with Elixir 1.4, and it's only been tested on OSX. If all you want is a good, stable Elixir editor, stick with whatever you're using now until ElixirLS is more mature.

The Elixir Language Server provides a server that runs in the background, providing IDEs, editors, and other tools with information about Elixir Mix projects. It adheres to the Language Server Protocol, a standard supported by Microsoft and Red Hat for frontend-independent IDE support. Debugger integration is accomplished through the similar VS Code Debug Protocol.

Features include:

• Debugger support!!!
• Inline reporting of build warnings and errors
• Documentation lookup on hover
• Go-to-definition
• Code completion

ElixirLS is intended to be frontend-independent, but at this point has only been tested with VS Code. You can install the VS Code extension by searching ElixirLS from the "extensions" pane. The plugin code resides in the vscode-elixir-ls repo.

ElixirLS includes debugger support adhering to the VS Code debugger protocol which is closely related to the Language Server Protocol. At the moment, only line breakpoints are supported.

When debugging in Elixir or Erlang, only modules that have been "interpreted" (using :int.ni/1 or :int.i/1) will accept breakpoints or show up in stack traces. The debugger in ElixirLS automatically interprets all modules in the Mix project and dependencies prior to launching the Mix task, so you can set breakpoints anywhere in your project or dependency modules.

In order to debug modules in .exs files (such as tests), they must be specified under requireFiles in your launch configuration so they can be loaded and interpreted prior to running the task. For example, the default launch configuration for "mix test" in the VS Code plugin looks like this:

{
  "type": "mix_task",
  "name": "mix test",
  "request": "launch",
  "task": "test",
  "taskArgs": ["--trace"],
  "projectDir": "${workspaceRoot}",
  "requireFiles": [
    "test/**/test_helper.exs",
    "test/**/*_test.exs"
  ]
}

In order to provide features like documentation look-up and code completion, ElixirLS needs to be able to load your project's compiled modules. ElixirLS attempts to compile your project automatically and reports build errors and warnings in the editor.

At the moment, this compilation is performed using a fork of Elixir 1.4's compiler. This is not a good long-term solution and replacing it is a high priority in the near future. See this blog post for an explanation.

To avoid interfering with the developer's CLI workflow, ElixirLS creates a folder .elixir_ls in the project root and saves its build output there, so add .elixir_ls to your gitignore file.

Note that compiling untrusted Elixir files can be dangerous. Elixir can execute arbitrary code at compile time, which is how it can support such extensive metaprogramming. Consequently, compiling untrusted Elixir code is a lot like executing an untrusted script. Since ElixirLS compiles your code automatically, opening a project in an ElixirLS-enabled editor has the same risks as running mix compile. It's an unlikely attack vector, but worth being aware of.

While you can develop ElixirLS on its own, it's easiest to test out changes if you clone the vscode-elixir-ls repository instead. It includes ElixirLS as a Git submodule, so you can make your changes in the submodule directory and launch the extension from the parent directory via the included "Launch Extension" configuration in launch.json. See the README on that repo for more details.

You can run the apps with mix run from the apps/language_server or apps/debugger directories. To build for release, run the release.sh script. It compiles these two apps to escripts in the release folder.

Elixir escripts typically embed Elixir, which is not what we want because users will want to run it against their own system Elixir installation. In order to support this, there are scripts in the bin folder that look for the system-wide Elixir installation and set the ERL_LIBS environment variable prior to running the escripts. These scripts are copied to the release folder by release.sh.

ElixirLS isn't the first frontend-independent server for Elixir language support. The original was Alchemist Server, which powers the Alchemist plugin for Emacs. Another project, Elixir Sense, builds upon Alchemist and powers the Elixir plugin for Atom as well as another VS Code plugin, VSCode Elixir. ElixirLS uses Elixir Sense for several code insight features. Credit for those projects goes to their respective authors.