486d80d8 — Steve Cohen 19 days ago
Fixing merge error
f7f7693d — Steve Cohen 19 days ago
Document Symbols support (#652)


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Lexical logo: Lexi the lynx


Lexical is a next-generation language server for the Elixir programming language.


  • Context aware code completion
  • As-you-type compilation
  • Advanced error highlighting
  • Code actions
  • Code Formatting
  • Go To Definition
  • Completely isolated build environment

#What makes Lexical different?

There are a couple things that lexical does differently than other language servers. Let's look at what separates it from the pack.


When lexical starts, it boots an erlang virtual machine that runs the language server and its code. It then boots a separate virtual machine that runs your project code and connects the two via distribution. This provides the following benefits:

  • None of lexical's dependencies will conflict with your project. This means that lexical can make use of dependencies to make developing in it easier without having to "vendor" them. It also means that you can use lexical to work on your project, even if lexical depends on your project.
  • Your project can depend on a different version of elixir and erlang than lexical itself. This means that lexical can make use of the latest versions of elixir and erlang while still supporting projects that run on older versions.
  • The build environment for your project is only aware of your project, which enables as-you-type compilation and error reporting.
  • In the future, there is a possibility of having the lexical vm instance control multiple projects
#Ease of contribution

Lexical has been designed to be easy to contribute to and work on. It features:

  • A consistent data model that represents the Language Server Protocol and mix tasks to generate new Language Server models.
  • A clearly defined separation between the language server and project code
  • A set of utilities that deal with manipulating code
  • A set of unit tests and test cases that make testing new features easy.
#Focus on developer productivity

Lexical is also built with an eye on increasing developer productivity, and approaches some common features a little bit differently. For example, Lexical's code completion is context aware, which means that if you type alias MyModule.| you will only receive completions for modules and not the names of functions in MyModule. This awareness will extend to other areas, which means:

  • You won't see completions for random functions and types in strings. In fact, when extended to string literals, Lexical will only show you completions if you're inside of an interpolation ("hello there #{na|}').
  • If you're inside of a struct reference (%StructModule.|), you will only see modules listed that define structs, or are the parents of modules that define structs.

Because of this focus, Lexical aims to deliver depth of features rather than breadth of them. We'll likely spend more time making sure each thing we add works and feels just right rather than adding a whole slew of features that people mostly won't use.

#As you type compilation

Because your project is run in a separate virtual machine, we can compile the code that you're working on as you type. This means you see errors immediately rather than having to wait for a save. The result is you see and fix typos, warnings, unused variables and a whole host of errors when they occur, which makes your code better, faster.


Follow the Detailed Installation Instructions

mix package

Lexical will now be available in _build/dev/package/lexical

If you would like to change the output directory, you can do so with the --path option

mix package --path /path/to/lexical

Lexical will be available in /path/to/lexical.


Lexical is intended to run on any version of Erlang 24+ and Elixir 1.13+. Before beginning development, you should install Erlang and Elixir 1.13.4 and use those versions when you're building code.

You should also look at the complete compatibility matrix do see which versions are supported.

You're going to need a local instance in order to develop lexical, so follow the Detailed Installation Instructions first.

Then, install the git hooks with

mix hooks

These are pre-commit hooks that will check for correct formatting and run credo for you.

After this, you're ready to put together a pull request for Lexical!


The remote_control project has a set of benchmarks that measure the speed of various internal functions and data structures. In order to use them, you first need to install git large file storage, and then run git pull. Benchmarks are stored in the benchmarks subdirectory, and can be run via

mix benchmark /benchmarks/<benchmark_file>.exs


When lexical starts up, it creates a .lexical directory in the root directory of a project. Inside that directory are two log files, lexical.log and project.log. The .lexical.log log file contains logging and OTP messages from the language server, while the project.log file contains logging and OTP messages from the project's node. While developing lexical, it's helpful to open up a terminal and tail both of these log files so you can see any errors and messages that lexical emits. To do that, run the following in a terminal while in the project's root directory:

tail -f .lexical/*.log

Note: These log files roll over when they reach 1 megabyte, so after a time, it will be necessary to re-run the above command.


Lexical supports a debug shell, which will connect a remote shell to a currently-running language server process. To use it, cd into your lexical installation directory and run

./bin/debug_shell.sh <name of project>

For example, if I would like to run the debug server for a server running in your lexical project, run:

./bin/debug_shell.sh lexical

...and you will be connected to a remote IEx session inside my language server. This allows you to investigate processes, make changes to the running code, or run :observer.

While in the debugging shell, all the functions in Lexical.Server.IEx.Helpers are imported for you, and some common modules, like Lexical.Project and Lexical.Document are aliased.

You can also start the lexical server in interactive mode via ./bin/start_lexical.sh iex. Combining this with the helpers that are imported will allow you to run projects and do completions entirely in the shell.

Note: The helpers assume that all of your projects are in folders that are siblings with your lexical project.

Consider the example shell session:

./bin/start_lexical.sh iex
iex(1)> start_project :other
# the project in the ../other directory is started
# the other project is compiled
iex(2)> complete :other, "defmo|"
    detail: "",
    insert_text: "defmacro ${1:name}($2) do\n  $0\nend\n",
    insert_text_format: :snippet,
    kind: :class,
    label: "defmacro (Define a macro)",
    sort_text: "093_defmacro (Define a macro)"
    detail: "",
    insert_text: "defmacrop ${1:name}($2) do\n  $0\nend\n",
    insert_text_format: :snippet,
    kind: :class,
    label: "defmacrop (Define a private macro)",
    sort_text: "094_defmacrop (Define a private macro)"
    detail: "",
    insert_text: "defmodule ${1:module name} do\n  $0\nend\n",
    insert_text_format: :snippet,
    kind: :class,
    label: "defmodule (Define a module)",
    sort_text: "092_defmodule (Define a module)"

The same kind of support is available when you run iex -S mix in the lexical directory, and is helpful for narrowing down issues without disturbing your editor flow.

#Other resources