Credo Language Server and the birth of elixir-tools

April 18, 2023 • 2 minute read • @mitchhanbergAnalytics

Last year I started working on gen_lsp, an abstraction for writing language servers in Elixir.

I gave a presentation on gen_lsp and writing OTP process abstractions at CodeBEAM America 2022. You can watch my talk on YouTube.

Today I’d like to announce the release of the first language server built with gen_lsp.

Credo Language Server

Credo Language Server is a persistent server that communicates with text editors via the Language Server Protocol.

This initial release comes with project wide diagnostics and a code action to add a # credo:disable-for-next-line magic comment above any Credo warning.

You can install it today using one of the elixir-tools family of editor extensions.


elixir-tools is the new home for Credo Language Server and the aforementioned editor extensions.


If you write Elixir and use Neovim, there is a chance you are already using my plugin, elixir-tools.nvim (formerly known as elixir.nvim).

The Neovim plugin has been renamed to help it better fit into it’s place in the ecosystem and has given rise to the new elixir-tools family.


The elixir-tools family keeps on growing!

The release of Credo Language Server naturally mean that there would need to a way to use it with Visual Studio Code, so elixir-tools.vscode was born.

This initial release brings support for Credo Language Server and Elixir filetype and highlighting support.

ElixirLS support has been omitted since it can happily coexist along side elixir-tools.vscode. If there is demand to add ElixirLS support, that can be done, but at this time there is no need.

Slow and Steady Wins the Race

Over the last 10 months I have been slowly chipping away at this project, making sure that every part is built with excellence in mind.

As I built out gen_lsp, I realized that the best way to achieve correctness was to generate most of the code from the official specification.

So, I built the lsp_codegen library.

It includes a handwritten generator that conforms to the LSP specification’s JSON Schema and is used to read the LSP metamodel and to generate Elixir code that includes typespecs and structs for all of the data structures.

While building the code generator, I realized that the specification’s “Or” type was easy to represent using typespecs, but was actually hard to deserialize from a JSON payload into the Elixir data structures.

Inspired by norm and my friend Chris Keathley, I built schematic. This allows me to fully validate, serialize, and deserialize complex data structures.

So, to build Credo Language Server, it just took 3 libraries, 2 editor extensions, and a conference talk. 😅

I will be writing more about gen_lsp and schematic in the future.

What’s Next

This is just the beginning! 🤗

I have some exciting ideas planned and can’t wait to be able to share them with the Elixir community. If you’d like to stay on the bleeding edge of elixir-tools, feel free to join the Discord.

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