Installing Elixir Language Server for Vim

ElixirLS is the Elixir Language Server that provides IDE features like autocomplete, diagnostics, go-to-definition, and Dialyzer integration. This guide covers setting it up in Vim using coc.nvim.

Prerequisites

• Vim or Neovim with coc.nvim installed
• Elixir and Erlang installed (ideally managed via asdf)

Step 1: Install ElixirLS

Clone and build ElixirLS:

git clone https://github.com/elixir-lsp/elixir-ls.git ~/.elixir-ls
cd ~/.elixir-ls
mix deps.get
MIX_ENV=prod mix compile
MIX_ENV=prod mix elixir_ls.release2 -o release

The last command builds the executable language server to ~/.elixir-ls/release/, and that is the path we need to use in the coc.nvim configuration.

Step 2: Configure coc.nvim

Open your coc settings in Vim with :CocConfig and add:

{
  "elixir.pathToElixirLS": "~/.elixir-ls/release/language_server.sh"
}

Step 3: Install coc-elixir

In Vim, run:

:CocInstall coc-elixir

This installs the coc.nvim extension that communicates with ElixirLS.

Step 4: Verify

Open an Elixir file in your project. You should see:

• Autocomplete suggestions
• Inline diagnostics and warnings
• Go-to-definition working (:call CocAction('jumpDefinition'))

Understanding the .elixir_ls Directory

When you first open an Elixir project, ElixirLS creates a .elixir_ls/ directory and compiles your project. You might wonder why it doesn’t just use _build/.

ElixirLS maintains its own build for several reasons:

1. Avoids conflicts - Your _build/ uses your development environment. ElixirLS compiles separately so running mix test doesn’t invalidate the language server’s build.

2. Different compile options - ElixirLS uses flags that capture additional metadata for IDE features. These would slow down normal development builds.

3. Dialyzer PLT files - ElixirLS caches Dialyzer’s Persistent Lookup Table here. Building PLTs is slow, so caching them per-project saves time.

4. Independence - If your _build/ breaks, ElixirLS keeps working.

Consider adding .elixir_ls/ to your global gitignore:

echo ".elixir_ls/" >> ~/.gitignore_global
git config --global core.excludesfile ~/.gitignore_global

Troubleshooting

If ElixirLS behaves unexpectedly, the simplest fix is to delete its cache and restart:

rm -rf .elixir_ls/

Then in Vim:

:CocRestart

ElixirLS automatically recreates the directory and recompiles when it restarts.

Common reasons to clear the cache:

• Upgraded Elixir or Erlang versions
• Dialyzer giving stale warnings
• Language server stuck or unresponsive
• Dependency changes not being picked up