Add a formatting pre-commit hook.

.envrc

@@ -1,3 +1,7 @@
 #!/usr/bin/env bash
 
+if command -v git > /dev/null && on_git_branch; then
+  git config --local core.hooksPath .githooks/
+fi
+
 use flake

.githooks/pre-commit

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+set -e
+
+filesToFormat=$(
+  git --no-pager diff --name-status --no-color --cached | \
+    awk '$1 != "D" && $2 ~ /\.exs?$/ {print $NF}'
+)
+
+for path in $filesToFormat
+do
+  mix format "$path"
+  git add "$path"
+done

README.md

@@ -34,11 +34,6 @@ and then start the local server:
 $> mix phx.server
 ```
 
-## Language Server
-
-The [elixir-ls](https://github.com/elixir-lsp/elixir-ls) LSP (version 0.17.10)
-is included in this flake.
-
 ## Dependencies
 
 This project pins Mix dependencies using [mix2nix](https://github.com/ydlr/mix2nix).
@@ -49,3 +44,19 @@ mix2nix > deps.nix
 As of now, `mix2nix` cannot handle git dependencies found inside the `mix.lock`
 file. If you have git dependencies, add them manually or use
 [FODs](https://nixos.org/manual/nixpkgs/stable/#packaging-beam-applications).
+
+## Language Server
+
+The [elixir-ls](https://github.com/elixir-lsp/elixir-ls) LSP (version 0.17.10)
+is included in this flake.
+
+## Formatting
+
+Formatting depends on the `mix format` task. A `pre-commit` hook is included in
+`.githooks` that can be used to format all `*.exs?` files prior to commit.
+Install via:
+```bash
+$> git config --local core.hooksPath .githooks/
+```
+If running [direnv](https://direnv.net/), this hook is installed automatically
+when entering the directory.