nix: setup pre commit hooks to format code

Author: Sorixelle

.gitignore

@@ -1,3 +1,4 @@
+# Nix devshells cached by direnv
 /.direnv/
 
 # The directory Mix will write compiled artifacts to.
@@ -26,3 +27,6 @@ memorable-*.tar
 
 # Memorable development databases
 /db/
+
+# git-hooks.nix generated configuration
+/.pre-commit-config.yaml

flake.lock

@@ -5,12 +5,17 @@
     nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
     flake-utils.url = "github:numtide/flake-utils";
     flake-compat.url = "github:edolstra/flake-compat";
+
+    git-hooks.url = "github:cachix/git-hooks.nix";
+    git-hooks.inputs.nixpkgs.follows = "nixpkgs";
   };
 
   outputs =
     {
+      self,
       nixpkgs,
       flake-utils,
+      git-hooks,
       ...
     }:
     flake-utils.lib.eachDefaultSystem (
@@ -19,7 +24,18 @@
         pkgs = nixpkgs.legacyPackages.${system};
       in
       {
-        devShell = import ./shell.nix { inherit pkgs; };
+        checks.pre-commit-check = git-hooks.lib.${system}.run {
+          src = ./.;
+          hooks = {
+            mix-format.enable = true;
+            nixfmt-rfc-style.enable = true;
+          };
+        };
+
+        devShell = import ./shell.nix {
+          inherit pkgs;
+          preCommitHook = self.checks.${system}.pre-commit-check.shellHook;
+        };
       }
     );
 }

flake.nix

@@ -1,5 +1,6 @@
 {
   pkgs ? import <nixpkgs> { },
+  preCommitHook ? "",
 }:
 
 pkgs.mkShell {
@@ -8,5 +9,12 @@ pkgs.mkShell {
   packages = with pkgs; [
     elixir
     elixir-ls
+
+    nixfmt-rfc-style
+    nil
   ];
+
+  shellHook = ''
+    ${preCommitHook}
+  '';
 }