ci: add nixpkgs-fmt check

default.nix

@@ -145,6 +145,7 @@ with rec
   # version from `package.yaml` and create a dummy module that we inject in the
   # `ghci` command.
   niv-devshell = haskellPackages.shellFor {
+    buildInputs = [ pkgs.nixpkgs-fmt ];
     packages = ps: [ ps.niv ];
     shellHook = ''
       repl_for() {

nix/default.nix

@@ -2,6 +2,7 @@
 import sources.nixpkgs {
   overlays = [
     (_: pkgs: { inherit sources; })
+    (_: pkgs: { nixpkgs-fmt = import pkgs.sources.nixpkgs-fmt {}; })
     (_: pkgs: { termtosvg = pkgs.callPackage ./termtosvg.nix {}; })
   ];
   config = {};

nix/sources.json

@@ -11,6 +11,18 @@
         "url": "https://github.com/NixOS/nixpkgs-channels/archive/d85e435b7bded2596d7b201bcd938c94d8a921c1.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
+    "nixpkgs-fmt": {
+        "branch": "master",
+        "description": "Nix code formatter for nixpkgs [maintainer=@zimbatm]",
+        "homepage": "https://nix-community.github.io/nixpkgs-fmt/",
+        "owner": "nix-community",
+        "repo": "nixpkgs-fmt",
+        "rev": "2a64f4b6a597d0d62004ffcec3ae4fd83cd1efc8",
+        "sha256": "01kx4y1d4c5fjvpcffk92pk9hdbgg5j9h2ryqy2hfm54s7mb0k63",
+        "type": "tarball",
+        "url": "https://github.com/nix-community/nixpkgs-fmt/archive/2a64f4b6a597d0d62004ffcec3ae4fd83cd1efc8.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
     "termtosvg": {
         "branch": "develop",
         "description": "Record terminal sessions as SVG animations",

script/test

@@ -2,6 +2,7 @@
 #!nix-shell -i bash
 #!nix-shell -I nixpkgs=./nix
 #!nix-shell -p nix
+#!nix-shell -p nixpkgs-fmt
 #!nix-shell --pure
 
 unset NIX_SSL_CERT_FILE
@@ -32,4 +33,11 @@ fi
 # Build and create a root
 nix-build ${nixargs[@]}
 
+echo "Formatting"
+if ! nixpkgs-fmt --check . ; then
+  echo
+  echo 'run `nixpkgs-fmt .` to fix this issue'
+  exit 1
+fi
+
 echo "all good"