build(nix): Introduce flake.formatter for nix fmt

flake.lock

@@ -1,5 +1,21 @@
 {
   "nodes": {
+    "flake-compat": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1673956053,
+        "narHash": "sha256-4gtG9iQuiKITOjNQQeQIpoIB6b16fm+504Ch3sNKLd8=",
+        "owner": "edolstra",
+        "repo": "flake-compat",
+        "rev": "35bb57c0c8d8b62bbfd284272c928ceb64ddbde9",
+        "type": "github"
+      },
+      "original": {
+        "owner": "edolstra",
+        "repo": "flake-compat",
+        "type": "github"
+      }
+    },
     "flake-parts": {
       "inputs": {
         "nixpkgs-lib": "nixpkgs-lib"
@@ -18,6 +34,48 @@
         "type": "github"
       }
     },
+    "flake-utils": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1694529238,
+        "narHash": "sha256-zsNZZGTGnMOf9YpHKJqMSsa0dXbfmxeoJ7xHlrt+xmY=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "ff7b65b44d01cf9ba6a71320833626af21126384",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixfmt": {
+      "inputs": {
+        "flake-compat": "flake-compat",
+        "flake-utils": "flake-utils",
+        "nixpkgs": [
+          "nixpkgs"
+        ],
+        "nixpkgs-stable": "nixpkgs-stable"
+      },
+      "locked": {
+        "lastModified": 1707402853,
+        "narHash": "sha256-d7a6xfqhbyLikHultxjHO33SqUcif/8eGyxhRZ9PvZk=",
+        "owner": "piegamesde",
+        "repo": "nixfmt",
+        "rev": "ca1fe3572ea6134551ee6af40ae735a1a1ed0d35",
+        "type": "github"
+      },
+      "original": {
+        "owner": "piegamesde",
+        "ref": "rfc101-style",
+        "repo": "nixfmt",
+        "type": "github"
+      }
+    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1708118438,
@@ -52,11 +110,43 @@
         "type": "github"
       }
     },
+    "nixpkgs-stable": {
+      "locked": {
+        "lastModified": 1696039360,
+        "narHash": "sha256-g7nIUV4uq1TOVeVIDEZLb005suTWCUjSY0zYOlSBsyE=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "32dcb45f66c0487e92db8303a798ebc548cadedc",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-23.05",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "root": {
       "inputs": {
         "flake-parts": "flake-parts",
+        "nixfmt": "nixfmt",
         "nixpkgs": "nixpkgs"
       }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
     }
   },
   "root": "root",

flake.nix

@@ -18,6 +18,10 @@
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
     flake-parts.url = "github:hercules-ci/flake-parts";
+    nixfmt = {
+        url = "github:piegamesde/nixfmt/rfc101-style";
+        inputs.nixpkgs.follows = "nixpkgs";
+    };
   };
 
   # There's an optional binary cache available. The details are below, but they're commented out.
@@ -131,6 +135,11 @@
             ...
           }:
           {
+            # For standardised reproducible formatting with `nix fmt`
+            # HELP: why does the per system formatter not work?
+            # formatter = inputs.nixfmt.packages.${system}.nixfmt;
+            formatter = if (system == "x86_64-linux") then inputs.nixfmt.packages.${system}.nixfmt else null;
+
             # Unlike `.#packages`, legacyPackages may contain values of
             # arbitrary types (including nested attrsets) and may even throw
             # exceptions. This attribute isn't recursed into by `nix flake