Allow overriding formatter entirely

Setting the `formatter` option now allows for setting the formatter
directly instead of using the provided formatting functionality with
`formatters`.

api_guide.md

@@ -577,6 +577,11 @@ For example:
 
 ### formatters
 
+The `formatter` option allows you to set `formatter.${system}` outputs. It can
+be set to a function that takes packages and returns the package to use. This
+overrides the `formatters` functionality described below though, so for
+configuring formatters for a file type, you likely want to use `formatters`.
+
 The `formatters` option allows you to configure formatting tools that will be
 used by `nix fmt`. If formatters are set, Flakelight will export
 `formatter.${system}` outputs which apply all the configured formatters.

builtinModules/builtinFormatters.nix

@@ -8,7 +8,8 @@ let
 in
 {
   options.flakelight.builtinFormatters =
-    mkEnableOption "default formatters" // { default = true; };
+    mkEnableOption "default formatters" //
+    { default = config.formatter == null; };
 
   config = mkIf config.flakelight.builtinFormatters {
     devShell.packages = pkgs: [

builtinModules/formatter.nix

@@ -4,19 +4,32 @@
 
 { config, src, lib, flakelight, ... }:
 let
-  inherit (lib) mkOption mkIf mapAttrsToList;
+  inherit (lib) mkDefault mkMerge mkOption mkIf mapAttrsToList;
-  inherit (lib.types) lazyAttrsOf nullOr str;
+  inherit (lib.types) lazyAttrsOf nullOr package str;
   inherit (flakelight.types) optFunctionTo;
 in
 {
-  options.formatters = mkOption {
+  options = {
+    formatter = mkOption {
+      type = nullOr (optFunctionTo package);
+      default = null;
+    };
+    formatters = mkOption {
       type = nullOr (optFunctionTo (lazyAttrsOf str));
       default = null;
     };
+  };
 
-  config = mkIf (config.formatters != null) {
+  config = mkMerge [
+    (mkIf (config.formatter != null) {
+      perSystem = pkgs: {
+        formatter = config.formatter pkgs;
+      };
+    })
+
+    (mkIf (config.formatters != null) {
       perSystem = { pkgs, lib, fd, coreutils, ... }: {
-      formatter = pkgs.writeShellScriptBin "formatter" ''
+        formatter = mkDefault (pkgs.writeShellScriptBin "formatter" ''
           PATH=${lib.makeBinPath (config.devShell.packages pkgs)}
           for f in "$@"; do
             if [ -d "$f" ]; then
@@ -29,13 +42,16 @@ in
             fi
           done &>/dev/null
           wait
-      '';
+        '');
       };
+    })
 
+    (mkIf ((config.formatters != null) || (config.formatter != null)) {
       checks.formatting = { lib, outputs', diffutils, ... }: ''
         ${lib.getExe outputs'.formatter} .
         ${diffutils}/bin/diff -qr ${src} . |\
           sed 's/Files .* and \(.*\) differ/File \1 not formatted/g'
       '';
-  };
+    })
+  ];
 }