fish: format user and generated .fish files

Adds a `fishIndent` wrapper to pass fish scripts to the built in
`fish_indent` function.

modules/programs/fish.nix

@@ -145,6 +145,16 @@ let
   aliasesStr = concatStringsSep "\n"
     (mapAttrsToList (k: v: "alias ${k} ${escapeShellArg v}") cfg.shellAliases);
 
+  fishIndent = name: text:
+    if cfg.formatFishScripts then
+      pkgs.runCommand name {
+        nativeBuildInputs = [ cfg.package ];
+        inherit text;
+        passAsFile = [ "text" ];
+      } "fish_indent < $textPath > $out"
+    else
+      pkgs.writeText name text;
+
 in {
   imports = [
     (mkRemovedOptionModule [ "programs" "fish" "promptInit" ] ''
@@ -279,6 +289,15 @@ in {
       '';
     };
 
+    programs.fish.formatFishScripts = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Whether to process fish configuration and scripts with
+        <literal>fish_indent</literal>.
+      '';
+    };
+
   };
 
   config = mkIf cfg.enable (mkMerge [
@@ -340,7 +359,7 @@ in {
         end
       '';
 
-      xdg.configFile."fish/config.fish".text = ''
+      xdg.configFile."fish/config.fish".source = fishIndent "config.fish" ''
         # ~/.config/fish/config.fish: DO NOT EDIT -- this file has been generated
         # automatically by home-manager.
 
@@ -379,7 +398,7 @@ in {
       xdg.configFile = mapAttrs' (name: def: {
         name = "fish/functions/${name}.fish";
         value = {
-          text = let
+          source = let
             modifierStr = n: v: optional (v != null) ''--${n}="${toString v}"'';
             modifierStrs = n: v: optional (v != null) "--${n}=${toString v}";
             modifierBool = n: v: optional (v != null && v) "--${n}";
@@ -397,9 +416,9 @@ in {
 
             modifiers = if isAttrs def then " ${toString mods}" else "";
             body = if isAttrs def then def.body else def;
-          in ''
+          in fishIndent "${name}.fish" ''
             function ${name}${modifiers}
-              ${body}
+              ${lib.strings.removeSuffix "\n" body}
             end
           '';
         };
@@ -410,7 +429,8 @@ in {
     # in the paths and any initialization scripts.
     (mkIf (length cfg.plugins > 0) {
       xdg.configFile = mkMerge ((map (plugin: {
-        "fish/conf.d/plugin-${plugin.name}.fish".text = ''
+        "fish/conf.d/plugin-${plugin.name}.fish".source =
+          fishIndent "${plugin.name}.fish" ''
             # Plugin ${plugin.name}
             set -l plugin_dir ${plugin.src}
 

tests/modules/programs/fish/default.nix

@@ -1,4 +1,5 @@
 {
+  fish-format-scripts = ./format-scripts.nix;
   fish-functions = ./functions.nix;
   fish-no-functions = ./no-functions.nix;
   fish-plugins = ./plugins.nix;

tests/modules/programs/fish/format-scripts.nix

@@ -0,0 +1,50 @@
+{ config, pkgs, ... }:
+
+let
+
+  expectedFunc = pkgs.writeText "func.fish" ''
+    function func
+        echo foo
+    end
+  '';
+
+  expectedFuncMulti = pkgs.writeText "func-multi.fish" ''
+    function func-multi
+        echo bar
+        if foo
+            bar
+            baz
+        end
+    end
+  '';
+
+in {
+  config = {
+    programs.fish = {
+      enable = true;
+
+      formatFishScripts = true;
+
+      functions = {
+        func = ''echo "foo"'';
+        func-multi = ''
+              echo bar
+          if foo
+              bar
+                  baz
+                end
+        '';
+      };
+    };
+
+    nmt.script = ''
+      assertFileExists home-files/.config/fish/functions/func.fish
+      echo ${expectedFunc}
+      assertFileContent home-files/.config/fish/functions/func.fish ${expectedFunc}
+
+      assertFileExists home-files/.config/fish/functions/func-multi.fish
+      echo ${expectedFuncMulti}
+      assertFileContent home-files/.config/fish/functions/func-multi.fish ${expectedFuncMulti}
+    '';
+  };
+}