fish: add babelfish translation option

Configuration may be ran through fenv at shell start time
(as previously) or translated to fish at build time with
the babelfish package.

nixos/modules/programs/fish.nix

@@ -13,6 +13,27 @@ let
       (filterAttrs (k: v: v != null) cfg.shellAliases)
   );
 
+  envShellInit = pkgs.writeText "shellInit" cfge.shellInit;
+
+  envLoginShellInit = pkgs.writeText "loginShellInit" cfge.loginShellInit;
+
+  envInteractiveShellInit = pkgs.writeText "interactiveShellInit" cfge.interactiveShellInit;
+
+  sourceEnv = file:
+  if cfg.useBabelfish then
+    "source /etc/fish/${file}.fish"
+  else
+    ''
+      set fish_function_path ${pkgs.fishPlugins.foreign-env}/share/fish-foreign-env/functions $fish_function_path
+      fenv source /etc/fish/foreign-env/${file} > /dev/null
+      set -e fish_function_path[1]
+    '';
+
+  babelfishTranslate = path: name:
+    pkgs.runCommand "${name}.fish" {
+      nativeBuildInputs = [ pkgs.babelfish ];
+    } "${pkgs.babelfish}/bin/babelfish < ${path} > $out;";
+
 in
 
 {
@@ -29,6 +50,15 @@ in
         type = types.bool;
       };
 
+      useBabelfish = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          If enabled, the configured environment will be translated to native fish using <link xlink:href="https://github.com/bouk/babelfish">babelfish</link>.
+          Otherwise, <link xlink:href="https://github.com/oh-my-fish/plugin-foreign-env">foreign-env</link> will be used.
+        '';
+      };
+
       vendor.config.enable = mkOption {
         type = types.bool;
         default = true;
@@ -105,11 +135,32 @@ in
     # Required for man completions
     documentation.man.generateCaches = lib.mkDefault true;
 
-    environment.etc."fish/foreign-env/shellInit".text = cfge.shellInit;
+    environment = mkMerge [
-    environment.etc."fish/foreign-env/loginShellInit".text = cfge.loginShellInit;
+      (mkIf cfg.useBabelfish
-    environment.etc."fish/foreign-env/interactiveShellInit".text = cfge.interactiveShellInit;
+      {
+        etc."fish/setEnvironment.fish".source = babelfishTranslate config.system.build.setEnvironment "setEnvironment";
+        etc."fish/shellInit.fish".source = babelfishTranslate envShellInit "shellInit";
+        etc."fish/loginShellInit.fish".source = babelfishTranslate envLoginShellInit "loginShellInit";
+        etc."fish/interactiveShellInit.fish".source = babelfishTranslate envInteractiveShellInit "interactiveShellInit";
+     })
 
-    environment.etc."fish/nixos-env-preinit.fish".text = ''
+      (mkIf (!cfg.useBabelfish)
+      {
+        etc."fish/foreign-env/shellInit".source = envShellInit;
+        etc."fish/foreign-env/loginShellInit".source = envLoginShellInit;
+        etc."fish/foreign-env/interactiveShellInit".source = envInteractiveShellInit;
+      })
+
+      {
+        etc."fish/nixos-env-preinit.fish".text =
+        if cfg.useBabelfish
+        then ''
+          # source the NixOS environment config
+          if [ -z "$__NIXOS_SET_ENVIRONMENT_DONE" ]
+            source /etc/fish/setEnvironment.fish
+          end
+        ''
+        else ''
           # This happens before $__fish_datadir/config.fish sets fish_function_path, so it is currently
           # unset. We set it and then completely erase it, leaving its configuration to $__fish_datadir/config.fish
           set fish_function_path ${pkgs.fishPlugins.foreign-env}/share/fish/vendor_functions.d $__fish_datadir/functions
@@ -122,15 +173,15 @@ in
           # clear fish_function_path so that it will be correctly set when we return to $__fish_datadir/config.fish
           set -e fish_function_path
         '';
+      }
 
-    environment.etc."fish/config.fish".text = ''
+      {
+        etc."fish/config.fish".text = ''
         # /etc/fish/config.fish: DO NOT EDIT -- this file has been generated automatically.
 
         # if we haven't sourced the general config, do it
         if not set -q __fish_nixos_general_config_sourced
-        set --prepend fish_function_path ${pkgs.fishPlugins.foreign-env}/share/fish/vendor_functions.d
+          ${sourceEnv "shellInit"}
-        fenv source /etc/fish/foreign-env/shellInit > /dev/null
-        set -e fish_function_path[1]
 
           ${cfg.shellInit}
 
@@ -142,9 +193,7 @@ in
         # if we haven't sourced the login config, do it
         status --is-login; and not set -q __fish_nixos_login_config_sourced
         and begin
-        set --prepend fish_function_path ${pkgs.fishPlugins.foreign-env}/share/fish/vendor_functions.d
+          ${sourceEnv "loginShellInit"}
-        fenv source /etc/fish/foreign-env/loginShellInit > /dev/null
-        set -e fish_function_path[1]
 
           ${cfg.loginShellInit}
 
@@ -158,9 +207,7 @@ in
         and begin
           ${fishAliases}
 
-        set --prepend fish_function_path ${pkgs.fishPlugins.foreign-env}/share/fish/vendor_functions.d
+          ${sourceEnv "interactiveShellInit"}
-        fenv source /etc/fish/foreign-env/interactiveShellInit > /dev/null
-        set -e fish_function_path[1]
 
           ${cfg.promptInit}
           ${cfg.interactiveShellInit}
@@ -171,23 +218,10 @@ in
           set -g __fish_nixos_interactive_config_sourced 1
         end
       '';
+      }
 
-    programs.fish.interactiveShellInit = ''
+      {
-      # add completions generated by NixOS to $fish_complete_path
+        etc."fish/generated_completions".source =
-      begin
-        # joins with null byte to acommodate all characters in paths, then respectively gets all paths before (exclusive) / after (inclusive) the first one including "generated_completions",
-        # splits by null byte, and then removes all empty lines produced by using 'string'
-        set -l prev (string join0 $fish_complete_path | string match --regex "^.*?(?=\x00[^\x00]*generated_completions.*)" | string split0 | string match -er ".")
-        set -l post (string join0 $fish_complete_path | string match --regex "[^\x00]*generated_completions.*" | string split0 | string match -er ".")
-        set fish_complete_path $prev "/etc/fish/generated_completions" $post
-      end
-      # prevent fish from generating completions on first run
-      if not test -d $__fish_user_data_dir/generated_completions
-        ${pkgs.coreutils}/bin/mkdir $__fish_user_data_dir/generated_completions
-      end
-    '';
-
-    environment.etc."fish/generated_completions".source =
         let
           patchedGenerator = pkgs.stdenv.mkDerivation {
             name = "fish_patched-completion-generator";
@@ -228,19 +262,40 @@ in
             ignoreCollisions = true;
             paths = map generateCompletions config.environment.systemPackages;
           };
+      }
 
       # include programs that bring their own completions
-    environment.pathsToLink = []
+      {
+        pathsToLink = []
         ++ optional cfg.vendor.config.enable "/share/fish/vendor_conf.d"
         ++ optional cfg.vendor.completions.enable "/share/fish/vendor_completions.d"
         ++ optional cfg.vendor.functions.enable "/share/fish/vendor_functions.d";
+      }
 
-    environment.systemPackages = [ pkgs.fish ];
+      { systemPackages = [ pkgs.fish ]; }
 
-    environment.shells = [
+      {
+        shells = [
           "/run/current-system/sw/bin/fish"
           "${pkgs.fish}/bin/fish"
         ];
+      }
+    ];
+
+    programs.fish.interactiveShellInit = ''
+      # add completions generated by NixOS to $fish_complete_path
+      begin
+        # joins with null byte to acommodate all characters in paths, then respectively gets all paths before (exclusive) / after (inclusive) the first one including "generated_completions",
+        # splits by null byte, and then removes all empty lines produced by using 'string'
+        set -l prev (string join0 $fish_complete_path | string match --regex "^.*?(?=\x00[^\x00]*generated_completions.*)" | string split0 | string match -er ".")
+        set -l post (string join0 $fish_complete_path | string match --regex "[^\x00]*generated_completions.*" | string split0 | string match -er ".")
+        set fish_complete_path $prev "/etc/fish/generated_completions" $post
+      end
+      # prevent fish from generating completions on first run
+      if not test -d $__fish_user_data_dir/generated_completions
+        ${pkgs.coreutils}/bin/mkdir $__fish_user_data_dir/generated_completions
+      end
+    '';
 
   };