option defaultEditor: Set {env}VISUAL on true (#8322)

The defaultEditor option sets {env}`EDITOR` however strictly speaking
{env}`EDITOR` is intended for editors that are fully compatible with
teletype terminals (ex: `ed` or `vi`'s `ex` mode).

The {env}`VISUAL variable is intended for modern "visual mode"
terminal editors (ex: `vi` or `emacs`).

Technically speaking editors that are assigned to {env}`EDITOR` should
be configured to operate in teletype compatible mode (see `vi -e` and
`vim -e`).

We don't do this currently because for most users this would be
unintuitive behavior when a script or program mistakenly checks
{env}`EDITOR` instead of first checking {env}`VISUAL`.

In the future it may be worthwhile to introduce an additional option to
these modules to configure {env}`EDITOR` in a strictly conforming manner
(i.e. using the teletype/`ex` mode flags or unsetting {env}`EDITOR`
entirely).

Related Issue: #8314

Signed-off-by: Jacob Abel <jacobabel@nullpo.dev>

modules/programs/helix.nix

@@ -49,7 +49,8 @@ in
       default = false;
       description = ''
         Whether to configure {command}`hx` as the default
-        editor using the {env}`EDITOR` environment variable.
+        editor using the {env}`EDITOR` and {env}`VISUAL`
+        environment variables.
       '';
     };
 
@@ -225,7 +226,10 @@ in
       else
         [ cfg.package ];
 
-    home.sessionVariables = mkIf cfg.defaultEditor { EDITOR = "hx"; };
+    home.sessionVariables = mkIf cfg.defaultEditor {
+      EDITOR = "hx";
+      VISUAL = "hx";
+    };
 
     xdg.configFile =
       let

modules/programs/kakoune.nix

@@ -705,7 +705,8 @@ in
         default = false;
         description = ''
           Whether to configure {command}`kak` as the default
-          editor using the {env}`EDITOR` environment variable.
+          editor using the {env}`EDITOR` and {env}`VISUAL`
+          environment variables.
         '';
       };
 
@@ -755,7 +756,10 @@ in
     programs.kakoune.finalPackage = lib.mkIf (cfg.package != null) kakouneWithPlugins;
 
     home.packages = lib.mkIf (cfg.finalPackage != null) [ cfg.finalPackage ];
-    home.sessionVariables = mkIf cfg.defaultEditor { EDITOR = "kak"; };
+    home.sessionVariables = mkIf cfg.defaultEditor {
+      EDITOR = "kak";
+      VISUAL = "kak";
+    };
     xdg.configFile = lib.mkMerge [
       { "kak/kakrc".source = configFile; }
       (mkIf (cfg.colorSchemePackage != null) {

modules/programs/neovim.nix

@@ -289,7 +289,8 @@ in
         default = false;
         description = ''
           Whether to configure {command}`nvim` as the default
-          editor using the {env}`EDITOR` environment variable.
+          editor using the {env}`EDITOR` and {env}`VISUAL`
+          environment variables.
         '';
       };
 
@@ -453,7 +454,10 @@ in
 
       home.packages = [ cfg.finalPackage ];
 
-      home.sessionVariables = mkIf cfg.defaultEditor { EDITOR = "nvim"; };
+      home.sessionVariables = mkIf cfg.defaultEditor {
+        EDITOR = "nvim";
+        VISUAL = "nvim";
+      };
 
       home.shellAliases = mkIf cfg.vimdiffAlias { vimdiff = "nvim -d"; };
 

modules/programs/vim.nix

@@ -162,7 +162,8 @@ in
         default = false;
         description = ''
           Whether to configure {command}`vim` as the default
-          editor using the {env}`EDITOR` environment variable.
+          editor using the {env}`EDITOR` and {env}`VISUAL`
+          environment variables.
         '';
       };
     };
@@ -213,7 +214,10 @@ in
 
       home.packages = [ cfg.package ];
 
-      home.sessionVariables = lib.mkIf cfg.defaultEditor { EDITOR = "vim"; };
+      home.sessionVariables = lib.mkIf cfg.defaultEditor {
+        EDITOR = "vim";
+        VISUAL = "vim";
+      };
 
       programs.vim = {
         package = vim;

modules/services/emacs.nix

@@ -113,7 +113,8 @@ in
       example = !default;
       description = ''
         Whether to configure {command}`emacsclient` as the default
-        editor using the {env}`EDITOR` environment variable.
+        editor using the {env}`EDITOR` and {env}`VISUAL`
+        environment variables.
       '';
     };
   };
@@ -121,11 +122,16 @@ in
   config = mkIf cfg.enable (
     lib.mkMerge [
       {
-        home.sessionVariables = mkIf cfg.defaultEditor {
-          EDITOR = lib.getBin (
-            pkgs.writeShellScript "editor" ''exec ${lib.getBin cfg.package}/bin/emacsclient "''${@:---create-frame}"''
-          );
-        };
+        home.sessionVariables =
+          let
+            editorBin = lib.getBin (
+              pkgs.writeShellScript "editor" ''exec ${lib.getBin cfg.package}/bin/emacsclient "''${@:---create-frame}"''
+            );
+          in
+          mkIf cfg.defaultEditor {
+            EDITOR = editorBin;
+            VISUAL = editorBin;
+          };
       }
 
       (mkIf pkgs.stdenv.isLinux {