neovim/basic: add undofile options

`vim.undoFile.enable` and `vim.undoFile.path` can be used to manipulate whether undofile will be enabled, and the location if it is enabled.

modules/neovim/init/basic.nix

@@ -3,11 +3,12 @@
   lib,
   ...
 }: let
-  inherit (lib.options) mkOption literalExpression;
+  inherit (lib.options) mkOption mkEnableOption literalExpression;
   inherit (lib.strings) optionalString;
-  inherit (lib.types) enum bool str int;
+  inherit (lib.types) enum bool str int nullOr either;
   inherit (lib.nvim.dag) entryAfter;
-  inherit (lib.nvim.lua) toLuaObject;
+  inherit (lib.nvim.lua) toLuaObject mkLuaInline;
+  inherit (lib.nvim.types) luaInline;
 
   cfg = config.vim;
 in {
@@ -158,9 +159,39 @@ in {
       default = "sensitive";
       description = "Set the case sensitivity of search";
     };
+
+    undoFile = {
+      enable = mkEnableOption ''
+        undofile for Neovim.
+
+        ::: {.warning}
+        [{option}`vim.undoFile.path`](#opt-vim.undoFile.path) **must** be set to a valid
+        path for this option to have any effect.
+        :::
+      '';
+
+      path = mkOption {
+        type = nullOr (either str luaInline);
+        default = null;
+        example = "os.getenv('XDG_DATA_HOME') .. '/nvf/undo'";
+        description = ''
+          Path to the file in which undo history will be saved. Must
+          an absolute path in a world-writable directory such as
+          {file}`~/.local/share`.
+
+          ::: {.tip}
+          You can use use variable substitution as you normally would in Neovim to
+          pick directories that conform with XDG spec. **nvf**, by, default creates
+          {file}`$XDG_DATA_HOME/nvf` - which you may choose to use for storing
+          state such as the undofile.
+          :::
+        '';
+      };
+    };
   };
 
-  config.vim.luaConfigRC.basic = entryAfter ["globalsScript"] ''
+  config = {
+    vim.luaConfigRC.basic = entryAfter ["globalsScript"] ''
       -- Settings that are set for everything
       vim.o.encoding = "utf-8"
       vim.o.hidden = true
@@ -178,6 +209,11 @@ in {
       vim.g.mapleader = ${toLuaObject cfg.leaderKey}
       vim.g.maplocalleader = ${toLuaObject cfg.leaderKey}
 
+      ${optionalString cfg.undoFile.enable ''
+        vim.o.undofile = true
+        vim.o.undodir = ${toLuaObject cfg.undoFile.path}
+      ''}
+
       ${optionalString cfg.splitBelow ''
         vim.o.splitbelow = true
       ''}
@@ -266,4 +302,16 @@ in {
         vim.o.ignorecase = false
       ''}
     '';
+
+    assertions = [
+      {
+        assertion = cfg.undoFile.enable -> cfg.undoFile.path != null;
+        message = ''
+          `vim.undoFile.enable` is set to true, but `vim.undoFile.path` is not set
+          as a valid path. Please set `vim.undoFile.path` to a valid path, or disable
+          the undoFile feature.
+        '';
+      }
+    ];
+  };
 }