spellcheck: allow adding arbitrary spellfiles from name-value pairs

modules/neovim/init/spellcheck.nix

@@ -1,11 +1,14 @@
 {
   config,
+  pkgs,
   lib,
   ...
 }: let
   inherit (lib.modules) mkIf mkRenamedOptionModule;
   inherit (lib.options) mkOption mkEnableOption literalExpression;
-  inherit (lib.types) listOf str;
+  inherit (lib.strings) concatLines;
+  inherit (lib.attrsets) mapAttrsToList;
+  inherit (lib.types) listOf str attrsOf;
   inherit (lib.nvim.lua) listToLuaTable;
   inherit (lib.nvim.dag) entryAfter;
 
@@ -24,10 +27,43 @@ in {
       description = ''
         A list of languages that should be used for spellchecking.
 
-        To add your own language files, you may place your `spell`
-        directory in either `~/.config/nvim` or the
-        [additionalRuntimePaths](#opt-vim.additionalRuntimePaths)
-        directory provided by **nvf**.
+        To add your own language files, you may place your `spell` directory in either
+        {file}`$XDG_CONFIG_HOME/nvf` or in a path that is included in the
+        [additionalRuntimePaths](#opt-vim.additionalRuntimePaths) list provided by nvf.
+      '';
+    };
+
+    extraSpellWords = mkOption {
+      type = attrsOf (listOf str);
+      default = {};
+      example = literalExpression ''{"en.utf-8" = ["nvf" "word_you_want_to_add"];}'';
+      description = ''
+        Additional words to be used for spellchecking. The names of each key will be
+        used as the language code for the spell file. For example
+
+        ```nix
+        "en.utf-8" = [ ... ];
+        ```
+
+        will result in `en.utf-8.add.spl` being added to Neovim's runtime in the
+        {file}`spell` directory.
+
+        ::: {.warning}
+        The attribute keys must be in `"<name>.<encoding>"` format for Neovim to
+        compile your spellfiles without mangling the resulting file names. Please
+        make sure that you enter the correct value, as nvf does not do any kind of
+        internal checking. Please see {command}`:help mkspell` for more details.
+
+        Example:
+
+        ```nix
+        # "en" is the name, and "utf-8" is the encoding. For most use cases, utf-8
+        # will be enough, however, you may change it to any encoding format Neovim
+        # accepts, e.g., utf-16.
+        "en.utf-8" = ["nvf" "word_you_want_to_add"];
+        => $out/spell/en-utf-8.add.spl
+        ```
+        :::
       '';
     };
 
@@ -38,8 +74,10 @@ in {
       description = ''
         A list of filetypes for which spellchecking will be disabled.
 
-        You may use `echo &filetype` in Neovim to find out the
+        ::: {.tip}
+        You may use {command}`:echo &filetype` in Neovim to find out the
         filetype for a specific buffer.
+        :::
       '';
     };
 
@@ -58,7 +96,41 @@ in {
   };
 
   config = mkIf cfg.enable {
-    vim.luaConfigRC.spellcheck = entryAfter ["basic"] ''
+    vim = {
+      additionalRuntimePaths = let
+        spellfilesJoined = pkgs.symlinkJoin {
+          name = "nvf-spellfiles-joined";
+          paths = mapAttrsToList (name: value: pkgs.writeTextDir "spell/${name}.add" (concatLines value)) cfg.extraSpellWords;
+          postBuild = ''
+            echo "Spellfiles joined"
+          '';
+        };
+
+        compileJoinedSpellfiles =
+          pkgs.runCommandLocal "nvf-compile-spellfiles" {
+            # Use the same version of Neovim as the user's configuration
+            nativeBuildInputs = [config.vim.package];
+          } ''
+            mkdir -p "$out/spell"
+
+            spellfilesJoined=$(find -L "${spellfilesJoined}/spell" -type f)
+            for spellfile in $spellfilesJoined; do
+                # Hacky way to ensure that the mangled extensions are omitted from the
+                # joined spellfiles. E.g.
+                local name=$(basename "$spellfile" ".add")
+                echo "Compiling spellfile: $spellfile"
+                nvim --headless --clean \
+                    --cmd "mkspell $out/spell/$name.add.spl $spellfile" -Es -n
+            done
+          '';
+      in
+        mkIf (cfg.extraSpellWords != {}) [
+          # If .outPath is missing, additionalRuntimePaths receives the *function*
+          # instead of a path, causing errors.
+          compileJoinedSpellfiles.outPath
+        ];
+
+      luaConfigRC.spellcheck = entryAfter ["basic"] ''
         vim.opt.spell = true
         vim.opt.spelllang = ${listToLuaTable cfg.languages}
 
@@ -72,4 +144,5 @@ in {
         })
       '';
     };
+  };
 }