standalone: Allow to extend a standalone derivation with a new module (#1142)

This adds the `nixvimExtend` attribute to the generated standalone
derivation, this attribute takes a module as an argument and returns a
new standalone derivation with the initial module & the extended module
merged together.

docs/man/default.nix

@@ -34,6 +34,7 @@
       cat \
         ${./nixvim-header-start.5} \
         ${mkMDSection ../user-guide/helpers.md} \
+        ${mkMDSection ../user-guide/extending-config.md} \
         ${mkMDSection ../user-guide/faq.md} \
         ${./nixvim-header-end.5} \
         >$out/nixvim-header.5

docs/mdbook/SUMMARY.md

@@ -4,6 +4,7 @@
 
 - [Installation](./user-guide/install.md)
 - [Helpers](./user-guide/helpers.md)
+- [Extending a standalone configuration](./user-guide/extending-config.md)
 - [FAQ](./user-guide/faq.md)
 
 # Options

docs/user-guide/extending-config.md

@@ -0,0 +1,26 @@
+# Extending a standalone configuration
+
+Given a `nvim` derivation obtained from `makeNixvim` or `makeNivxmiWithModule` it is possible to create a new derivation with additional options.
+
+This is done through the `nvim.nixvimExtend` function. This function takes a NixOS module that is going to be merged with the currently set options.
+
+This attribute is recursive, meaning that it can be applied an arbitrary number of times.
+
+## Example
+
+```nix
+{makeNixvimWithModule}: let
+    first = makeNixvimWithModule {
+        module = {
+            extraConfigLua = "-- first stage";
+        };
+    };
+
+    second = first.nixvimExtend {extraConfigLua = "-- second stage";};
+    
+    third = second.nixvimExtend {extraConfigLua = "-- third stage";};
+in
+    third
+```
+
+This will generate a `init.lua` that will contain the three comments from each stages.

flake-modules/tests.nix

@@ -25,6 +25,10 @@
         inherit makeNixvimWithModule;
       };
 
+      extend = import ../tests/extend.nix {
+        inherit pkgs makeNixvimWithModule;
+      };
+
       enable-except-in-tests = import ../tests/enable-except-in-tests.nix {
         inherit pkgs makeNixvimWithModule;
         inherit (self.lib.${system}.check) mkTestDerivationFromNixvimModule;

tests/extend.nix

@@ -0,0 +1,28 @@
+{
+  makeNixvimWithModule,
+  pkgs,
+}: let
+  firstStage = makeNixvimWithModule {
+    module = {
+      extraConfigLua = "-- first stage";
+    };
+  };
+
+  secondStage = firstStage.nixvimExtend {extraConfigLua = "-- second stage";};
+
+  generated = secondStage.nixvimExtend {extraConfigLua = "-- third stage";};
+in
+  pkgs.runCommand "extend-test" {
+    printConfig = "${generated}/bin/nixvim-print-init";
+  } ''
+    config=$($printConfig)
+    for stage in "first" "second" "third"; do
+      if ! "$printConfig" | grep -q -- "-- $stage stage"; then
+        echo "Missing $stage stage in config"
+        echo "$config"
+        exit 1
+      fi
+    done
+
+    touch $out
+  ''

wrappers/standalone.nix

@@ -16,19 +16,20 @@ default_pkgs: {
     config = {};
   };
 
-  eval = lib.evalModules {
-    modules =
-      [
-        module
-        {wrapRc = true;}
-      ]
-      ++ shared.topLevelModules;
-    specialArgs =
-      {
-        inherit helpers;
-      }
-      // extraSpecialArgs;
-  };
+  mkEval = mod:
+    lib.evalModules {
+      modules =
+        [
+          mod
+          {wrapRc = true;}
+        ]
+        ++ shared.topLevelModules;
+      specialArgs =
+        {
+          inherit helpers;
+        }
+        // extraSpecialArgs;
+    };
 
   handleAssertions = config: let
     failedAssertions = map (x: x.message) (lib.filter (x: !x.assertion) config.assertions);
@@ -37,15 +38,21 @@ default_pkgs: {
     then throw "\nFailed assertions:\n${builtins.concatStringsSep "\n" (map (x: "- ${x}") failedAssertions)}"
     else lib.showWarnings config.warnings config;
 
-  config = handleAssertions eval.config;
+  mkNvim = mod: let
+    config = handleAssertions (mkEval mod).config;
+  in
+    (pkgs.symlinkJoin {
+      name = "nixvim";
+      paths =
+        [
+          config.finalPackage
+          config.printInitPackage
+        ]
+        ++ pkgs.lib.optional config.enableMan self.packages.${pkgs.system}.man-docs;
+      meta.mainProgram = "nvim";
+    })
+    // {
+      nixvimExtend = extension: mkNvim {imports = [mod extension];};
+    };
 in
-  pkgs.symlinkJoin {
-    name = "nixvim";
-    paths =
-      [
-        config.finalPackage
-        config.printInitPackage
-      ]
-      ++ pkgs.lib.optional config.enableMan self.packages.${pkgs.system}.man-docs;
-    meta.mainProgram = "nvim";
-  }
+  mkNvim module