post: self implementing module systems

content/posts/2024-03-30-self-implementing-module-systems.md

@@ -0,0 +1,49 @@
+---
+title: "Nix module trick: Re-evaluating yourself"
+date: 2024-03-30
+draft: false
+markup: "mmark"
+---
+
+This is a cool trick for those playing around with Nix's module
+system. While developing
+[nix-config-modules](https://github.com/chadac/nix-config-modules/), I
+ran into a semi-unique issue: In the module system I defined a set of
+*hosts* and a separate, general set of *apps*, and hosts could define
+which *apps* they could import.
+
+To generate this subset of `app` configurations, I wanted to be able
+to re-evaluate all of my `app` modules with a host argument. In other
+words, I wanted to be able to write modules like:
+
+```nix
+{ host, ... }:
+{
+  app.emacs = {
+    enable = host.kind == "nixos";
+  };
+}
+```
+
+And then essentially I would call `lib.evalModules` on the apps
+modules with the `host` argument substituted with each host I had
+configured.
+
+What I needed was, in my module system I needed to grab all the
+modules that defined my modules. Fortunately Nix makes this easy:
+every module gets a `moduleType` argument that includes the definition
+of all modules. So if you need to reevaluate all of your modules with
+a special additional configuration, all you need to do is:
+
+```nix
+{ lib, moduleType, ... }:
+# < ... >
+lib.evalModules {
+  modules = moduleType.getSubModules ++ [
+    { _module.args.host = myHost; }
+  ];
+};
+```
+
+Of course you'd need to be extremely careful as there's a huge risk of
+infinite recursion here, but it can be pretty handy.