docs: home-manager module design spec

2026-10-03 03:35:32 +03:00
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+# valw home-manager module (design)
+
+Date: 2026-10-03
+Status: approved in brainstorming, pending written-spec review
+Builds on: every phase on `main`; the config format in `src/config.rs`
+
+## 1. Goal
+
+Manage valw from home-manager: install it and write its config from Nix,
+with mistakes in the settings caught when the configuration is built, not
+when valw runs.
+
+### Out of scope
+
+- A NixOS module (valw is per user; `packages.default` installs it).
+- niri key bindings (they stay in the user's niri config, i.e. copland).
+- The toolbar's remembered choices (`~/.local/state/valw/toolbar.toml`),
+  which stay writable state.
+
+## 2. Usage
+
+```nix
+programs.valw = {
+  enable = true;
+  settings = {
+    preview.timeout_secs = 3;
+    sound.volume = 0.4;
+    capture.window_shadow = true;
+  };
+};
+```
+
+## 3. Options
+
+| Option | Type | Default | Meaning |
+|---|---|---|---|
+| `programs.valw.enable` | bool | `false` | Install valw (`home.packages`) |
+| `programs.valw.package` | package | this flake's `packages.${system}.default` | The valw to install and to check the settings with |
+| `programs.valw.settings` | `(pkgs.formats.toml { }).type` | `{ }` | Written to `$XDG_CONFIG_HOME/valw/config.toml`; nothing is written when empty, so valw uses its defaults |
+
+## 4. Checking at build time
+
+- `settings` becomes a TOML file with `pkgs.formats.toml`.
+- The file installed as `xdg.configFile."valw/config.toml".source` is a
+  derivation that runs `${package}/bin/valw __check-config <file>` and
+  copies the file to `$out` only if the check passes.
+- `valw __check-config <file>` (hidden): parses the file exactly as valw
+  does at run time. An invalid value prints valw's own error (for example
+  `sound.volume must be between 0 and 1`) and exits non-zero. An unknown
+  key, which valw only warns about at run time, is an error here, listing
+  every unknown key (`unknown config key: preview.timout_secs`). A valid
+  file prints nothing and exits 0. It needs no Wayland, niri or home
+  directory.
+
+## 5. Structure
+
+| File | Change |
+|---|---|
+| `src/main.rs` | hidden `Command::CheckConfig { file }` |
+| `src/config.rs` | `check(path) -> Result<()>`: parse + unknown keys as an error |
+| `nix/home-module.ulu.nix` (new) | `flake.homeModules.default` |
+| `nix/checks.ulu.nix` | `home-module` (evaluates the module with stub `home.packages` / `xdg.configFile` options and builds the checked config) and `home-module-rejects` (a bad setting fails to build, via `pkgs.testers.testBuildFailure`) |
+| `README.md` (new) | what valw is, install (flake package or the module), the module example, niri bind examples, the commands |
+
+No new flake input: the checks evaluate the module with `lib.evalModules`
+and stubs instead of pulling in home-manager.
+
+## 6. Testing
+
+- Rust (`cargo nextest`): `check` accepts a valid file and the defaults,
+  rejects an invalid value with valw's message, rejects unknown keys and
+  names them; the CLI parses `__check-config FILE` and hides it from help.
+- Nix (`nix flake check`): the two module checks above.
+
+Manual: wiring it into copland happens in the end-of-project test pass
+(checklist section).
+
+## 7. Definition of done
+
+`cargo clippy -- -D warnings`, `cargo fmt --check`, `cargo nextest run`
+and `nix flake check` pass; README written; checklist section added.