Add transposition.<name>.adHoc

README.md

@@ -128,3 +128,13 @@ Why so many ways?
 
 1. Flakes counterintuitively handles `system` by enumerating all of them in attribute sets. `flake-parts` does not impose this restriction, but does need to support it.
 2. `flake-parts` provides an extensible structure that is richer than the flakes interface alone. 
+
+# How do I define my own flake output attribute?
+
+Have a look at the [source](https://github.com/hercules-ci/flake-parts/tree/main/modules) for some examples.
+
+Whether directly or indirectly, you'll be defining an attribute inside [the `flake` option](https://flake.parts/options.html#opt-flake).
+
+If you want the attribute to be derived from [`perSystem`](https://flake.parts/options.html#opt-perSystem) you can start with [`packages.nix`](https://github.com/hercules-ci/flake-parts/blob/main/modules/packages.nix) as an example, or [`formatter.nix`](https://github.com/hercules-ci/flake-parts/blob/main/modules/formatter.nix) if you need to do some filtering.
+
+If you really don't care about your attribute, you may temporarily use [`transposition.<name>.adHoc = true`](https://flake.parts/options.html#opt-transposition._name_.adHoc) to create and expose a `perSystem` option without merging support, type checking or documentation.

modules/transposition.nix

@@ -11,6 +11,23 @@ let
     mkSubmoduleOptions
     mkPerSystemOption
     ;
+
+  transpositionModule = {
+    options = {
+      adHoc = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Whether to provide a stub option declaration for <option>perSystem.&lt;name></option>
+
+          The stub option declaration does not support merging and lacks
+          documentation, so you are recommended to declare the <option>perSystem.&lt;name></option>
+          option yourself and avoid <option>adHoc</option>.
+        '';
+      };
+    };
+  };
+
 in
 {
   options = {
@@ -30,7 +47,7 @@ in
       '';
       type =
         types.lazyAttrsOf
-          (types.submoduleWith { modules = [ ]; });
+          (types.submoduleWith { modules = [ transpositionModule ]; });
     };
   };
 
@@ -52,5 +69,15 @@ in
           (attrName: attrConfig: flake?${attrName}.${system})
           config.transposition
         );
+
+    perSystem = { ... }: {
+      options =
+        mapAttrs
+          (k: v: lib.mkOption { })
+          (filterAttrs
+            (k: v: v.adHoc)
+            config.transposition
+          );
+    };
   };
 }