example refactor snowflake module system

nix/snow/flake/modules/transposition.nix

@@ -0,0 +1,132 @@
+{ config, lib, flake-parts-lib, ... }:
+
+let
+  inherit (lib)
+    filterAttrs
+    mapAttrs
+    mkOption
+    types
+    ;
+  inherit (lib.strings)
+    escapeNixIdentifier
+    ;
+
+  transpositionModule = {
+    options = {
+      adHoc = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Whether to provide a stub option declaration for {option}`perSystem.<name>`.
+
+          The stub option declaration does not support merging and lacks
+          documentation, so you are recommended to declare the {option}`perSystem.<name>`
+          option yourself and avoid {option}`adHoc`.
+        '';
+      };
+    };
+  };
+
+  perInputAttributeError = { flake, attrName, system, attrConfig }:
+    # This uses flake.outPath for lack of a better identifier.
+    # Consider adding a perInput variation that has a normally-redundant argument for the input name.
+    # Tested manually with
+    # perSystem = { inputs', ... }: {
+    #   packages.extra = inputs'.nixpkgs.extra;
+    #   packages.default = inputs'.nixpkgs.packages.default;
+    #   packages.veryWrong = (top.config.perInput "x86_64-linux" inputs'.nixpkgs.legacyPackages.hello).packages.default;
+    # };
+    # transposition.extra = {};
+    let
+      attrPath = "${escapeNixIdentifier attrName}.${escapeNixIdentifier system}";
+      flakeIdentifier =
+        if flake._type or null != "flake"
+        then
+          throw "An attempt was made to access attribute ${attrPath} on a value that's supposed to be a flake, but may not be a proper flake."
+        else
+          builtins.addErrorContext "while trying to find out how to describe what is supposedly a flake, whose attribute ${attrPath} was accessed but does not exist" (
+            toString flake.outPath
+          );
+      # This ought to be generalized by extending attrConfig, but this is the only known and common mistake for now.
+      alternateAttrNameHint =
+        if attrName == "packages" && flake?legacyPackages
+        then # Unfortunately we can't just switch them out, because that will put packages *sets* where single packages are expected in user code, resulting in potentially much worse and more confusing errors down the line.
+          "\nIt does define legacyPackages; try that instead?"
+        else "";
+    in
+    if flake?${attrName}
+    then
+      throw ''
+        Attempt to access ${attrPath} of flake ${flakeIdentifier}, but it does not have it.
+        It does have attribute ${escapeNixIdentifier attrName}, so it appears that it does not support system type ${escapeNixIdentifier system}.
+      ''
+    else
+      throw ''
+        Attempt to access ${attrPath} of flake ${flakeIdentifier}, but it does not have attribute ${escapeNixIdentifier attrName}.${alternateAttrNameHint}
+      '';
+
+
+in
+{
+  options = {
+    transposition = lib.mkOption {
+      description = ''
+        A helper that defines transposed attributes in the flake outputs.
+
+        When you define `transposition.foo = { };`, definitions are added to the effect of (pseudo-code):
+
+        ```nix
+        flake.foo.''${system} = (perSystem system).foo;
+        perInput = system: inputFlake: inputFlake.foo.''${system};
+        ```
+
+        Transposition is the operation that swaps the indices of a data structure.
+        Here it refers specifically to the transposition between
+
+        ```plain
+        perSystem: .''${system}.''${attribute}
+        outputs:   .''${attribute}.''${system}
+        ```
+
+        It also defines the reverse operation in [{option}`perInput`](#opt-perInput).
+      '';
+      type =
+        types.lazyAttrsOf
+          (types.submoduleWith { modules = [ transpositionModule ]; });
+    };
+  };
+
+  config = {
+    flake =
+      lib.mapAttrs
+        (attrName: attrConfig:
+          mapAttrs
+            (system: v: v.${attrName} or (
+              abort ''
+                Could not find option ${attrName} in the perSystem module. It is required to declare such an option whenever transposition.<name> is defined (and in this instance <name> is ${attrName}).
+              ''))
+            config.allSystems
+        )
+        config.transposition;
+
+    perInput =
+      system: flake:
+      mapAttrs
+        (attrName: attrConfig:
+          flake.${attrName}.${system} or (
+            throw (perInputAttributeError { inherit system flake attrName attrConfig; })
+          )
+        )
+        config.transposition;
+
+    perSystem = {
+      options =
+        mapAttrs
+          (k: v: lib.mkOption { })
+          (filterAttrs
+            (k: v: v.adHoc)
+            config.transposition
+          );
+    };
+  };
+}