example refactor snowflake module system

nix/snow/flake/modules/perSystem.nix

@@ -0,0 +1,159 @@
+{ config, lib, flake-parts-lib, self, ... }:
+let
+  inherit (lib)
+    genAttrs
+    mapAttrs
+    mkOption
+    types
+    ;
+  inherit (lib.strings)
+    escapeNixIdentifier
+    ;
+  inherit (flake-parts-lib)
+    mkPerSystemType
+    ;
+
+  rootConfig = config;
+
+  # Stubs for self and inputs. While it'd be possible to define aliases
+  # inside perSystem, that is not a general solution, and it would make
+  # top.config harder to discover, stretching the learning curve rather
+  # than flattening it.
+
+  throwAliasError' = param:
+    throw ''
+      `${param}` (without `'`) is not a `perSystem` module argument, but a
+      module argument of the top level config.
+
+      The following is an example usage of `${param}`. Note that its binding
+      is in the `top` parameter list, which is declared by the top level module
+      rather than the `perSystem` module.
+
+        top@{ config, lib, ${param}, ... }: {
+          perSystem = { config, ${param}', ... }: {
+            # in scope here:
+            #  - ${param}
+            #  - ${param}'
+            #  - config (of perSystem)
+            #  - top.config (note the `top@` pattern)
+          };
+        }
+    '';
+
+  throwAliasError = param:
+    throw ''
+      `${param}` is not a `perSystem` module argument, but a module argument of
+      the top level config.
+
+      The following is an example usage of `${param}`. Note that its binding
+      is in the `top` parameter list, which is declared by the top level module
+      rather than the `perSystem` module.
+
+        top@{ config, lib, ${param}, ... }: {
+          perSystem = { config, ... }: {
+            # in scope here:
+            #  - ${param}
+            #  - config (of perSystem)
+            #  - top.config (note the `top@` pattern)
+          };
+        }
+    '';
+
+  /**
+    We primarily use `systems` to help memoize the per system context, but that
+    doesn't extend to arbitrary `system`s.
+    For that, we use the slightly less efficient, but perfectly acceptable
+    `memoizeStr` function.
+   */
+  otherMemoizedSystems = flake-parts-lib.memoizeStr config.perSystem;
+
+in
+{
+  options = {
+    systems = mkOption {
+      description = ''
+        All the system types to enumerate in the flake output subattributes.
+
+        In other words, all valid values for `system` in e.g. `packages.<system>.foo`.
+      '';
+      type = types.listOf types.str;
+    };
+
+    perInput = mkOption {
+      description = ''
+        A function that pre-processes flake inputs.
+
+        It is called for users of `perSystem` such that `inputs'.''${name} = config.perInput system inputs.''${name}`.
+
+        This is used for [`inputs'`](../module-arguments.html#inputs) and [`self'`](../module-arguments.html#self).
+
+        The attributes returned by the `perInput` function definitions are merged into a single namespace (per input), 
+        so each module should return an attribute set with usually only one or two predictable attribute names. Otherwise,
+        the `inputs'` namespace gets polluted.
+      '';
+      type = types.functionTo (types.functionTo (types.lazyAttrsOf types.unspecified));
+    };
+
+    perSystem = mkOption {
+      description = ''
+        A function from system to flake-like attributes omitting the `<system>` attribute.
+
+        Modules defined here have access to the suboptions and [some convenient module arguments](../module-arguments.html).
+      '';
+      type = mkPerSystemType ({ config, system, ... }: {
+        _file = ./perSystem.nix;
+        config = {
+          _module.args.inputs' =
+            mapAttrs
+              (inputName: input:
+                builtins.addErrorContext "while retrieving system-dependent attributes for input ${escapeNixIdentifier inputName}" (
+                  if input._type or null == "flake"
+                  then rootConfig.perInput system input
+                  else
+                    throw "Trying to retrieve system-dependent attributes for input ${escapeNixIdentifier inputName}, but this input is not a flake. Perhaps flake = false was added to the input declarations by mistake, or you meant to use a different input, or you meant to use plain old inputs, not inputs'."
+                )
+              )
+              self.inputs;
+          _module.args.self' =
+            builtins.addErrorContext "while retrieving system-dependent attributes for a flake's own outputs" (
+              rootConfig.perInput system self
+            );
+
+          # Custom error messages
+          _module.args.self = throwAliasError' "self";
+          _module.args.inputs = throwAliasError' "inputs";
+          _module.args.getSystem = throwAliasError "getSystem";
+          _module.args.withSystem = throwAliasError "withSystem";
+          _module.args.moduleWithSystem = throwAliasError "moduleWithSystem";
+        };
+      });
+      apply = modules: system:
+        (lib.evalModules {
+          inherit modules;
+          prefix = [ "perSystem" system ];
+          specialArgs = {
+            inherit system;
+          };
+          class = "perSystem";
+        }).config;
+    };
+
+    allSystems = mkOption {
+      type = types.lazyAttrsOf types.unspecified;
+      description = "The system-specific config for each of systems.";
+      internal = true;
+    };
+  };
+
+  config = {
+    allSystems = genAttrs config.systems config.perSystem;
+    _module.args.getSystem = system: config.allSystems.${system} or (otherMemoizedSystems system);
+
+    # The warning is there for a reason. Only use this in situations where the
+    # performance cost has already been incurred, such as in `flakeModules.easyOverlay`,
+    # where we run in the context of an overlay, and the performance cost of the
+    # extra `pkgs` makes the cost of running `perSystem` probably negligible.
+    _module.args.getSystemIgnoreWarning = system: config.allSystems.${system} or (config.perSystem system);
+  };
+
+}