example refactor snowflake module system

nix/snow/default.nix

@@ -12,180 +12,23 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 {
-  this,
-  self,
-  inputs,
-  systems,
   nt,
   mix,
   ...
 } @ args: let
-  inherit
-    (builtins)
-    all
-    attrNames
-    elem
-    mapAttrs
-    warn
-    ;
-
-  inherit (inputs.nixpkgs) lib;
-
   inherit (nt) findImport;
 in
   mix.newMixture args (mixture: let
     inherit (mixture) mapNodes;
   in {
-    includes.private = [
-      ./lib/nodes.nix
-    ];
+    includes = {
+      private = [
+        ./lib/nodes.nix
+      ];
+      public = [
+        ./flake
+      ];
+    };
 
     inherit findImport;
-
-    # snow.flake
-    flake = flakeInputs: root: let
-      module = lib.evalModules {
-        class = "snowflake";
-        # TODO: abort if inputs contains reserved names
-        specialArgs =
-          (flakeInputs
-            // {
-              inherit systems root;
-              inherit (this) snow;
-              inputs = flakeInputs;
-            })
-          |> (x: builtins.removeAttrs x ["self" "nodes"]);
-
-        modules = [
-          ./module.nix
-          ({config, ...}: {
-            _module.args = {
-              self = config;
-              nodes = config.nodes.nodes;
-            };
-          })
-        ];
-      };
-
-      nodes = module.config.nodes;
-    in rec {
-      nixosConfigurations = mapNodes nodes (
-        {
-          base,
-          lib,
-          name,
-          node,
-          groupModules,
-          ...
-        }: let
-          homeManager =
-            if node.homeManager != null
-            then node.homeManager
-            else if nodes.homeManager != null
-            then nodes.homeManager
-            else
-              warn ''
-                [snowflake] Neither `nodes.homeManager` nor `nodes.nodes.${name}.homeManager` were specified!
-                [snowflake] home-manager will NOT be used! User configuration will be ignored!
-              ''
-              null;
-
-          userArgs = nodes.args // node.args;
-          ceruleanArgs = {
-            inherit systems root base nodes node;
-            inherit (node) system;
-            inherit (this) snow;
-            hostname = name;
-
-            _cerulean = {
-              inherit inputs userArgs ceruleanArgs homeManager;
-              specialArgs = userArgs // ceruleanArgs;
-            };
-          };
-          specialArgs = assert (userArgs
-            |> attrNames
-            |> all (argName:
-              ! ceruleanArgs ? argName
-              || abort ''
-                `specialArgs` are like super important to Cerulean my love... </3
-                But `args.${argName}` is a reserved argument name :(
-              ''));
-            ceruleanArgs._cerulean.specialArgs;
-        in
-          lib.nixosSystem {
-            inherit (node) system;
-            inherit specialArgs;
-            modules =
-              [
-                self.nixosModules.default
-                (findImport /${root}/hosts/${name})
-              ]
-              ++ (groupModules root)
-              ++ node.modules
-              ++ nodes.modules;
-          }
-      );
-
-      deploy.nodes = mapNodes nodes ({
-        name,
-        node,
-        ...
-      }: let
-        inherit
-          (node.deploy)
-          ssh
-          user
-          interactiveSudo
-          remoteBuild
-          rollback
-          autoRollback
-          magicRollback
-          activationTimeout
-          confirmTimeout
-          ;
-
-        nixosFor = system: inputs.deploy-rs.lib.${system}.activate.nixos;
-      in {
-        hostname =
-          if ssh.host != null
-          then ssh.host
-          else "";
-
-        profilesOrder = ["default"]; # profiles priority
-        profiles.default = {
-          path = nixosFor node.system nixosConfigurations.${name};
-
-          user = user;
-          sudo = "sudo -u";
-          interactiveSudo = interactiveSudo;
-
-          fastConnection = false;
-
-          autoRollback = autoRollback -> rollback;
-          magicRollback = magicRollback -> rollback;
-          activationTimeout = activationTimeout;
-          confirmTimeout = confirmTimeout;
-
-          remoteBuild = remoteBuild;
-          sshUser = ssh.user;
-          sshOpts =
-            ssh.opts
-            ++ (
-              if elem "-p" ssh.opts
-              then []
-              else ["-p" (toString ssh.port)]
-            )
-            ++ (
-              if elem "-A" ssh.opts
-              then []
-              else ["-A"]
-            );
-        };
-      });
-
-      checks =
-        inputs.deploy-rs.lib
-        |> mapAttrs (system: deployLib:
-          deployLib.deployChecks deploy);
-    };
   })

nix/snow/flake/default.nix

@@ -0,0 +1,38 @@
+{
+  this,
+  inputs,
+  systems,
+  ...
+}: let
+  inherit (inputs.nixpkgs) lib;
+in {
+  # snow.flake
+  flake = flakeInputs: root: let
+    snowflake = lib.evalModules {
+      class = "snowflake";
+      # XXX: TODO: abort if inputs contains reserved names
+      specialArgs =
+        (flakeInputs
+          // {
+            inherit (this) snow;
+            inherit systems root;
+            inputs = flakeInputs;
+          })
+        # XXX: TODO:
+        # |> (x: builtins.removeAttrs x ["self" "nodes"]);
+        |> (x: builtins.removeAttrs x ["self"]);
+
+      modules = [
+        ./module.nix
+        ({config, ...}: {
+          _module.args = {
+            self = config;
+            # XXX: TODO:
+            # nodes = config.nodes.nodes;
+          };
+        })
+      ];
+    };
+  in
+    snowflake.config.outputs;
+}

nix/snow/flake/lib.nix.bak

@@ -0,0 +1,318 @@
+{ lib
+  # Optionally a string with extra version info to be included in the error message
+  # in case is lib is out of date. Empty or starts with space.
+, revInfo ? ""
+}:
+let
+  inherit (lib)
+    mkOption
+    mkOptionType
+    defaultFunctor
+    isAttrs
+    isFunction
+    showOption
+    throwIf
+    types
+    warnIf
+    getAttrFromPath
+    setAttrByPath
+    attrByPath
+    optionalAttrs
+    ;
+  inherit (lib.modules)
+    mkAliasAndWrapDefsWithPriority;
+  inherit (lib.types)
+    path
+    submoduleWith
+    ;
+
+  # Polyfill isFlake until Nix with https://github.com/NixOS/nix/pull/7207 is common
+  isFlake = maybeFlake:
+    if maybeFlake ? _type
+    then maybeFlake._type == "flake"
+    else maybeFlake ? inputs && maybeFlake ? outputs && maybeFlake ? sourceInfo;
+
+  /**
+    Deprecated for any use except type-merging into `perSystem`.
+    Use `lib.types.deferredModuleWith` instead, and add `apply = m: [ m ];` if needed.
+
+    The deferredModule type was pioneered in flake-parts for the `perSystem` option.
+    The Nixpkgs version has an improved merge function that returns a single module,
+    whereas this version returns a list. The flake-parts version was not updated to
+    match this improvement in Nixpkgs.
+
+    # History
+
+    This predates `lib.types.deferredModuleWith`, added in Nixpkgs 22.11
+    (https://github.com/NixOS/nixpkgs/pull/163617).
+    Documented as deprecated in flake-parts in January 2026.
+  */
+  deferredModuleWith =
+    attrs@{ staticModules ? [ ] }: mkOptionType {
+      name = "deferredModule";
+      description = "module";
+      check = x: isAttrs x || isFunction x || path.check x;
+      merge = loc: defs: staticModules ++ map (def: lib.setDefaultModuleLocation "${def.file}, via option ${showOption loc}" def.value) defs;
+      inherit (submoduleWith { modules = staticModules; })
+        getSubOptions
+        getSubModules;
+      substSubModules = m: deferredModuleWith (attrs // {
+        staticModules = m;
+      });
+      functor = defaultFunctor "deferredModuleWith" // {
+        type = deferredModuleWith;
+        payload = {
+          inherit staticModules;
+        };
+        binOp = lhs: rhs: {
+          staticModules = lhs.staticModules ++ rhs.staticModules;
+        };
+      };
+    };
+
+  # Internal: preserves legacy list-merge behavior for perSystem type-merging.
+  mkLegacyDeferredModuleType =
+    module:
+    deferredModuleWith {
+      staticModules = [ module ];
+    };
+
+  errorExample = ''
+    For example:
+
+        outputs = inputs@{ flake-parts, ... }:
+          flake-parts.lib.mkFlake { inherit inputs; } { /* module */ };
+
+    To avoid an infinite recursion, *DO NOT* pass `self.inputs` and
+    *DO NOT* pass `inherit (self) inputs`, but pass the output function
+    arguments as `inputs` like above.
+  '';
+
+  flake-parts-lib = rec {
+    evalFlakeModule =
+      args@
+      { inputs ? self.inputs
+      , specialArgs ? { }
+
+        # legacy
+      , self ? inputs.self or (throw ''
+          When invoking flake-parts, you must pass all the flake output arguments,
+          and not just `self.inputs`.
+
+          ${errorExample}
+        '')
+      , moduleLocation ? "${self.outPath}/flake.nix"
+      }:
+      let
+        inputsPos = builtins.unsafeGetAttrPos "inputs" args;
+        errorLocation =
+          # Best case: user makes it explicit
+          args.moduleLocation or (
+            # Slightly worse: Nix does not technically commit to unsafeGetAttrPos semantics
+            if inputsPos != null
+            then inputsPos.file
+            # Slightly worse: self may not be valid when an error occurs
+            else if args?inputs.self.outPath
+            then args.inputs.self.outPath + "/flake.nix"
+            # Fallback
+            else "<mkFlake argument>"
+          );
+      in
+      throwIf
+        (!args?self && !args?inputs) ''
+        When invoking flake-parts, you must pass in the flake output arguments.
+
+        ${errorExample}
+      ''
+        warnIf
+        (!args?inputs) ''
+        When invoking flake-parts, it is recommended to pass all the flake output
+        arguments in the `inputs` parameter. If you only pass `self`, it's not
+        possible to use the `inputs` module argument in the module `imports`.
+
+        Please pass the output function arguments. ${errorExample}
+      ''
+
+        (module:
+        lib.evalModules {
+          specialArgs = {
+            inherit self flake-parts-lib moduleLocation;
+            inputs = args.inputs or /* legacy, warned above */ self.inputs;
+          } // specialArgs;
+          modules = [ ./all-modules.nix (lib.setDefaultModuleLocation errorLocation module) ];
+          class = "flake";
+        }
+        );
+
+    # Function to extract the default flakeModule from
+    # what may be a flake, returning the argument unmodified
+    # if it's not a flake.
+    #
+    # Useful to map over an 'imports' list to make it less
+    # verbose in the common case.
+    defaultModule = maybeFlake:
+      if isFlake maybeFlake
+      then maybeFlake.flakeModules.default or maybeFlake
+      else maybeFlake;
+
+    mkFlake = args: module:
+      let
+        eval = flake-parts-lib.evalFlakeModule args module;
+      in
+      eval.config.flake;
+
+    /**
+      Deprecated. Declare options directly, e.g. `options.foo.bar = mkOption { ... }`,
+      provided that `foo` is already declared as a submodule option.
+
+      In flake-parts, `flake` is declared as a submodule option by the core modules,
+      so `options.flake.<name>` declarations work directly.
+
+      This function wraps option declarations in a submodule, allowing them to
+      be merged into an existing submodule option. For example, if `foo` is
+      already declared as a submodule option, using
+      `options.foo = mkSubmoduleOptions { bar = mkOption {...}; }` would add
+      `bar` to the `foo` submodule.
+
+      # History
+
+      This was a workaround for https://github.com/NixOS/nixpkgs/issues/146882,
+      fixed in Nixpkgs 22.05 by https://github.com/NixOS/nixpkgs/pull/156533.
+      With the fix, declaring `options.foo.bar` directly works when `foo` is
+      already a submodule option. Documented as deprecated in flake-parts in January 2026.
+    */
+    mkSubmoduleOptions =
+      options:
+      mkOption {
+        type = types.submoduleWith {
+          modules = [{ inherit options; }];
+        };
+      };
+
+    /**
+      Deprecated. Use mkPerSystemType/mkPerSystemOption for `perSystem` type-merging, or
+      use Nixpkgs `types.deferredModule` directly, noting the lack of list wrapping;
+      see `deferredModuleWith` docs.
+    */
+    mkDeferredModuleType = mkLegacyDeferredModuleType;
+
+    /**
+      Given a module, construct an option type suitable for type-merging into `perSystem`'s type.
+    */
+    mkPerSystemType = mkLegacyDeferredModuleType;
+
+    /**
+      Deprecated. Use mkPerSystemOption for `perSystem` type-merging, or
+      use `mkOption` and Nixpkgs `types.deferredModule` directly, noting the
+      lack of list wrapping; see `deferredModuleWith` docs.
+    */
+    mkDeferredModuleOption =
+      module:
+      mkOption {
+        type = flake-parts-lib.mkPerSystemType module;
+      };
+
+    /**
+      Given a module, construct an option declaration suitable for merging into the core `perSystem` module.
+    */
+    mkPerSystemOption = mkDeferredModuleOption;
+
+    # Polyfill https://github.com/NixOS/nixpkgs/pull/344216
+    # Nixpkgs master 2024-12-09, Nixpkgs 25.05
+    attrsWith = types.attrsWith or ({ elemType, lazy ? false, placeholder ? "name" }:
+      if lazy then types.attrsOf elemType else types.lazyAttrsOf elemType);
+
+    # Helper function for defining a per-system option that
+    # gets transposed by the usual flake system logic to a
+    # top-level flake attribute.
+    mkTransposedPerSystemModule = { name, option, file }: {
+      _file = file;
+
+      options = {
+        flake.${name} = mkOption {
+          type = attrsWith {
+            elemType = option.type;
+            lazy = true;
+            placeholder = "system";
+          };
+          default = { };
+          description = ''
+            See {option}`perSystem.${name}` for description and examples.
+          '';
+        };
+
+        perSystem = flake-parts-lib.mkPerSystemOption {
+          _file = file;
+
+          options.${name} = option;
+        };
+      };
+
+      config = {
+        transposition.${name} = { };
+      };
+    };
+
+    # Needed pending https://github.com/NixOS/nixpkgs/pull/198450
+    mkAliasOptionModule = from: to: { config, options, ... }:
+      let
+        fromOpt = getAttrFromPath from options;
+        toOf = attrByPath to
+          (abort "Renaming error: option `${showOption to}' does not exist.");
+        toType = let opt = attrByPath to { } options; in opt.type or (types.submodule { });
+      in
+      {
+        options = setAttrByPath from (mkOption
+          {
+            visible = true;
+            description = "Alias of {option}`${showOption to}`.";
+            apply = x: (toOf config);
+          } // optionalAttrs (toType != null) {
+          type = toType;
+        });
+        config = mkAliasAndWrapDefsWithPriority (setAttrByPath to) fromOpt;
+      };
+
+    # Helper function for importing while preserving module location. To be added
+    # in nixpkgs: https://github.com/NixOS/nixpkgs/pull/230588
+    # I expect these functions to remain identical. This one will stick around
+    # for a while to support older nixpkgs-lib.
+    importApply =
+      modulePath: staticArgs:
+      lib.setDefaultModuleLocation modulePath (import modulePath staticArgs);
+
+    inherit (import ./lib/memoize/memoize.nix {
+      inherit lib;
+    }) memoizeStr;
+
+    /**
+      `importAndPublish name module` returns a module that both imports the `module`, and exposes it as flake attribute `modules.flake.${name}`.
+
+      This also imports the optional [`modules`](https://flake.parts/options/flake-parts-modules.html) module to support that.
+    */
+    importAndPublish = name: module: { lib, ... }: {
+      _class = "flake";
+      imports = [
+        module
+        ./extras/modules.nix
+      ];
+      flake.modules.flake.${name} = module;
+    };
+  };
+
+  # A best effort, lenient estimate. Please use a recent nixpkgs lib if you
+  # override it at all.
+  minVersion = "23.05pre-git";
+
+in
+
+if builtins.compareVersions lib.version minVersion < 0
+then
+  abort ''
+    The nixpkgs-lib dependency of flake-parts was overridden but is too old.
+    The minimum supported version of nixpkgs-lib is ${minVersion},
+    but the actual version is ${lib.version}${revInfo}.
+  ''
+else
+
+  flake-parts-lib

nix/snow/flake/modules/apps.nix

@@ -0,0 +1,61 @@
+{ lib, flake-parts-lib, ... }:
+let
+  inherit (lib)
+    mkOption
+    types
+    ;
+  inherit (flake-parts-lib)
+    mkTransposedPerSystemModule
+    ;
+
+  programType = lib.types.coercedTo derivationType lib.getExe lib.types.str;
+
+  derivationType = lib.types.package // {
+    check = lib.isDerivation;
+  };
+
+  appType = lib.types.submodule {
+    options = {
+      type = mkOption {
+        type = lib.types.enum [ "app" ];
+        default = "app";
+        description = ''
+          A type tag for `apps` consumers.
+        '';
+      };
+      program = mkOption {
+        type = programType;
+        description = ''
+          A path to an executable or a derivation with `meta.mainProgram`.
+        '';
+      };
+      meta = mkOption {
+        type = types.lazyAttrsOf lib.types.raw;
+        default = { };
+        # TODO refer to Nix manual 2.25
+        description = ''
+          Metadata information about the app.
+          Standardized in Nix at <https://github.com/NixOS/nix/pull/11297>.
+
+          Note: `nix flake check` is only aware of the `description` attribute in `meta`.
+        '';
+      };
+    };
+  };
+in
+mkTransposedPerSystemModule {
+  name = "apps";
+  option = mkOption {
+    type = types.lazyAttrsOf appType;
+    default = { };
+    description = ''
+      Programs runnable with nix run `<name>`.
+    '';
+    example = lib.literalExpression ''
+      {
+        default.program = "''${config.packages.hello}/bin/hello";
+      }
+    '';
+  };
+  file = ./apps.nix;
+}

nix/snow/flake/modules/checks.nix

@@ -0,0 +1,21 @@
+{ lib, flake-parts-lib, ... }:
+let
+  inherit (lib)
+    mkOption
+    types
+    ;
+  inherit (flake-parts-lib)
+    mkTransposedPerSystemModule
+    ;
+in
+mkTransposedPerSystemModule {
+  name = "checks";
+  option = mkOption {
+    type = types.lazyAttrsOf types.package;
+    default = { };
+    description = ''
+      Derivations to be built by [`nix flake check`](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-flake-check.html).
+    '';
+  };
+  file = ./checks.nix;
+}

nix/snow/flake/modules/debug.nix

@@ -0,0 +1,78 @@
+{ config, flake-parts-lib, lib, options, getSystem, extendModules, ... }:
+let
+  inherit (lib)
+    mapAttrs
+    mkIf
+    mkOption
+    optionalAttrs
+    types
+    ;
+  inherit (flake-parts-lib)
+    mkPerSystemOption
+    ;
+
+  mkDebugConfig = { config, options, extendModules }: config // {
+    inherit config;
+    inherit (config) _module;
+    inherit options;
+    inherit extendModules;
+  };
+in
+{
+  options = {
+    debug = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Whether to add the attributes `debug`, `allSystems` and `currentSystem`
+        to the flake output. When `true`, this allows inspection of options via
+        `nix repl`.
+
+        ```
+        $ nix repl
+        nix-repl> :lf .
+        nix-repl> currentSystem._module.args.pkgs.hello
+        «derivation /nix/store/7vf0d0j7majv1ch1xymdylyql80cn5fp-hello-2.12.1.drv»
+        ```
+
+        Each of `debug`, `allSystems.<system>` and `currentSystem` is an
+        attribute set consisting of the `config` attributes, plus the extra
+        attributes `_module`, `config`, `options`, `extendModules`. So note that
+        these are not part of the `config` parameter, but are merged in for
+        debugging convenience.
+
+         - `debug`: The top-level options
+         - `allSystems`: The `perSystem` submodule applied to the configured `systems`.
+         - `currentSystem`: Shortcut into `allSystems`. Only available in impure mode.
+           Works for arbitrary system values.
+
+        See [Expore and debug option values](../debug.html) for more examples.
+      '';
+    };
+    perSystem = mkPerSystemOption
+      ({ options, config, extendModules, ... }: {
+        _file = ./formatter.nix;
+        options = {
+          debug = mkOption {
+            description = ''
+              Values to return in e.g. `allSystems.<system>` when
+              [`debug = true`](#opt-debug).
+            '';
+            type = types.lazyAttrsOf types.raw;
+          };
+        };
+        config = {
+          debug = mkDebugConfig { inherit config options extendModules; };
+        };
+      });
+  };
+
+  config = mkIf config.debug {
+    flake = {
+      debug = mkDebugConfig { inherit config options extendModules; };
+      allSystems = mapAttrs (_s: c: c.debug) config.allSystems;
+    } // optionalAttrs (builtins?currentSystem) {
+      currentSystem = (getSystem builtins.currentSystem).debug;
+    };
+  };
+}

nix/snow/flake/modules/devShells.nix

@@ -0,0 +1,30 @@
+{ lib, flake-parts-lib, ... }:
+let
+  inherit (lib)
+    mkOption
+    types
+    literalExpression
+    ;
+  inherit (flake-parts-lib)
+    mkTransposedPerSystemModule
+    ;
+in
+mkTransposedPerSystemModule {
+  name = "devShells";
+  option = mkOption {
+    type = types.lazyAttrsOf types.package;
+    default = { };
+    description = ''
+      An attribute set of packages to be used as shells.
+      [`nix develop .#<name>`](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-develop.html) will run `devShells.<name>`.
+    '';
+    example = literalExpression ''
+      {
+        default = pkgs.mkShell {
+          nativeBuildInputs = with pkgs; [ wget bat cargo ];
+        };
+      }
+    '';
+  };
+  file = ./devShells.nix;
+}

nix/snow/flake/modules/flake.nix

@@ -0,0 +1,47 @@
+{
+  lib,
+  config,
+  ...
+}: let
+  inherit
+    (lib)
+    mkOption
+    types
+    ;
+
+  flake = mkOption {
+    type = types.submoduleWith {
+      modules = [
+        {
+          freeformType =
+            types.lazyAttrsOf
+            (types.unique
+              {
+                message = ''
+                  No option has been declared for this flake output attribute, so its definitions can't be merged automatically.
+                  Possible solutions:
+                    - Load a module that defines this flake output attribute
+                      Many modules are listed at https://flake.parts
+                    - Declare an option for this flake output attribute
+                    - Make sure the output attribute is spelled correctly
+                    - Define the value only once, with a single definition in a single module
+                '';
+              }
+              types.raw);
+        }
+      ];
+    };
+    description = ''
+      Raw flake output attributes. Any attribute can be set here, but some
+      attributes are represented by options, to provide appropriate
+      configuration merging.
+    '';
+  };
+in {
+  options = {
+    inherit flake;
+    output = {inherit flake;};
+  };
+
+  config = {inherit (config) flake;};
+}

nix/snow/flake/modules/formatter.nix

@@ -0,0 +1,52 @@
+{ config, lib, flake-parts-lib, ... }:
+let
+  inherit (lib)
+    filterAttrs
+    mapAttrs
+    mkOption
+    optionalAttrs
+    types
+    ;
+  inherit (flake-parts-lib)
+    mkPerSystemOption
+    ;
+in
+{
+  options = {
+    flake.formatter = mkOption {
+      type = types.lazyAttrsOf types.package;
+      default = { };
+      description = ''
+        An attribute set of per system a package used by [`nix fmt`](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-fmt.html).
+      '';
+    };
+
+    perSystem = mkPerSystemOption {
+      _file = ./formatter.nix;
+      options = {
+        formatter = mkOption {
+          type = types.nullOr types.package;
+          default = null;
+          description = ''
+            A package used by [`nix fmt`](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-fmt.html).
+          '';
+        };
+      };
+    };
+  };
+  config = {
+    flake.formatter =
+      mapAttrs
+        (k: v: v.formatter)
+        (filterAttrs
+          (k: v: v.formatter != null)
+          config.allSystems
+        );
+
+    perInput = system: flake:
+      optionalAttrs (flake?formatter.${system}) {
+        formatter = flake.formatter.${system};
+      };
+
+  };
+}

nix/snow/flake/modules/legacyPackages.nix

@@ -0,0 +1,21 @@
+{ lib, flake-parts-lib, ... }:
+let
+  inherit (lib)
+    mkOption
+    types
+    ;
+  inherit (flake-parts-lib)
+    mkTransposedPerSystemModule
+    ;
+in
+mkTransposedPerSystemModule {
+  name = "legacyPackages";
+  option = mkOption {
+    type = types.lazyAttrsOf types.raw;
+    default = { };
+    description = ''
+      An attribute set of unmergeable values. This is also used by [`nix build .#<attrpath>`](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-build.html).
+    '';
+  };
+  file = ./legacyPackages.nix;
+}

nix/snow/flake/modules/moduleWithSystem.nix

@@ -0,0 +1,32 @@
+{ withSystem, ... }:
+{
+  config = {
+    _module.args = {
+      moduleWithSystem =
+        module:
+
+        { config, ... }:
+        let
+          system =
+            config._module.args.system or
+              config._module.args.pkgs.stdenv.hostPlatform.system or
+                (throw "moduleWithSystem: Could not determine the configuration's system parameter for this module system application.");
+
+          allArgs = withSystem system (args: args);
+
+          lazyArgsPerParameter = f: builtins.mapAttrs
+            (k: v: allArgs.${k} or (throw "moduleWithSystem: module argument `${k}` does not exist."))
+            (builtins.functionArgs f);
+
+          # Use reflection to make the call lazy in the argument.
+          # Restricts args to the ones declared.
+          callLazily = f: a: f (lazyArgsPerParameter f);
+        in
+        {
+          imports = [
+            (callLazily module allArgs)
+          ];
+        };
+    };
+  };
+}

nix/snow/flake/modules/nixosConfigurations.nix

@@ -0,0 +1,36 @@
+{ lib, ... }:
+let
+  inherit (lib)
+    mkOption
+    types
+    literalExpression
+    ;
+in
+{
+  options = {
+    flake.nixosConfigurations = mkOption {
+      type = types.lazyAttrsOf types.raw;
+      default = { };
+      description = ''
+        Instantiated NixOS configurations. Used by `nixos-rebuild`.
+
+        `nixosConfigurations` is for specific machines. If you want to expose
+        reusable configurations, add them to [`nixosModules`](#opt-flake.nixosModules)
+        in the form of modules (no `lib.nixosSystem`), so that you can reference
+        them in this or another flake's `nixosConfigurations`.
+      '';
+      example = literalExpression ''
+        {
+          my-machine = inputs.nixpkgs.lib.nixosSystem {
+            # system is not needed with freshly generated hardware-configuration.nix
+            # system = "x86_64-linux";  # or set nixpkgs.hostPlatform in a module.
+            modules = [
+              ./my-machine/nixos-configuration.nix
+              config.nixosModules.my-module
+            ];
+          };
+        }
+      '';
+    };
+  };
+}

nix/snow/flake/modules/nixosModules.nix

@@ -0,0 +1,26 @@
+{ self, lib, moduleLocation, ... }:
+let
+  inherit (lib)
+    mapAttrs
+    mkOption
+    types
+    ;
+in
+{
+  options = {
+    flake.nixosModules = mkOption {
+      type = types.lazyAttrsOf types.deferredModule;
+      default = { };
+      apply = mapAttrs (k: v: {
+        _class = "nixos";
+        _file = "${toString moduleLocation}#nixosModules.${k}";
+        imports = [ v ];
+      });
+      description = ''
+        NixOS modules.
+
+        You may use this for reusable pieces of configuration, service modules, etc.
+      '';
+    };
+  };
+}

nix/snow/flake/modules/nixpkgs.nix

@@ -0,0 +1,26 @@
+#
+# Nixpkgs module. The only exception to the rule.
+#
+# Provides a `pkgs` argument in `perSystem`.
+#
+# Arguably, this shouldn't be in flake-parts, but in nixpkgs.
+# Nixpkgs could define its own module that does this, which would be
+# a more consistent UX, but for now this will do.
+#
+# The existence of this module does not mean that other flakes' logic
+# will be accepted into flake-parts, because it's against the
+# spirit of Flakes.
+#
+{
+  config = {
+    perSystem = { inputs', lib, ... }: {
+      config = {
+        _module.args.pkgs = lib.mkOptionDefault (
+          builtins.seq
+            (inputs'.nixpkgs or (throw "flake-parts: The flake does not have a `nixpkgs` input. Please add it, or set `perSystem._module.args.pkgs` yourself."))
+            inputs'.nixpkgs.legacyPackages
+        );
+      };
+    };
+  };
+}

nix/snow/flake/modules/overlays.nix

@@ -0,0 +1,32 @@
+{ lib, ... }:
+let
+  inherit (lib)
+    mkOption
+    types
+    ;
+in
+{
+  options = {
+    flake.overlays = mkOption {
+      # uniq -> ordered: https://github.com/NixOS/nixpkgs/issues/147052
+      # also update description when done
+      type = types.lazyAttrsOf (types.uniq (types.functionTo (types.functionTo (types.lazyAttrsOf types.unspecified))));
+      # This eta expansion exists for the sole purpose of making nix flake check happy.
+      apply = lib.mapAttrs (_k: f: final: prev: f final prev);
+      default = { };
+      example = lib.literalExpression ''
+        {
+          default = final: prev: {};
+        }
+      '';
+      description = ''
+        An attribute set of [overlays](https://nixos.org/manual/nixpkgs/stable/#chap-overlays).
+
+        Note that the overlays themselves are not mergeable. While overlays
+        can be composed, the order of composition is significant, but the
+        module system does not guarantee sufficiently deterministic
+        definition ordering, across versions and when changing `imports`.
+      '';
+    };
+  };
+}

nix/snow/flake/modules/packages.nix

@@ -0,0 +1,23 @@
+{ lib, flake-parts-lib, ... }:
+let
+  inherit (lib)
+    mkOption
+    types
+    ;
+  inherit (flake-parts-lib)
+    mkTransposedPerSystemModule
+    ;
+in
+mkTransposedPerSystemModule {
+  name = "packages";
+  option = mkOption {
+    type = types.lazyAttrsOf types.package;
+    default = { };
+    description = ''
+      An attribute set of packages to be built by [`nix build`](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-build.html).
+
+      `nix build .#<name>` will build `packages.<name>`.
+    '';
+  };
+  file = ./packages.nix;
+}

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);
+  };
+
+}

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
+          );
+    };
+  };
+}

nix/snow/flake/modules/withSystem.nix

@@ -0,0 +1,37 @@
+{ lib, flake-parts-lib, getSystem, ... }:
+let
+  inherit (lib)
+    mkOption
+    types
+    ;
+  inherit (flake-parts-lib)
+    mkPerSystemOption
+    ;
+in
+{
+  options = {
+    perSystem = mkPerSystemOption ({ config, options, specialArgs, ... }: {
+      _file = ./perSystem.nix;
+      options = {
+        allModuleArgs = mkOption {
+          type = types.lazyAttrsOf (types.raw or types.unspecified);
+          internal = true;
+          readOnly = true;
+          description = "Internal option that exposes _module.args, for use by withSystem.";
+        };
+      };
+      config = {
+        allModuleArgs = config._module.args // specialArgs // { inherit config options; };
+      };
+    });
+  };
+
+  config = {
+    _module.args = {
+      withSystem =
+        system: f:
+        f
+          (getSystem system).allModuleArgs;
+    };
+  };
+}

nix/snow/flake/outputs/checks.nix

@@ -0,0 +1,5 @@
+    checks =
+      inputs.deploy-rs.lib
+      |> mapAttrs (system: deployLib:
+        deployLib.deployChecks deploy);
+

nix/snow/flake/outputs/deploy.nix

@@ -0,0 +1,57 @@
+    deploy.nodes = mapNodes nodes ({
+      name,
+      node,
+      ...
+    }: let
+      inherit
+        (node.deploy)
+        ssh
+        user
+        interactiveSudo
+        remoteBuild
+        rollback
+        autoRollback
+        magicRollback
+        activationTimeout
+        confirmTimeout
+        ;
+
+      nixosFor = system: inputs.deploy-rs.lib.${system}.activate.nixos;
+    in {
+      hostname =
+        if ssh.host != null
+        then ssh.host
+        else "";
+
+      profilesOrder = ["default"]; # profiles priority
+      profiles.default = {
+        path = nixosFor node.system nixosConfigurations.${name};
+
+        user = user;
+        sudo = "sudo -u";
+        interactiveSudo = interactiveSudo;
+
+        fastConnection = false;
+
+        autoRollback = autoRollback -> rollback;
+        magicRollback = magicRollback -> rollback;
+        activationTimeout = activationTimeout;
+        confirmTimeout = confirmTimeout;
+
+        remoteBuild = remoteBuild;
+        sshUser = ssh.user;
+        sshOpts =
+          ssh.opts
+          ++ (
+            if elem "-p" ssh.opts
+            then []
+            else ["-p" (toString ssh.port)]
+          )
+          ++ (
+            if elem "-A" ssh.opts
+            then []
+            else ["-A"]
+          );
+      };
+    });
+

nix/snow/flake/outputs/nixosConfigurations.nix

@@ -0,0 +1,67 @@
+# {
+#   _module = { ... };
+#   _type = "configuration";
+#   class = null;
+#   config = { ... };
+#   extendModules = «lambda extendModules @ /nix/store/9hfp0agnm43kz72l5lpfn9var5p0x2fa-source/lib/modules.nix:340:9»;
+#   graph = [ ... ];
+#   options = { ... };
+#   type = { ... };
+# }
+    nixosConfigurations = mapNodes nodes (
+      {
+        base,
+        lib,
+        name,
+        node,
+        groupModules,
+        ...
+      }: let
+        homeManager =
+          if node.homeManager != null
+          then node.homeManager
+          else if nodes.homeManager != null
+          then nodes.homeManager
+          else
+            warn ''
+              [snowflake] Neither `nodes.homeManager` nor `nodes.nodes.${name}.homeManager` were specified!
+              [snowflake] home-manager will NOT be used! User configuration will be ignored!
+            ''
+            null;
+
+        userArgs = nodes.args // node.args;
+        ceruleanArgs = {
+          inherit systems root base nodes node;
+          inherit (node) system;
+          inherit (this) snow;
+          hostname = name;
+
+          _cerulean = {
+            inherit inputs userArgs ceruleanArgs homeManager;
+            specialArgs = userArgs // ceruleanArgs;
+          };
+        };
+        specialArgs = assert (userArgs
+          |> attrNames
+          |> all (argName:
+            ! ceruleanArgs ? argName
+            || abort ''
+              `specialArgs` are like super important to Cerulean my love... </3
+              But `args.${argName}` is a reserved argument name :(
+            ''));
+          ceruleanArgs._cerulean.specialArgs;
+      in
+        lib.nixosSystem {
+          inherit (node) system;
+          inherit specialArgs;
+          modules =
+            [
+              self.nixosModules.default
+              (this.findImport /${root}/hosts/${name})
+            ]
+            ++ (groupModules root)
+            ++ node.modules
+            ++ nodes.modules;
+        }
+    );
+