tests for PoC!

modules/dynamicism/default.nix

@@ -1,38 +1,42 @@
 { pkgs, lib, config, options, ... }:
 let
-  inherit (lib.modules)
-    mkIf
-  ;
   inherit (lib.options)
     mkOption
-    mkEnableOption
-    literalExpression
     showOption
   ;
   t = lib.types;
-  cfg = config.dynamicism;
+
+  inherit (import ./lib.nix { inherit lib; })
+    typeCheck
+    convenientAttrPath
+    concatFoldl
+    recUpdateFoldl
+    recUpdateFoldlAttrs
+  ;
+
+  evalNixos = import (pkgs.path + "/nixos");
+
   opts = options.dynamicism;
+
   subOpts = lib.mapAttrs (_: metaAttr: metaAttr.configuration.options) options.dynamicism.for.valueMeta.attrs;
   settingsFormat = pkgs.formats.yaml { };
 
-  concatFoldl = f: list: lib.foldl' (acc: value: acc ++ (f value)) [ ] list;
-  recUpdateFoldlAttrs = f: attrs: lib.foldlAttrs (acc: name: value: lib.recursiveUpdate acc (f name value)) { } attrs;
-
-  finalSettingsFor = { ... }@submod: lib.foldl (acc: optPath: let
-    next =
-      assert lib.isList optPath;
-      lib.setAttrByPath optPath (lib.getAttrFromPath optPath config);
-  in lib.recursiveUpdate acc next) { } submod.source-options;
+  finalSettingsFor = { ... }@submod: recUpdateFoldl (optPath:
+    lib.setAttrByPath optPath (lib.getAttrFromPath optPath config)
+  ) submod.source-options;
 
   ourAssertions = lib.concatAttrValues {
-    unitsExist = concatFoldl (submod: let
-      next = lib.map (unit: assert lib.isString unit; {
+    unitsExist = subOpts
+    |> lib.attrValues
+    |> concatFoldl (submod: submod.systemd-services-updated.value
+      |> lib.map (unit: {
         assertion = config.systemd.units.${unit}.enable or false;
         message = ''
-          '${showOption submod.systemd-services-updated.loc}' specified non-existent unit '${unit}'
+          ${showOption submod.systemd-services-updated.loc}' specified non-existent unit '${unit}'
         '';
-      }) submod.systemd-services-updated.value;
-    in lib.optionals submod.enable.value next) (lib.attrValues subOpts);
+      })
+      |> lib.optionals submod.enable.value
+    );
 
     optsExist = concatFoldl (submod: lib.optionals submod.enable.value (lib.map (optPath: {
       assertion = lib.hasAttrByPath optPath options;
@@ -62,6 +66,14 @@ in
         Attrset of each `source-options` tree to their actual values.
       '';
     };
+
+    doChange = mkOption {
+      type = t.functionTo t.pathInStore;
+      readOnly = true;
+      description = ''
+        The function to call to Do The Thing.
+      '';
+    };
   };
 
   # Assertions.
@@ -70,6 +82,39 @@ in
   #
   # Generic implementation.
   #
+  config.dynamicism.doChange = {
+    option,
+    value,
+    configuration ? builtins.getEnv "NIXOS_CONFIG",
+  }: let
+    loc = opts.doChange.loc ++ [ "(function argument)" "value" ];
+    option' = typeCheck loc convenientAttrPath option;
+    nixosAfter = evalNixos {
+      configuration = { config, ... }: {
+        imports = [
+          configuration
+          (lib.setAttrByPath option' (lib.mkOverride (-999) value))
+        ];
+
+        environment.systemPackages = [
+          config.dynamicism.for.gotosocial.activate
+        ];
+      };
+    };
+
+    allActivations = lib.mapAttrsToList (name: submod: submod.activate) config.dynamicism.for;
+    allActivationScripts = pkgs.writeShellApplication {
+      name = "dynamicism-activate";
+      runtimeInputs = allActivations;
+      text = nixosAfter.config.dynamicism.for
+      |> lib.mapAttrsToList (name: submod: ''
+        echo "Activating dynamicism for ${name}"
+        ${lib.getExe submod.activate}
+      '')
+      |> lib.concatStringsSep "\n";
+    };
+  in allActivationScripts;
+
   config.dynamicism.finalSettings = lib.asserts.checkAssertWarn ourAssertions [ ] (
     recUpdateFoldlAttrs (name: { ... }@submod: finalSettingsFor submod) config.dynamicism.for
   );

modules/dynamicism/lib.nix

@@ -0,0 +1,24 @@
+{
+  lib ? import <nixpkgs/lib>,
+}: let
+  t = lib.types;
+in lib.fix (self: {
+  /** Perform module-system type checking and resolving on a single option value. */
+  typeCheck = loc: option: value:
+    assert lib.isOptionType option;
+    assert lib.isList loc;
+    assert lib.all lib.isString loc;
+  let
+    merged = lib.modules.mergeDefinitions loc option [ {
+      inherit value;
+      file = "«inline»";
+    } ];
+  in merged.mergedValue;
+
+  /** Either a list of strings, or a dotted string that will be split. */
+  convenientAttrPath = t.coercedTo t.str (lib.splitString ".") (t.listOf t.str);
+
+  concatFoldl = f: list: lib.foldl' (acc: value: acc ++ (f value)) [ ] list;
+  recUpdateFoldl = f: list: lib.foldl' (acc: value: lib.recursiveUpdate acc (f value)) { } list;
+  recUpdateFoldlAttrs = f: attrs: lib.foldlAttrs (acc: name: value: lib.recursiveUpdate acc (f name value)) { } attrs;
+})