factor out do_append in prep for delta subcommands

dynamic-options.nix

@@ -0,0 +1,61 @@
+{ pkgs, lib, config, ... }:
+let
+  inherit (lib.modules)
+    mkIf
+  ;
+  inherit (lib.options)
+    mkOption
+    mkEnableOption
+    literalExpression
+    showOption
+  ;
+  t = lib.types;
+  cfg = config.dynamicism;
+
+  settingsFormat = pkgs.formats.yaml { };
+
+  assertionFor = submodName: unitName: let
+    optName = [ "dynamicism" "for" submodName "systemd-services-updated" ];
+  in {
+    assertion = config.systemd.units.${unitName}.enable or false;
+    message = "'${showOption optName}' specified non-existentant unit '${unitName}'";
+  };
+
+  assertionsFor =
+    submodName:
+    submod:
+    lib.map (assertionFor submodName) submod.systemd-services-updated;
+in
+{
+  options.dynamicism.for = mkOption {
+    type = t.attrsOf (t.submoduleWith {
+      modules = [ ./dynamic-submodule.nix ];
+      shorthandOnlyDefinesConfig = false;
+    });
+    default = { };
+  };
+
+  options.dynamicism.finalSettings = mkOption {
+    type = t.attrsOf t.raw;
+    internal = true;
+    readOnly = true;
+  };
+
+  config.assertions = lib.foldlAttrs (acc: name: submod: let
+    next = lib.optionals submod.enable (assertionsFor name submod);
+  in acc ++ next) [ ] cfg.for;
+
+  config.dynamicism.for = {
+    gotosocial = {
+      source-parameters = [ "services" "gotosocial" "settings" ];
+      systemd-services-updated = [ "gotosocial.service" ];
+    };
+  };
+
+    # FIXME: this should be a fold.
+  config.dynamicism.finalSettings = lib.mapAttrs (name: submod: let
+    #optValue = lib.getAttrFromPath submod.
+    # FIXME: this should be a fold.
+    optValue = lib.getAttrFromPath submod.source-parameters config;
+  in optValue) cfg.for;
+}