lib/clan: add checks

flakeModules/clan.nix

@@ -39,23 +39,32 @@ in
           };
           modules = [
             clan-core.modules.clan.default
+            {
+              checks.minNixpkgsVersion = {
+                assertion = lib.versionAtLeast nixpkgs.lib.version "25.11";
+                message = ''
+                  Nixpkgs version: ${nixpkgs.lib.version} is incompatible with clan-core. (>= 25.11 is recommended)
+                  ---
+                  Your version of 'nixpkgs' seems too old for clan-core.
+                  Please read: https://docs.clan.lol/guides/nixpkgs-flake-input
+
+                  You can ignore this check by setting:
+                  clan.checks.minNixpkgsVersion.ignore = true;
+                  ---
+                '';
+              };
+            }
           ];
         };
         apply =
           config:
-          # TOOD:
+          lib.deepSeq (lib.mapAttrs (
-          # - allow to disable version check?
+            id: check:
-          # - self-incrementing version?
+            if check.ignore || check.assertion then
-          if !lib.versionAtLeast nixpkgs.lib.version "25.11" then
+              null
-            throw ''
+            else
-              Nixpkgs version: ${nixpkgs.lib.version} is incompatible with clan-core. (>= 25.11 is required)
+              throw "clan.checks.${id} failed with message\n${check.message}"
-              ---
+          ) config.checks) config;
-              Your version of 'nixpkgs' seems too old for clan-core.
-              Please read: https://docs.clan.lol/guides/nixpkgs-flake-input
-              ---
-            ''
-          else
-            config;
       };
 
     # Mapped flake toplevel outputs

lib/modules/clan/interface.nix

@@ -9,6 +9,36 @@
 }:
 let
   types = lib.types;
+
+  checkType = types.attrsOf (
+    types.submodule {
+      # Skip entire evaluation of this check
+      options.ignore = lib.mkOption {
+        type = types.bool;
+        default = false;
+        description = "Ignores this check entirely";
+      };
+
+      # Can only be defined once
+      options.assertion = lib.mkOption {
+        type = types.bool;
+        readOnly = true;
+        description = ''
+          The assertion that must hold true.
+
+          If false, the message is shown.
+        '';
+      };
+      # Message shown when the assertion is false
+      options.message = lib.mkOption {
+        type = types.str;
+        description = "Message shown when the assertion is false";
+      };
+
+      # TODO: add severity levels?
+      # Fail, Warn, Log
+    }
+  );
 in
 {
   options = {
@@ -18,6 +48,17 @@ in
       visible = false;
       default = [ ];
     };
+
+    # id :: { assertion, message }
+    checks = lib.mkOption {
+      type = checkType;
+      default = { };
+      description = ''
+        Assertions that must hold true when evaluating the clan.
+        When the assertion fails, the message is shown and the evaluation is aborted.
+      '';
+    };
+
     self = lib.mkOption {
       type = types.raw;
       default = self;