use mix module system

flake.nix

@@ -47,11 +47,12 @@
         config.allowUnfree = false;
       };
     };
-  in rec {
+
+    cerulean = import ./cerulean {inherit inputs lib sys;};
+  in {
     overlays = [
-      # deploy-rs is built from the flake input, not from nixpkgs!
-      # To take advantage of the nixpkgs binary cache,
-      # the deploy-rs package can be overwritten:
+      # build deploy-rs as a package not from the flake input,
+      # hence we can rely on a nixpkg binary cache.
       deploy-rs.overlays.default
       (self: super: {
         deploy-rs = {
@@ -60,123 +61,5 @@
         };
       })
     ];
-
-    mkNexusConfig = config: let
-      # abstract node instance that stores all default values
-      templateNode = name: system: let
-        Terminal = nib.types.Terminal;
-
-        missing = msg: path:
-          Terminal (abort ''
-            Each Cerulean Nexus node is required to specify ${msg}!
-            Ensure `cerulean.nexus.nodes.${name}.${path}` exists under your call to `cerulean.mkNexus`.
-          '');
-      in {
-        system = missing "its system type" "system"; # intentionally left missing!! (to raise errors)
-        modules = missing "its required modules" "modules";
-        specialArgs = Terminal {};
-
-        deploy = {
-          user = "root";
-          sudo = "sudo -u";
-          interactiveSudo = false;
-
-          remoteBuild = false; # prefer local builds for remote deploys
-
-          autoRollback = true; # reactivate previous profile if activation fails
-          magicRollback = true;
-
-          activationTimeout = 500; # timeout in seconds for profile activation
-          confirmTimeout = 30; # timeout in seconds for profile activation confirmation
-
-          ssh = {
-            host = missing "an SSH hostname (domain name or ip address) for deployment" "deploy.ssh.host";
-            user = missing "an SSH username for deployment" "deploy.ssh.user";
-            port = 22;
-            opts = [];
-          };
-        };
-      };
-
-      parseNode = name: nodeAttrs:
-        if !(builtins.isAttrs nodeAttrs)
-        then
-          # fail if node is not an attribute set
-          abort ''
-            Cerulean Nexus nodes must be provided as an attribute set, got "${builtins.typeOf nodeAttrs}" instead!
-            Ensure all `cerulean.nexus.nodes.${name}` declarations are attribute sets under your call to `cerulean.mkNexus`.
-          ''
-        else let
-          templateAttrs = templateNode name nodeAttrs.system;
-        in
-          nib.parse.mergeStructs templateAttrs nodeAttrs;
-
-      # mapNodes = f: builtins.mapAttrs f (builtins.mapAttrs parseNode config.nexus.nodes);
-      mapNodes = f:
-        builtins.mapAttrs
-        (nodeName: nodeAttrs: f nodeName (parseNode nodeName nodeAttrs))
-        config.nexus.nodes;
-    in rec {
-      nixosConfigurations = mapNodes (
-        nodeName: node:
-          lib.nixosSystem {
-            system = node.system;
-            modules = node.modules;
-
-            # nix passes these to every single module
-            specialArgs =
-              node.specialArgs
-              // {
-                inherit inputs;
-                pkgs = sys.pkgsFor node.system;
-                upkgs = sys.upkgsFor node.system;
-              };
-          }
-      );
-
-      deploy.nodes = mapNodes (nodeName: node: let
-        nixosFor = system: deploy-rs.lib.${system}.activate.nixos;
-      in {
-        hostname = node.deploy.ssh.host;
-
-        profilesOrder = ["default"]; # profiles priority
-        profiles.default = {
-          path = nixosFor node.system nixosConfigurations.${nodeName};
-
-          user = node.deploy.user;
-          sudo = node.deploy.sudo;
-          interactiveSudo = node.deploy.interactiveSudo;
-
-          fastConnection = false;
-
-          autoRollback = node.deploy.autoRollback;
-          magicRollback = node.deploy.magicRollback;
-          activationTimeout = node.deploy.activationTimeout;
-          confirmTimeout = node.deploy.confirmTimeout;
-
-          remoteBuild = node.deploy.remoteBuild;
-          sshUser = node.deploy.ssh.user;
-          sshOpts =
-            node.deploy.ssh.opts
-            ++ (
-              if builtins.elem "-p" node.deploy.ssh.opts
-              then []
-              else ["-p" (toString node.deploy.ssh.port)]
-            )
-            ++ (
-              if builtins.elem "-A" node.deploy.ssh.opts
-              then []
-              else ["-A"]
-            );
-        };
-      });
-
-      checks = builtins.mapAttrs (system: deployLib: deployLib.deployChecks deploy) deploy-rs.lib;
-    };
-
-    mkNexus = outputs: let
-      config = outputs.cerulean;
-    in
-      (mkNexusConfig config) // (removeAttrs outputs ["cerulean"]);
   };
 }