nix: start darwin configuration

preparation for macOS

modules/darwin/nix.nix

@@ -0,0 +1,34 @@
+{ config, lib, pkgs, ... }:
+
+{
+  # auto upgrade nix package and the daemon service.
+  services.nix-daemon.enable = true;
+
+  # nix auto garbage collection
+  nix.gc = {
+    automatic = true;
+    interval = { Weekday = 0; Hour = 0; Minute = 0; };
+    options = "--delete-older-than 7d";
+  };
+
+  # make nix applications use alias instead of symlinks for compatability
+  system.activationScripts.applications.text = let
+    env = pkgs.buildEnv {
+      name = "system-applications";
+      paths = config.environment.systemPackages;
+      pathsToLink = "/Applications";
+    };
+  in
+    pkgs.lib.mkForce ''
+    # Set up applications.
+    echo "setting up /Applications..." >&2
+    rm -rf /Applications/Nix\ Apps
+    mkdir -p /Applications/Nix\ Apps
+    find ${env}/Applications -maxdepth 1 -type l -exec readlink '{}' + |
+    while read -r src; do
+      app_name=$(basename "$src")
+      echo "copying $src" >&2
+      ${pkgs.mkalias}/bin/mkalias "$src" "/Applications/Nix Apps/$app_name"
+    done
+        '';
+}