diff --git a/callUnitRoot.nix b/callUnitRoot.nix
new file mode 100644
index 0000000..5490d15
--- /dev/null
+++ b/callUnitRoot.nix
@@ -0,0 +1,10 @@
+{ pkgs, lib ? pkgs.lib, unitDir ? "unit", packageFun ? "package.nix", root ? "${./pkgs}/${unitDir}" }:
+let
+  shards = lib.attrNames (builtins.readDir root);
+  namesForShard = shard: lib.mapAttrs'
+    (name: _: { inherit name; value = "${root}/${shard}/${name}"; })
+    (builtins.readDir "${root}/${shard}");
+  namesToPath = lib.foldl' lib.recursiveUpdate { } (map namesForShard shards);
+  units = lib.mapAttrs (_: path: pkgs.callPackage "${path}/${packageFun}" { }) namesToPath;
+in
+units
diff --git a/default.nix b/default.nix
index ef496d5..42a0f68 100644
--- a/default.nix
+++ b/default.nix
@@ -9,9 +9,9 @@
 { pkgs ? import <nixpkgs> { } }:
 
 import ./pkgs/top-level/all-packages.nix { inherit pkgs; }
-  // {
+// {
   # The `lib`, `modules`, and `overlay` names are special
   lib = import ./lib { inherit pkgs; }; # functions
   modules = import ./modules; # NixOS modules
   overlays = import ./overlays; # nixpkgs overlays
-}
+} // import ./callUnitRoot.nix { inherit pkgs; }
diff --git a/flake.nix b/flake.nix
index a21531a..71090fc 100644
--- a/flake.nix
+++ b/flake.nix
@@ -20,23 +20,11 @@
           # Unstable names are variables.
           packages =
             let
-              unitDir = "unit";
-              packageFun = "package.nix";
 
-              callUnitRoot = root:
-                let
-                  shards = lib.attrNames (builtins.readDir root);
-                  namesForShard = shard: lib.mapAttrs'
-                    (name: _: { inherit name; value = "${root}/${shard}/${name}"; })
-                    (builtins.readDir "${root}/${shard}");
-                  namesToPath = lib.foldl' lib.recursiveUpdate { } (map namesForShard shards);
-                  units = lib.mapAttrs (_: path: pkgs.callPackage "${path}/${packageFun}" { }) namesToPath;
-                in
-                units;
               legacyPackages = import ./pkgs/top-level/all-packages.nix { inherit pkgs; };
               onlyAvailable = lib.filterAttrs (_: drv: builtins.elem system (drv.meta.platforms or [ ]));
             in
-            onlyAvailable (legacyPackages // callUnitRoot "${./pkgs}/${unitDir}");
+            onlyAvailable (legacyPackages // import ./callUnitRoot.nix { inherit pkgs; });
 
           checks = {
             nixpkgs-fmt = pkgs.writeShellScriptBin "nixpkgs-fmt-check" ''