diff --git a/lib.nix b/lib.nix
index 1eac279..989b3e5 100644
--- a/lib.nix
+++ b/lib.nix
@@ -4,28 +4,56 @@
 }:
 let
   _mkTinyShell =
-    mkShell:
+    mkShell: name:
     args@{
       shellHook ? "",
+      dataDir ? ".data",
+      setDataDir ? dataDir != null,
+      createDataDir ? setDataDir,
       ...
     }:
+
+    assert lib.assertMsg (builtins.isString shellHook) "${name}: `shellHook` must be a string";
+    assert lib.assertMsg (
+      builtins.isString dataDir || dataDir == null
+    ) "${name}: `dataDir` must be a string or null";
+    assert lib.assertMsg (builtins.isBool setDataDir) "${name}: `setDataDir` must be a boolean";
+    assert lib.assertMsg (builtins.isBool createDataDir) "${name}: `createDataDir` must be a boolean";
+
+    assert lib.assertMsg (
+      dataDir != null -> (!lib.hasPrefix "/" dataDir)
+    ) "${name}: `dataDir` must be a relative path";
+    assert lib.assertMsg (
+      dataDir == null -> (!setDataDir && !createDataDir)
+    ) "${name}: `dataDir` must be not be `null` in order to set or create the data directory";
+    assert lib.assertMsg (
+      !createDataDir || setDataDir
+    ) "${name}: `setDataDir` must be set to `true` in order to create the data directory";
+
     mkShell (
-      args
+      (removeAttrs args [
+        "shellHook"
+        "dataDir"
+        "setDataDir"
+        "createDataDir"
+        "gitPackage"
+      ])
       // {
         shellHook = ''
-          # start -- tinyshell hook
+          # start -- tinyshell hook (${name})
           export PRJ_ROOT="$(${lib.getExe pkgs.git} rev-parse --show-toplevel 2>/dev/null || printf '%s\n' "$PWD")"
+          ${lib.optionalString setDataDir ''export PRJ_DATA_DIR="$PRJ_ROOT/${dataDir}"''}
+          ${lib.optionalString createDataDir ''mkdir -vp "$PRJ_DATA_DIR"''}
+          # end -- tinyshell hook (${name})
+          ${lib.optionalString (shellHook != "") ''
 
-          export PRJ_DATA_DIR="$PRJ_ROOT/.data"
-          mkdir -vp "$PRJ_DATA_DIR"
-          # end -- tinyshell hook
-
-          ${shellHook}
+            ${shellHook}
+          ''}
         '';
       }
     );
 in
 {
-  mkTinyShell = _mkTinyShell pkgs.mkShell;
-  mkTinyShellNoCC = _mkTinyShell pkgs.mkShellNoCC;
+  mkTinyShell = _mkTinyShell pkgs.mkShell "mkTinyShell";
+  mkTinyShellNoCC = _mkTinyShell pkgs.mkShellNoCC "mkTinyShellNoCC";
 }