diff --git a/Core/lib/array/map.php b/Core/lib/array/map.php
index c3f718862..7e0a3d5a5 100644
--- a/Core/lib/array/map.php
+++ b/Core/lib/array/map.php
@@ -32,6 +32,13 @@
  * Now the function accepts an array as $object.
  * Even in this case it differs from @see array_column(): array_column() misses the keys: https://3v4l.org/llMrL
  * df_column() preserves the keys.
+ * 2024-06-03
+ * 1.1) "Use the `iterable` type": https://github.com/mage2pro/core/issues/403
+ * 1.2) `iterable` is supported by PHP ≥ 7.1: https://3v4l.org/qNX1j
+ * 1.3) https://php.net/manual/en/language.types.iterable.php
+ * 2) We still can not use «Union Types» (e.g. `callable|iterable`) because they require PHP ≥ 8 (we need to support PHP ≥ 7.1):
+ * 2.1) https://php.watch/versions/8.0/union-types
+ * 2.2) https://3v4l.org/AOWmO
  * @used-by df_category_names()
  * @used-by \Df\Config\Backend\ArrayT::processI()
  * @used-by \Df\Core\GlobalSingletonDestructor::process()
@@ -39,7 +46,7 @@
  * @param string|callable $f
  * @param mixed ...$p
  */
-function df_each($c, $f, ...$p):array {return df_map(function($v) use($f, $p) {return df_call($v, $f, $p);}, $c);}
+function df_each(iterable $c, $f, ...$p):array {return df_map(function($v) use($f, $p) {return df_call($v, $f, $p);}, $c);}
 
 /**
  * 2015-02-11