diff --git a/src/collections_me.c b/src/collections_me.c
index 58bcdad..e2bd2bf 100644
--- a/src/collections_me.c
+++ b/src/collections_me.c
@@ -114,17 +114,6 @@ ZEND_BEGIN_ARG_INFO(get_arginfo, 0)
     ZEND_ARG_INFO(0, default)
 ZEND_END_ARG_INFO()
 
-ZEND_BEGIN_ARG_INFO(group_by_arginfo, 0)
-    ZEND_ARG_CALLABLE_INFO(0, key_selector, 0)
-    ZEND_ARG_CALLABLE_INFO(0, value_transform, 0)
-ZEND_END_ARG_INFO()
-
-ZEND_BEGIN_ARG_INFO(group_by_to_arginfo, 0)
-    ZEND_ARG_OBJ_INFO(0, destination, Collection, 0)
-    ZEND_ARG_CALLABLE_INFO(0, key_selector, 0)
-    ZEND_ARG_CALLABLE_INFO(0, value_transform, 0)
-ZEND_END_ARG_INFO()
-
 ZEND_BEGIN_ARG_INFO(to_collection_arginfo, 0)
     ZEND_ARG_OBJ_INFO(0, destination, Collection, 0)
 ZEND_END_ARG_INFO()
@@ -164,8 +153,8 @@ const zend_function_entry collection_methods[] = {
     PHP_ME(Collection, foldRight, initial_operation_arginfo, ZEND_ACC_PUBLIC)
     PHP_ME(Collection, forEach, action_arginfo, ZEND_ACC_PUBLIC)
     PHP_ME(Collection, get, get_arginfo, ZEND_ACC_PUBLIC)
-    PHP_ME(Collection, groupBy, group_by_arginfo, ZEND_ACC_PUBLIC)
-    PHP_ME(Collection, groupByTo, group_by_to_arginfo, ZEND_ACC_PUBLIC)
+    PHP_ME(Collection, groupBy, transform_arginfo, ZEND_ACC_PUBLIC)
+    PHP_ME(Collection, groupByTo, destination_transform_arginfo, ZEND_ACC_PUBLIC)
     PHP_ME(Collection, indexOf, element_arginfo, ZEND_ACC_PUBLIC)
     PHP_ME(Collection, indexOfFirst, predicate_arginfo, ZEND_ACC_PUBLIC)
     PHP_ME(Collection, indexOfLast, predicate_arginfo, ZEND_ACC_PUBLIC)
diff --git a/stubs/Collection.php b/stubs/Collection.php
index f7db537..f4d1de4 100644
--- a/stubs/Collection.php
+++ b/stubs/Collection.php
@@ -314,27 +314,29 @@ class Collection implements ArrayAccess, Countable
     function get($key, $default) {}
 
     /**
-     * Groups values returned by the value_transform function applied to each element of the
-     * original collection by the key returned by the given key_selector function applied to the element
+     * Groups values by the key returned by the given transform function applied to the element
      * and returns a collection where each group key is associated with a list of corresponding values.
+     * 
+     * Key should be either string or integer. If the transform function returns a Pair, the
+     * original value will be mapped to the new value.
      *
-     * @param callable $key_selector ($value, $key) -> $new_key
-     * @param callable $value_transform[optional] ($value) -> $new_value
+     * @param callable $transform ($value, $key) -> $new_key | Pair($new_key, $new_value)
      * @return Collection
      */
-    function groupBy($key_selector, $value_transform) {}
+    function groupBy($transform) {}
 
     /**
-     * Groups values returned by the value_transform function applied to each element of the original
-     * collection by the key returned by the given key_selector function applied to the element and
+     * Groups values by the key returned by the given transform function applied to the element and
      * puts to the destination collection each group key associated with a list of corresponding values.
+     * 
+     * Key should be either string or integer. If the transform function returns a Pair, the
+     * original value will be mapped to the new value.
      *
      * @param Collection $destination
-     * @param callable $key_selector ($value, $key) -> $new_key
-     * @param callable $value_transform[optional] ($value) -> $new_value
+     * @param callable $transform ($value, $key) -> $new_key | Pair($new_key, $new_value)
      * @return Collection
      */
-    function groupByTo($destination, $key_selector, $value_transform) {}
+    function groupByTo($destination, $transform) {}
 
     /**
      * Returns first key of element, or null if the collection does not contain element.