diff --git a/.DS_Store b/.DS_Store
new file mode 100644
index 00000000..7a6ca9d9
Binary files /dev/null and b/.DS_Store differ
diff --git a/src/.DS_Store b/src/.DS_Store
new file mode 100644
index 00000000..42fc6281
Binary files /dev/null and b/src/.DS_Store differ
diff --git a/src/Functional/CompareObjectHashOn.php b/src/Functional/CompareObjectHashOn.php
index c4194479..faf6c2e2 100644
--- a/src/Functional/CompareObjectHashOn.php
+++ b/src/Functional/CompareObjectHashOn.php
@@ -18,7 +18,7 @@
  * @return callable
  * @no-named-arguments
  */
-function compare_object_hash_on(callable $comparison, callable $keyFunction = null)
+function compare_object_hash_on(callable $comparison, ?callable $keyFunction = null)
 {
     $keyFunction = $keyFunction ? compose($keyFunction, 'spl_object_hash') : 'spl_object_hash';
 
diff --git a/src/Functional/CompareOn.php b/src/Functional/CompareOn.php
index b0e5cda0..eec4a2d2 100644
--- a/src/Functional/CompareOn.php
+++ b/src/Functional/CompareOn.php
@@ -18,7 +18,7 @@
  * @return callable
  * @no-named-arguments
  */
-function compare_on(callable $comparison, callable $reducer = null)
+function compare_on(callable $comparison, ?callable $reducer = null)
 {
     if ($reducer === null) {
         return static function ($left, $right) use ($comparison) {
diff --git a/src/Functional/Every.php b/src/Functional/Every.php
index e4d8ff62..667cd72b 100644
--- a/src/Functional/Every.php
+++ b/src/Functional/Every.php
@@ -22,7 +22,7 @@
  * @return bool
  * @no-named-arguments
  */
-function every($collection, callable $callback = null)
+function every($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/First.php b/src/Functional/First.php
index 87db945e..f9e30afd 100644
--- a/src/Functional/First.php
+++ b/src/Functional/First.php
@@ -23,7 +23,7 @@
  * @return mixed
  * @no-named-arguments
  */
-function first($collection, callable $callback = null)
+function first($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Head.php b/src/Functional/Head.php
index c6de5e86..48f8bedd 100644
--- a/src/Functional/Head.php
+++ b/src/Functional/Head.php
@@ -21,7 +21,7 @@
  * @return mixed
  * @no-named-arguments
  */
-function head($collection, callable $callback = null)
+function head($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Last.php b/src/Functional/Last.php
index 8989f945..4ffa2e4e 100644
--- a/src/Functional/Last.php
+++ b/src/Functional/Last.php
@@ -22,7 +22,7 @@
  * @return mixed
  * @no-named-arguments
  */
-function last($collection, callable $callback = null)
+function last($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Memoize.php b/src/Functional/Memoize.php
index 3813e3ce..aca716b5 100644
--- a/src/Functional/Memoize.php
+++ b/src/Functional/Memoize.php
@@ -21,7 +21,7 @@
  * @return mixed
  * @no-named-arguments
  */
-function memoize(callable $callback = null, $arguments = [], $key = null)
+function memoize(?callable $callback = null, $arguments = [], $key = null)
 {
     static $storage = [];
     if ($callback === null) {
diff --git a/src/Functional/None.php b/src/Functional/None.php
index edbd33a9..c90e4055 100644
--- a/src/Functional/None.php
+++ b/src/Functional/None.php
@@ -22,7 +22,7 @@
  * @return bool
  * @no-named-arguments
  */
-function none($collection, callable $callback = null)
+function none($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Pick.php b/src/Functional/Pick.php
index b4be3c13..7d29f92c 100644
--- a/src/Functional/Pick.php
+++ b/src/Functional/Pick.php
@@ -24,7 +24,7 @@
  * @return mixed
  * @no-named-arguments
  */
-function pick($collection, $index, $default = null, callable $callback = null)
+function pick($collection, $index, $default = null, ?callable $callback = null)
 {
     InvalidArgumentException::assertArrayAccess($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Poll.php b/src/Functional/Poll.php
index 7c632d77..a7a54b82 100644
--- a/src/Functional/Poll.php
+++ b/src/Functional/Poll.php
@@ -26,7 +26,7 @@
  * @return boolean
  * @no-named-arguments
  */
-function poll(callable $callback, $timeout, Traversable $delaySequence = null)
+function poll(?callable $callback, $timeout, ?Traversable $delaySequence = null)
 {
     InvalidArgumentException::assertIntegerGreaterThanOrEqual($timeout, 0, __FUNCTION__, 2);
 
diff --git a/src/Functional/Reject.php b/src/Functional/Reject.php
index 505acc91..0efddeee 100644
--- a/src/Functional/Reject.php
+++ b/src/Functional/Reject.php
@@ -22,7 +22,7 @@
  * @return array
  * @no-named-arguments
  */
-function reject($collection, callable $callback = null)
+function reject($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Retry.php b/src/Functional/Retry.php
index 40ef0fa7..43660182 100644
--- a/src/Functional/Retry.php
+++ b/src/Functional/Retry.php
@@ -29,7 +29,7 @@
  * @return mixed Return value of the function
  * @no-named-arguments
  */
-function retry(callable $callback, $retries, Traversable $delaySequence = null)
+function retry(?callable $callback, $retries, ?Traversable $delaySequence = null)
 {
     InvalidArgumentException::assertIntegerGreaterThanOrEqual($retries, 1, __FUNCTION__, 2);
 
diff --git a/src/Functional/Select.php b/src/Functional/Select.php
index 93420357..eb0ab73d 100644
--- a/src/Functional/Select.php
+++ b/src/Functional/Select.php
@@ -22,7 +22,7 @@
  * @return array
  * @no-named-arguments
  */
-function select($collection, callable $callback = null)
+function select($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Some.php b/src/Functional/Some.php
index df27e0a8..9ae81ade 100644
--- a/src/Functional/Some.php
+++ b/src/Functional/Some.php
@@ -22,7 +22,7 @@
  * @return bool
  * @no-named-arguments
  */
-function some($collection, callable $callback = null)
+function some($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Tail.php b/src/Functional/Tail.php
index 495131d2..621b174d 100644
--- a/src/Functional/Tail.php
+++ b/src/Functional/Tail.php
@@ -22,7 +22,7 @@
  * @return array
  * @no-named-arguments
  */
-function tail($collection, callable $callback = null)
+function tail($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Unique.php b/src/Functional/Unique.php
index 656ac2be..31b845b7 100644
--- a/src/Functional/Unique.php
+++ b/src/Functional/Unique.php
@@ -22,7 +22,7 @@
  * @return array
  * @no-named-arguments
  */
-function unique($collection, callable $callback = null, $strict = true)
+function unique($collection, ?callable $callback = null, $strict = true)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);