Add keys operation

src/Nexus/Collection/Collection.php

@@ -98,6 +98,15 @@ public function getIterator(): \Traversable
         yield from $this->innerIterator->getIterator();
     }
 
+    public function keys(): CollectionInterface
+    {
+        return new self(static function (iterable $collection): iterable {
+            foreach ($collection as $key => $_) {
+                yield $key;
+            }
+        }, [$this]);
+    }
+
     public function values(): CollectionInterface
     {
         return new self(static function (iterable $collection): iterable {

src/Nexus/Collection/CollectionInterface.php

@@ -19,12 +19,14 @@
  *
  * @extends \IteratorAggregate<TKey, T>
  * @extends Operation\All<TKey, T>
+ * @extends Operation\Keys<TKey, T>
  * @extends Operation\Values<TKey, T>
  */
 interface CollectionInterface extends
     \Countable,
     \IteratorAggregate,
     Operation\All,
+    Operation\Keys,
     Operation\Values
 {
     /**

src/Nexus/Collection/Operation/Keys.php

@@ -0,0 +1,36 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * This file is part of the Nexus framework.
+ *
+ * (c) John Paul E. Balandan, CPA <paulbalandan@gmail.com>
+ *
+ * For the full copyright and license information, please view
+ * the LICENSE file that was distributed with this source code.
+ */
+
+namespace Nexus\Collection\Operation;
+
+use Nexus\Collection\CollectionInterface;
+
+/**
+ * @template TKey
+ * @template T
+ */
+interface Keys
+{
+    /**
+     * Returns a new collection from keys of the original collection as
+     * the new values.
+     *
+     * ```
+     * Collection::wrap(['banana' => 1, 'apple' => 2])->keys();
+     * => Collection(['banana', 'apple'])
+     * ```
+     *
+     * @return CollectionInterface<int, TKey>
+     */
+    public function keys(): CollectionInterface;
+}

tests/Collection/AbstractCollectionTestCase.php

@@ -55,6 +55,19 @@ public function testCount(): void
         self::assertCount(2, $this->collection([1, 2]));
     }
 
+    public function testKeys(): void
+    {
+        $collection = $this->collection(static function (): \Generator {
+            yield 'bananas' => 5;
+
+            yield 'apples' => 4;
+
+            yield 'oranges' => 7;
+        });
+
+        self::assertSame(['bananas', 'apples', 'oranges'], $collection->keys()->all());
+    }
+
     public function testValues(): void
     {
         $collection = $this->collection(static function (): \Generator {