Illuminate\Support\Collection::sum PHP Method

    public function sum($callback = null)
    {
        if (is_null($callback)) {
            return array_sum($this->items);
        }
        $callback = $this->valueRetriever($callback);
        return $this->reduce(function ($result, $item) use($callback) {
            return $result + $callback($item);
        }, 0);
    }

 public function testPipe()
 {
     $collection = new Collection([1, 2, 3]);
     $this->assertEquals(6, $collection->pipe(function ($collection) {
         return $collection->sum();
     }));
 }