Twig `compact` function

CompactFunction.php

<?php 

class CompactFunction extends Twig_Extension
{

    /**
     * @inheritdoc
     */
    public function getFunctions()
    {
        return [
            new Twig_SimpleFunction('compact', [$this, 'compact'],
                [
                    'needs_context' => true,
                    'is_variadic'   => true,
                ]
            ),
        ];
    }

    /**
     * Return a compacted array, grabbing only the items from the
     * context that are specified in the given keys.
     *
     * @param array $context
     * @param array $keys
     *
     * @return array
     */
    public function compact(array $context, array $keys = [])
    {
        return array_intersect_key($context, array_flip((array) $keys));
    }

}

See twigphp/Twig#2532 for context.