HalServiceProvider.php

<?php

namespace Drupal\hal;

use Drupal\Core\DependencyInjection\ContainerBuilder;
use Drupal\Core\DependencyInjection\ServiceModifierInterface;

/**
 * Adds hal+json as known format.
 */
class HalServiceProvider implements ServiceModifierInterface {
    
    /**
     * {@inheritdoc}
     */
    public function alter(ContainerBuilder $container) {
        if ($container->has('http_middleware.negotiation') && is_a($container->getDefinition('http_middleware.negotiation')
            ->getClass(), '\\Drupal\\Core\\StackMiddleware\\NegotiationMiddleware', TRUE)) {
            $container->getDefinition('http_middleware.negotiation')
                ->addMethodCall('registerFormat', [
                'hal_json',
                [
                    'application/hal+json',
                ],
            ]);
        }
    }

}