code: protected function getConnectionInitializer(OptionsInterface $options, $callable)
      {
          if (!is_callable($callable)) {
              $class = get_called_class();
  
              throw new \InvalidArgumentException("$class expects a valid callable");
          }
  
          $option = $this;
  
          return function ($parameters = null) use ($callable, $options, $option) {
              $connection = call_user_func($callable, $options, $parameters);
  
              if (!$connection instanceof AggregateConnectionInterface) {
                  $class = get_class($option);
  
                  throw new \InvalidArgumentException("$class expects a valid connection type returned by callable initializer");
              }
  
              return $connection;
          };
      }
docs: Wraps a callable to ensure that the returned value is a valid connection.
  
  @param OptionsInterface $options  Client options.
  @param mixed            $callable Callable initializer.
  
  @return \Closure