// Return a copy of the array with falsy values removed
  public function compact($collection=null) {
    list($collection) = self::_wrapArgs(func_get_args(), 1);
    
    $collection = self::_collection($collection);
    
    $__ = new self;
    return self::_wrap($__->select($collection, function($val) {
      return (bool) $val;
    }));
  }

function($val) {

代码来自php underscore, 这个地方报错, 似乎不能做参数.