/**
* @param Iterator $iterator
* @param callable $fn($key, $value)
* @return Sequence
*/
public static function filterKeys(Iterator $iterator, $fn)
{
return Sequence::make(new FilteredSequence($iterator, FnGen::fnSwapParamsPassThrough($fn)));
}
/**
* Returns the key of the first element where $fnTest returns true.
*
* @param callable|null $fnTest($value, $key)
* @return mixed
*/
public function firstKey($fnTest = null)
{
$fnTest = $fnTest ?: fn\fnTrue();
return $this->filter($fnTest)->limit(1)->keys()->reduce(null, FnGen::fnSwapParamsPassThrough(FnGen::fnIdentity()));
}
