Retain

:::info[Note] This is a point-in-time snapshot of the API documentation from January 2026. Going forward, we will not be maintaining a public copy of these references, and recommend users to refer to the built-in signature helpers available in the Hack LSP instead for complete and up-to-date information. :::

Alters the current Set so that it only contains the values that meet a supplied condition on each value

public function retain(
  (function(Tv): bool) $callback,
): Set<Tv>;

This method is like filter(), but mutates the current Set too in addition to returning the current Set.

Future changes made to the current Set ARE reflected in the returned Set, and vice-versa.

Parameters

• (function(Tv): bool) $callback

Returns

• Set<Tv> - Returns itself.

Examples

$s = Set {'red', 'green', 'blue', 'yellow'};

// Only keep values beginning with 'r' or 'b'
$s->retain($color ==> $color[0] === 'r' || $color[0] === 'b');
\var_dump($s);