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filter

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.

Returns a new keyset containing only the values for which the given predicate returns true

namespace HH\Lib\Keyset;

function filter<Tv as arraykey>(
  Traversable<Tv> $traversable,
  ?(function(Tv): bool) $value_predicate = NULL,
): keyset<Tv>;

The default predicate is casting the value to boolean.

To remove null values in a typechecker-visible way, see Keyset\filter_nulls().

Time complexity: O(n * p), where p is the complexity of $value_predicate Space complexity: O(n)

Parameters​

  • Traversable<Tv>$traversable
  • ?(function(Tv): bool) $value_predicate = NULL

Returns​

  • keyset<Tv>

Examples​

$result = Keyset\filter(keyset[1,2,3,4], $val ==> $val%2==0);
print_r($result);
//result: keyset[2,4]

$result = Keyset\filter(keyset[1,2,3,4], $val ==> $val==0);
print_r($result);
//result: keyset[]

$result = Keyset\filter(keyset[1,2,3,4],  $val ==> $val == $val);
print_r($result);
//result: keyset[1,2,3,4]