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filter_with_key

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_with_key<Tk, Tv as arraykey>(
  KeyedTraversable<Tk, Tv> $traversable,
  (function(Tk, Tv): bool) $predicate,
): keyset<Tv>;

If you don't need access to the key, see Keyset\filter().

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

Parameters​

Returns​

  • keyset<Tv>

Examples​

$result = Keyset\filter_with_key(dict[1 => 2, 2 => 4, 3 => 5], ($key, $val) ==> $val%2==0);
print_r($result);
//result: keyset[2,4]

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

$result = Keyset\filter_with_key(dict[1 => 2, 2 => 4, 3 => 5],  ($key, $val) ==> $val == $val);
print_r($result);
//result: keyset[2,4,5]