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partition

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 2-tuple containing keysets for which the given predicate returned true and false, respectively

namespace HH\Lib\Keyset;

function partition<Tv as arraykey>(
  Traversable<Tv> $traversable,
  (function(Tv): bool) $predicate,
): (keyset<Tv>, keyset<Tv>);

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

Parameters​

Returns​

  • (keyset<Tv>, keyset<Tv>)

Examples​

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

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