Partition Async

:::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. :::

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

namespace HH\Lib\Keyset;

function partition_async<Tv as arraykey>(
  Container<Tv> $container,
  (function(Tv): Awaitable<bool>) $value_predicate,
): Awaitable<(keyset<Tv>, keyset<Tv>)>;

For non-async predicates, see Keyset\partition().

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

The IO operations for each of the calls to $value_predicate will happen in parallel.

Parameters

• Container<Tv>$container
• (function(Tv): Awaitable<bool>) $value_predicate

Returns

• Awaitable<(keyset<Tv>,keyset<Tv>)>