Reduce With Key

:::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. :::

Reduces the given KeyedTraversable into a single value by applying an accumulator function against an intermediate result and each key/value

namespace HH\Lib\C;

function reduce_with_key<Tk, Tv, Ta>(
  KeyedTraversable<Tk, Tv> $traversable,
  (function(Ta, Tk, Tv): Ta) $accumulator,
  Ta $initial,
): Ta;

Time complexity: O(n) Space complexity: O(1)

Parameters

• KeyedTraversable<Tk,Tv> $traversable
• (function(Ta, Tk, Tv): Ta) $accumulator
• Ta $initial

Returns

• Ta

Examples

$dict = dict["a" => 1, "b" => 2, "c" => 3];
$list = vec["a","b"];
$reduce_with_key_result = C\reduce_with_key($dict, ($a, $k, $v) ==> $a + (C\contains($list, $k) ? 0 : $v), 0);
echo "Reduce with key result: $reduce_with_key_result\n";
//Output: Reduce with key result: 3