Reduce

:::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 Traversable into a single value by applying an accumulator function against an intermediate result and each value

namespace HH\Lib\C;

function reduce<Tv, Ta>(
  Traversable<Tv> $traversable,
  (function(Ta, Tv): Ta) $accumulator,
  Ta $initial,
): Ta;

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

Parameters

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

Returns

• Ta

Examples

$values = vec[1,2,3,4,5];
$reduce_result = C\reduce($values, ($total, $value) ==> $total + $value, 0);
echo "Reduce result: $reduce_result\n";
//Output: Reduce result: 15