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map_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 where each value is the result of calling the given function on the original key and value

namespace HH\Lib\Keyset;

function map_with_key<Tk, Tv1, Tv2 as arraykey>(
  KeyedTraversable<Tk, Tv1> $traversable,
  (function(Tk, Tv1): Tv2) $value_func,
): keyset<Tv2>;

Time complexity: O(n * f), where f is the complexity of $value_func Space complexity: O(n)

Parameters​

Returns​

  • keyset<Tv2>

Examples​

$result = Keyset\map_with_key(dict[1 => 2, 2 => 4, 3 => 5], ($key, $val) ==> $val*$key);
print_r($result);
//result: keyset[2,8,15]

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