MapWithKey

:::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 MutableSet containing the values after an operation has been applied to each "key" and value in the current Set

public function mapWithKey<Tu as arraykey>(
  (function(arraykey, Tv): Tu) $fn,
): MutableSet<Tu>;

Since sets don't have keys, the callback uses the values as the keys as well.

Every value in the current MutableSet is affected by a call to mapWithKey(), unlike filterWithKey() where only values that meet a certain criteria are affected.

Parameters

• (function(arraykey, Tv): Tu) $fn - The callback containing the operation to apply to the current MutableSet "keys" and values.

Returns

• MutableSet<Tu> - a MutableSet containing the values after a user-specified operation on the current MutableSet's values is applied.