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mapWithKey

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 ConstSet 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,
): ConstSet<Tu>;

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

Every value in the current ConstSet 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 ConstSet "keys" and values.

Returns​