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

public function mapWithKey<Tu as arraykey>(
  (function(arraykey, Tv): Tu) $callback,
): ImmSet<Tu>;

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

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

Parameters​

  • (function(arraykey, Tv): Tu) $callback

Returns​

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

Examples​

See Set::mapWithKey for usage examples.