Map - Hhvm — ContextQMD

:::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 value in the current MutableSet

Hack

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

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

Guide

Examples

Parameters

(function(Tv): Tu) $fn - The callback containing the operation to apply to the current MutableSet values.

Returns

MutableSet<Tu> - a MutableSet containing the values after a user-specified operation is applied.