Flatten

:::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 new dict formed by merging the KeyedContainer elements of the given Traversable

namespace HH\Lib\Dict;

function flatten<Tk as arraykey, Tv>(
  Traversable<KeyedContainer<Tk, Tv>> $traversables,
): dict<Tk, Tv>;

In the case of duplicate keys, later values will overwrite the previous ones.

For a fixed number of KeyedTraversables, see Dict\merge().

Time complexity: O(n), where n is the combined size of all the $traversables Space complexity: O(n), where n is the combined size of all the $traversables

Parameters

• Traversable<KeyedContainer<Tk,Tv>> $traversables

Returns

• dict<Tk, Tv>