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 vec formed by joining the Traversable elements of the given Traversable

namespace HH\Lib\Vec;

function flatten<Tv>(
  Traversable<Container<Tv>> $traversables,
): vec<Tv>;

For a fixed number of Traversables, see Vec\concat().

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<Container<Tv>>$traversables

Returns

• vec<Tv>

Examples

$example_vec = vec[vec[1,2,3,4,5],vec[98,99]];
$result = Vec\flatten($example_vec);
print_r($result);
//result: [1,2,3,4,5,98,99]

$example_vec = vec[vec[1,2,3,4,5,98,99]];
$result = Vec\flatten($example_vec);
print_r($result);
//result: [1,2,3,4,5,98,99]

$example_vec = vec[];
$result = Vec\flatten($example_vec);
print_r($result);
//result: []