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firstx

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 the first element of the given Traversable, or throws if the Traversable is empty

namespace HH\Lib\C;

function firstx<T>(
  Traversable<T> $traversable,
): T;
  • For possibly empty Traversables, see C\first.
  • For possibly null Traversables, see C\nfirst.
  • For single-element Traversables, see C\onlyx.
  • For Awaitables that yield Traversables, see C\firstx_async.

Time complexity: O(1) Space complexity: O(1)

Parameters​

Returns​

  • T

Examples​

$strings = vec["a", "b", "c"];
$first_string = C\firstx($strings);
echo "First string in traversable: $first_string \n";
//Output: First string in traversable: a 

$empty_traversable = vec[];
$first_element = C\firstx($empty_traversable);
//Output: Hit a php exception : exception 'InvariantViolationException' with message
//'HH\Lib\C\firstx: Expected at least one element.'