Onlyx

:::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 the first and only element of the given Traversable, or throws if the Traversable is empty or contains more than one element

namespace HH\Lib\C;

function onlyx<T>(
  Traversable<T> $traversable,
  ?\HH\Lib\Str\SprintfFormatString $format_string = NULL,
  mixed ...$format_args,
): T;

An optional format string (and format arguments) may be passed to specify a custom message for the exception in the error case.

For Traversables with more than one element, see C\firstx.

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

Parameters

• Traversable<T>$traversable
• ?\HH\Lib\Str\SprintfFormatString $format_string = NULL
• mixed ...$format_args

Returns

• T

Examples

$string = vec["a"];
$onlyx_result_1 = C\onlyx($string);
echo "First onlyx result: $onlyx_result_1\n";
//Output: First onlyx result: a

$strings = vec["a", "b"];
$onlyx_result_2 = C\onlyx($strings);
//Output: Hit a php exception : exception 'InvariantViolationException' with message
//'Expected exactly one element but got 2.'

$empty_strings = vec[];
$onlyx_result_3 = C\onlyx($empty_strings);
//Output: Hit a php exception : exception 'InvariantViolationException' with message
//'Expected non-empty Traversable.'