twigphp/Twig · error · Twig\Error\RuntimeError

Unable to format the given list

Error message

Unable to format the given list: %s

What it means

After constructing an IntlListFormatter, formatList calls ->format($strings). IntlListFormatter signals failure by returning false, and its getErrorMessage() holds the reason (often an intl/ICU error such as U_MEMORY_ALLOCATION_ERROR or bad input). The extension surfaces that message wrapped in this RuntimeError.

Solutions

1. Read the embedded message from formatter->getErrorMessage() in the exception text to identify the underlying ICU failure
2. Verify the host's ICU/intl installation is healthy (php -i intl section, matching ICU data)
3. Reduce the size of the input list or ensure all items are strings/stringable and non-recursive
4. Test the same format() call in a plain PHP script to rule out Twig-specific wrapping

Example fix

// before
try {
    echo $twig->render('t.twig', ['items' => $hugeList]);
} catch (\Twig\Error\RuntimeError $e) {
    // "Unable to format the given list: ..."
}
// after
$items = \count($hugeList) > 5000 ? \array_slice($hugeList, 0, 5000) : $hugeList;
try {
    echo $twig->render('t.twig', ['items' => $items]);
} catch (\Twig\Error\RuntimeError $e) {
    error_log('format_list failed: '.$e->getMessage());
    echo implode(', ', $items);
}

if (!function_exists('intl_get_error_code') || intl_get_error_code() !== 0) { /* intl broken, avoid format_list */ }

try { $out = $item->formatList($strings); } catch (\Twig\Error\RuntimeError $e) { $out = implode(', ', $strings); }

When it happens

Trigger: Calling IntlListFormatter::format() via the format_list filter and receiving false — e.g. intl/ICU internal failure reported by getErrorMessage(), or invalid underlying data the formatter cannot handle.

Common situations: Broken or misconfigured ICU library on the host; extremely large input lists exhausting memory; corrupted locale data causing the formatter to fail during formatting.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• The "format_list" filter requires the "IntlListFormatter"…
• The date format " " does not exist, known formats are: " ".
• The time format " " does not exist, known formats are: " ".
• The style " " does not exist, known styles are: " ".
• The number formatter attribute

AI-assisted analysis of twigphp/Twig@a414c3a491 (2026-09-13). Data as JSON: /api/errors/0056a8f48134f6d1. Report an issue: GitHub.

Appendix: source

Thrown at extra/intl-extra/IntlExtension.php:456

     * @param \DateTimeZone|string|false|null $timezone The target timezone, null to use the default, false to leave unchanged
     */
    public function formatTime(Environment $env, $date, ?string $timeFormat = null, string $pattern = '', $timezone = null, ?string $calendar = null, ?string $locale = null): string
    {
        return $this->formatDateTime($env, $date, 'none', $timeFormat, $pattern, $timezone, $calendar, $locale);
    }

    /**
     * @param array<string|\Stringable> $strings A list of items to be joined into a formatted list
     */
    public function formatList(array $strings, string $type = 'and', string $width = 'wide', ?string $locale = null): string
    {
        if (!class_exists('IntlListFormatter')) {
            throw new RuntimeError('The "format_list" filter requires the "IntlListFormatter" class, which is available since PHP 8.5.');
        }

        $formatter = $this->createListFormatter($locale, $type, $width);
        if (false === $ret = $formatter->format($strings)) {
            throw new RuntimeError(\sprintf('Unable to format the given list: %s', $formatter->getErrorMessage()));
        }

        return $ret;
    }

    private function createDateFormatter(?string $locale, ?string $dateFormat, ?string $timeFormat, string $pattern, ?\DateTimeZone $timezone, ?string $calendar): \IntlDateFormatter
    {
        $dateFormats = self::availableDateFormats();

        if (null !== $dateFormat && !isset($dateFormats[$dateFormat])) {
            throw new RuntimeError(\sprintf('The date format "%s" does not exist, known formats are: "%s".', $dateFormat, implode('", "', array_keys($dateFormats))));
        }

        if (null !== $timeFormat && !isset(self::TIME_FORMATS[$timeFormat])) {
            throw new RuntimeError(\sprintf('The time format "%s" does not exist, known formats are: "%s".', $timeFormat, implode('", "', array_keys(self::TIME_FORMATS))));
        }

        if (null === $locale) {

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