twigphp/Twig · error · SyntaxError

The arrow function argument must be a list of variables or…

Error message

The arrow function argument must be a list of variables or a single variable.

What it means

ArrowFunctionExpression's constructor requires its parameter list to be a ListExpression of AssignContextVariable nodes (or a single ContextVariable, which it wraps automatically). Anything else — a constant, expression, or malformed node — throws this SyntaxError, because arrow function arguments must be plain variables to bind.

Solutions

1. Ensure the left side of => is a variable or a parenthesized list of variables: sort((a, b) => a - b), filter(v => v > 1).
2. Wrap multiple arguments in parentheses: (a, b) => ..., not a, b => ....
3. Remove any literal or expression used as an arrow-function parameter; only variable names are allowed.
4. If constructing ArrowFunctionExpression in PHP, pass a ListExpression of AssignContextVariable (or a ContextVariable) as $names.

Example fix

// before (Twig template)
{{ numbers|sort(1 => a) }} {# SyntaxError #}
// after
{{ numbers|sort((a, b) => a <=> b) }}

// validate arrow-function callbacks in template source before render
if (preg_match('/\d+\s*=>|["\']\s*=>/', $templateSource)) {
    throw new InvalidArgumentException('Arrow function argument must be variable(s).');
}

function isValidArrowFnArgs($names): bool {
    return $names instanceof \Twig\Node\Expression\ListExpression
        || $names instanceof \Twig\Node\Expression\ContextVariable;
}

try {
    $twig->parse($twig->tokenize(new \Twig\Source($code, 'tpl')));
} catch (\Twig\Error\SyntaxError $e) {
    if (str_contains($e->getMessage(), 'arrow function argument')) {
        // fix the callback to use variable(s) left of =>
    }
    throw $e;
}

When it happens

Trigger: In a template, writing an arrow function whose left side of => is not a variable or variable list, e.g. map(1 => v) or map('x' => v); multi-argument callbacks without parentheses (a, b => ...); programmatically constructing ArrowFunctionExpression with a non-list/non-variable $names node.

Common situations: Typos in Twig arrow-function callbacks passed to filters like sort/filter/map/reduce; forgotten parentheses around multiple parameters; PHP extensions building arrow function nodes with wrong argument nodes.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

• Empty array elements are only allowed in destructuring…
• The "output_strategy" argument of the "render_sandboxed"…
• Unknown " " configuration.
• The "html_classes" function argument
• Block " " on template " " does not exist.

AI-assisted analysis of twigphp/Twig@a414c3a491 (2026-09-13). Data as JSON: /api/errors/9b183aa7bbd016d9. Report an issue: GitHub.

Appendix: source

Thrown at src/Node/Expression/ArrowFunctionExpression.php:34

use Twig\Node\Expression\Variable\AssignContextVariable;
use Twig\Node\Expression\Variable\ContextVariable;
use Twig\Node\Node;

/**
 * Represents an arrow function.
 *
 * @author Fabien Potencier <fabien@symfony.com>
 */
class ArrowFunctionExpression extends AbstractExpression
{
    public function __construct(AbstractExpression $expr, Node $names, $lineno)
    {
        if ($names instanceof ContextVariable) {
            $names = new ListExpression([new AssignContextVariable($names->getAttribute('name'), $names->getTemplateLine())], $lineno);
        }

        if (!$names instanceof ListExpression) {
            throw new SyntaxError('The arrow function argument must be a list of variables or a single variable.', $names->getTemplateLine(), $names->getSourceContext());
        }

        parent::__construct(['expr' => $expr, 'names' => $names], [], $lineno);
    }

    public function compile(Compiler $compiler): void
    {
        $compiler
            ->addDebugInfo($this)
            ->raw('function (')
            ->subcompile($this->getNode('names'))
            ->raw(') use ($context, $macros) { ')
        ;
        foreach ($this->getNode('names') as $name) {
            $compiler
                ->raw('$context["')
                ->raw($name->getAttribute('name'))
                ->raw('"] = $__')

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