phacility/phabricator · error · Exception

Value for "function" argument must be a list with a…

Error message

Value for "function" argument must be a list with a function name; got an empty list.

What it means

After confirming a 'function' argument is a natural list, PhabricatorChartFunctionArgument::newValue() requires the list to be non-empty because the first element is the function name. An empty array throws this Exception.

Solutions

Omit the argument entirely if there is no nested function to apply.
Default empty optional values to a meaningful function (e.g. array('constant', 0)).
Filter empty arrays out of dynamically built argument lists before passing them to chart APIs.
Validate count($value) > 0 during spec construction.

Example fix

// before
$arguments[] = array(); // empty list -> Exception

// after: omit empty nested functions
if ($nested) {
  $arguments[] = $nested;
}

Defensive patterns

Strategy: validation

Validate before calling

if (is_array($value) && !count($value)) {
  // skip the argument or supply a meaningful default function
  $value = array('constant', 0);
}

Prevention

Filter empty lists out of dynamically built argument lists.
Default optional nested functions to an explicit function definition.
Check count($value) > 0 when composing specs programmatically.

When it happens

Trigger: Passing array() (or []) as a nested function definition, commonly from an empty optional parameter, an array_map over an empty set, or a default value that was never filled in.

Common situations: Building argument lists dynamically where a sub-expression produced no elements; empty request parameters flowing into chart construction; JSON specs containing [] as a function node.

Related errors

AI-assisted analysis of phacility/phabricator@5720a38cfe (2026-08-21). Data as JSON: /api/errors/331b92a34e9f21b1. Report an issue: GitHub.

Appendix: source

Thrown at src/applications/fact/chart/PhabricatorChartFunctionArgument.php:101

        if (!is_array($value)) {
          throw new Exception(
            pht(
              'Value for "function" argument must be a function definition, '.
              'formatted as a list, like: [fn, arg1, arg, ...]. Actual value '.
              'is %s.',
              phutil_describe_type($value)));
        }

        if (!phutil_is_natural_list($value)) {
          throw new Exception(
            pht(
              'Value for "function" argument must be a natural list, not '.
              'a dictionary. Actual value is "%s".',
              phutil_describe_type($value)));
        }

        if (!$value) {
          throw new Exception(
            pht(
              'Value for "function" argument must be a list with a function '.
              'name; got an empty list.'));
        }

        $function_name = array_shift($value);

        if (!is_string($function_name)) {
          throw new Exception(
            pht(
              'Value for "function" argument must be a natural list '.
              'beginning with a function name as a string. The first list '.
              'item has the wrong type, %s.',
              phutil_describe_type($function_name)));
        }

        $functions = PhabricatorChartFunction::getAllFunctions();
        if (!isset($functions[$function_name])) {

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