public function resolve(array $args, HandlerList $list)
    {
        $args['config'] = [];
        foreach ($this->argDefinitions as $key => $a) {
            // Add defaults, validate required values, and skip if not set.
            if (!isset($args[$key])) {
                if (isset($a['default'])) {
                    // Merge defaults in when not present.
                    if (is_callable($a['default'])
                        && (
                            is_array($a['default'])
                            || $a['default'] instanceof \Closure
                        )
                    ) {
                        $args[$key] = $a['default']($args);
                    } else {
                        $args[$key] = $a['default'];
                    }
                } elseif (empty($a['required'])) {
                    continue;
                } else {
                    $this->throwRequired($args);
                }
            }

            // Validate the types against the provided value.
            foreach ($a['valid'] as $check) {
                if (isset(self::$typeMap[$check])) {
                    $fn = self::$typeMap[$check];
                    if ($fn($args[$key])) {
                        goto is_valid;
                    }
                } elseif ($args[$key] instanceof $check) {
                    goto is_valid;
                }
            }

            $this->invalidType($key, $args[$key]);

            // Apply the value
            is_valid:
            if (isset($a['fn'])) {
                $a['fn']($args[$key], $args, $list);
            }

            if ($a['type'] === 'config') {
                $args['config'][$key] = $args[$key];
            }
        }

        return $args;
    }