private function wrapExpression($from, callable $expr, array $types)
    {
        list($fn, $pos) = explode(':', $from);
        $from = "The expression return value of argument {$pos} of {$fn}";
        return function ($value) use ($from, $expr, $types) {
            $value = $expr($value);
            $this->validateType($from, $value, $types);
            return $value;
        };
    }