Interesting enough, but this misses most of the point. In any case, its not really that interesting if the value you looked for is missing or not, its way more important that I can see _which_ value was not found. Any solution that does not capture which value was missing, is kind of besides the point for most practical applications.
You can get that by implementing an error type, or otherwise constructing a new error every time one is returned. But doing this adds to the cost of returning the error.
This is how we use most errors in our code base. But we tend to not treat them as sentinels, we are only interested in the descriptive error messages.
