"That is not possible" is a valid answer

Question

"No" is a valid answer to "is this possible?". Sometimes it can be quite useful. A recent example: How to ban all connections to .se and .ru in the hosts.deny file

Or not?

• https://twitter.com/srostedt/status/1132009918444318721
• Change format of syscall event trace output to ftrace

EDIT: In the two links above, you can currently see a "no" answer which was deleted, and converted into a comment. But looking at When to "Comment", and when to "Answer"? , I would think it fits better as an answer, and does not fit well as a comment.

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This type of answer might be most useful when worded "It is not possible in version 42. The current features (or architecture) are exhaustively documented <here>, and none of them will let you do this."

I think there is no written rule that requires that wording. I can imagine a moderator wanting to avoid the "this answer is not useful" scoring process etc, for an inexperienced StackExchange user. But I am not convinced this is the best approach. As an example of an alternative, I saw another short, new answer today, which received a comment response "Do you have any references for this?"

In this particular case, an edit like "citation: I am the first name listed in the MAINTAINERS document for this code" could make the answer more useful. The author has since attempted to provide this information.

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This answer is not simply a pedantic mis-understanding of a common idiom. It would still apply when question posts are worded "I want to do X, and I need to do it using tool Y because reason Z". The correct answer may be "tool Y cannot do X".

The caveat here is that when you cannot tell what reason Z is, it can be unclear what the intended question is. Does the asker require answers to use tool Y, or did they just assume that tool Y would have a good way to do X? That is what we have question comments for. And ultimately, the ability to put a question on hold. I.e. if a post is not a good fit for this site's definition of a question: a post that can have a definitive answer.

Answer 1

I agree with you. "You can't" is a valid answer -- and one that I've used! I agree also that such a claim should be backed up by evidence of some kind (a demonstration, documentation, or as in this case, the author of the software) that indicates the current limits. Thank you, sourcejedi, for doing the right thing and bringing the situation up here on Meta.

I converted the Answer when I saw it while going through the Late Answer review queue ("late answers from new users"). At that time, the Answer lacked the attribution claim, which was added about a day later, so the text I was judging read:

Unfortunately, there is currently not a way to do this. But perhaps in the future I may add it, if I can figure out a sane interface and implementation to do such a thing. Maybe I will add a trigger that will make the output show differently.

... which read to me less like an Answer and more like a wish. It invoked the "I'm having this problem, too" reaction from me, which isn't fair in this case, given what we've now learned about the author.

I'd like to apologize to nevets for my reaction to their post and would like to welcome them to join the U&L Stack Exchange. As I've learned, it's a great community with a lot of features and functionality -- and great users! The tour is an easy place to start. I will reverse the deletion so that it can show the current state of the Question.

Answer 2

Of course it is a valid answer. Not everything is possible to do. If the person wants to know how to do something, but that thing is impossible, they're better off knowing that it's impossible so they can confidently abandon their approach and look for an alternative.

However there is a difference between "impossible" and "impossible as far as I know". So I suspect that only in a minority of cases will be possible to give a quality "impossible" answer, one which is actually backed up by argument and evidence. In other cases the question is better left unanswered so that a more knowledgeable person can later come along and answer.

There is also "possible, but very impractical, so it might as well be possible". Technically the correct answer in such situations is to see how to do the thing being asked. But if the solution is impractical, readers are unlikely to implement it. The main object becomes not to provide a solution but to convince the reader that it is indeed impractical. Alternatively you could simply ask "is it possible to do this easily", but then you get the nag message about subjective questions. In any case, we still have the issue that just because you don't see how its possible, doesn't mean it isn't.

Lastly there is "possible, but only a tiny number of people know this topic well enough to say how". In this case, the correct answer may be of dubious value. If working with such an obscure setup that you cannot easily get help, it is likely that solving the one problem will not be enough. There will be many more problems and for those it may take even longer to find someone who knows a solution. Rather than persisting in stubbornly fighting this obscure system, the asker is arguably better advised to abandon it and find a more practical alternative. Unlike saying for sure that something is impossible, I think it is more constructive to say:

I am moderately knowledgeable in this topic, and if there is a way to do this it then it is not easy to find. However, you could instead try this alternative approach, which does not have the problems you suffer.