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"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?

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.


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.


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.

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    The motivating answer from the Twitter post has been (wrongly) deleted so it's 10k-only, but for everyone else the text was: 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. and post-deletion edit Although I may be new to StackExchange, I am the author of ftrace (real name Steven Rostedt - look up the git history). The "real answer" will happen when I write the code! – Michael Homer May 25 at 8:43
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    Please note that the answer wasn't simply deleted. It was converted to a comment. – terdon May 25 at 11:08
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    @terdon That should be simply undeleted. And not just because it's the author -- if you know better than a random poster who says that it's not possible, then add your answer where you explain how to actually do it. – mosvy May 25 at 13:51
  • @mosvy I had nothing to do with this, so let's wait for the person who took action to explain their rationale. – terdon May 25 at 13:54
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    No need to explain anything -- mistakes happen. Other moderators can undelete answers too, right? – mosvy May 25 at 13:58
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    +10000. Nothing drives me more than people trying to dance around your question without answering it. It's frustrating you can't tell if it's because they don't know the answer or if they know the answer is no. People seriously need to learn that This Is Not Possible is a perfectly good answer. – Mehrdad Jun 7 at 4:19
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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.

  • Thanks for the undelete. That seemed the clear way forward :-). The second “agree” does not follow. In my question I tried to not assert whether or not evidence was required in such answers. The question could be more clear, but at least the quickest clarification would be to remove the second “agree” here :-). I used the word "useful" mostly to refer to answer upvotes or downvotes. Distinct from “valid” v.s. “not an answer”, which would be eligible for flagging. – sourcejedi May 25 at 17:20
  • Reading this, I can't tell whether nevets will see it. I think "I'd like to apologize" is awkward phrasing in that context. I'm not sure I would use the word if you can't say you have offered (or given) it? - that would be more clear. The phrase used seems more appropriate in a context where it is unambiguous that they would see it. (Or in public statements that address a large group of people). – sourcejedi May 25 at 17:34
  • I’d like to think about this more later - I have other stuff to do at the moment. It’s a good intention to want to apologize, but if it’s qualified by “given what we've now learned”, it’s not clear to me what you think needs to be apologized for. – sourcejedi May 25 at 17:35
  • The question I have in mind is, if someone who works with the notoriously cut and thrust Linux mailing lists is taken aback by our response (and is not able to find any way to contest the response with additional information), is there anything we can learn from it? – sourcejedi May 25 at 17:41
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    The context about the Late Answer review queue is helpful. – sourcejedi May 25 at 17:53
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    @sourcejedi I think the mechanics of SE are sufficiently different to LKML and such to be surprising in any case; on LKML at least, messages aren’t deleted (or “downgraded”), nor are they edited... This doesn’t undermine your question though, which is certainly worth thinking about. As for the main question, I find “that is not possible” answers particularly difficult to write, because I always wonder if it’s that I don’t know something rather than that it isn’t possible. – Stephen Kitt May 25 at 20:13
  • I agree with the spirit of the answer. This post was caught by a type of moderation that, on balance, probably helps maintain the distinct value of StackExchange. Thank you for your service :-). I just wouldn't point to this answer without being slightly more explicit about what we're balancing against. Also, while it would be great if all moderation was accompanied by a clear objective explanation, I do not believe that is a realistic demand. To the extent that it was explained in this case, it appears to have helped move towards a resolution. – sourcejedi May 29 at 11:25
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    To be fair, Jeff's action was perfectly valid, considering that the answer originally had no mention that the poster is the author of ftrace in question. They've not stated that anywhere in their profile either. From what I've learned over the past 5 or so years of being here and on AskUbuntu, one has to add a small statement saying "Full disclosure: I am the author of the software xyz". I know - that's not gonna be a popular opinion. The author didn't do that, which to me looks like not following SE rules. And fairly enough, it does read as a comment. – Sergiy Kolodyazhnyy May 31 at 8:05
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    @SergiyKolodyazhnyy sorry for repeating myself, it does not matter: If some random user says that something's not possible, then go on and explain how it's possible. Do no "moderate" him or her, especially when you're not familiar with the subject. And do not invent rules or interpret the (non-binding!) guidelines overly broadly: the "full-disclosure rule" should only be wielded against obnoxious shills and snake-oil sellers, anything else is simply abuse. – mosvy Jun 1 at 19:08
  • @SergiyKolodyazhnyy As to your opinion being unpopular, I'm aftraid it's just the contrary: that combo of appeal-to-authoritiy accepted as a valid argument and deference to mean rules is WAY too popular, more than it should be for people pretending to live in a free society. – mosvy Jun 1 at 19:13
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    @mosvy I have no problem with possible/impossible types of answers so long as everything is backed up bu evidence. What I have problem with is that the user did not follow the proper process set up on Stackexchange, did not go on meta but rather to twitter. Sourcejedi did the right thing, the user - did not. This of course stems from no one reading the help center when they sign up or reach out to peers to clarify. – Sergiy Kolodyazhnyy Jun 1 at 20:14
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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.

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    +1. Regarding the last case - I agree you should only say "impossible" when that is what you really think. But I might hesitate to offer what you suggest as an answer. I think it can work as a comment (using a link to a resource about the more common approach). The implied edit to improve the question is: "I think it would be easier if I could do [this] instead. However that is ruled out by [this unusual reason] / I do not have the power to change it / I am interested if anyone is willing to demonstrate specifically what level of effort this would require". – sourcejedi May 28 at 19:02

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