This has also been discussed at math.SE, I'll quote a part of Willie Wong's answer there:
Another possibility [...] is that the user is only
leaving a short sketch of the complete answer in the comments. For
those with sufficient mathematical maturity, those short sketches may
appear to be a complete answer. But for a self-conscious educator,
those sketches may not be sufficiently pedagogical as an answer.
On the flip side, because of the Q&A (emphasis on the A part) nature
of this site, we desire answers. So if a user decides to post an
answer as a comment, I think he or she should not complain when
somebody else decides to incorporate the content of said comment into
a "more complete" answer.
Which leads to...
If you see an "unanswered question" with the answer already appearing
in the comments, you can either:
- Ping the author of that comment and request that it be posted as
- Just copy the comment and post it as an answer yourself
(perhaps with proper attribution, and/or marking it as community wiki
if you don't want the credit); or
- Expand upon that comment and
write a better, clearer answer.
I agree with the fact that some users have a bad feeling about leaving an answer that, to them, seems incomplete or sketchy. But that's what SE is about: You don't post an ultimate answer. There won't be the need for an erratum. You can edit and improve your post anytime. If you think it's too sketchy but you don't feel like elaborating further, post it as answer, turn it community wiki and leave a comment what you feel is missing. Other users are encouraged to add details. And you don't even have to feel bad about earning rep for a sketch since CW is rep-neutral.
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