Does a wikicode exist – why, why not, etc.
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Thus one could point to examples "existing", but the question would be why are they not popular? And I'd hazard a hypothesis that is because most code programmers are going to write is highly context sensitive and the rest is going to come from libraries.
NPM, Packagist, PyPI, RubyGems, etc. are popular and could be considered "wikicodes" in a sense. StackOverflow and StackExchange cover most everything else.
* good find on that stub / proposal
would probably add that the badging of stackoverflow goes a long way to offset any intrinsic motivation to do a wikicode like thing -- people love their points and badges :)
And to a lesser extent StackOverflow: https://stackoverflow.com/questions
SO is halfway there -- it lacks the collaborative edit mode -- would be interesting if SO added ability to make groups that could edit posts together, e.g., this post by TheFabFourSvelters, etc.