Adding `language: lang-el` to code blocks is not adding syntax highlighting for elisp

Question

Per What is syntax highlighting and how does it work?, it would appear that adding

<!-- language: lang-el -->

prior to code blocks in Emacs should enable syntax highlighting. Unfortunately it does not yet appear to be enabled, or is missing something. As example, I added the above to the answer in https://emacs.stackexchange.com/a/81/104, and it is not highlighted. Is there a specific tag that is also required, and could we just presume that Elisp should be the default highlighting for the emacs community?

Answer 1

Syntax highlighting is activated on a site-by-site basis.

Should we have syntax highlighting for Emacs Lisp by default? I don't expect the presence of Emacs Lisp to be correlated with tags: any question can be answered by “put this little chunk of code in your .emacs”. However code blocks are sometimes used to present buffer contents (example).

Ideally there would be a heuristic: in the absence of a magic comment, highlight for Emacs Lisp if the code block starts with one of the characters `(;' (skipping leading whitespace), otherwise don't highlight. I don't know if this is possible with the current code; treat this answer as a feature request.

I had a go at extracting all code blocks from the site (questions and answers), classifying them according to their first non-whitespace character, and visually inspecting each category.

#        31     1 Lisp
'         3     all Lisp
(       342     all but 2 Lisp
;        30     all Lisp
[        14     none Lisp
`         4     all Lisp
other   199     1 Lisp

The Lisp snippet beginning with # is #(…) syntax. The “other” Lisp snippet begins with .... The two non-Lisp blocks beginning with ( are part of a LaTeX error log and a regular expression (not in Emacs syntax). The non-Lisp blocks are as you'd expect a mix of content with no coherent theme, mainly anything that one could edit (LaTeX, C, Org headers, ASCII art tables, etc.).

22 Lisp blocks start with whitespace. They're probably over-indented but I didn't investigate further.

Thus highlighting blocks as Lisp by default would work about 63% of the time. The heuristic above would work with a very high success rate: 0.5% false positive and 1% false negative (my original proposal of just (; would miss about 2% of the blocks to be highlighted).

Conclusion: we should have syntax highlighting. Preferably with the heuristic above, otherwise with Elisp syntax by default.

I haven't studied the correlation with tags. I don't expect the tags to give much information since just about any question could have a Lisp snippet in an answer.

Answer 2

Clearly there's no reason for syntax highlighting to be unavailable here. So I turned it on.

That said, it's currently off by default unless a language is explicitly specified. My gut feeling is that it'd make more sense to format all code as lisp by default, and force folks to override that when using code blocks for anything else - but after reviewing the numbers Gilles came up with in his answer, it doesn't appear this would be a clear win either.

So for now, use the language hints (you may have to re-save existing posts that used them, since they're evaluated when the post is saved). If, on down the line, this becomes too much of a burden... We can always turn it on everywhere by default.

Answer 3

It seems that syntax highlighting simply doesn't work on emacs.stackexchange.com (I added the <-- language: foo --> blocks everytime):

C:

int main()
{
   printf("not working for c");
}

Python:

def foo():
    print "not working for python"

Ruby:

def foo
    print "not working for ruby"
end

Emacs-lisp:

(defun foo ()
  (message "not working for emacs-lisp"))

This is fairly annoying. When is it planned to enable it?