Comment AST element?

[jondo]

I am using Pandoc 1.13.0.1 to convert Markdown to LaTeX. My Markdown documents contain comments of the form <!-- some comment -->, that are currently stripped by Pandoc.

It would be useful to create LaTeX comments instead: % some comment.

[rgaiacs]

Hi @jondo,

thanks for the feedback.

Pandoc's internal AST doesn't have an representation for comments. For example, in HTML

$ pandoc -f html -t json <<EOF
<!-- Foo -->
EOF
[{"unMeta":{}},[]]

and in LaTeX

$ pandoc -f latex -t json <<EOF
% Foo
EOF
[{"unMeta":{}},[]]

Because of this there is no to preserve comments right now.

[jgm]

Exactly. Comments are just represented as raw HTML, so they don't appear in non-HTML formats. In principle, pandoc could add a native Comment element, but this would be quite an involved change. (Every reader and writer would need to be modified.) And even if we did this, we'd get complaints if we made it the default to convert comments to the target format, since some people may be relying on this not happening.

[mhkeller]

I just came across this since I'm looking for similar functionality. To address one of your points, a flag like --preserve-comments could keep it hidden for most users. My use case is I want to convert markdown files to word docs so people can more easily share them among non-markdown users.

If at all possible, this would be a valuable feature.

I am also after preservation of comments across formats. My current interest is in having latex comments converted to comments in odt output. Issue #1561 raises the related point of annotations, but I'm not sure how that could be applied to tex formats.

[philbarresi]

As for:

In principle, pandoc could add a native Comment element, but this would be quite an involved change. (Every reader and writer would need to be modified.)

and:

To address one of your points, a flag like --preserve-comments could keep it hidden for most users

If someone can point me in the right direction, I can take a crack at these; comments would be a massive bonus for me, personally.

[jgm]

You could write a pandoc filter that passes your HTML comments through to latex as latex comments. Something like ``` import Text.Pandoc.JSON main = toJSONFilter commentHtmlToTeX commentHtmlToTeX :: Block -> Block commentHtmlToTeX (RawBlock (Format "html") ('<':'!':'-':'-':xs)) = RawBlock (Format "latex") (unlines $ map ("% "++) $ lines $ take (length xs - 3) xs commentHtmlToTeX x = x ``` This would allow you to pass through comments without any change in pandoc itself.

[bamcdougall]

JGM--That is a clever solution!

[jrk]

(Transferring/merging from #3187 as requested.)

I realize most uses of Pandoc are one-way and display-format-oriented, but it is such a rich transformation system that it can be very valuable to capture all available information where possible in readers, rather than dropping or flattening it during reading.

Pandoc readers already do this to a very large degree, and even for comments, the Markdown reader reads comments as raw HTML blocks which can be suppressed by default when going to other targets. The LaTeX reader, however, does not seem to have a way to preserve comments.

It would be useful for (in my case) LaTeX <==> (extended) Markdown round tripping to be able to capture comments when reading LaTeX. I'm glad to handle them in my own desired way using my own filter scripts, but even that is not currently possible since they're elided on reading. It's not obvious why they could not also be parsed as raw TeX strings, starting with %, just as Markdown/HTML comments are raw HTML nodes internally enclosed by the text <!--…-->.

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In short, as a useful half-step still well short of supporting comments as a general new node type, it would be valuable for the LaTeX reader/writer, in particular, to support comments as Raw TeX blocks as Markdown/HTML do with raw HTML comments.