The problem is that basically anything is valid Markdown,
it's just a matter of how it is interpreted.  It does seem
to me that in the second case we might want to at least emit
a warning -- if not downright fail.  It's just so unlikely
that someone would write a block beginning with `---`
and ending with `...` and intend it to be something other
than YAML metadata.  If you'd like, you could submit an
issue for this.  I'd rather not suggest avoiding complex
YAML blocks -- sometimes they're fine, if the syntax is
correct!

William Lupton <wlupton-QSt+ys/nuMyEUIsrzH9SikB+6BGkLq7r@public.gmane.org> writes:

> Thanks. Yes, I guess I realised it had something to do with ambiguous
> grammars, but I hadn't really considered that my input was a valid table
> (mea culpa).
>
> So I thought aha! I'll change the terminating "---" to "..." (in the hope
> of an error), but this of course gives the output shown below.
>
> I wonder whether there could be a warning in the manual (or maybe there is
> and I've missed it?) and/or perhaps a suggestion that it might be a good
> idea to avoid complex YAML metadata blocks (in favour of metadata files)?
> I'd be happy to create an issue and/or a MANUAL.md pull request.
>
> % cat bad.md
> ---
> history:
> - number: Release 1.2
>   changes: |
>   - TBD
> ...
>
> % pandoc bad.md
> <hr />
> <p>history: - number: Release 1.2 changes: | - TBD …</p>
>
>
> On Wed, 2 Jun 2021 at 17:47, John MacFarlane <jgm-TVLZxgkOlNX2fBVCVOL8/A@public.gmane.org> wrote:
>
>>
>> Well, this actually IS a valid "simple table".  So how is
>> pandoc to know that you intended it as a YAML metadata block?
>> Pandoc tries first to parse things as YAML blocks, and then as
>> tables, if either works, the parse succeeds with no error.
>>
>> William Lupton <wlupton-QSt+ys/nuMyEUIsrzH9SikB+6BGkLq7r@public.gmane.org> writes:
>>
>> > Should I expect syntax errors in YAML blocks to be detected and reported?
>> >
>> > This has a syntax error in the YAML block:
>> >
>> > % cat bad.md
>> > ---
>> > history:
>> > - number: Release 1.2
>> >   changes: |
>> >   - TBD
>> > ---
>> >
>> > This fixes it:
>> >
>> > % diff bad.md good.md
>> > 5c5
>> > <   - TBD
>> > ---
>> >>     - TBD
>> >
>> > The latest pandoc:
>> >
>> > % ~/Downloads/pandoc-2.14.0.1/bin/pandoc -v
>> > pandoc 2.14.0.1
>> > ...
>> >
>> > Pandoc thinks the bad YAML is a table:
>> >
>> > % ~/Downloads/pandoc-2.14.0.1/bin/pandoc bad.md
>> > <table>
>> > <tbody>
>> > <tr class="odd">
>> > <td>history:</td>
>> > </tr>
>> > <tr class="even">
>> > <td>- number: Release 1.2</td>
>> > </tr>
>> > <tr class="odd">
>> > <td>changes: |</td>
>> > </tr>
>> > <tr class="even">
>> > <td>- TBD</td>
>> > </tr>
>> > </tbody>
>> > </table>
>> >
>> > But it's OK with the good YAML:
>> >
>> > % ~/Downloads/pandoc-2.14.0.1/bin/pandoc good.md
>> >
>> > Parsing the file as YAML reports the error:
>> >
>> > % ~/Downloads/pandoc-2.14.0.1/bin/pandoc --metadata-file bad.md
>> > Error at "bad.md" (line 5, column 0):
>> > Unexpected ' '
>> >
>> > % ~/Downloads/pandoc-2.14.0.1/bin/pandoc --metadata-file good.md
>> > ^C
>> >
>> > --
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>> > To view this discussion on the web visit
>> https://groups.google.com/d/msgid/pandoc-discuss/CAEe_xxjF3Un2J-xg5UOs7uReZ5cO27TATA%2BfDY%2BAtghAsVhShA%40mail.gmail.com
>> .
>>

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