Thank you for your insight.

1. I'm not 100% satisfied with your solution since it does not work with 
latex environments which open math-mode (such as a align). These produce 
errors because of the double-opening of math-mode. It's also pretty 
cumbersome compared to usual md.

I see that this is how pandoc parses typst equations (I should have looked!)
[ Math
DisplayMath "\\begin{aligned}\n1 + 1 & = 2 \\\\\n1 + 1 & = 
2\n\\end{aligned}" ]
Is that now going to be the accepted format for multiline equations?

2. I had overlooked the possibility of writing a filter.
The principle of the filter would then be:

parse DisplayMath
if there are & and/or \\ present, modify the block
depending on writer:
- latex, HTML: transform to align
- everything else: transform to accepted format(??) \begin{aligned}

Is that correct? If so, I'll start hacking at it in the near future.

Best regards,
Guillaume
On Thursday, October 19, 2023 at 6:11:41 PM UTC+2 John MacFarlane wrote:

> > Pandoc provides a solution for almost all of these. The only issue is 
> multi-line equations.
> > A work-around I am aware of is to just use raw latex code like:
> > ```latex
> > \begin{align}
> > 1 + 1 &= 2 \label{first_line} \\
> > 1 + 2 &= 3 \label{second_line}
> > \end{align}
> > ```
> > which works fine when exporting to both HTML and latex: it is added as 
> is to the HTML where it is parsed by both mathjax and katex.
>
> Note that there's another way to do this that will create a native Math 
> element in pandoc:
>
> $$
> \begin{aligned}
> 1 + 1 &= 2 \\
> 1 + 2 &= 3
> \end{aligned}
> $$
>
>
> > However, I propose that formatting equations like typst might be a good 
> way to make multiline math native to Pandoc, instead of piggy-backing on 
> raw latex.
> > 
> > Multiline equations could be written as in latex, wrapped in `$$ $$`:
> > ```
> > $$
> > 1 + 1 &= 2 \\
> > 1 + 2 &= 3 
> > $$
> > ```
> > 
> > which could be parsed as a new DisplayMathMultiline type in the AST.
> > (This is a mockup: I really don't understand how the AST is written)
> > ```
> > [ Math DisplayMathMultiline
> > [
> > Math DisplayMathLine ["1+1", "=2"]
> > Math DisplayMathLine ["1+2", "=3"]
> > ]
> > ]
> > ```
> > 
> > This type can be written to:
> > - HTML and latex: as an `align` block
> > - typst: as a multiline math block
> > - pandoc markdown: as a multiline math block
> > - other: as multiple successive displayMath blocks
> > 
> > I do not see how modifying the AST in this way would produce an issue: 
> it's an additive feature as far as I can tell.
>
> Any change to our algebraic data types is a breaking change that would 
> require modifications throughout the ecosystem (all writers and readers, 
> filters, etc.). That's why we're very conservative about such changes and 
> make them infrequently and only with very strong reasons.
>
> I suppose one possibility would be to parse this syntax into
>
> \begin{aligned}
> 1 + 1 &= 2 \\
> 1 + 2 &= 3
> \end{aligned}
>
> which should work with all the formats you mention...
>
> You could even create a filter that does this, without changes to pandoc 
> itself.
>
>
>

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