Re: User story: typst multiline math in pandoc markdown and pandoc AST

[John MacFarlane <fiddlosopher-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

> 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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