[v4] Math syntax: GFM vs Pandoc based

[yhatt]

Marp Core is following a lot of features from GitHub Flavored Markdown, such as table and strikethrough. And today, GitHub has started Math support powered by MathJax: $ax^2+bc+c$ https://github.blog/2022-05-19-math-support-in-markdown/

$$ I_{xx}=\int\int_Ry^2f(x,y)\cdot{}dydx $$

$$ f(x) = \int_{-\infty}^\infty \hat f(\xi)\,e^{2 \pi i \xi x} \,d\xi $$

Marp has already supported MathJax math rendering, and in upcoming stable release v3, MathJax will become a default math library on behalf of KaTeX.

On the other hand, Marp's math syntax spec is actually based on tex_math_dollars Pandoc extension, and it has some of heuristic parsing:

The opening $ must have a non-space character immediately to its right, while the closing $ must have a non-space character immediately to its left, and must not be followed immediately by a digit. Thus, $20,000 and $30,000 won’t parse as math. If for some reason you need to enclose text in literal $ characters, backslash-escape them and they won’t be treated as math delimiters.

_— https://pandoc.org/MANUAL.html#extension-tex_math_dollars_

GitHub's Math syntax parsing is just simpler than Marp/Pandoc, and all of these rules are not applicable to GitHub Flavored Markdown. For example, backslash-escape \$ is working in Marp but not in GitHub. GitHub docs is instructing to use <span> raw HTML tag to escape (https://docs.github.com/ja/get-started/writing-on-github/working-with-advanced-formatting/writing-mathematical-expressions#writing-dollar-signs-in-line-with-and-within-mathematical-expressions):

To split <span>$</span>100 in half, we calculate $100/2$

Following GFM spec might make high compatibility with Markdown document, that have popular syntax used by a familiar platform.

This is just an idea, and not yet decided to take GFM spec. If changed, we should work carefully because exist math slides may become broken.