marp-cli async rules

[GuillaumeDesforges]

In order to run some rendering, in my case renderMermaid from mermaid, one needs to be able to run async rules.

However markdown-it's author is very clear about async rules: they are not and won't be supported.

References:

• https://github.com/markdown-it/markdown-it/blob/master/docs/development.md#i-need-async-rule-how-to-do-it
• Async operations markdown-it/markdown-it#248

It states:

Alternatively, you can render HTML, then parse it to DOM

I've tried to make my own engine:

class PostprocessMarpitEngine extends Marpit {
    constructor(options, postprocess) {
        super(options)
        this.postprocess = postprocess
    }

    withPostprocess(postprocess) {
        this.postprocess = postprocess
        return this
    }

    async render(markdown, env = {}) {
        const { html, css, comments } = super.render(markdown, env)
        if (this.postprocess) {
            return await this.postprocess(markdown, env, html, css, comments)
        }
        return { html, css, comments }
    }
}

which I hope to use like so

export default {
    engine: async (constructorOptions) =>
        new PostprocessMarpitEngine(constructorOptions)
            .use(marpMermaidPlugin)
            .withPostprocess(async (markdown, env, html, css, comments) => {
                // do async things
                return { html, css, comments }
            }),
}

However the current implementation does not support an async render method.

I'm wondering if changing this line like so would be enough
https://github.com/marp-team/marp-cli/blob/8c7a09bfe8e983ca3bad709a15d42b5cf94ea4ac/src/converter.ts#L174

-        const ret = engine.render(stripBOM(markdown))
+        const ret = await engine.render(stripBOM(markdown))

I don't see any blocker, since the wrapping function is already async
https://github.com/marp-team/marp-cli/blob/8c7a09bfe8e983ca3bad709a15d42b5cf94ea4ac/src/converter.ts#L168C7-L168C15

Would this change be possible?

EDIT: I went ahead and proposed the change.
marp-team/marp-cli#540
I'll try to use it on my own project.

[yhatt]

Shipped as Marp CLI v3.2.1. However, keep in mind returning Promise in Marpit.prototype.render is still out of the spec of current Marpit framework. The custom engine with Promise will not become working on other Marp/Marpit tools that are strictly following API interface of the framework.

[GuillaumeDesforges]

Thanks for the reminder. What are the preferred steps to make it part of the spec?