Support HTMLx

(I removed the issue boilerplate because it doesn’t seem to pertain to feature requests — hope that’s okay.)

@vjeux sent me here! On the Svelte project, we’re often asked if it it’s possible to use Prettier to format single-file components. At present the answer is ‘no, not reliably’, because we typically write those components in .html files, and components are written in a superset of HTML called HTMLx.

Since Prettier supports Angular and Vue flavours of HTML already, I wondered what it would take to support HTMLx in addition? (We’re considering adopting and promoting an .htmlx extension similar to .vue, if that’s necessary.) We’d obviously be happy to do the work, I’m just raising this issue to see if a PR adding support would be likely to be accepted.

(Note that HTMLx will change slightly for Svelte 3, out soon.)

[jamesbirtles]

I’ve made prettier-plugin-svelte. Is it preferred that things are built in to prettier, or provided as plugins? I don’t know how hard it would be to integrate what I’ve done into prettier but willing to give it a go if I find some time

[suchipi]

Assuming our HTML/Vue/Angular parser can already parse everything you need, add an entry for htmlx here and then adjust the printing to support htmlx-specific semantics in this file. The way the printer works is it returns an intermediate representation of the text called a “document” that is comprised of various nodes which are informally documented here. The logic here where we handle vue’s v-for attribute may be a good place to start.