Move `prettier-plugin-hermes-parser` to Prettier repo

[fisker]

I'm working on adding hermes parser in Prettier repo prettier/prettier#17520, currently it passed all Prettier tests except some tests are disabled due to parser issues, I'll open issues about them.

I can see most changes are in this repo https://github.com/pieterv/prettier/tree/hermes-v2-backport ?

1. What do you think about moving to Prettier?
2. Still need Prettier v2 support? (This commit is turning async apis to sync)
3. Are there additional supports required from Prettier?
4. Are you willing to upstream changes? (Most likely hook and match syntax support)

[pieterv]

What do you think about moving to Prettier?

Moving to prettier upstream would be nice. Last time we discussed this the hermes-parser size was a blocker, is that no longer the case?

Still need Prettier v2 support?

Yes i believe we do but no reason we can't maintain a small fork with just that commit while we move to the latest (at some point in the future).

Are there additional supports required from Prettier?

I don't believe so, the plugin works with v3 today so we should have everything we need.

Are you willing to upstream changes? (Most likely hook and match syntax support)

Should be possible, we will just need to discuss as a team when we will have time to work on this. I assume it shouldn't be much work though since most stuff has been upstreamed.

[fisker]

is that no longer the case?

It will be published as a seperate package, need install and config step to make it work https://github.com/fisker/prettier/tree/hermes/packages/plugin-hermes#prettierplugin-hermes

[fisker]

@pieterv The diff from hermes-v2-backport branch seems small, should I pick the changes myself or have you decided how to process?

[pieterv]

should I pick the changes myself

Yes, if you can. That would be fantastic.

[fisker]

@prettier/plugin-hermes already published.

https://prettier.io/blog/2025/06/23/3.6.0

[fisker]

Hello, I copied code from hermes-v2-backport, but there are no tests included, so I copied some tests from https://github.com/facebook/hermes/tree/main/test/Parser/flow/match,

Looking at https://github.com/facebook/hermes/blob/main/test/Parser/flow/match/expression.js, the following code should be valid?

const e = match (a) {1: 1}

But when I pass to hermes-parser

> require('hermes-parser').parse('const e = match (a) {1: 1}')
Uncaught [SyntaxError: '=>' expected after match pattern (1:22)
const e = match (a) {1: 1}
                     ~^
] {
  loc: { line: 1, column: 22 }
}

Did I do something wrong?

PR on Prettier prettier/prettier#17681

[pieterv]

cc @gkz

[gkz]

So, the hermes team moved their main development to the static_h branch and haven't deleted the main branch yet. So whenever looking at the source, you have to make sure you are on static_h (I'm sorry, I know it is confusing).

https://github.com/facebook/hermes/tree/static_h/test/Parser/flow/match

[fisker]

Okay, thanks!

[fisker]

Everything works now prettier/prettier#17681 , thanks!

Looks like flow-parser also supports "matching" behind pattern_matching: true, but it's undocumented, is it missing in https://github.com/facebook/flow/tree/main/packages/flow-parser#options or intentionally not to?