Switch to oxidized flow parser by SamChou19815 · Pull Request #19398 · prettier/prettier

SamChou19815 · 2026-06-17T15:14:40Z

Description

This PR switches over the flow parsing in prettier to the upcoming flow parser powered by wasm based on our rust port.

Background

Historically, Flow is always written in OCaml, which has the ability to be compiled to JS. The current flow-parser used by prettier is produced by js_of_ocaml which is serious perf issues. We are unhappy about the perf, so we also separately maintain hermes-parser powered by wasm compiled from hermes c++ code. So Prettier also ends up with both flow-parser and hermes-parser which are mostly in sync but not exactly in sync.

Current Status

We have completed our rust port and we have just released flow-bin based on the rust port binaries. We have also released the rust port based parser in 'flow-parser/oxidized' (while the ocaml based one still remain importable via 'flow-parser' before we migrate the Prettier use case away from it). 'flow-parser/oxidized' aims to be mostly a drop in replacement for hermes-parser, apart from a few newer syntax we haven't ported to hermes-parser yet and a few expected AST shape changes.

I believe now 'flow-parser/oxidized' is good for prettier use, so it will eventually just becomes 'flow-parser'. At that point, we will stop updating the hermes-parser package, and I believe the hermes-parser based plugin in Prettier will also stop being useful.

This PR

The code changes were initially generated by AI using the prompt

bump flow-parser to latest, and use flow-parser/oxidized instead of just flow-parser. the new one has similar api to the hermes-parser.

It went through several rounds of revision, and it's now a clean replacement except a few parsing error differences.

Checklist

I’ve added tests to confirm my change works.
(If changing the API or CLI) I’ve documented the changes I’ve made (in the docs/ directory).
(If the change is user-facing) I’ve added my changes to changelog_unreleased/*/XXXX.md file following changelog_unreleased/TEMPLATE.md.
I’ve read the contributing guidelines.
I did not use AI to generate this PR.
(If the above is not checked) I have reviewed the AI-generated content before submitting.

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pkg-pr-new · 2026-06-17T15:16:16Z

Open in StackBlitz

yarn add https://pkg.pr.new/@prettier/plugin-hermes@19398.tgz

yarn add https://pkg.pr.new/@prettier/plugin-oxc@19398.tgz

yarn add https://pkg.pr.new/prettier@19398.tgz

commit: dd9a379

fisker · 2026-06-17T19:01:12Z

Any plan to ship ESM version?

SamChou19815 · 2026-06-17T20:24:40Z

Any plan to ship ESM version?

This would be something we could do in the future, but right now we are just focusing on shipping the rust port powered the one and delete the ocaml code.

with this we can kill some of the ugly patches. if it's good we can eventually remove these vendoring and cut a new release on the flow side

SamChou19815 · 2026-06-19T21:47:06Z

In the latest version with a temporarily checked-in latest build of flow-parser, we are able to completely remove those ast patching hack in earlier commits. The newly disabled tests are due to the intentional decision of dropping support for special casing comment-syntax to align with hermes-parser, and hermes-parser have same behavior on these two files.

@fisker let me know if the implementation looks good for you. If it's good, we can cut a new parser release on the flow side so that we can remove the temp vendor and have a proper PR.

fisker · 2026-06-19T22:33:57Z

-        // https://github.com/facebook/flow/blob/HEAD/Changelog.md#03130
        case "TypeParameter":
-          if (astType !== "flow" && node.bound?.type === "TypeAnnotation") {
+          if (node.bound?.type === "TypeAnnotation") {


This change still needed? If it's needed we can just revert #19127

fisker · 2026-06-19T22:40:32Z

Looks good except #19398 (comment)

fisker · 2026-06-21T10:56:26Z

How should we process?

We are close to release v3.9, should we include this in v3.9 or merge after it?

What's the future of hermes-parser? Should Prettier deprecate support for it?

SamChou19815 · 2026-06-21T13:00:58Z

We are close to release v3.9, should we include this in v3.9 or merge after it?

I don't have a strong opinion on this: we are not in a rush wanting a new release. On the Flow side, we are mostly interested in deleting the old OCaml powered parser code, so once we have the confirmation on the Prettier side that this PR is good we will go ahead and delete the old OCaml based parser.

However, based on my understanding of the changes in v3.9, this PR does fit in. It is probably far less disruptive than the other formatting changes.

What's the future of hermes-parser? Should Prettier deprecate support for it?

We have already de-facto stopped adding new syntax support to hermes-parser for a while, knowing that the Rust based flow-parser will be available soon. For example, you can see that syntax support like abstract class was added to flow-parser but never made it to the hermes-parser.

Therefore, given these facts on the ground, Prettier should probably deprecate support for it.

fisker · 2026-06-22T04:49:51Z

Can you fix this script? https://github.com/prettier/prettier/blob/main/scripts/generate-flow-estree-type-definition.js

fisker · 2026-06-22T06:41:53Z

@SamChou19815 Can you add a changelog entry? Instructions

It would be great if there were some data about performance improvements.

[experimental][wip] Switch to oxidized flow parser

b7e7701

SamChou19815 commented Jun 17, 2026

View reviewed changes

Comment thread src/language-js/parse/postprocess/index.js Outdated

Comment thread src/language-js/parse/flow.js Outdated

SamChou19815 added 3 commits June 17, 2026 11:37

fix

2b1ee3b

shim fix

d55c219

disable from failing tests for now

576996c

SamChou19815 added 3 commits June 19, 2026 17:02

include temporary vendoring of latest flow-parser.tgz built from flow ci

de0d8ae

with this we can kill some of the ugly patches. if it's good we can eventually remove these vendoring and cut a new release on the flow side

temp hack to include the vendor files, will revert eventually

69250b3

re-add to unstable tests

7317299

fisker reviewed Jun 19, 2026

View reviewed changes

SamChou19815 added 2 commits June 19, 2026 21:16

revert 99bdbf3

f62845f

switch to proper 0.320.0 release

4a74da9

SamChou19815 changed the title ~~[experimental][wip] Switch to oxidized flow parser~~ Switch to oxidized flow parser Jun 21, 2026

SamChou19815 marked this pull request as ready for review June 21, 2026 02:29

github-actions Bot mentioned this pull request Jun 21, 2026

v3.9 prettier/prettier-regression-testing#702

Closed

fisker reviewed Jun 22, 2026

View reviewed changes

Comment thread scripts/build/esbuild-plugins/throw-warnings.js Outdated

SamChou19815 and others added 6 commits June 21, 2026 22:09

fix script

115c7fc

Clean

cc5a317

Minor tweak to build script

9f52497

Clean generate-flow-estree-type-definition.js

470d6d4

Add cause

9b9581e

Snapshots

a1a1d50

fisker added 4 commits June 22, 2026 14:23

Another one

71ee364

Revert Visitor Keys change

0f6ebcc

Update link

caa633e

Snapshot

5456ea5

fisker added this to the 3.9 milestone Jun 22, 2026

kachkaev mentioned this pull request Jun 22, 2026

Add blog post for v3.9 #19414

Merged

6 tasks

Create 19398.md

dd9a379

fisker merged commit 9c0e48c into prettier:main Jun 23, 2026
40 checks passed

SamChou19815 deleted the flow-parser-oxidized branch June 23, 2026 02:14

fisker mentioned this pull request Jun 30, 2026

Question: will these two AST changes apply to the oxidized parser? facebook/flow#9449

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