Flow type checker being ported to rust
2 points · Open discussionmeta
flow
Rust2014–2026
meta / flow
flow
Adds static typing to JavaScript to improve developer productivity and code quality.
55Distinct people
64Mentions
1Explicit recommenders
22.3KGitHub stars
What to know before exploring
What it is
Adds static typing to JavaScript to improve developer productivity and code quality.
GitHub
Where it is used
No documented product use was verified in the last source check.
What people are saying
I'm using Flow but I'd recommend TypeScript. I love Flow but TypeScript just seems to be more seriously maintained and less buggy. I've been waiting some bug fixes for 2 years…
ggregoire used a recommendation phrase. 2 indexed comments match. This is a phrase match, not a verdict.
No classified warning is attached to this page. Read the discussions before treating silence as approval.
Also in these discussions
- microsoft/typescript · linked in 7 of the same discussions
- meta/infer · linked in 3 of the same discussions
- reasonml/reason · linked in 3 of the same discussions
Mentioned in the same Hacker News threads. That is not proof they are substitutes.
This week on Hacker News
Nov 7–Nov 14 UTC · Direct repository links and separately labeled related project stories. Indexed through 11/14/2026, 12:54:37 PM UTC.
No direct repository links were found in the indexed HN discussions over the past seven days. Older discussions are available below.
Mentions over time
HN mentions by year64 dated mentions
Observed mentions: 2014-11-18 18:29:48 UTC to 2026-06-07 04:00:52 UTC.
* An incomplete calendar period is marked with an outlined bar. Its count is not directly comparable with a full period.
Counts come from the repository’s dated source evidence. Index cutoff: 2026-11-02 08:16:59 UTC.
View mention counts and date ranges
| Period | Start (UTC) | End (exclusive, UTC) | Mentions | Submissions | Comments |
|---|---|---|---|---|---|
| 2014 | 2014-01-01 00:00:00 UTC | 2015-01-01 00:00:00 UTC | 5 | 0 | 5 |
| 2015 | 2015-01-01 00:00:00 UTC | 2016-01-01 00:00:00 UTC | 13 | 0 | 13 |
| 2016 | 2016-01-01 00:00:00 UTC | 2017-01-01 00:00:00 UTC | 11 | 1 | 10 |
| 2017 | 2017-01-01 00:00:00 UTC | 2018-01-01 00:00:00 UTC | 10 | 2 | 8 |
| 2018 | 2018-01-01 00:00:00 UTC | 2019-01-01 00:00:00 UTC | 9 | 0 | 9 |
| 2019 | 2019-01-01 00:00:00 UTC | 2020-01-01 00:00:00 UTC | 6 | 0 | 6 |
| 2020 | 2020-01-01 00:00:00 UTC | 2021-01-01 00:00:00 UTC | 1 | 0 | 1 |
| 2021 | 2021-01-01 00:00:00 UTC | 2022-01-01 00:00:00 UTC | 1 | 0 | 1 |
| 2022 | 2022-01-01 00:00:00 UTC | 2023-01-01 00:00:00 UTC | 0 | 0 | 0 |
| 2023 | 2023-01-01 00:00:00 UTC | 2024-01-01 00:00:00 UTC | 2 | 0 | 2 |
| 2024 | 2024-01-01 00:00:00 UTC | 2025-01-01 00:00:00 UTC | 2 | 0 | 2 |
| 2025 | 2025-01-01 00:00:00 UTC | 2026-01-01 00:00:00 UTC | 2 | 1 | 1 |
| 2026 (partial) | 2026-01-01 00:00:00 UTC | 2027-01-01 00:00:00 UTC | 2 | 2 | 0 |
From the project README
An excerpt from the repository, fetched Nov 13, 2026.
Read the project’s introduction
Flow
Flow is a static typechecker for JavaScript. To find out more about Flow, check out flow.org.
Contents
Requirements
Using Flow
Using Flow's parser from JavaScript
Building Flow from source
Join the Flow community
License
Requirements
Flow works with:
macOS (arm64)
Linux (x86_64 and arm64)
Windows (x86_64, Windows 10 recommended)
There are binary distributions for each of these platforms and you can also build it from source on any of them as well.
Using Flow
Check out the installation instructions, and then the usage docs.
Using Flow's parser from JavaScript
Flow's parser is available as a compiled-to-JavaScript module published to npm, named flow-parser. Most end users of Flow
will not need to use this parser directly, but JavaScript packages which make use of parsing
Flow-typed JavaScript can use this to generate Flow's syntax tree with annotated types attached.
Building Flow from source
Flow is written in Rust. GitHub CI builds Flow from the rust_port workspace with nightly Rust.
Install system dependencies:
Install Rust via rustup.
Install the nightly Rust toolchain:
rustup toolchain install nightly
Build the Rust workspace:
cd rust_port
cargo +nightly build
Run the Rust tests:
cargo +nightly test
Build an optimized flow binary:
cargo +nightly build --release --bin flow_cli
Build flow.js (optional):
The flow.js build also uses the Rust port. Install Emscripten, Node, Yarn, and a Rust nightly toolchain with the wasm32-unknown-emscripten target and rust-src component first. GitHub CI uses Emscripten 3.1.44 and nightly Rust for this build.
rustup toolchain install nightly-2026-04-14 --target wasm32-unknown-emscripten --component rust-src
RUSTUP_TOOLCHAIN=nightly-2026-04-14 make js FLOW_JS_IMPL=rust-wasm
This produces bin/flow.js.
To build a faster, larger local development version instead, run:
RUSTUP_TOOLCHAIN=nightly-2026-04-14 make js FLOW_JS_IMPL=rust-wasm FLOW_DOT_JS_WASM_PROFILE=dev
The Flow parser can also be compiled to JavaScript. Read how here.
Running the tests
See tests/README.md for detailed testing documentation.
Join the Flow community
Website: https://flow.org
Discord: https://discord.gg/8ezwRUK
irc: #flowtype on Freenode
Twitter: follow @flowtype and #flowtype to keep up with the latest Flow news.
Stack Overflow: Ask a question with the flowtype tag
License
Flow is MIT-licensed (LICENSE). The website and documentation are licensed under the Creative Commons Attribution 4.0 license (website/LICENSE-DOCUMENTATION).
Read the full README on GitHubWhat Hacker News says
64 sourcesFacebook's Flow is being ported to Rust
3 points · Open discussionThe circle of possible contributors doesn't really matter. It's a Meta project, they have others written in OCaml and to this day they manage to have contributors eg https://github.com/facebook/flow because they hire and pay them.
In Pyrefly: A new type checker and IDE experience for PythonFlow is much better with opaque types.
Also nominal types for classes.
And correct variance.
And adhering to liskov substitution principles.
And exact object types.
And spread on types matching runtime behavior.
And proper no transpilation mode with full access to the language.
And has 10x less LoC than ts.
ps. before somebody says "flow is dead" have a look at flow contributions [0] vs typescript contributions [1]
[0] https://github.com/facebook/flow/graphs/contributors
[1] https://github.com/microsoft/TypeScript/graphs/contributors
In TypeScript: Branded TypesIt's not as popular as typescript but not dead, it's consistently active for a decade [0].
Compare it with typescript [1] contributions if you want.
[0] https://github.com/facebook/flow/graphs/contributors
[1] https://github.com/microsoft/TypeScript/graphs/contributors
In Eloquent JavaScript 4th edition (2024)https://github.com/facebook/pyre-check
https://github.com/facebook/flow
https://github.com/facebook/infer
https://github.com/facebook/hhvm/tree/master/hphp/hack
In Should I Rust or Should I Go?I am still attracted by Flow which seems to have a simpler and stronger type system. However, Flow seems a bit unstable and the lack of support for TypeScript declaration files and widely used TypeScript syntaxes are stopping me from switching to Flow. I opened an issue for bridging the syntax gap between Flow and TypeScript [0]. Flow v0.201.0 renamed `$Partial` to `Partial` making Flow a bit more compatible with TypeScript. I hope it is just the beginning...
[0] https://github.com/facebook/flow/issues/8989
In TypeScript 5.0It seems like Facebook doesn't have problem with releasing open source project [0] while keeping centralised control over it.
Why not just say they want to monetize it?
Arguably even pure, single monetizing argument can be challanged as their adoption would increase tenfold, if not more and there are tons of ways to monetize popular technology.
[0] https://github.com/facebook/flow
In Why Wolfram tech isn’t open source (2019)You're comparing a tool which you have source for and control over with a service which is out of your control. The comparison is of apples and oranges.
Compare something like flow [1], reason [2], or prepack[3] as it's tools which serves you rather than the company behind it with access to the source which is the same as the case for weboob if you want to compare against facebook. I don't see an issue with using them though as they don't collect data on you.
[1]: https://github.com/facebook/flow
[2]: https://github.com/facebook/reason
[3]: https://github.com/facebook/prepack
[4]: https://github.com/majestrate/XD
In WeboobPlus Facebook with their Flow[1], Infer[2][3], Airbus with their BinCAT[4], and Frama-C[5].
[1] https://github.com/facebook/flow
[2] https://fbinfer.com/
[3] https://github.com/facebook/infer
[4] https://github.com/airbus-seclab/bincat
[5] http://frama-c.com/
In Machine and Deep Learning with OCaml NativelyI'd bet Flow is basically being replaced by ReasonML at Facebook[1]. Bolting a sound typesystem onto Javascript is really hard, and Flow is making a truly valiant effort, but it seems to have stalled out. Instead, ReasonML is based on starting from a "cleaned up" semantics and type system, then translating it into raw Javascript. This avoids all the weird edge cases and type inference problems one runs into when trying to use Flow[2].
(I don't ever have to have any `any` in my ReasonML code, but it was unavoidable with Flow.)
1. https://reasonml.github.io/blog/2017/09/08/messenger-50-reas...
2. https://github.com/facebook/flow/issues?utf8=&q=is%3Aissue+i...
In TypeScript vs. ReasonMLUgh yeah, the same thing happened with Flow. Massive immediate adoption by react developers and then FB basically saying "screw you" no roadmap and we're never going to add the features you want. I think I still have an open issue in the flow repo for a roadmap.
EDIT: haha here it is https://github.com/facebook/flow/issues/4785
In TypeScript vs. ReasonMLThis is actually huge and will probably only get better, especially considering now they own their whole JS stack: the framework (React/RN), the language (Flow [1]) and now the VM.
It sounds like Hermes is exclusively for running React Native on mobile. They could start building React directly into Hermes— the reconciling algorithm for example, or maybe even some real parallelism.
Very curious to see how all this keeps evolving.
[1]: https://github.com/facebook/flow
In Hermes – A small JavaScript engine optimized for running React Native on AndroidFacebook (excuse me, bunch of FB employees from the Flow team) says no: https://github.com/facebook/flow/issues/7365#issuecomment-45.... Of course, that's exactly what I'd say if I was in charge of a dying project.
Edit: but, their main codebase is in Flow, and that sounds like a mess to migrate, so I wouldn't be _too_ worried. It might slow down, but I doubt it will become unmaintained anytime before Facebook gets sued out of business :-P
In Yarn's Future – v2 and beyondShowing the 15 most recent of 64 sources. Enable JavaScript to search and browse every indexed mention.