Neil Mitchell (neildmitchell @🪡)

Pyrefly is awesome. Anyone using Pyright (the default Python support in Cursor) should switch today.

@swyx @mattpocockuk @jeremyphoward And it's really fun to watch all the cool stuff you've done since!

@mattpocockuk @jeremyphoward due to a weird quirk of my finance career i was basically a desk quant who somehow ended up getting mentored by @ndm_haskell before i ever learned javascript OR python. still one of my favorite jobs ever

For when I need to make a REALLY good prompt I'm going to start using this function

@seanparsons 30 million. Getting to a reproducible example was about a week long exercise from a colleague. I managed to reduce it to 500 lines, but not any shorter.

@ndm_haskell I'm sorry, a how many line example?

Today I spent 8 hours to change 2 lines of code (github.com/facebook/pyref…). That involved: * 30,342,810 line example that caused a crash * 97 compilations * 1.3Gb of log files * 6 historic diffs * 3 computers * 2 whiteboards of notes It's like a day long logic puzzle :)

New-ish @galois blog: “What Works (and Doesn't) Selling Formal Methods”. The boring truth: engineers are rational and adoption is all about cost/benefit tradeoffs

We landed a Zed extension for `pyrefly`, @facebook's new Python language server written in Rust. "Type check over 1.85 million lines of code per second" and receive other goodies, like go-to-definition.

🚀New Project Alert🚀 We recently launched an open source project, Pyrefly. It's a new type checker and IDE experience for Python. Lightweight, accurate, and designed for scale, Pyrefly is an excellent fit for your Python workflow! Learn more: engineering.fb.com/2025/05/15/dev…

Pyrefly (our Python type checker) got a blog post (engineering.fb.com/2025/05/15/dev…) and podcast (open.spotify.com/episode/4JhEid…) today. Available from pyrefly.org if you want to give it a try.

@dsp_ @charliermarsh They both aim to follow the same spec, so shouldn't follow that path.

@ndm_haskell Curious how this is going to fit into what @charliermarsh is working on with TY pypi.org/project/ty/. I hope it doesn’t end up like flow and typescript

Heading off to PyCon to talk about Pyrefly, the Python type checker I've been working on for the past 9 months. us.pycon.org/2025/schedule/…

@noteed Don't think so, although restrict might possibly work - but it mostly applies at the value level

@ndm_haskell Is it possible to configure HLint to warn when IO appears in a type signature? I don't see something similar in #restricting-items" target="_blank" rel="nofollow noopener">github.com/ndmitchell/hli…

I'll be talking at this about Buck2

@yawaramin @SuppaYami Not yet. I'll try and get something added to the FAQ

@ndm_haskell @SuppaYami Is there something written up about why this rewrite was needed?

TypeScript's three main competitors: Flow, Haxe, and ReScript--are all implemented in OCaml. TypeScript is kind of the odd one out by using Go...

@yawaramin @SuppaYami But the in-development successor pyrefly.org is written in Rust

@SuppaYami Doesn't change the language they're implemented in. Incidentally the main Mypy competitor, Pyre, is also written in OCaml pyre-check.org

@ChShersh Thanks for the invite! It was great fun and amazing views from the office.

I had a great pleasure hosting @ndm_haskell at Bloomberg office in London! Neil talked about Buck2, and I learned a lot about challenges of solving build tooling at scale. On a personal note, it feels amazing to meet Haskell legends in real life.

Rust 1.85.0 has been released! 🌈🦀✨ Not only does this release add async closures, it also includes a whole new Rust Edition, Rust 2024! 🎆🚀 Check out the blog post for an overview of all the changes and additions: blog.rust-lang.org/2025/02/20/Rus…

HLint (Haskell linter) v3.10 out, which now works with GHC 9.8-9.12. A few little hints here and there too. hackage.haskell.org/package/hlint

I don't recommend developing two build systems, and then switching from one to the other, but that's what we at Jane Street accidentally did. And what a journey it was! In the end, we're finally using Dune to build all our 70M+ lines of OCaml & Python 🙌 blog.janestreet.com/how-we-acciden…

@tomjaguarpaw I haven't finished debugging it yet. github.com/ndmitchell/sha… shows the things I tried and the currently active failure.

@ndm_haskell Interesting! Is the code public so I can see what broke? Or are there more details you can share about how the change broke your code?

Hi Haskell fans, have any of your come across any non-trivial changes needed to upgrade your code to GHC 9.12? I haven't found any so far!