
Neil Mitchell (neildmitchell @🪡)
@ndm_haskell
Haskell/Rust programmer. Views expressed are my own.
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Pyrefly is awesome. Anyone using Pyright (the default Python support in Cursor) should switch today.
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.
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/…
us.pycon.org
High-Performance Python: Faster Type Checking and Free Threaded Execution (Sponsor: Meta)
Meta's Commitment to the Python Community: Empowering Innovation and Collaboration At Meta, we're dedicated to advancing the Python ecosystem through … Presented by: Sam Gross, Neil Mitchell
I'll be talking at this about Buck2
🎉 Build Meetup London - May 22 @ 11 AM! Expert insights on Bazel migration & scaling, Buck2, & build strategies. Plus, a hands-on unconference session. Register: share.hsforms.com/2-kAtpya7SouXm… ❤️ Hosted by EngFlow, @JaneStreetGroup, and @tweagio. #Bazel #Buck2 #BuildSystems #DevOps

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…
This Linux-kernel-RCU bug fought well: people.kernel.org/8q9a9dt4q3
Very cool talk on building D binaries with Buck2 - youtube.com/live/AzezZhvIy… by Max Haughton
youtube.com
YouTube
DConf '24 Day Two Livestream
I’ve done a lot of formal methods “technical sales” - trying to scope projects for Galois clients that solve their problem and fit into a budget. I gave a talk last week on some things that I learned along the way: mikedodds.github.io/files/talks/20…

Released the Haskell extra library 1.8, now with support for the upcoming GHC 9.12. Mostly a lot of handy list functions - hackage.haskell.org/package/extra/…
My Haskell Uniplate library got ported to Rust - crates.io/crates/uniplate. Allows doing traversals by type, eg making it easy to work with big syntax trees. I've been using these ideas in Rust a while (eg github.com/facebook/starl…) but happily someone made them a proper library!
I finally wrote down my thoughts on the design of the LSP, both good and bad: michaelpj.com/blog/2024/09/0…
Got starlark-rust working with WASM/JS, so you can now run Starlark (approximately simplified Python) in your browser. Based on code from april.dev. PR at github.com/facebook/starl….

Haskell XML parsing library `hexml` (github.com/ndmitchell/hex…) gets an update after 6 years! Now supports <![CDATA[ ]]>, thanks to a contribution from mniip.com. Hexml is a very fast XML parser written in C, designed for easy wrapping as a fast Haskell XML library.
My 11yo made a platformer scratch game - scratch.mit.edu/projects/10098… - requires keyboard control. Finally a scratch game from him with a reasonable difficulty curve!
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