Pony

Last Week in Pony - June 14, 2026

Sean T. Allen — Sun, 14 Jun 2026 07:00:06 -0400

This week's theme song is The Rubberbandits' "Horse Outside", and it's completely brilliant. It's a man at a wedding telling everyone to keep their cars. Fuck your Honda Civic, he's got a horse outside. Pony spent the week in that exact mood. Fuck your external linker, it's gone. Fuck your second build system, ponyc compiles your C shims itself now. That last one came together so fast it just felt good. Pony is the fucking horse outside.

Last Week in Pony - June 7, 2026

Sean T. Allen — Sun, 07 Jun 2026 07:00:06 -0400

This week's theme song is Willie Nelson's "Uncloudy Day". Last week shipped with some brimstone in it, and this week the skies cleared. No releases went out, but main kept filling up, and the headline is that RFC 86 is implemented. The stdlib json package now has a way to turn any JsonValue back into a string.

Last Week in Pony - May 31, 2026

Sean T. Allen — Sun, 31 May 2026 07:00:06 -0400

This week's theme song is 16 Horsepower's "Black Soul Choir", all banjo and brimstone, because we shipped a release with some brimstone in it. ponyc 0.64.0 is out. Three breaking changes, two long-standing compiler bugs put to bed, and the recursive type alias work all landed in one drop. The whole networking stack moved over with it. Red also shipped the first release of a timezone library.

Pony's Errors Stop Unwinding the Stack

Sean T. Allen — Tue, 26 May 2026 14:30:00 -0400

Here's how it is: for a few years, off and on, I went looking for a bug.

My aarch64 testing machine, a sturdy little Raspberry Pi, was the site of so very many segfaults. The same tests would fail run after run, and I could boil the crash down to a handful of lines of try and error. It came out of the machinery Pony uses to unwind the stack when error fires. So I'd pull that code up and compare it to the spec. It looked right. I'd run out of leads and put it down. Months later I'd pick it up and start over.

I never found it.

I kept coming back to it. I'd talk it through with Sylvan, and we'd end up in the same place. I'd talk it through with Joe, and we'd end up there too. The bug probably isn't ours. It's probably down in the guts of LLVM, somewhere we don't own. Probably.

So I stopped chasing the bug and looked at where it lived: the stack unwinding. Pony doesn't have to unwind the stack to raise an error. Take the unwinding away, and the bug has nowhere left to be. Soon, that's how it'll work — there's a pull request open against the compiler that takes stack unwinding out of Pony's errors, and that bug is part of why I wrote it. But only a part.

Round and Round

Sean T. Allen — Sun, 24 May 2026 14:00:00 -0400

What comes around goes around. I'll tell you why, why, why, why.

That is, near enough, how the Pony compiler's subtype check works on a type that refers to itself. It follows the type down into its own structure, and for an ordinary self-referential type it soon comes back around to a pair it is already checking further up. That return is the exit. What comes around closes, and the check finishes.

This is a story about two types where the check went down and never came back around. One of them sent the compiler into a loop with no end. The other ran it off the end of its stack and knocked it over. One was reported in 2016, the other in 2021, and both closed in a single pull request.

Last Week in Pony - May 24, 2026

Sean T. Allen — Sun, 24 May 2026 07:00:06 -0400

One theme song wasn't going to cover this week, so you're getting three, and all of them are gospel. PR 5246 merged. Finite recursive type aliases are in ponyc, and the oldest open issue in the repository, eleven years on the books, is closed. I have been buried in this for weeks. It's done. So we open with Johnny Cash, "It Was Jesus" and "Swing Low, Sweet Chariot", and then the Blind Boys of Alabama, "Jesus Gonna Be Here Soon". Hallelujah. It was a glorious week and I'm in the mood to celebrate.

There's more than the merge. Three blog posts went up telling the whole story behind the alias work, and we put our stance on AI-assisted contributions in writing. An exploratory port of Pony to Haiku opened, which made two old BeOS hands very happy. And the old HTTP libraries are on their way out.

Making Finite Recursive Type Aliases Compilation Fast

Sean T. Allen — Thu, 21 May 2026 21:00:00 -0400

This is the third post in a short series on finite recursive type aliases in Pony. The first post told the story of the eleven years it took to allow them. The second post laid out the algorithm that decides which recursive aliases are legal.

That algorithm is correct. Correct isn't enough. Past a certain size, a tangle of type aliases that all refer to each other sent the compiler's type checker into exponential work. Slow at first. Then, on a bigger tangle, eleven minutes of churning with no end in sight. The compiler wasn't rejecting these programs — the algorithm accepts them. It just couldn't finish checking them in any reasonable time.

Inside Pony's Finite Recursive Type Aliases

Sean T. Allen — Sun, 17 May 2026 20:35:00 -0400

This is the second post in a short series on finite recursive type aliases in Pony. The first post told the story of why this took eleven years. As I write this, the pull request that adds the feature is open and in review on ponyc. It hasn't merged yet. Details may shift before it does, but everything in this post is foundational. It should all hold.

So how does the compiler tell a finite recursive type alias from an infinite one?

The algorithm is Tarjan's strongly connected components. I'll walk through it. Then I'll show you the two checks I built on top of it.

Eleven years to a finite recursive type alias

Sean T. Allen — Sun, 17 May 2026 16:00:00 -0400

There's a pull request open against the Pony compiler. It's in review right now. When it lands, and it will land soon, you'll be able to write this:

```pony use "collections"

Today the compiler rejects it. JsonValue mentions Array[JsonValue], which mentions JsonValue, and ponyc throws up its hands: type aliases can't be recursive. That has been true for the entire history of the language. It's about to stop being true, and the pull request that changes it closes out the oldest open issue in the ponyc repository.

Last Week in Pony - May 17, 2026

Sean T. Allen — Sun, 17 May 2026 07:00:06 -0400

This week's theme song is "You Wreck Me" by Tom Petty. Tom Petty! It's been on rotation while I've been buried in PR 5246, the finite recursive type aliases work. That's the big job I warned you about last week. The one eating my coding hours and keeping the Pony news drumbeat quiet. Tonight we ride. Because... Tom Petty!

Quiet doesn't mean nothing. A new blog post went up on why I pulled documentation generation out of the compiler. Office Hours had Adrian taking a CLI-based LLM tool for its first spin. And I filed an official RFC request for someone to design optimization options into ponyc.

pony-doc: From the back pew to the front pew

Sean T. Allen — Wed, 13 May 2026 22:30:00 -0400

ponyc --docs is how you used to generate Pony API documentation. For more than a decade, you'd run that command, point it at a package, and the compiler would write you documentation in MkDocs-compatible format. It was quiet. It was reliable. It was boring.

Last month I deleted the documentation pass from the compiler. pony-doc, a separate Pony program, generates Pony documentation now. It creates the same output as the old documentation pass did. We switched all our sites over from one to the other and no one noticed.

So why move documentation generation out of the compiler? Why do work that has no discernible change for the user? Why now, after a decade of ponyc --docs working just fine?

Last Week in Pony - May 10, 2026

Sean T. Allen — Sun, 10 May 2026 07:00:06 -0400

This week's theme song is "My Bucket's Got a Hole in It" by Tim Timebomb. Looks like Pony's bucket has sprung a leak this week, doesn't it? It hasn't.

You've been getting a steady drumbeat of Pony news for months now. That drumbeat goes quiet this week, and it'll stay quiet for a stretch. Don't read it as the momentum dropping off. The water's all running into one place — a big job that's eaten most of my coding hours, more on it below.

pony-lint: Codifying the Style Guide

Sean T. Allen — Mon, 04 May 2026 08:00:00 -0400

A couple months ago I wrote about teaching Claude to write Pony. The shorthand version: treat the LLM like a junior developer with no memory, build up a CLAUDE.md, refine it as you find where Claude falls short. The CLAUDE.md got Claude most of the way there.

The piece it never quite covered was the style guide. There's one for the projects under the ponylang GitHub org. It has rules for indentation, line length, naming, where blank lines go. Claude would write code that compiled and worked but didn't follow the rules. Indentation that should have been two spaces would land on three. A type name would pick up an underscore. A function that belonged on three lines would end up jammed onto one. Stray trailing whitespace. Each one I'd correct. And correct it again. And again.

Last Week in Pony - May 3, 2026

Sean T. Allen — Sun, 03 May 2026 07:00:06 -0400

This week's theme song is "Voodoo Child" by Monica Valli. Trust me on this one.

Big release week. ponyc 0.63.4 ships two new pony-lsp features. Signature help pops up the parameters of the method you're calling and highlights the one you're filling in. Type hierarchy navigation lets your editor walk between a type, its supertypes, and its subtypes. The release also fixes a link failure on Fedora-family distributions, a multilib crt1.o gotcha, and a match exhaustiveness hole on Bool tuples. The RFC front was busy too: three new proposals and one across the finish line. Let's get into it.

Last Week in Pony - April 26, 2026

Sean T. Allen — Sun, 26 Apr 2026 07:00:06 -0400

This week's theme song is from the man himself, Johnny Cash: "Take Me Home". I've been on the road for two weeks and I'm finally back home, so this one fits.

Plenty to dig into. ponyc 0.63.3 ships another round of pony-lsp work and tightens the runtime's memory ordering on its hottest queues. ponylang/ssl picked up OpenSSL 4.0.x support and the matching builder image is live. There's also Ubuntu 26.04 support, a new blog post on the design of pony-lsp, and a couple of RFC moves to chew on. Let's get into it.

pony-lsp: An Actor and a Callback

Sean T. Allen — Sat, 25 Apr 2026 08:00:00 -0400

Embed You a ponyc for Great Good introduced libponyc-standalone, a static compiler library you can link your tools against, and a Pony wrapper called pony-ast that exposes the compiler as a callable function. This post is about the Pony language server we built on top of it.

A quick disclaimer before I get going. Almost none of pony-lsp is my work. Matthias Wahl built it from scratch. He wrote the actor architecture, the message dispatch, and the original feature set. He also wrote pony-ast. Orien Madgwick has been pushing it forward; most of the new features over the past several months are his. My contribution is mostly fixing things that broke when I imported the project into ponyc, plus a small feature here and there. The clever stuff is theirs.

Last Week in Pony - April 19, 2026

Sean T. Allen — Sun, 19 Apr 2026 07:00:06 -0400

Two theme songs this week. The topical pick is "New York Groove", because I spent most of the week back in NYC. Apropos. But the real theme song is "Dollar Bill Bar" because OMFG, I love that little damn song so much. You go Sierra. You fucking go. Right, umm, where was I. Oh yeah. Pony shit.

Big week. ponyc 0.63.2 is out with a pile of pony-lsp work that makes your editor a lot more useful. ponylang/postgres 0.5.0 is a security-hardening release that closes a SCRAM mutual-authentication bypass, requires SCRAM by default, and routes a stack of protocol failures to your application instead of crashing the driver. contact-red/sensitive shipped its first release and gives you a clean way to keep secrets out of your log files. And three networking libraries picked up timer failure callbacks so you actually find out when your timer subsystem has fallen over. Let's dig in.

Embed You a ponyc for Great Good

Sean T. Allen — Tue, 14 Apr 2026 08:00:00 -0400

The ponyc command you run every day is a main() function with a terminal-width detector glued to it. The actual compiler is a library called libponyc. ponyc is a wrapper around that library, and the wrapper is 149 lines of C.

That's the setup for this post. The Pony compiler is a library and you can link against it. And because you can link against it, you can build your own tools. And if you want your tool to be one binary instead of a ball of loose dependencies, you want libponyc-standalone.

Last Week in Pony - April 12, 2026

Sean T. Allen — Sun, 12 Apr 2026 07:00:06 -0400

I'm going to be traveling a lot during the next couple weeks, so our theme song this week is Willie Nelson's "On the Road Again". A true classic. And for a true story: once upon a time I was a young child and met another kid while we were both sitting on Willie's lap at his 4th of July picnic. Years later, I met him again at a bar in NYC when he came up from Texas to hang out with a good friend who was one of my best friends. Small world, right? Willie Nelson, bringing together awesome people since the time of the dinosaurs.

All that aside, we have some great stuff as usual happening in Pony land. ponyc 0.63.1 is out and has safety-related fixes you need. ponylang/postgres 0.3.0 is a monster of a release. A pile of networking libraries went out that you'll want to grab. And there's a whole pitch to make about LLMs and Pony. Let's dig in, because it all just fucking rocks. Get on board this train while there is still time. That's what I'm saying. Get the fuck on board. Oh, and Cloudsmith has throttled us until the end of the day today UTC so, sorry, you can't download any prebuilt binaries until that has lifted.

Claude You Some Pony for Great Good

Sean T. Allen — Fri, 10 Apr 2026 08:00:00 -0400

Here's how it is:

Two months ago I wrote about teaching Claude to write Pony. That post was about the process — mentoring an LLM like a junior developer, building up engineering principles, establishing review loops. Today, I'm here to talk about the LLM skills I've been working on. They're in ponylang/llm-skills, and if you're working on Pony with an LLM, you should try them.