New post: Test-case Reducers Are Underappreciated Debugging Tools. The more I've used these tools, the more ways I realise they can be used! This post starts at the basics and works up to things like reducing on properties other than just the length of the input. tratt.net/laurie/blog/...

A confluence of events meant that, unusually, I've travelled three times to the EU in the last month. The new Entry & Exit System has failed 5 times out of 6. Each country/airport implements it differently, some comically incompetently. One, clearly, was ignoring it.

pizauth-1.1.0 out with a number of features: systemd users will want to enable the `systemd` feature. I've also updated my web site with signed ssh signatures, so you can tell that I did (or didn't!) make a given release. tratt.net/laurie/src/p...

This weekend I visited The Herschel Museum of Astronomy in Bath: a small but perfectly formed museum in the building where he discovered Uranus. What I did not know is that he discovered infrared; if I had known, I still would not have expected to see the actual device he used!

My simple shell version of "ETA for rsync" found more users than I expected. After some useful suggestions, I've rewritten it in Rust, and it now shows sends (green)/dels (red) as it progresses, doesn't stall on big files, and has a (better) sliding-window ETA.

As a long-term sceptic of debuggers, I am still astonished at how often, when I have a weird problem, two minutes of rr (the reverse debugger) pinpoints the problem.

I haven't worked out a reliable way of detecting "AI writing", but there is one thing I really notice about "AI code": lots more error checks. This is, mostly, not bad, but some of the checks clearly can't trigger because of invariants elsewhere.

New version of hk: now with a test suite! tratt.net/laurie/src/hk/

rsync is great but doesn't provide ETAs. rsyncp is a simple tool to do just that: it remembers the previous rsync's elapsed time and total files and reuses those on the next run. Here's an example: on the first run it has no idea how long things might take but it does thereafter.

I've used rsync for many years, and these days use it heavily. Astonishingly today is the first time I've encountered an actual bug in it: it's excellent software (albeit with enough flags to sink a ship)! github.com/RsyncProject...

Dr Johnson on what made universities so effective (quote circa 1768).

Fundamentally, our software security setup has focussed on stopping external attackers, not autonomous actors already on the inside. I fear that we'll see more and more of this sort of thing occurring until we address that fact. x.com/lifeof_jer/s...

I am not quite sure what I did before `shellcheck`, but I think the answer is "a lot of my shell scripts were subtly broken".

C interpreters underlie many of our most widely used language implementations -- but they're slow. Wouldn't it be great if we could turn them into JIT compiling VMs? This video shows what happens when we do just that to the normal Lua VM (first) and "yklua" (Lua w/JIT, second).

Last week I upgraded the encryption I use to be post-quantum. I then realised that if enough people do the same, "using quantum f/decryptt" probably becomes a meaningless use case. Is there a word which captures the concept of "changing X effects the future of Y, even though X and Y seem distinct"?

At Wells this morning, we saw a Palm Sunday procession that - to the extent anything can trace itself back this far - seems to have started around 800 years ago, including hidden singers in the Cathedral's front. There was certainly plenty of incense which was distributed around by the strong winds!

We're running a user study, maybe you can help! We're studying the effect of new ways to show code diffs to programmers, and looking for participants for a 1-1.5hr virtual user study. In the study you will be shown several diffs and asked questions about them.

I was a judge for STEM for Britain yesterday, and I have to say: I was genuinely impressed by what I saw. Much was in fields I have no expertise in, but I still ended up learning things, because the explanations were so good. The winners are thoroughly deserving! bsky.app/profile/raen...

----- "Automatically Retrofitting JIT Compilers", @ltratt.bsky.social, #QConLondon About taking existing language implementations and automatically generating just-in-time compilers for improved performance. Demoing a Mandelbrot in Lua, which takes 3.2 seconds, on standard impl

pizauth-1.0.11 is out, and ensures it keeps retrying in the face of transitory HTTP errors tratt.net/laurie/src/p...

Sometimes the best way to predict the future is to wait and see what happens.

One of the interesting consequences of LLMs/agents is that the short-term costs of boilerplate in code have plummeted. An interesting question is whether those costs are simply shunted to the future or whether they are permanently reduced.

It seems that I have been blessed with my first clawdbot (or similar) interactions on one of my repositories. I really hope this sort of noise doesn't end up (a) wasting maintainer's time (b) putting people off trying the underlying technology where it's actually useful.

In the futurementary Terminator, I don't remember Skynet starting off with open-source repositories theshamblog.com/an-ai-agent-... (I also strongly suggest following the link to the GitHub PR, because it makes much more sense with that as context).

In the past fortnight, 50% of my conversations have seen me say "today's AI cannot do that and we have no idea if and when it will". In the other 50% I've said "today's AI can do that, really, and you just need to try it". Our collective calibration is currently very wonky!

Scratched a Rust itch with an experimental new crate `test_stubs`. I have traits with lots of methods. "Proper" code should implement all methods, but test code need not. `test_stubs` means that test code doesn't have to manually create `todo!()` methods.

There's nothing more surprising than an agent, in the middle of doing some useful refactoring, asking if it can run a Python script which solely prints "HI" (yes, in uppercase) to stdout.

If you're thinking of applying to PLISS, you've got three days left! pliss.org/2026/registr...

A first: I awoke to a PR on one of the Rust projects I maintain that results from a proposed fix to a rustc oversight. Our code should never have compiled, IMHO, so the rustc fix is a good one! For those interested in the rustc change: github.com/rust-lang/ru...

When I read Walter Bagehot's (d. 1877) "The English Constitution" years ago, I was surprised at how much time he spent contrasting it to the US constitution. I now realise he had thought deeply about the plausible futures of both and how they could adapt and/or go wrong.

I've never been a fan of debuggers. Examining the state at the point of a crash rarely helps me: the problem is nearly always earlier in time. `printf` is more useful to follow the trail forwards.

A reminder for those interested in PLISS 2026: there are 10 days left to register your interest. pliss.org/2026/registr...

Even though I know from past experience how important it can be to avoid small memory allocations in performance critical code, the measurable effect of removing them still often surprises me.

I was recently asked how I choose what podcasts I listen to, and paused. Then I realised it's similar to how I choose what to read. In particular, for current affairs, I actively try to avoid just reinforcing my current opinions. I think this has helped make me wrong less often!

Low-needle-drop Christmas trees are willing to delay their revenge: they wait until you remove them from the house before dropping dry, sharp, needles everywhere.

A few weeks ago I gave a talk "Some Things I've Learned About Software" that was unusual for me. I didn't program in it once. I didn't mention specific technologies. Instead I tried to look at the high-level things about software that I've slowly understood. www.youtube.com/watch?v=RJgD...

More PLISS speakers announced! pliss.org/2026/

I can't quite believe that we're now announcing the eighth Programming Language Implementation Summer School (PLISS), but we are --- in May next year, if you want to learn more about programming language implementations, this is the place to come!

email has many detractors -- and I certainly grumble from time to time when I'm ploughing my way through mine -- but we still haven't come up with a sufficiently superior option for asynchronous communication that would make the pain of transition worth it.

For those interested in hearing me waffle about software in a public talk this Thursday in London, there's still a few hours left to register! www.eventbrite.co.uk/e/informatic...

This year's weather has apparently decided it's not going to muck around with any of that nonsense in the 5-12C range. On the plus side, some of my "summer" walks are doable again and they look lovely in the cold mornings!

Amongst the pleasures of living in Somerset is our village and street names. On Sunday, for example, I cycled through Gurney Slade and up Binegar Bottom. Note: I have checked this message carefully for typos.

This weekend my wife wanted to watch a film called The Princess Bride. I hadn't heard of it. I was thus, to put it mildly, surprised to gradually realise that it has generated as many widespread memes as franchises seemingly 100x better known!

New post: "Async and finaliser deadlocks", based on an accidental poke from a recent episode of the excellent @oxide.computer podcast tratt.net/laurie/blog/...

Somewhat delayed because of Covid, I'm finally going to give my "inaugural lecture", which is a short, hopefully accessible, public talk: in my case "Some things I've learned about software"! All are welcome!

One thing pizauth could really do with is zsh completion: we have bash and fish but not yet zsh. PRs welcome! github.com/ltratt/pizau...

New post: "What Context Can Bring to Terminal Mouse Clicks", explaining how one can click on filenames with line numbers in my terminal and have the "correct" editor instance jump to that file and line. A hack, or pragmatic use of existing tools? You decide! tratt.net/laurie/blog/...

grmtools-0.14.0 is out github.com/softdevteam/... -- mostly "quality of life" changes that e.g. make understanding errors easier, but there are a few minor breaking changes that meant the version bump was necessary.

I've been using Alacritty for a fair while, but a very useful feature doesn't seem likely to be accepted (github.com/alacritty/al...) so it's time to move on. I need a modern terminal which run commands on matched text and sends me the terminal title. Any suggestions?

As someone who came to modal editing rather late, I'm still unsure whether it's a good idea -- but I'm used to it now! This post has an interesting argument, pointing out how few programs are modal, and what we might learn from that. buttondown.com/hillelwayne/...