alt.hn

6/9/2026 at 11:27:49 AM

Test-case reducers are underappreciated debugging tools

 https://tratt.net/laurie/blog/2026/test_case_reducers_are_underappreciated_debugging_tools.html

by ltratt

6/9/2026 at 8:26:26 PM

> To make things even worse, the community that has most thoroughly embraced them are compiler authors, who many programmers think of as being an impossibly skilled elite

The article's approach seems super ad-hoc, leaving you to have to think hard, do all the work, and make all the mistakes.

If you were to go down the other path, you might try dividing and conquering the problem. An arbitrary Pair<A,B> is trivially constructed from an arbitrary A and an arbitrary B. So if you can generate a string, and a number, you could generate a User full of number and string fields. If your generate function accepts a number describing how complex a string to make, then you can also choose how complicated to make your User. That's all shrinking needs to be. Repeatedly trying smaller Ns while the problem still happens (the problem being one of your unit tests - not an additional "interestingness" test you need to write.)

You'll probably way more likely to hit boundary cases by using the structure of the input and making interesting variations that way, rather than hoping you can permute the right bytes from the CLI.

by mrkeen

6/9/2026 at 8:39:43 PM

Property-based testing frameworks will often do test case reduction as well (called shrinking).

by skybrian

6/9/2026 at 9:16:41 PM

Shrink Ray, described in the article, is developed by D.R. MacIver who also developed Hypothesis. I remember when it was announced a while back but had forgotten about it, I guess I have something to play with tonight.

by Jtsummers

6/9/2026 at 9:34:02 PM

These days, he’s also working on Hegel - bringing test case reduction and PBT to more languages.

https://hegel.dev

by akshayshah

6/9/2026 at 9:20:18 PM

Brilliant tools, well worth investigating for any system-critical applications. They don't seem to get enough attention outside of the FP community.

by macintux

6/9/2026 at 9:09:46 PM

I read the first part of this article, then gave up and Googled "Test-case Reducers".

I'm not sure if that's an article failure (that I didn't want to read a whole ton of text and C code details), or a success (as it got me interested in the topic). I guess both?

by hungryhobbit

6/10/2026 at 3:03:10 AM

I read the whole article, and I am still confused. I get that test case reducers find the smallest error causing string. I don't understand why that is particularly valuable.

Also, do test case reducers work on integers or other numbers? What about reducing some other complexity? Is this for developing unit tests or just debugging?

by Hnrobert42

6/10/2026 at 3:16:11 AM

A reduced test case means you run less code to process the test case, which means your breakpoints trigger less frequently (and the remaining breakpoint triggers are more likely to be relevant to the actual bug). It also means all your debugging steps are likely to run faster and produce less data to sort through. Your log files will be shorter and easier to read/grep, etc.

Imagine being handed a sheet of 10 equations and being told "1 of these equations is wrong." Now imagine that someone came in and erased 8 of the correct equations - they just saved you a bunch of time.

by kg

6/9/2026 at 9:14:51 PM

> I read the first part of this article, then gave up and Googled "Test-case Reducers".

It's answered pretty early on:

>> Test-case reducers try to reduce the length of an input

If that still doesn't answer the question, try this extension:

>> Test-case reducers try to reduce the length of an [error causing or interesting] input

by Jtsummers

6/9/2026 at 11:19:36 PM

I have a similar tool to shrink ray, called [bonsai](https://github.com/nnunley/bonsai). I designed it to allow me to try to inline and reduce code for both simplifying single file examples, as well working across multiple files. It uses Tree-Sitter for syntax awareness, and the [Perses algorithm](https://doi.org/10.1109/ICSE.2018.00046) as the methodology for simplification.

I'd love to get some feedback if anyone's interested.

by nnunley

6/9/2026 at 8:20:41 PM

I've only ever known about these through compilers, very cool.

On one project, through a variety of circumstances, dead code elimination was straight up not working, but we wanted to show the theoretical improvement of some approach - but we couldn't figure out why at the moment (we did spend a whole week chasing down the root cause after - maybe worth in hindsight...).

We were doing it by hand at one point, but someone suggested using CReduce for shrinking the code. Definitely was an interesting test-iterate loop...

by sigbottle