waseigo
I let LLMs write an Elixir NIF in C; it mostly worked
This post documents how I built a cross-platform Elixir NIF in C to get on-demand up-to-date disk-usage stats without relying on os_mon and its disksup service. I had Grok 3 generate the initial C code and Makefile, then iterated through multiple code reviews by Gemini 2.5 Flash and GPT-5 to make it work on Linux, macOS, Windows, and the BSDs (except DragonFlyBSD).
Along the way, I ran into typical LLM hiccups that speak volumes about the breathless hyperbole often peddled by LLM vendors, compute providers, and over-enthusiastic consultants, middle managers and executives on LinkedIn. Nevertheless, the result is a working, cross-platform Elixir package on Hex.pm, plus a real-world case study in where LLMs shine, where they fail, and what “human-in-the-loop” can mean in practice.
Spoiler alert: the hype is exactly that; even so, we ended up with working code that is, at the very least, a solid starting point for further improvements by actual general intelligence.
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krasenyp
It mostly worked means it didn’t work. Good job getting it there though.
dimitarvp
Should have asked me in a DM, would have told you to use Rust from the get go. 
Now if I can only fix the strange out-of-memory in the GitHub CI runners, I’d be able to finally release my library… (It’s not because of Rust though, my tests so far have put it somewhere in the native building stack in a previous stage of the entire process, or a compile-time switch.)
If you have questions how to best work with Rustler, just ask.
waseigo
Are you kidding me? After seeing the output of splint, I’m in awe at Linux, if anything 
I’m not surprised. I’m very positively surprised by Rust though.
But I’ll stick with Elixir!
dimitarvp
Why not actively use both, like me?
Making stuff work with C is nearly the same as making stuff work with Python: it is working despite the language. Not because of it.
dimitarvp
I love Elixir and so far to me it is irreplaceable (9.5 years after discovering it). It covers aspects that so many other languages / runtimes don’t even dare begin tackling (Rust and Go are exceptions but they just don’t take the problem seriously enough and as such kind of accidentally try to invent their own BEAM, badly).
I am saying that my attention can be on multiple things. Focus is hugely important and it’s been a lackluster part of my methods (one that I am actively fixing lately) but I also find it extremely important to have more tools in my bag so I can practice the “right tool for the job” confidently.
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