I'd also emphasize the point here is that if a systems-level programming language is going to call itself pure, it just needs a really, really careful definitio

I'd also emphasize the point here is that if a systems-level programming language is going to call itself pure, it just needs a really, really careful definition of what exactly it means by pure. Purity is intrinsically relative [1]. That doesn't make it a bad goal, or a bad thing, and there's definitely a significant difference between a language striving for any meaning of "purity" versus one that doesn't care at all, but whatever definition the language designer is using should be very carefully defined. Particularly for a systems language, if by "systems language" one means "the sort of language that allows poking at low level details", because having lots of "low level" access greatly expands the scope of "things my code may be able to witness the stack for". To give a degenerate-but-simple example of that, a low-level systems language striving for "purity" but that also allowed arbitrary memory reading for whatever reason ("deep low-level custom stack trace functionality") would technically be able to witness the effects of the stack changing due to function calls. You can just define that away as an effect (and honestly would probably have to), but I'd suggest being clear about it. A denegenerate-but-often-relevant example is that a "pure" function in a language that considers memory allocation "pure" can crash the entire OS process by running it out of memory. That's so impure that not only can the execution context (thread, async context, whatever) that is running the program out of memory witness it, so can every other execution context in the process, indeed, whether they want to or not they have to! We generally consider memory allocation "pure" for pragmatic reasons, because we really have no choice, the alternative is to create such a restrictive definition of "pure" as to be effectively useless, but that is almost the largest possible "effect" we're glossing over! [1]: https://jerf.org/iri/post/2025/fp_lessons_purity/#purity-is-relative https://jerf.org/iri/post/2025/fp_lessons_purity/#purity-is-...