> Whether the implementation of a function performs IO is in principle an implementation detail that can change in the future.

I think that's where your perspective differs from Zig developers.

Performing IO, in my opinion, is categorically not an implementation detail. In the same way that heap allocation is not an implementation detail in idiomatic Zig.

I don't want to find out my math library is caching results on disk, or allocating megabytes to memoize. I want to know what functions I can use in a freestanding environment, or somewhere resource constrained.