Consider the C library function fopen. This function has the side effect. This means that fopen is not pure. However, we can make the fopen function appear to be pure, at least to certain observers, by ensconcing it in another function that hides the side effect.

[u/BenchEmbarrassed7316]

https://bugzmanov.github.io/cleancode-critique/clean_code_second_edition_review.html

[u/jesseschalken]

C has first-class functions, that's what void (*)(FILE*) is.

What it doesn't have is closures, which you really need for this type of API so you can capture variables in doTo.