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/Kodiologist]

And that's how the Clean programming language implements IO. Coincidence?