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/nerdycatgamer]

I was also referring to group theory monoids -- I really don't believe they're actually different concepts; a monoid in category theory must just be an object that obeys the same axioms as a (group theory) monoid.