So, I decided to write a little thing, only to practice writing, it's about SOLID being compared between class-based-OO and FP, and there will be grammar errors and there will be a lot of formatting errors, 'bear' with me as I type this as fast as I can. Also correct if I'm wrong. Warning, wall of text in English.
" a class should have only a single responsibility "
A pure function is something with a single responsibility, turning its input into an output. What could be simpler?.