Classes and objects don't have this issue because there is no underlying formal definition for them. They're also less general. Most descriptions of basic OOP i

Classes and objects don't have this issue because there is no underlying formal definition for them. They're also less general. Most descriptions of basic OOP ideas are on the same level as "side effects might happen here"; there's just nobody to correct them and nothing to correct them against. Don't get me started about pointers. They're at least as difficult to grasp for most beginners than monads ever were. Hell, people have days of trouble just with references as in Java! Functions do have this issue, and most of what different languages call "functions" aren't. It's just that unlike monads, functions have entered the common mind and people talking about them informally far outnumber those interested in correctness. And many people have plenty of problems with things like higher-order functions.