A monad isn’t a category. Tbh I think the whole “category theory” obsession one sees in some parts of online FP evangelism needs to die. Haskell has (endo)func

A monad isn’t a category. Tbh I think the whole “category theory” obsession one sees in some parts of online FP evangelism needs to die. Haskell has (endo)functors which are a useful concept for which one needs to know zero category theory (similarly for monads). But otherwise, FP basically only ever has one category which has all the nice properties one could want (ok if you have a weird type system based on a weird logic you might have a slightly different category), so the thing people call category theory is just the theory of the one category you live in. One doesn’t say group theory is category theory just because there is a category of groups. Actual category theory isn’t much about specific categories so much as it is about the relationships between them, and ideas common to many categories (natural transformations, limits, terminal objects, etc). I’m not saying category theory is bad, but I think to get much out of it one needs more than one category (and hopefully one can think of a category which isn’t a topos too), and one doesn’t tend to come across categories in day to day functional programming. Some definitions and constructions from category theory may be useful in constructing a type system for a new ML-family programming language.