Counterpoint: I learned about Applicative, Functor and Monad by reading docs and tutorials, and I haven't the faintest idea about category theory.
These are the building blocks of Haskell. You learn them as you go, just as you learn what a "method" is when learning OOP. You don't need to know category theory.
Learning what Applicative, Functor and Monad are is category theory. It's like saying "you can learn how to do unions, intersections, and differences on collections of unique objects without understanding set theory".
That seems a little silly to me and I think we're splitting hairs with what it means to do category/set theory. I don't know category or set theory, so I hope you will forgive me for using yet another allegory.
Let's say I make a type class for Groups (in the abstract algebra sense). The rationale behind this is that there's an algorithm for exponentiation which is O(log(n)) versus the naive O(n) algorithm. So if you make an instance that's a Group, you get to use this fast exponentiation.
Sure, to understand and use this type class you have to understand what a Group is. However, I think it's a bit of a stretch to tell someone "in order to use Group you must first learn abstract algebra" because they'll think you're telling them to take a university level course. In actuality, they don't have to know much at all (they don't even need to understand _why_ the exponentiation algorithm works) - they just need to know what is a lawful Group.
The first day of the intro to abstract algebra course I took introduced much more information than you'd need to use this made up type class, and I expect the same of the others.
Like this is my understanding of Functors/Applicatives/Monads. I kind of know their shape and how to use them. If you asked me about any of the underlying math I would shrug and maybe draw a pig (https://bartoszmilewski.com/2014/10/28/category-theory-for-p...).
That's not really true. When some people claim you must learn category theory to use Haskell, they mean they have to learn the math-subject-capital-letter Category Theory, which goes way beyond programming and comes with a lot of baggage.
They never mean you have to learn Functor, Applicative, etc as used in Haskell and taught in every tutorial. If they did mean the latter, it would be a tautology, which is not very useful.
Compare "in order to do OOP you have to learn what an object truly is, its essence, and possibly take some courses on the philosophy of being" vs "you have to learn about methods and objects".
> It's like saying "you can learn how to do unions, intersections, and differences on collections of unique objects without understanding set theory".
You can totally do unions, intersections and differences without knowing set theory.
These are the building blocks of Haskell. You learn them as you go, just as you learn what a "method" is when learning OOP. You don't need to know category theory.