I'm the author of Scala.js, so supposedly a Scala "expert", and I still don't understand category theory.

Does that answer the question?

My understanding is that, just like the abstract algebra, category theory is a useful tool to help you recognize patterns and provide you some level of certainty about their behavior. Rather than figuring out yourself if you should do something one way or the other, you can say: Hey, I know it's X, and people smarter than me have proven that X will behave in a certain way and that I can do certain stuff with it. When building complex systems this can save you a lot of time and nerves. However, with a common, simpler software that most of us build, it's more about writing the eloquent and stylish code, than an absolute necessity for a programmer. You definitely can work in scala for years without knowing anything about it.

Basing on how much it is important to understand categories to know mathematics or to know other languages, I would say nothing. But probably it can be fun, and definitely you can then brag about it.

However, if you still have to learn X, don't try to learn categories to learn X. First learn X decently, then if you want learn categories (or any algebraic abstractions that people say it is useful) and you have some hope that categories will give you some additional insights on X.

You can get by with Scala, or Functional Programming in general, without an understanding of category theory, although naturally some CT concepts will creep into your understanding without you realizing it.

Category Theory basically provides a framework for talking about composability in rigorous language. It's important if you want to grok Functional Programming concepts, but you can get by without it just fine.

Not at all or absolutely essential, depending on wether or not you work with developers (or with their libraries) who use this as the language to talk about code.

Since Scala is a hybrid language, you can still program in it and do things as you would do in Java, for example. You will see common patterns emerging, such as seen a `map` method on List, Option, Try, etc.. Now, you could ask yourself "What is this pattern and why do I see it everywhere?", or you can just continue with your life being satisfied with what you know today, and no harm will be done.

I've been doing Scala for the past two years, and the amount of things I've learned feels amazing. I've watched (and still am watching) Bartosz Milewskis "Category Theory for Programmers" which takes you from the very fundamentals of CT all the way to things like Adjunctions and Kan Extensions. I find it fascinating.

I wrote introductory blog post answer that. https://tech.iheart.com/why-fp-its-the-composition-f585d17b0... TLDR; It's about the ability to compose. Category theory enables more general and powerful composition of components. That is not to say that without CT you won't be able to compose, it's just more cumbersome and less flexible.