Scala with Cats is a free ebook put together to help introduce the Cats library and it’s programming style to developers. It is targeted to Scala developers with about a year of experience with the language, but if you were using the language in a very Java-like way you may not be prepared for this book. That caveat aside, it brings an accessible introduction to the library and it’s style of programming.
I can’t go back and have read this before having read Functional Programming in Scala but it seems like either order would work fine. They both talk about the same basic concepts around purely functional programming. They come at it from two different perspectives; Scala with Cats is about how the category theory-inspired structures in the library can be used to solve problems, whereas Functional Programming in Scala is leading you towards those same category theory-inspired structures but getting you to find the patterns yourself.
I really appreciated the last set of exercises in Scala with Cats where it had you implement this concept. It starts out out as a fully concrete class then converting it into more and more generic structures. First, by adding some type classes to become generic to the specific types. Then, by abstracting over the intermediate data structure and converted the structure to its own type class. Finally, by abstracting over the data structure even further by replacing it with another type class.
I think this style of programming has some definitive pros. The idea behind the vocabulary is good, even if the terms chosen obscure some of the intent. The extensive usage of type classes adds an additional layer of polymorphism that lets a library author abstract over portions of the implementation to make it future-proof. The Scala implementation of type classes makes this feel awkward at points since the imports around implicit instances are less obvious around what is happening. I feel like I need to spend some time with a real application written in this style to try to see what the negatives are to working with it. I can see the issues with learning to work in this style, but I’m uncertain about what the negatives are once you’ve gotten used to this style.