> the language itself is not that complicated

I find Haskell quite complicated, involving abstract concepts you would likely not consciously encounter in other languages (lenses, monad transformers, free monads, etc).

Lenses and monad transformers are libraries, not part of the language. It doesn't look like the book uses them, so you don't need to understand them in order to follow it:

https://nostarch.com/learn-physics-functional-programming

It doesn't need to be.

You can do so much with just data declarations and typeclasses.

https://www.simplehaskell.org/

(disclaimer: not a huge fan of the sight design, but I fully support the concept)

> and some elements of it carry over to other languages, for example, type classes and Rust’s traits.

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