Really puts me off learning it, other than dipping my toes in functional programming and being experienced in C# and the .NET Core/Framework libraries. I thought it may be easier to start with F# before moving to another functional language.
As for learning a functional language, I recommend this Haskell tutorial[0], and accompanying video series of an experienced haskeller (Brian McKenna) running through it[1]. I've read countless texts and tutorials explaining Haskell and FP to me but it didn't fully click until I saw someone with experience using the language and tooling effectively.
[0]: https://github.com/system-f/fp-course [1]: https://www.youtube.com/playlist?list=PLly9WMAVMrayYo2c-1E_r...
There are a few other video series from other experienced haskellers available that may be better, too, I just haven't watched them so I cannot comment.
The foundational concept is functoriality, i.e. mapping over the argument of a type. Whether a datatype has an instance of
1. (covariant) Functor
2. Contravariant functor
3. Functor+Contravariant (phantom argument), or
4. neither
says a lot about its structure and tells me what further questions I can ask (what hierarchies I can expect): A datatype can only be a Monad if is covariant. It can only be Divisible and Decidable if it is contravariant. If it is both (3.) then its argument is not used (phantom) and can be mapped to any type. If it is neither (4.) it can be invariant (where the argument appears in both positive and negative position: like the Endo datatype) or a more complicated datatype like GADT, which would require a more complicated functor.