I find functional programming reasonably straightforward to learn but difficult to master.

For example, this Common Lisp cheat sheets covers a large part of the language and isnt hard to follow

https://github.com/ashok-khanna/lisp-notes

I was doing some Haskell and Racket and I found basics not too hard to learn, but I did then too hard to master, but I think thats to do more with the complexities of the problems I tried to solve and less to do with the languages themselves