[jetti]

I have a couple for various reasons:

The hardest that I gave up on was Groovy. The language itself seems really nice, it was just hard because of the lack of documentation that I was able to find so I just switched to Scala since I had to learn it for a class anyways.

The hardest that I still use is Haskell. While there documentation is phenomenal compared to other languages, I have found that it isn't something that you can just pick up and start using and be able to write idiomatic programs without studying theory. For me, when I get to monads I struggled a bit. Then I get to the monadic bind and other operators like $ and . and I find I have to keep going back to tutorials to see when and how to use them properly.