If you're writing Haskell code you need to be able to understand Haskell libraries and other people's code. You definitely need to understand the type system and what monads are, unless you're main objective is to just write a toy program

The evidence is overwhelming, considering how many people want to learn and write Haskell and how many people struggle with it. If people could ignore all of this stuff they would've figured that out by now, but they can't ignore it to really be productive. I like Haskell a lot btw, just don't think it's very practical, and it's most important contributions to FP are already being adopted in other languages without the additional complexity