parsley VS Fast Parse

Or alternatively my own https://github.com/j-mie6/parsley for a more Haskell-style library - it has a wiki that discusses a lot of the main ideas, including how to deal with Def/Val/lazy val

Depends on your existing knowledge of Haskell and stuff like monads, applicatives, etc. I haven't gotten around to writing a tutorial for Parser Combinators yet (I'd actually like to write a book about them at some point) in Haskell, but I do have this wiki here ( https://github.com/j-mie6/Parsley/wiki/Guide-to-Parser-Combinators ) for my parser combinator library in Scala, that might be of some help. A Haskell version of a lot of the later material there can be found in this paper https://dl.acm.org/doi/10.1145/3471874.3472984. The paper assumes some familiarity with Parser Combinators, the wiki does not (but is written in Scala): it's the resource I use to teach my 2nd year undergrads about Parser Combinators for their compilers project. It doesn't talk about monads/applicatives at all. I'm more than happy to answer any questions you have about either of those two.

Perhaps use Li Haoyi's fastparse instead? https://github.com/com-lihaoyi/fastparse

You might want to adapt Li Haoyi’s XML parser for fastparse.

Mostly a mess of pattern matching. I really need to make some generic tree utilities. Haven't been able to find a decent parser combinator that works in Scala 3 (I usually use fastparse which depends heavily on Scala 2 macros, and scala-parser-combinators works in Scala 3, but I've had a lot of trouble getting it to not be too greedy), so I used the state monad from cats to parse at the bottom of the file, which I think turned out fairly nice.

Agreed! I would say that parser combinators are the sweet spot and the right choice in most cases.

Scala has them as well, e.g.: https://com-lihaoyi.github.io/fastparse/

And the good thing is, you don't have to learn a completely new language/syntax, you can use the host language's syntax and you have full IDE support as well.