[ffreire]

I wouldn't consider them rules inasmuch as they're things to keep in mind when working in a Haskell codebase, in much the same way GoF has been useful for OO programmers for decades.

Can you write code without using a pattern from GoF? Sure. Will it work? Probably. Does that make GoF useless? No, I'd think not. This type of content is immensely valuable for beginner and practitioner alike because it starts to build a shared language with which to talk about how codebases are organized :)