I have a commit on GHC (a very modest one). The compiler is written in a mix of somewhat old Haskell style and new which makes sense since it's one of the oldes

I have a commit on GHC (a very modest one). The compiler is written in a mix of somewhat old Haskell style and new which makes sense since it's one of the oldest and largest codebases in the language. There is a constant drive to modernize it and some new techniques come out of GHC architectural changes [0]. I didn't work with the parser, mostly the constraint solver in the type checker. Still, I browsed quite a bit of code. It's one of the most pleasant and well documented codebases I've worked in in any language. In particular, the [note] system is an excellent way to embed long form documentation that touches many components without using fancy doc tools: grep suffices. If you want to see what a large, old, industrial grade Haskell project looks like I think GHC is a good place to look. And if you ever run into trouble with GHC then I encourage you not to be afraid to hack on it. The code won't bite and the maintainers have no shortage of patience for newcomers. [0] https://arxiv.org/abs/1610.04799 https://arxiv.org/abs/1610.04799