That's cool, but question: why is the language-ext library documentation using Haskell's Haddock to render the pages? Wouldn't C#'s native documentation system be better?

It isn't Haddock, it's simply inspired by Haddock. It's a library I wrote called best-form [1]. The README gives context:

"This is a C# doc-gen tool. It was primarily built to support my Language-Ext project. Which is non-idiomatic in its approach. I couldn't find documentation generators that did it justice. I also really liked the Hackage documentation style from Haskell, so have taken a styling approach from there (even if it looks a little dated now, it was always the documentation I felt most comfortable reading)."

language-ext isn't idiomatic C# and so idiomatic C# doc-gens have quite a bad time with this library. One of the major benefits over other doc-gens is it grabs the README.md from each folder and prepends it to the relevant section - allowing me to write some contextual documentation outside of the auto-generated API documentation. This makes it a much more approachable reference.

It also supports markdown in the XML documentation, avoiding a lot of the need for lots of ugly XML special characters [in code] and extending the layout options compared to the common C# doc-gens.

[1] https://github.com/louthy/best-form