I'm not sure I'll call xmonad popular. Also I know quite a lot of people who hate it -- because it's configuration language is "write some Haskell and recompile", so it tends to be very fragile (people who use Haskell love it)

Also found it crashed a lot, which I would have hoped would be one of the things Haskell would solve (the problems revolved it having to integrate into C code for linking to X, but that's part of living in the real world).

Is there anything actually interesting about what they do, though? Converting markup is not a complex task.

Grandparent sold pandoc short. It is actually a really nifty utility capable of converting a plethora of formats from/to each other. It is a godsend for situations when you have to create MS Office-compatible documents, and want to avoid the inevitable muscle cramps you'd get if you were to actually use Word.

You can create your own document templates, and tweak the resulting output in a million ways. At my job, I have to create a lot of documentation, often in PDF or docx, and Pandoc has become an integral part of my workflow. I can create documents within the comfortable environment of Emacs, and use Pandoc to produce output in any old format that $pointy_haired_boss may desire.

Parsing is something that's very pleasant in Haskell. Other languages can use parser combinators too, of course. It's good at it though.

C-to-Rust converter is written ... in Haskell https://github.com/jameysharp/corrode

Yup! There was already a great, ready to use C parser in Haskell, and we don't have one in Rust yet.

Great example!

Yeah, fair point. That'll be a function of the fact that Haskell has tended to prioritize the concerns and problems of writing the compiler.

Complex transformations of data structures are part of the compilation activity too, but they're more difficult to spot in the wild of applications.

I'd disagree. Most business logic is just transformation of data structures in disguise. Once you have a sufficiently rich set of data structures this becomes abundantly clear.

Pandoc actually converts between a wide range of document formats.

Moving goalposts detected.

I didn't ask the original question.