Yes, it is the only program coded in Haskell I have ever used for anything practical, to my knowledge.

I have heard of others, like git-annex, but not used them myself. I wonder if there are any I just didn't know were.

I also wonder if anything about Haskell makes it particularly suited as the implementation language for Pandoc. It must have a lot of parsers in it, and Haskell is supposed to be good for coding parsers.

There are parser generation libraries and meta-libraries for certain other languages, notably C++. I wonder what Pandoc in C++ would look like. Probably a pretty good parser meta-library could be spun out of such a project.