https://github.com/bazqux/bazqux-urweb/tree/master/crawler
Off the top of my head I also know of Freckle (https://www.freckle.com/) and Co-Star (https://www.costarastrology.com/).
More importantly, this is not a Haskell book! I'm sure the author would be happy to see these ideas applied using Javascript or Scala or Java 14 or anything else.
> Haskell is considered a difficult language to learn, which is certainly true if you come from traditional procedural languages. But rest assured, this is not a Haskell book. You won’t need more than a passing understanding of the language’s syntax and a few high-level idioms. Everything you’ll need to know will be shown later in this section. The ideas present in these pages are adaptable to any piece of technology you’d like, though they might require strictly more discipline to maintain than is necessary in Haskell.
- Chapter 1.3.1, page 6
I'm not going to try to list all the user-facing applications written in any functional language, but I'm sure we can agree it is a much longer list.