At REA we happily use a variety of programming languages. Teams are given the freedom to choose a fitting language for a given project. Mostly this ends up being one of Ruby, Java, or Scala. However, there are some languages that we as developers and ops people get excited about, but the viability as a mainstream REA language hasn't yet been established.
For me personally, Haskell is what I code on the weekends and I've been looking for a way to shoehorn it into my regular work 😉
Recently I learnt that REA does in fact have some Haskell in prod. Who owns it? What does it do? No one will ever know. However as the story goes, it was automating a manual task and as such it was simply a value add that became a useful tool, and avoided questions like 'just what is a monad anyway!?'.
Jim Gaylard and I, Haskell acolytes, attempted something similar on hack day.