I'd guess Scala is the most popular choice, but for nearly every language you'll find a company using it for at least parts of their products.
I like Scala, but it's still got mutable state, for loops, and lots of other imperative trappings. It's got some functional features, but I don't consider it to be a functional language (IMHO). If Scala is functional, then so are Ruby, JavaScript, and a host of other languages with higher order functions and the ability to be immutable, so long as you make the effort to not mutate anything (though anything is immutab…
Immutability != FP And "It's got some functional features..." isn't fair to say because other FP languages contain lesser features compared to Scala - which is an other thing.
> According to the Red Monk rankings, Haskell is close to Scala in usage. The learning curve is much higher, but I love writing it. I love the clean, simple code that Haskell pushes me towards, and it usually does the right thing the first time I run it. It seems to be gaining traction lately, and Stack has me excited about the possibility of things ramping up even further.
Haskell would be good if it could handle state and some basic features for code reusage - so we could create uis and everyday stuff seemlessly. But the real-real-world haskell code is beyond ugliness. It's nice when you start with it - creating fibonacci, Monad, little game loops etc. But when you want to create something...