Comment by mplanchard

There is lots of functional language code in production.

As a recent example GitHub uses this Haskell application for code analysis: https://github.com/github/semantic

Also, Erlang powers a huge amount of the US’ cellular infrastructure, as well as RabbitMQ, which is used in a ton of production workloads.

There’s actually a pretty decent list on Wikipedia: https://en.m.wikipedia.org/wiki/Functional_programming