When learning TDD, you usually start by testing small, individual classes. Exercises like these are designed to get you into the workflow quickly. When you transition from small exercises to real-world work, however, things quickly get hairy.
Are Elm and Phoenix/Elixir ready for prime time? I'll let you decide: they were both used in live during the main French political show called "L'émission Politique" to help generate a word cloud base.