I've been extremely pleased with the Nerves project. It's Elixir (or any flavor of Erlang, really) top to bottom. The toolkit will build and flash the SD card, so you don't need to worry about anything except your app.

Very nice to be able to ssh to my Pi and get dropped immediately into an IEX repl (no auth, no command prompt, it's just IEX over SSH)

That's incredible, thanks for sharing! Here's the links for the lazy:

https://nerves-project.org/

https://github.com/nerves-project/nerves

Is nerves running without an OS on the Pi (baremetal)?

It's running on Linux. I am not privy to the specifics.

Yes; or, for greater ease-of-use, you could just write your logic as a Linux kernel driver, leaving the rest of the OS to do its job. After all, from the perspective of the controller, you're interacting with a peripheral—and that's exactly what device drivers are designed to do.

Yes, can do bare metal https://www.raspberrypi.org/forums/viewforum.php?f=72

Though I'm sure many `purests` will say this is not bare metal and it's impossible upon modern CPU's due to the ISA microcode.........