How to exit the elm repl?

To exit the elm repl you need to type :exit. You can also use Ctrl + d, but I prefer typed commands instead of multi-key holds. I found this answer here as part of the (unofficial) getting started with elm programming guide.

Just to add a little more detail, after launching the REPL in your terminal/command line, you are prompted to visit the elm-lang site for further information regarding use of the interactive shell:

To exit the REPL, you can type :exit.

You can also press ctrl-d or ctrl-c on some platforms.

-- https://elm-lang.org/0.19.0/repl

Just for future reference, Evan (Elm's creator) has created a trove of 'hints' in the elm/compiler repo.