If after installing High Sierra (the latest version of OSX for Macs) you notice issues with Elixir or Phoenix, the fix I’ve found is to update Erlang.
If you installed Erlang with homebrew, you can update by running;
I’d then suggest forcing a recompile of your project.
For me the issue presented itself after update when I tried to start my Phoenix server.
It was crashing with the error
segmentation fault mix phx.server.
After the Erlang upgrade and the recompile everything was good.