There are a number of tutorials on how to add Elm to Phoenix apps. Many were for Elm 0.16. They're broken for Elm 0.17. This repo will help you translate from then to now.

Here are pointers to the files that matter. In all cases, look for "elm":

My (trivial) use of Elm is in the root page.

You might also be interested in the brunch-config and .gitignore files.