A simple script to download the Elm binary in Heroku
To build on M1 we want to avoid installing Elm via npm, because it doesn't have binaries for all architectures. Hence we need a way to install Elm on Heroku. I didn't understand what the other buildpacks were doing with Docker and AWS (see this buildpack for an example), seemed really complicated for something rather simple. They were also unmaintained and not touched since 2017.
A Heroku buildpack is really just a little bit of bash that runs during deployment. It has three scripts: detect, compile and release.
Checks to see whether elm in the /app/bin/ folder returns a non zero status code. If it's there (because maybe you checked it into the repo) this buildpack won't do anything. Probably could be improved to parse the output of elm --version and compare that with the ELM_VERSION to see if it needs installing...
Doesn't compile anything :) Just downloads Elm from GitHub, unzips it and puts it into /app/bin/elm. Can't write it into /usr/local/bin so we'll set the PATH in the next step...
Adds the $BUILD_DIR/bin/ (e.g. /app/bin) to the $PATH so we find elm.