This is a Rotor to compile Sass files. It uses the Sass.ex library to do the Sass compilation.
The SassRotor module provides the sass rotor. And this is how you call it
SassRotor.sass(files)
# This goes in the place you are configuring Rotor
import Rotor.BasicRotors
import SassRotor
output_path = "priv/static/assets/app.css"
Rotor.watch :stylesheets, ["priv/assets/stylesheets/*.scss"], fn(_changed_files, all_files)->
  read_files(all_files)
  |> sass
  |> concat
  |> output_to(output_path)
end