Mill is a JVM build tool that lets you write pure-functional build code to compile Java  projects 2-10x faster than existing tools like Maven or Gradle. This talk while explore how Mill uses pure functional programming to provide a uniquely intuitive developer experience, while at the same time moving heaven and earth to bridge the gap between pure-functional elegance and the messy filesystem IO, subprocess spawning, concurrency and state management necessary in any real-world build system.