Mill: A JVM Build Tool based on Pure Functional Programming, Functional Conf 24 Jan 2025

Haoyi Li explains how the Mill build tool models JVM builds as pure functional programs to achieve faster builds and better IDE support.

Build systems are often messy programs: full of weird frameworks and unrestricted side effects. This talk will show how the underlying structure of a build actually looks much more like a pure functional program, and how the Mill build tool uses this fact to provide a uniquely simple & principled build experience for projects on the JVM and other platforms. Lastly we discuss how these theoretical foundations result in quantitative improvements, with 2-10x faster builds and a much richer IDE experience than existing tools like Maven, Gradle, or Bazel Project Website mill-build.org/