If you run `~` before any command, sbt watches the directory for any source changes, and on detecting source changes, redoes the command. So `sbt ~compile` will re-compile any new sources as soon as they are saved.

The use of the ~ here illustrates the problem in the scala world: a lot of surprising bells and whistles. Why not call it watch instead of ~?

Or just use IntelliJ and you can reload classes while your program is running if you want. Sbt is a very powerful tool but it's not written with readability for new users in mind.

Wait, how does that help build time exactly ? It may help with the perceived build time, but your CI builds are still taking the same (long) time.

CI, sure, but on dev machines this means you're not throwing away the JVM and restarting it, reloading all the classes etc. on every build