There is basically no DSL. You simply write what a build needs, e.g. you write a function `collectCFiles()` that collects every file with extension `.c`. You then issue a command like `gcc ${collectCFiles()}`. And pretty much that’s it - you can use shell commands, or do anything in scala (or java or whathaveyou). You simply have your functions return either a value (e.g. a checksum) or a location, which is the only mill-specific logic. So your compileC() function will simply invoke your collectCFiles() function, and this invocation implicitly creates a dependency between these tasks. You have written literally the simplest way to describe your build logic. But in the background mill will cache your functions’ inputs outputs and parallelize those that need re-run, which is what a build tool should do.

The implementation may not be the theoretical best, but I think the idea is pretty much the perfect build system out there.