Is it possible to extend a dynamic cross module in package.mill?

[asarkar]

Given the following directory structure, I set up immediate subdirectories as dynamic cross modules.

.
├── simple-linked-list
│   ├── build.sbt
│   └── src
│       ├── main
│       │   └── scala
│       │       └── B.scala
│       └── test
│           └── scala
│               └── BTest.scala
├── <more directories>
└── build.mill
└── versions.mill

build.mill

package build

import mill._, scalalib._, scalafmt._
import $packages._
import $file.{versions => v}

def isSbtProject(p: os.Path) = os.exists(p / "build.sbt")
def moduleNames = interp.watchValue(
    os.walk(millSourcePath, !isSbtProject(_), maxDepth = 1)
      .map(_.last)
)

object modules extends Cross[MyModule](moduleNames)

trait MyModule extends SbtModule with Cross.Module[String] with ScalafmtModule {
  outer =>

  val scalaVersion = v.scalaVersion

  // Ends with 'modules' that need to be removed
  def millSourcePath = super.millSourcePath / os.up / crossValue

  def scalacOptions: T[Seq[String]] = Seq(
    "-encoding", "UTF-8",
    "-feature",
    "-Werror",
    "-explain",
    "-deprecation",
    "-unchecked",
    "-Wunused:all",
    "-rewrite",
    "-indent",
    "-source", "future",
  )

  object test extends SbtTests with TestModule.ScalaTest {
   def scalacOptions: T[Seq[String]] = Seq("-encoding", "UTF-8")

   val commonDeps = Seq(ivy"org.scalatest::scalatest:${v.scalatestVersion}")
   val scDeps =  Seq(ivy"org.scalatestplus::scalacheck-1-18:${v.scalacheckVersion}")
   // Alternatively, separate this logic out.
   // https://mill-build.org/mill/scalalib/build-examples.html
   def ivyDeps = Task{commonDeps ++ (if (crossValue == "simple-linked-list") scDeps else Seq())}
  }
}

As you can see towards the end, one of the modules gets an additional dependency added depending on its name. This is ok for now, but if more and more modules need special configuration, the root build file will become messy.

My question is, can a module-level package.mill add a dependency instead of doing it in the root build? Note that while it can possibly override the root dependencies, for the sake of DRY, I want to add an additional dependency instead of redefining the whole list.

[lihaoyi]

No, module-level package.mills always translate into normal Modules, not Cross modules. And modules can only be defined in one place, so if it's defined in build.mill it cannot be defined in a nested package.mill, and vice versa

If you are finding a lot of modules needing special configuration, then instead of using a cross module you can use normal modules and traits to extract out the commonalities. That would let you more easily mix and max things, e.g. having each module inherit different sets of traits, or easily adding overrides specific to each specific module