Scala Module Configuration

This page goes into more detail about the various configuration options for ScalaModule.

Many of the APIs covered here are listed in the Scaladoc:

mill.scalalib.ScalaModule

Compilation & Execution Flags

import mill._, scalalib._

object foo extends RootModule with ScalaModule{
  def scalaVersion = "2.13.8"
  def scalacOptions = Seq("-Ydelambdafy:inline")
  def forkArgs = Seq("-Xmx4g", "-Dmy.jvm.property=hello")
  def forkEnv = Map("MY_ENV_VAR" -> "WORLD")
}

You can pass flags to the Scala compiler via scalacOptions. By default, run runs the compiled code in a subprocess, and you can pass in JVM flags via forkArgs or environment-variables via forkEnv.

You can also run your code via

mill foo.runLocal

Which runs it in-process within an isolated classloader. This may be faster since you avoid the JVM startup, but does not support forkArgs or forkEnv.

If you want to pass main-method arguments to run or runLocal, simply pass them after the foo.run/foo.runLocal:

mill foo.run arg1 arg2 arg3
mill foo.runLocal arg1 arg2 arg3

> ./mill run
hello WORLD

Adding Ivy Dependencies

build.sc (download, browse)

import mill._, scalalib._

object foo extends RootModule with ScalaModule {
  def scalaVersion = "2.12.17"
  def ivyDeps = Agg(
    ivy"com.lihaoyi::upickle:3.1.0",
    ivy"com.lihaoyi::pprint:0.8.1",
    ivy"${scalaOrganization()}:scala-reflect:${scalaVersion()}"
  )
}

You can define the ivyDeps field to add ivy dependencies to your module.

Single : syntax (e.g. "ivy"org.testng:testng:6.11") defines Java dependencies
Double :: syntax (e.g. ivy"com.lihaoyi::upickle:0.5.1") defines Scala dependencies
Triple ::: syntax (e.g. ivy"org.scalamacros:::paradise:2.1.1") defines dependencies cross-published against the full Scala version e.g. 2.12.4 instead of just 2.12. These are typically Scala compiler plugins or similar.

To select the test-jars from a dependency use the following syntax:

ivy"org.apache.spark::spark-sql:2.4.0;classifier=tests.

Please consult the Library Dependencies in Mill section for even more details.

> ./mill run i am cow
pretty-printed using PPrint: Array("i", "am", "cow")
serialized using uPickle: ["i","am","cow"]

Runtime and Compile-time Dependencies

If you want to use additional dependencies at runtime or override dependencies and their versions at runtime, you can do so with runIvyDeps.

build.sc (download, browse)

import mill._, scalalib._

object foo extends ScalaModule {
  def scalaVersion = "2.13.8"
  def moduleDeps = Seq(bar)
  def runIvyDeps = Agg(
    ivy"javax.servlet:servlet-api:2.5",
    ivy"org.eclipse.jetty:jetty-server:9.4.42.v20210604",
    ivy"org.eclipse.jetty:jetty-servlet:9.4.42.v20210604"
  )
  def mainClass = Some("bar.Bar")
}

You can also declare compile-time-only dependencies with compileIvyDeps. These are present in the compile classpath, but will not propagated to the transitive dependencies.

object bar extends ScalaModule {
  def scalaVersion = "2.13.8"
  def compileIvyDeps = Agg(
    ivy"javax.servlet:servlet-api:2.5",
    ivy"org.eclipse.jetty:jetty-server:9.4.42.v20210604",
    ivy"org.eclipse.jetty:jetty-servlet:9.4.42.v20210604"
  )
}

Typically, Mill assumes that a module with compile-time dependencies will only be run after someone includes the equivalent run-time dependencies in a later build step. e.g. in the case above, bar defines the compile-time dependencies, and foo then depends on bar and includes the runtime dependencies. That is why we can run foo as show below:

> ./mill foo.runBackground

> curl http://localhost:8079
<html><body>Hello World!</body></html>

Compile-time dependencies are translated to provided-scoped dependencies when publish to Maven or Ivy-Repositories.

Keeping up-to-date with Scala Steward

It’s always a good idea to keep your dependencies up-to-date.

If your project is hosted on GitHub, GitLab, or Bitbucket, you can use Scala Steward to automatically open a pull request to update your dependencies whenever there is a newer version available.

Scala Steward can also keep your Mill version up-to-date.

Test Dependencies

Mill has no test-scoped dependencies!

You might be used to test-scoped dependencies from other build tools like Maven, Gradle or sbt. As test modules in Mill are just regular modules, there is no special need for a dedicated test-scope. You can use ivyDeps and runIvyDeps to declare dependencies in test modules, and test modules can use their moduleDeps to also depend on each other

build.sc (download, browse)

import mill._, scalalib._

object qux extends ScalaModule {
  def scalaVersion = "2.13.8"
  def moduleDeps = Seq(baz)

  object test extends ScalaTests {
    def ivyDeps = Agg(ivy"com.lihaoyi::utest:0.7.11")
    def testFramework = "utest.runner.Framework"
    def moduleDeps = super.moduleDeps ++ Seq(baz.test)
  }
}


object baz extends ScalaModule {
  def scalaVersion = "2.13.8"

  object test extends ScalaTests {
    def ivyDeps = Agg(ivy"com.lihaoyi::utest:0.7.11")
    def testFramework = "utest.runner.Framework"
  }
}

In this example, not only does qux depend on baz, but we also make qux.test depend on baz.test. That lets qux.test make use of the BazTestUtils class that baz.test defines, allowing us to re-use this test helper throughout multiple modules' test suites

> ./mill qux.test
-------------------------------- Running Tests --------------------------------
Using BazTestUtils.bazAssertEquals
+ qux.QuxTests.simple ...

> ./mill baz.test
-------------------------------- Running Tests --------------------------------
Using BazTestUtils.bazAssertEquals
+ baz.BazTests.simple ...

Scala Compiler Plugins

build.sc (download, browse)

import mill._, scalalib._

object foo extends RootModule with ScalaModule {
  def scalaVersion = "2.13.8"

  def compileIvyDeps = Agg(ivy"com.lihaoyi:::acyclic:0.3.6")
  def scalacOptions = Seq("-P:acyclic:force")
  def scalacPluginIvyDeps = Agg(ivy"com.lihaoyi:::acyclic:0.3.6")
}

You can use Scala compiler plugins by setting scalacPluginIvyDeps. The above example also adds the plugin to compileIvyDeps, since that plugin’s artifact is needed on the compilation classpath (though not at runtime).

Remember that compiler plugins are published against the full Scala version (eg. 2.13.8 instead of just 2.13), so when including them make sure to use the ::: syntax shown above in the example.

> ./mill compile
...
error: Unwanted cyclic dependency
error: ...src/Foo.scala...
error:   def y = Bar.z
error: ...src/Bar.scala...
error:   def x = Foo.y

Scaladoc Config

To generate API documenation you can use the docJar task on the module you’d like to create the documenation for, configured via scalaDocOptions or javadocOptions:

build.sc (download, browse)

import mill._, scalalib._

object foo extends ScalaModule {
  def scalaVersion = "3.1.3"

  def scalaDocOptions = Seq("-siteroot", "mydocs", "-no-link-warnings")
}

> ./mill show foo.docJar

> unzip -p out/foo/docJar.dest/out.jar foo/Foo.html
...
...<p>My Awesome Scaladoc for class Foo</p>...

When using Scala 3 you’re also able to use Scaladoc to generate a full static site next to your API documention. This can include general documenation for your project and even a blog. While you can find the full documenation for this in the Scala 3 docs, below you’ll find some useful information to help you generate this with Mill.

By default, Mill will consider the site root as it’s called in Scala 3 docs, to be the value of docResources(). It will look there for your _docs/ and your _blog/ directory if any exist. Given a project called bar:

object bar extends ScalaModule {
  def scalaVersion = "3.1.3"
}

Your project structure for this would look something like this:

.
├── build.sc
├── bar
│  ├── docs
│  │  ├── _blog
│  │  │  ├── _posts
│  │  │  │  └── 2022-08-14-hello-world.md
│  │  │  └── index.md
│  │  └── _docs
│  │     ├── getting-started.md
│  │     ├── index.html
│  │     └── index.md
│  └── src
│     └── example
│        └── Hello.scala

After generating your docs with mill example.docJar you’ll find by opening your out/app/docJar.dest/javadoc/index.html locally in your browser you’ll have a full static site including your API docs, your blog, and your documentation.

> ./mill show bar.docJar

> unzip -p out/bar/docJar.dest/out.jar bar/Bar.html
...
...<p>My Awesome Scaladoc for class Bar</p>...

Unmanaged Jars

build.sc (download, browse)

import mill._, scalalib._

object foo extends RootModule with ScalaModule {
  def scalaVersion = "2.13.8"
  def unmanagedClasspath = T {
    if (!os.exists(millSourcePath / "lib")) Agg()
    else Agg.from(os.list(millSourcePath / "lib").map(PathRef(_)))
  }
}

You can override unmanagedClasspath to point it at any jars you place on the filesystem, e.g. in the above snippet any jars that happen to live in the lib/ folder.

> ./mill run '{"name":"John","age":30}'     # mac/linux
Key: name, Value: John
Key: age, Value: 30

Specifying the Main Class

build.sc (download, browse)

import mill._, scalalib._

object foo extends RootModule with ScalaModule {
  def scalaVersion = "2.13.8"
  def mainClass = Some("foo.Qux")
}

Mill’s foo.run by default will discover which main class to run from your compilation output, but if there is more than one or the main class comes from some library you can explicitly specify which one to use. This also adds the main class to your foo.jar and foo.assembly jars.

> ./mill run
Hello Qux

Downloading Non-Maven Jars

build.sc (download, browse)

import mill._, scalalib._

object foo extends RootModule with ScalaModule {
  def scalaVersion = "2.13.8"
  def unmanagedClasspath = T {
    os.write(
      T.dest / "fastjavaio.jar",
      requests.get.stream(
        "https://github.com/williamfiset/FastJavaIO/releases/download/1.1/fastjavaio.jar"
      )
    )
    Agg(PathRef(T.dest / "fastjavaio.jar"))
  }
}

You can also override unmanagedClasspath to point it at jars that you want to download from arbitrary URLs. Note that targets like unmanagedClasspath are cached, so your jar is downloaded only once and re-used indefinitely after that.

> ./mill run "textfile.txt" # mac/linux
I am cow
hear me moo
I weigh twice as much as you

Customizing the Assembly

build.sc (download, browse)

import mill._, scalalib._
import mill.scalalib.Assembly._

object foo extends ScalaModule {
  def moduleDeps = Seq(bar)
  def scalaVersion = "2.13.8"
  def ivyDeps = Agg(ivy"com.lihaoyi::os-lib:0.9.1")
  def assemblyRules = Seq(
    // all application.conf files will be concatenated into single file
    Rule.Append("application.conf"),
    // all *.conf files will be concatenated into single file
    Rule.AppendPattern(".*\\.conf"),
    // all *.temp files will be excluded from a final jar
    Rule.ExcludePattern(".*\\.temp"),
    // the `shapeless` package will be shaded under the `shade` package
    Rule.Relocate("shapeless.**", "shade.shapless.@1")
  )
}

object bar extends ScalaModule {
  def scalaVersion = "2.13.8"
}

When you make a runnable jar of your project with assembly command, you may want to exclude some files from a final jar (like signature files, and manifest files from library jars), and merge duplicated files (for instance reference.conf files from library dependencies).

By default mill excludes all *.sf, *.dsa, *.rsa, and META-INF/MANIFEST.MF files from assembly, and concatenates all reference.conf files. You can also define your own merge/exclude rules.

> ./mill foo.assembly

> unzip -p ./out/foo/assembly.dest/out.jar application.conf
Bar Application Conf
Foo Application Conf

> java -jar ./out/foo/assembly.dest/out.jar\
Loaded application.conf from resources:...
...Foo Application Conf
...Bar Application Conf

Repository Config

By default, dependencies are resolved from maven central, but you can add your own resolvers by overriding the repositoriesTask task in the module:

build.sc (download, browse)

import mill._, scalalib._
import mill.define.ModuleRef
import coursier.maven.MavenRepository

val sonatypeReleases = Seq(
  MavenRepository("https://oss.sonatype.org/content/repositories/releases")
)

object foo extends ScalaModule {
  def scalaVersion = "2.13.8"

  def repositoriesTask = T.task {
    super.repositoriesTask() ++ sonatypeReleases
  }
}

Mill read coursier config files automatically.

It is possible to setup mirror with mirror.properties

central.from=https://repo1.maven.org/maven2
central.to=http://example.com:8080/nexus/content/groups/public

Note theses default config file locatations:

Linux: ~/.config/coursier/mirror.properties
MacOS: ~/Library/Preferences/Coursier/mirror.properties
Windows: C:\Users\<user_name>\AppData\Roaming\Coursier\config\mirror.properties

You can also set the environment variable COURSIER_MIRRORS or the jvm property coursier.mirrors to specify config file location.

To add custom resolvers to the initial bootstrap of the build, you can create a custom ZincWorkerModule, and override the zincWorker method in your ScalaModule by pointing it to that custom object:

object CustomZincWorkerModule extends ZincWorkerModule with CoursierModule {
  def repositoriesTask = T.task { super.repositoriesTask() ++ sonatypeReleases }
}

object bar extends ScalaModule {
  def scalaVersion = "2.13.8"
  def zincWorker = ModuleRef(CustomZincWorkerModule)
  // ... rest of your build definitions

  def repositoriesTask = T.task {super.repositoriesTask() ++ sonatypeReleases}
}

> ./mill bar.compile

Maven Central: Blocked!

Under some circumstances (e.g. corporate firewalls), you may not have access maven central. The typical symptom will be error messages which look like this;

1 targets failed
mill.scalalib.ZincWorkerModule.classpath
Resolution failed for 1 modules:
--------------------------------------------
  com.lihaoyi:mill-scalalib-worker_2.13:0.11.1
        not found: C:\Users\partens\.ivy2\local\com.lihaoyi\mill-scalalib-worker_2.13\0.11.1\ivys\ivy.xml
        download error: Caught java.io.IOException (Server returned HTTP response code: 503 for URL: https://repo1.maven.org/maven2/com/lihaoyi/mill-scalalib-worker_2.13/0.11.1/mill-scalalib-worker_2.13-0.11.1.pom) while downloading https://repo1.maven.org/maven2/com/lihaoyi/mill-scalalib-worker_2.13/0.11.1/mill-scalalib-worker_2.13-0.11.1.pom

It is expected that basic commands (e.g. clean) will not work, as Mill saying it is unable to resolve it’s own, fundamental, dependancies. Under such circumstances, you will normally have access to some proxy, or other corporate repository which resolves maven artefacts. The strategy is simply to tell mill to use that instead.

The idea is to set an environment variable COURSIER_REPOSITORIES (see coursier docs). The below command should set the environment variable for the current shell, and then run the mill command.

 COURSIER_REPOSITORIES=https://packages.corp.com/artifactory/maven/ mill resolve _

If you are using millw, a more permanent solution could be to set the environment variable at the top of the millw script, or as a user environment variable etc.

Backticked Names

build.sc (download, browse)

import mill._
import mill.scalalib._

object `hyphenated-module` extends Module {
  def `hyphenated-target` = T{
    println("hyphenated target in a hyphenated module.")
  }
}

object unhyphenatedModule extends Module {
  def unhyphenated_target = T{
    println("unhyphenated target in an unhyphenated module.")
  }
}

Mill modules and tasks may be composed of the following character types:

Alphanumeric (A-Z, a-z, and 0-9)
Underscore (_)
Hyphen (-)

Due to Scala naming restrictions, module and task names with hyphens must be surrounded by back-ticks (`).

Using hyphenated names at the command line is unaffected by these restrictions.

> ./mill hyphenated-module.hyphenated-target
hyphenated target in a hyphenated module.

> ./mill unhyphenatedModule.unhyphenated_target
unhyphenated target in an unhyphenated module.

Unidoc

build.sc (download, browse)

import mill._, scalalib._

object foo extends ScalaModule with UnidocModule{
  def scalaVersion = "2.13.8"
  def moduleDeps = Seq(bar, qux)

  object bar extends ScalaModule{
    def scalaVersion = "2.13.8"
  }

  object qux extends ScalaModule {
    def scalaVersion = "2.13.8"
    def moduleDeps = Seq(bar)
  }

  def unidocVersion = Some("0.1.0")
  def unidocSourceUrl = Some("https://github.com/lihaoyi/test/blob/master")
}

This example demonstrates use of mill.scalalib.UnidocModule. This can be mixed in to any ScalaModule, and generates a combined Scaladoc for the module and all its transitive dependencies. Two targets are provided:

.unidocLocal: this generates a site suitable for local browsing. If unidocSourceUrl is provided, the scaladoc provides links back to the local sources
.unidocSite: this generates a site suitable for local browsing. If unidocSourceUrl is provided, the scaladoc provides links back to the sources as browsable from the unidocSourceUrl base (e.g. on Github)

> ./mill show foo.unidocLocal
".../out/foo/unidocLocal.dest"

> cat out/foo/unidocLocal.dest/foo/Foo.html
...
...My Eloquent Scaladoc for Foo...

> cat out/foo/unidocLocal.dest/foo/qux/Qux.html
...
...My Excellent Scaladoc for Qux...

> cat out/foo/unidocLocal.dest/foo/bar/Bar.html
...
...My Lucid Scaladoc for Bar...

> ./mill show foo.unidocSite

Reformatting your code

Mill supports code formatting via scalafmt out of the box.

To have a formatting per-module you need to make your module extend mill.scalalib.scalafmt.ScalafmtModule:

build.sc

import mill._, scalalib._, scalafmt._

object foo extends ScalaModule with ScalafmtModule {
  def scalaVersion = "2.13.12"
}

Now you can reformat code with mill foo.reformat command, or only check for misformatted files with mill foo.checkFormat.

You can also reformat your project’s code globally with mill mill.scalalib.scalafmt.ScalafmtModule/reformatAll __.sources command, or only check the code’s format with mill mill.scalalib.scalafmt.ScalafmtModule/checkFormatAll __.sources. It will reformat all sources that matches __.sources query.

If you add a .scalafmt.conf file at the root of you project, it will be used to configure formatting. It can contain a version key to specify the scalafmt version used to format your code. See the scalafmt configuration documentation for details.

Using the Ammonite Repl / Scala console

All ScalaModules have a console and a repl target, to start a Scala console or an Ammonite Repl.

When using the console, you can configure its scalac options using the consoleScalacOptions target.

For example, you may want to inherit all of your regular scalacOptions but disable -Xfatal-warnings:

Example: Using consoleScalacOptions to disable fatal warnings

import mill._, scalalib._

object foo extends ScalaModule {
  def consoleScalacOptions = scalacOptions().filterNot(o => o == "-Xfatal-warnings")
}

To use the repl, you can (and sometimes need to) customize the Ammonite version to work with your selected Scala version. Mill provides a default Ammonite version, but depending on the Scala version you are using, there may be no matching Ammonite release available. In order to start the repl, you may have to specify a different available Ammonite version.

Example: Overriding ammoniteVersion to select a release compatible to the scalaVersion

import mill._. scalalib._

object foo extends ScalaModule {
  def scalaVersion = "2.12.6"
  def ammoniteVersion = "2.4.0"
}

Why is Ammonite tied to the exact Scala version?

This is because Ammonite depends on the Scala compiler. In contrast to the Scala library, compiler releases do not guarantee any binary compatibility between releases. As a consequence, Ammonite needs full Scala version specific releases.

The older your used Mill version or the newer the Scala version you want to use, the higher is the risk that the default Ammonite version will not match.

Disabling incremental compilation with Zinc

By default all `ScalaModule`s use incremental compilation via Zinc to only recompile sources that have changed since the last compile, or ones that have been invalidated by changes to upstream sources.

If for any reason you want to disable incremental compilation for a module, you can override and set zincIncrementalCompilation to false

build.sc

import mill._, scalalib._

object foo extends ScalaModule {
  def zincIncrementalCompilation = false
}