Configuring Mill

You can pass flags to the Scala compiler via scalacOptions. By default, foo.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

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:

You can define the ivyDeps field to add ivy dependencies to your module. The ivy"com.lihaoyi::upickle:0.5.1" syntax (with ::) represents Scala dependencies; for Java dependencies you would use a single : e.g. "ivy"com.lihaoyi:upickle:0.5.1". If you have dependencies cross-published against the full Scala version (eg. 2.12.4 instead of just 2.12), you can use ::: as in ivy"org.scalamacros:::paradise:2.1.1".

To select the test-jars from a dependency use the following syntax: ivy"org.apache.spark::spark-sql:2.4.0;classifier=tests.

By default, these are resolved from maven central, but you can add your own resolvers by overriding the repositories definition in the module:

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:

For convenience, you can also use one of the predefined test frameworks:

You can define a test suite by creating a nested module extending Tests, and specifying the ivy coordinates and name of your test framework. This expects the tests to be laid out as follows:

The above example can be run via

By default, tests are run in a subprocess, and forkArg and forkEnv can be overriden to pass JVM flags & environment variables. You can also use

To run tests in-process in an isolated classloader.

If you want to pass any arguments to the test framework, simply put them after foo.test in the command line. e.g. uTest lets you pass in a selector to decide which test to run, which in Mill would be:

You can define multiple test suites if you want, e.g.:

Each of which will expect their sources to be in their respective foo/test and foo/integration folder.

Tests modules are ScalaModules like any other, and all the same configuration options apply.

Integrating with test frameworks like Scalatest or specs2 is simply a matter of adding it to ivyDeps and specifying the testFramework you want to use.

Scalatest example:

Specs2 example:

After that, you can follow the instructions in Adding a Test Suite, and use mill foo.test as usual, or pass args to the test suite via mill foo.test arg1 arg2 arg3.

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).

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:

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.

You can extract out configuration common to multiple modules into a trait that those modules extend. This is useful for providing convenience & ensuring consistent configuration: every module often has the same scala-version, uses the same testing framework, etc. and all that can be extracted out into the trait.

Mill builds on ammonite which allows you to define global configuration. Depending on how you start mill, one of two files will be loaded. For the build REPL (--repl or -i without specifying a target), ~/.mill/ammonite/predef.sc will be loaded, and for builds from the command line the file ~/.mill/ammonite/predefScript.sc will be included. You might want to create a symlink from one to the other to avoid duplication.

Example ~/.mill/ammonite/predef.sc

Everything declared in the above file will be available to any build you run.

You can define new cached Targets using the T {…​} syntax, depending on existing Targets e.g. foo.sources via the foo.sources() syntax to extract their current value, as shown in lineCount above. The return-type of a Target has to be JSON-serializable (using uPickle) and the Target is cached when first run until its inputs change (in this case, if someone edits the foo.sources files which live in foo/src. Cached Targets cannot take parameters.

You can print the value of your custom target using show, e.g.

You can define new un-cached Commands using the T.command {…​} syntax. These are un-cached and re-evaluate every time you run them, but can take parameters. Their return type needs to be JSON-writable as well, or (): Unit if you want to return nothing.

Your custom targets can depend on each other using the def bar = T {…​ foo() …​} syntax, and you can create arbitrarily long chains of dependent targets. Mill will handle the re-evaluation and caching of the targets' output for you, and will provide you a T.dest folder for you to use as scratch space or to store files you want to return.

Custom targets and commands can contain arbitrary code. Whether you want to download files (e.g. using mill.modules.Util.download), shell-out to Webpack to compile some Javascript, generate sources to feed into a compiler, or create some custom jar/zip assembly with the files you want (e.g. using mill.modules.Jvm.createJar), all of these can simply be custom targets with your code running in the T {…​} block.

Not every Module needs to be a ScalaModule; sometimes you just want to group things together for neatness. In the above example, you can run foo and bar namespaced inside qux:

You can also define your own module traits, with their own set of custom tasks, to represent other things e.g. Javascript bundles, docker image building,:

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

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.

You can re-define targets and commands to override them, and use super if you want to refer to the originally defined task. The above example shows how to override compile and run to add additional logging messages, but you can also override ScalaModule#generatedSources to feed generated code to your compiler, ScalaModule#prependShellScript to make your assemblies executable, or ScalaModule#console to use the Ammonite REPL instead of the normal Scala REPL.

In Mill builds the override keyword is optional.

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 foo/lib/ folder.

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.

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.

To exclude Scala library from assembly

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.