## Hello Mill

The simplest Mill build for a Scala project looks as follows:

```scala
import mill._
import mill.scalalib._

object foo extends ScalaModule {
  def scalaVersion = "2.12.4"
}
```

This would build a project laid out as follows:

```
build.sc
foo/
    src/
        Main.scala
    resources/
        ...
out/
    foo/
        ... 
```

The source code for this module would live in the `foo/src/` folder, matching
the name you assigned to the module. Output for this module (compiled files,
resolved dependency lists, ...) would live in `out/foo/`.

This can be run from the Bash shell via:

```bash
$ mill foo.compile        # compile sources into classfiles

$ mill foo.run            # run the main method, if any

$ mill foo.jar            # bundle the classfiles into a jar

$ mill foo.assembly       # bundle the classfiles and all dependencies into a jar 
```

The most common **tasks** that Mill can run are cached **targets**, such as
`compile`, and un-cached **commands** such as `foo.run`. Targets do not
re-evaluate unless one of their inputs changes, where-as commands re-run every
time.


### Multiple Modules

```scala
import mill._
import mill.scalalib._
object foo extends ScalaModule {
  def scalaVersion = "2.12.4"
}
object bar extends ScalaModule {
  def moduleDeps = Seq(foo)
  def scalaVersion = "2.12.4"
}
```

You can define multiple modules the same way you define a single module, using
`def moduleDeps` to define the relationship between them. The above build
expects the following project layout:

```
build.sc
foo/
    src/
        Main.scala
    resources/
        ...
bar/
    src/
        Main2.scala
    resources/
        ...
out/
    foo/
        ... 
    bar/
        ... 
```

And can be built/run using:

```bash
$ mill foo.compile        
$ mill bar.compile        

$ mill foo.run            
$ mill bar.run            

$ mill foo.jar            
$ mill bar.jar            

$ mill foo.assembly        
$ mill bar.assembly        
```

Mill's evaluator will ensure that the modules are compiled in the right order,
and re-compiled as necessary when source code in each module changes.

Modules can also be nested:

```scala
import mill._
import mill.scalalib._
object foo extends ScalaModule {
  def scalaVersion = "2.12.4"
  object bar extends ScalaModule {
    def moduleDeps = Seq(foo)
    def scalaVersion = "2.12.4"
  }
}
```

Which would result in a similarly nested project layout:

```
build.sc
foo/
    src/
        Main.scala
    resources/
        ...
    bar/
        src/
            Main2.scala
        resources/
            ...
out/
    foo/
        ...
        bar/
            ...
```

Where the nested modules can be run via:

```bash
$ mill foo.compile        
$ mill foo.bar.compile        

$ mill foo.run            
$ mill foo.bar.run            

$ mill foo.jar            
$ mill foo.bar.jar            

$ mill foo.assembly        
$ mill foo.bar.assembly        
```

### Watch and Re-evaluate

You can use the `--watch` flag to make Mill watch a task's inputs, re-evaluating
the task as necessary when the inputs change:

```bash
$ mill --watch foo.compile 
$ mill --watch foo.run 
```

### Command-line Tools

Mill comes built in with a small number of useful command-line utilities:

#### all

```bash
mill all foo.{compile,run}
mill all "foo.{compile,run}"
mill all foo.compile foo.run
mill all _.compile # run compile for every top-level module
mill all __.compile  # run compile for every module
```

`all` runs multiple tasks in a single command

#### resolve

```bash
mill resolve foo.{compile,run}
mill resolve "foo.{compile,run}"
mill resolve foo.compile foo.run
mill resolve _.compile          # list the compile tasks for every top-level module
mill resolve __.compile         # list the compile tasks for every module
mill resolve _                  # list every top level module or task
mill resolve foo._              # list every task directly within the foo module
mill resolve __                 # list every module or task recursively
mill resolve foo._              # list every task recursively within the foo module
```

`resolve` lists the tasks that match a particular query, without running them.
This is useful for "dry running" an `mill all` command to see what would be run
before you run them, or to explore what modules or tasks are available from the
command line using `resolve _`, `resolve foo._`, etc.


#### describe

```bash
$ mill describe core.run

core.run(ScalaModule.scala:211)
Inputs:
    core.mainClass
    core.runClasspath
    core.forkArgs
    core.forkEnv
```

`describe` is a more verbose version of [resolve](#resolve). In addition to
printing out the name of one-or-more tasks, it also display's it's source
location and a list of input tasks. This is very useful for debugging and
interactively exploring the structure of your build from the command line.

`describe` also works with the same `_`/`__` wildcard/query syntaxes that
[all](#all)/[resolve](#resolve) do:


```bash
mill describe foo.compile
mill describe foo.{compile,run}
mill describe "foo.{compile,run}"
mill describe foo.compile foo.run
mill describe _.compile
mill describe __.compile
mill describe _
mill describe foo._
mill describe __
mill describe foo._
```

#### show

By default, Mill does not print out the metadata from evaluating a task. Most
people would not be interested in e.g. viewing the metadata related to
incremental compilation: they just want to compile their code! However, if you
want to inspect the build to debug problems, you can make Mill show you the
metadata output for a task using the `show` flag:

You can also ask Mill to display the metadata output of a task using `show`:

```bash
$ mill show foo.compile
{
    "analysisFile": "/Users/lihaoyi/Dropbox/Github/test/out/foo/compile/dest/zinc",
    "classes": {
        "path": "/Users/lihaoyi/Dropbox/Github/test/out/foo/compile/dest/classes"
    }
}
```

This also applies to tasks which hold simple configurable values:

```bash
$ mill show foo.sources
[
    {"path": "/Users/lihaoyi/Dropbox/Github/test/foo/src"}
]

$ mill show foo.compileDepClasspath
[
    {"path": ".../org/scala-lang/scala-compiler/2.12.4/scala-compiler-2.12.4.jar"},
    {"path": ".../org/scala-lang/scala-library/2.12.4/scala-library-2.12.4.jar"},
    {"path": ".../org/scala-lang/scala-reflect/2.12.4/scala-reflect-2.12.4.jar"},
    {"path": ".../org/scala-lang/modules/scala-xml_2.12/1.0.6/scala-xml_2.12-1.0.6.jar"}
]
```

`show` is also useful for interacting with Mill from external tools, since the
JSON it outputs is structured and easily parsed & manipulated.

### IntelliJ Support

Mill supports IntelliJ by default. Use `mill mill.scalalib.GenIdea/idea` to
generate an IntelliJ project config for your build.

This also configures IntelliJ to allow easy navigate & code-completion within
your build file itself.


Any flags passed *before* the name of the task (e.g. `foo.compile`) are given to
Mill, while any arguments passed *after* the task are given to the task itself.
For example:

### The Build Repl

```bash
$ mill
Loading...
@ foo
res1: foo.type = ammonite.predef.build#foo:2
Commands:
    .runLocal(args: String*)()
    .run(args: String*)()
    .runMainLocal(mainClass: String, args: String*)()
    .runMain(mainClass: String, args: String*)()
    .console()()
Targets:
    .allSources()
    .artifactId()
    .artifactName()
...

@ foo.compile
res3: mill.package.T[mill.scalalib.CompilationResult] = mill.scalalib.ScalaModule#compile:152
Inputs:
    foo.scalaVersion
    foo.allSources
    foo.compileDepClasspath
...
    
@ foo.compile()
res2: mill.scalalib.CompilationResult = CompilationResult(
  root/'Users/'lihaoyi/'Dropbox/'Github/'test/'out/'foo/'compile/'dest/'zinc,
  PathRef(root/'Users/'lihaoyi/'Dropbox/'Github/'test/'out/'foo/'compile/'dest/'classes, false)
)
```

You can run `mill` alone to open a build REPL; this is a Scala console with your
`build.sc` loaded, which lets you run tasks interactively. The task-running
syntax is slightly different from the command-line, but more in-line with how
you would depend on tasks from within your build file.

You can use this REPL to run build commands quicker, due to keeping the JVM warm
between runs, or to interactively explore your build to see what is available.


## Configuring Mill

You can configure your Mill build in a number of ways:

### Compilation & Execution Flags

```scala
import mill._
import mill.scalalib._
object foo extends ScalaModule {
  def scalaVersion = "2.12.4"
  
  def scalacOptions = Seq("-Ydelambdafy:inline")
  
  def forkArgs = Seq("-Xmx4g")
  
  def forkEnv = Map("HELLO_MY_ENV_VAR" -> "WORLD")
}
```

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

```bash
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`.

### Adding Ivy Dependencies

```scala
import mill._
import mill.scalalib._
object foo extends ScalaModule {
  def scalaVersion = "2.12.4"
  def ivyDeps = Agg(
    ivy"com.lihaoyi::upickle:0.5.1",
    ivy"com.lihaoyi::pprint:0.5.2",
    ivy"com.lihaoyi::fansi:0.2.4"
  )
}
```

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"`.

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

```scala
def repositories = super.repositories ++ Seq(
  MavenRepository("https://oss.sonatype.org/content/repositories/releases")
)
```

### Adding a Test Suite

```scala
import mill._
import mill.scalalib._
object foo extends ScalaModule {
  def scalaVersion = "2.12.4"

  object test extends Tests{ 
    def ivyDeps = Agg(ivy"com.lihaoyi::utest:0.6.0")
    def testFramework = "mill.UTestFramework"
  }
}
```

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:

```
build.sc
foo/
    src/
        Main.scala
    resources/
        ...
    test/
        src/
            MainTest.scala
        resources/
            ...
out/
    foo/
        ...
        test/
            ...
```

The above example can be run via

```bash
mill foo.test
```

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

```bash
mill foo.test.testLocal
```

To run tests in-process in an isolated classloader.

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

```scala
import mill._
import mill.scalalib._
object foo extends ScalaModule {
  def scalaVersion = "2.12.4"

  object test extends Tests{ 
    def ivyDeps = Agg(ivy"com.lihaoyi::utest:0.6.0")
    def testFramework = "mill.UTestFramework"
  }
  object integration extends Tests{ 
    def ivyDeps = Agg(ivy"com.lihaoyi::utest:0.6.0")
    def testFramework = "mill.UTestFramework"
  }
}
```

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

`Tests` modules are `ScalaModule`s like any other, and all the same
configuration options apply.


### Scala Compiler Plugins

```scala
import mill._
import mill.scalalib._
object foo extends ScalaModule {
  def scalaVersion = "2.12.4"
  
  def compileIvyDeps = Agg(ivy"com.lihaoyi::acyclic:0.1.7")
  def scalacOptions = Seq("-P:acyclic:force")
  def scalacPluginIvyDeps = Agg(ivy"com.lihaoyi::acyclic:0.1.7")
}
```

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).

### Common Configuration

```scala
import mill._
import mill.scalalib._
trait CommonModule extends ScalaModule{
  def scalaVersion = "2.12.4"
}
 
object foo extends CommonModule
object bar extends CommonModule {
  def moduleDeps = Seq(foo)
}
```

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`.

### Custom Tasks

```scala
import mill._
import mill.scalalib._
object foo extends ScalaModule {
  def scalaVersion = "2.12.4"
}

def lineCount = T{
  import ammonite.ops._
  foo.sources().flatMap(ref => ls.rec(ref.path)).flatMap(read.lines).size
}

def printLineCount() = T.command{
  println(lineCount())
}
```

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](https://github.com/lihaoyi/upickle)) and the Target is cached when
first run until it's 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.

```bash
mill run show lineCount
```

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.

### Custom Modules

```scala
import mill._
import mill.scalalib._
object qux extends Module{
  object foo extends ScalaModule {
    def scalaVersion = "2.12.4"
  }
  object bar extends ScalaModule {
    def moduleDeps = Seq(foo)
    def scalaVersion = "2.12.4"
  }
}
```

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`:

```bash
mill qux.foo.compile
mill qux.bar.run
```

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,:

```scala
trait MySpecialModule extends Module{
  ...
}
object foo extends MySpecialModule
object bar extends MySpecialModule
```

### Overriding Tasks

```scala
import mill._
import mill.scalalib._

object foo extends ScalaModule {
  def scalaVersion = "2.12.4"
  def compile = T{
    println("Compiling...")
    super.compile()
  }
  def run(args: String*) = T.command{
    println("Running... + args.mkString(" "))
    super.run(args:_*)
  }
}
```

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.

In Mill builds the `override` keyword is optional.

## Common Project Layouts

Above, we have shown how to work with the Mill default Scala module layout. Here
we will explore some other common project layouts that you may want in your
Scala build:

### Cross Scala-Version Modules

```scala
import mill._
import mill.scalalib._
object foo extends Cross[FooModule]("2.10.6", "2.11.11", "2.12.4")
class FooModule(val crossScalaVersion: String) extends CrossScalaModule{
   ...
}
```

Mill provides a `CrossScalaModule` template, which can be used with `Cross` to
cross-build Scala modules across different versions of Scala. The default
configuration for `CrossScalaModule` expects a filesystem layout as follows:

```text
build.sc
foo/
    src/
    src-2.10/
    src-2.11/
    src-2.12/
    test/
        src/
        src-2.10/
        src-2.11/
        src-2.12/
```

Code common to all Scala versions lives in `src`, while code specific to one
version lives in `src-x.y`.

### Scala.js Modules

```scala
import mill._
import mill.scalajslib._

object foo extends ScalaJSModule {
  def scalaVersion = "2.12.4"
  def scalaJSVersion = "0.6.22"
}
```

`ScalaJSModule` is a variant of `ScalaModule` that builds your code using
Scala.js. In addition to the standard `foo.compile` and `foo.run` commands (the
latter of which runs your code on Node.js, which must be pre-installed)
`ScalaJSModule` also exposes the `foo.fastOpt` and `foo.fullOpt` tasks for
generating the optimized Javascript file.

### SBT-Compatible Modules

```scala
import mill._
import mill.scalalib._

object foo extends SbtModule {
  def scalaVersion = "2.12.4"
}
```

These are basically the same as normal `ScalaModule`s, but configured to follow
the SBT project layout:

```text
build.sc
foo/
    src/
        main/
            scala/
        test/
            scala/
```

Useful if you want to migrate an existing project built with SBT without having
to re-organize all your files


### SBT-Compatible Cross Scala-Version Modules

```scala
import mill._
import mill.scalalib._
object foo extends Cross[FooModule]("2.10.6", "2.11.11", "2.12.4")
class FooModule(val crossScalaVersion: String) extends CrossSbtModule{
   ...
}
```

A `CrossSbtModule` is a version of `CrossScalaModule` configured with the SBT
project layout:

```text
build.sc
foo/
    src/
        main/
            scala/
            scala-2.10/
            scala-2.11/
            scala-2.12/
        test/
            scala/
            scala-2.10/
            scala-2.11/
            scala-2.12/
```

### Publishing
```scala
import mill._
import mill.scalalib._
object foo extends ScalaModule with PublishModule{
  def scalaVersion = "2.12.4"
  def publishVersion = "0.0.1
  def pomSettings = PomSettings(
    description = "My first library",
    organization = "com.lihaoyi",
    url = "https://github.com/lihaoyi/mill",
    licenses = Seq(
      License("MIT license", "http://www.opensource.org/licenses/mit-license.php")
    ),
    scm = SCM(
      "git://github.com/lihaoyi/mill.git",
      "scm:git://github.com/lihaoyi/mill.git"
    ),
    developers = Seq(
      Developer("lihaoyi", "Li Haoyi","https://github.com/lihaoyi")
    )
  )
}
```

You can make a module publishable by extending `PublishModule`.

`PublishModule` then needs you to define a `publishVersion` and `pomSettings`.
The `artifactName` defaults to the name of your module (in this case `foo`) but
can be overriden. The `organization` is defined in `pomSettings`.

Once you've mixed in `PublishModule`, you can publish your libraries to maven
central  via:

```bash
target/bin/mill mill.scalalib.PublishModule/publishAll \
        lihaoyi:$SONATYPE_PASSWORD \
        $GPG_PASSWORD \ 
        foo.publishArtifacts
```

This uploads them to `oss.sonatype.org` where you can log-in and stage/release
them manually. You can also pass in the `--release true` flag to perform the
staging/release automatically:

```bash
target/bin/mill mill.scalalib.PublishModule/publishAll \
        lihaoyi:$SONATYPE_PASSWORD \
        $GPG_PASSWORD \ 
        foo.publishArtifacts \
        --release true
```

If you want to publish/release multiple modules, you can use the `_` or `__`
wildcard syntax:

```bash
target/bin/mill mill.scalalib.PublishModule/publishAll \
        lihaoyi:$SONATYPE_PASSWORD \
        $GPG_PASSWORD \ 
        __.publishArtifacts \
        --release true
```


## Example Builds

Mill comes bundled with example builds for existing open-source projects, as
integration tests and examples:


### Acyclic

- [Mill Build](https://github.com/lihaoyi/mill/blob/master/integration/test/resources/acyclic/build.sc#L1)

A small single-module cross-build, with few sources minimal dependencies, and
wired up for publishing to Maven Central


### Better-Files

- [Mill Build](https://github.com/lihaoyi/mill/blob/master/integration/test/resources/better-files/build.sc#L1)

A collection of small modules compiled for a single Scala version.

Also demonstrates how to define shared configuration in a `trait`, enable Scala
compiler flags, and download artifacts as part of the build.

### Jawn

- [Mill Build](https://github.com/lihaoyi/mill/blob/master/integration/test/resources/jawn/build.sc#L1)

A collection of relatively small modules, all cross-built across the same few
versions of Scala.


### Ammonite

- [Mill Build](https://github.com/lihaoyi/mill/blob/master/integration/test/resources/ammonite/build.sc#L1)

A relatively complex build with numerous submodules, some cross-built across
Scala major versions while others are cross-built against Scala minor versions.

Also demonstrates how to pass one module's compiled artifacts to the
`run`/`test` commands of another, via their `forkEnv`.