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Mill: Better Scala Builds

Posted 2018-02-22
Build Tools as Pure Functional Programs

Mill is a new build tool for Scala: it compiles your Scala code, packages it, runs it, and caches things to avoid doing unnecessary work. Mill aims to be better than Scala's venerable old SBT build tool, learning from it's mistakes and building upon ideas from functional programming to come up with a build tool that is fast, flexible, and easy to understand and use. This post will explore what makes Mill interesting to a Scala developer who is likely already using SBT

The main Mill documentation already covers getting started with Mill and how to use it, and early blog posts cover in detail how we can improve upon with SBT and design intuitive build tools. This post will cover the highlights of what makes Mill a nice build tool, in practice, to someone in-the-trenches trying to make their build do the right thing .

A build tool like SBT contains a lot of functionality. Mill supports most of the common workflows in SBT, but in many cases does them better, faster, and in a more user-friendly way.

For many of these examples, I will be comparing the SBT and Mill builds for the Ammonite project.

Command-line Friendliness

SBT has always had a bit of awkwardness passing arguments to main methods or test suites from the command line:

sbt "main/run arg1 arg2 arg3"
sbt "main/test-only arg1 arg2 arg3"

With Mill, it's a lot cleaner:

mill main.run arg1 arg2 arg3
mill main.test arg1 arg2 arg3

Mill follows the command line convention that every other interpreter follows:

interpreter [interpreter-flags] interpreted-command [command-flags]

java        -cp Foo.jar         foo.Main             arg1 arg2 arg3
python      -u                  hello.py             arg1 arg2 arg3

For example, here is a comparison of how you pass a test selector to the uTest test framework using SBT and Mill:

sbt "core/test-only -- mypackage.MySuite.myTestCase"
mill core.test mypackage.MySuite.myTestCase

And turning on watch-and-re-evaluate:

sbt "~core/test-only -- mypackage.MySuite.myTestCase"
mill --watch core.test mypackage.MySuite.myTestCase

Thus, when evaluating targets or commands using Mill, you tend to have to worry a whole lot less about quoting, escaping or syntax than you do using SBT. Rather than re-inventing it's own command-line convention, Mill falls in line with the conventions you are likely already familiar with. You should hopefully find using Mill from the command line simple and predictable, with much less quoting and escaping that you have to worry about.

External-tool Friendliness

Unlike SBT, Mill does not aim to be the center of your world: you can happily evaluate Mill tasks from bash or external scripts without worrying too much about fixed overhead or performance. You do not need to integrate everything into SBT, but rather can call out to Mill from whatever other scripts or tools you are already using.

This philosophy extends to far more than just performance!

Mill's out/ folder is simple and predictable:

This output includes both a meta.json as well as a dest/ folder that contains any output files. As an external tool, you trivially parse this meta.json file to find exactly the results you want from the output of a command.

If you want to figure out why a build is "going wrong", you can inspect the task graph and open up the relevant JSON files in Vim to see exactly what data is being passed where, without needing to open up a special Mill console for debugging.

You can also ask Mill to print out the JSON output for a particular task directly via the mill show foo.bar.baz syntax, e.g.

Ammonite$ mill show ops[2.12.4].scalaVersion

Ammonite$ mill show ops[2.12.4].allSourceFiles
    {"path": "~/Github/Ammonite/ops/src/main/scala/ammonite/ops/Extensions.scala"},
    {"path": "~/Github/Ammonite/ops/src/main/scala/ammonite/ops/FileOps.scala"},
    {"path": "~/Github/Ammonite/ops/src/main/scala/ammonite/ops/Model.scala"},
    {"path": "~/Github/Ammonite/ops/src/main/scala/ammonite/ops/Path.scala"},
    {"path": "~/Github/Ammonite/ops/src/main/scala/ammonite/ops/PathUtils.scala"},
    {"path": "~/Github/Ammonite/ops/src/main/scala/ammonite/ops/Shellout.scala"},
    {"path": "~/Github/Ammonite/ops/src/main/scala/ammonite/ops/package.scala"}

This is in contrast with SBT which outputs showed values in an ad-hoc hard-to-parse format

lihaoyi Ammonite$ sbt "show ops/sources"
[warn] Executing in batch mode.
[warn]   For better performance, hit [ENTER] to switch to interactive mode, or
[warn]   consider launching sbt without any commands, or explicitly passing 'shell'
[info] Loading project definition from /Users/lihaoyi/Dropbox/Github/Ammonite/project
[info] Set current project to ammonite (in build file:/Users/lihaoyi/Dropbox/Github/Ammonite/)
[info] * /Users/lihaoyi/Dropbox/Github/Ammonite/ops/src/main/scala/ammonite/ops/Model.scala
[info] * /Users/lihaoyi/Dropbox/Github/Ammonite/ops/src/main/scala/ammonite/ops/FileOps.scala
[info] * /Users/lihaoyi/Dropbox/Github/Ammonite/ops/src/main/scala/ammonite/ops/Extensions.scala
[info] * /Users/lihaoyi/Dropbox/Github/Ammonite/ops/src/main/scala/ammonite/ops/PathUtils.scala
[info] * /Users/lihaoyi/Dropbox/Github/Ammonite/ops/src/main/scala/ammonite/ops/Path.scala
[info] * /Users/lihaoyi/Dropbox/Github/Ammonite/ops/src/main/scala/ammonite/ops/package.scala
[info] * /Users/lihaoyi/Dropbox/Github/Ammonite/ops/src/main/scala/ammonite/ops/Shellout.scala
[info] * /Users/lihaoyi/Dropbox/Github/Ammonite/project/Constants.scala
[success] Total time: 0 s, completed 22 Feb, 2018 9:15:09 PM

Mill also automatically redirects all unnecessary debug output to stderr, leaving the pristine un-contaminated JSON output to stdout for you to parse or pipe around.

Thus, external tools do not need a "Mill integration" or "Mill plugin" in order to extract data from a Mill build. Anyone with a JSON parser can shell out to Mill and extract the data they need, and with Mill's low fixed overhead such an operation is both fast and convenient.

Jump to Definition

Imagine you see a piece of SBT code in your IDE:

// Aggregate source jars into the assembly classpath, so that the
// `source` macro can find their sources and highlight/display them.
(fullClasspath in Runtime) ++= {
  (updateClassifiers in Runtime).value
    .find(_.configuration == Runtime.name)
    .collect{case (a, f) if a.classifier == Some("sources") => f}

You aren't familiar with this code, so you wonder: what was fullClasspath before I added this stuff to it? And what does updateClassifiers do?

Your IDE likely has jump-to-definition, and you heard it has SBT support, so you put your mouse on fullClasspath and updateClassifiers and hit Ctrl-B (IntelliJ's hotkey) to jump to it's definition and see what their values are.

You are then faced with the following results in keys.scala:

val externalDependencyClasspath = TaskKey[Classpath]("external-dependency-classpath", "The classpath consisting of library dependencies, both managed and unmanaged.", BMinusTask)
val dependencyClasspath = TaskKey[Classpath]("dependency-classpath", "The classpath consisting of internal and external, managed and unmanaged dependencies.", BPlusTask)
val fullClasspath = TaskKey[Classpath]("full-classpath", "The exported classpath, consisting of build products and unmanaged and managed, internal and external dependencies.", BPlusTask)
val trackInternalDependencies = SettingKey[TrackLevel]("track-internal-dependencies", "The level of tracking for the internal (inter-project) dependency.", BSetting)
val exportToInternal = SettingKey[TrackLevel]("export-to-internal", "The level of tracking for this project by the internal callers.", BSetting)
val evictionWarningOptions = SettingKey[EvictionWarningOptions]("eviction-warning-options", "Options on eviction warnings after resolving managed dependencies.", DSetting)
val transitiveUpdate = TaskKey[Seq[UpdateReport]]("transitive-update", "UpdateReports for the internal dependencies of this project.", DTask)
val updateClassifiers = TaskKey[UpdateReport]("update-classifiers", "Resolves and optionally retrieves classified artifacts, such as javadocs and sources, for dependency definitions, transitively.", BPlusTask, update)
val transitiveClassifiers = SettingKey[Seq[String]]("transitive-classifiers", "List of classifiers used for transitively obtaining extra artifacts for sbt or declared dependencies.", BSetting)
val updateSbtClassifiers = TaskKey[UpdateReport]("update-sbt-classifiers", "Resolves and optionally retrieves classifiers, such as javadocs and sources, for sbt, transitively.", BPlusTask, updateClassifiers)

You can see their type, maybe a short comment saying what it's meant to do, but where is the code? After all, the code is what tells you what something actually does!

Depending on how expert you are at SBT, you may find yourself in Defaults.scala

If you are lucky, you may spend some time grepping around before you find the relevant definition:

updateClassifiers := (Def.task {
  val s = streams.value
  val is = ivySbt.value
  val mod = (classifiersModule in updateClassifiers).value
  val c = updateConfiguration.value
  val app = appConfiguration.value
  val out = is.withIvy(s.log)(_.getSettings.getDefaultIvyUserDir)
  val uwConfig = (unresolvedWarningConfiguration in update).value
  val depDir = dependencyCacheDirectory.value
  withExcludes(out, mod.classifiers, lock(app)) { excludes =>
    val uwConfig = (unresolvedWarningConfiguration in update).value
    val logicalClock = LogicalClock(state.value.hashCode)
    val depDir = dependencyCacheDirectory.value
    val artifacts = update.value.toSeq.toVector
    IvyActions.updateClassifiers(is, GetClassifiersConfiguration(mod, excludes, c, ivyScala.value), uwConfig, LogicalClock(state.value.hashCode), Some(depDir), artifacts, s.log)
} tag (Tags.Update, Tags.Network)).value

Now, if you're curious for any of the other things that it's calling .value on (streams, ivySbt, mod, updateConfiguration, ...) you have to repeat the same dance: maybe one or two jumps, and then mostly Cmd-F and praying you find what you want. Grep, grep, grep...

Why is that?

The reason why jump-to-definition doesn't take you immediately to the relevant piece of code, is that SBT's execution model is basically the same as assigning null to a variable and then assigning the real value to it later, somewhere else.

Of course, your "variable" is called a TaskKey instead of a var, your assignment is called := instead of =, "null" is called something else.

But fundamentally you are assigning null to a variable and filling in the real value later, somewhere else.

Given that execution model, it's not surprising jump-to-definition is useless and you find yourself grepping. If you programmed with vars initialized to null in your own code, you would find yourself grepping too!

You will undoubtedly meet Mill code that is initially as confusing as the SBT code you met above:

def repositories = super.repositories ++ Seq(

def devAssembly = T{
    (scalalib.testArgs() ++ scalajslib.testArgs() ++ scalaworker.testArgs()).mkString(" ")

But once you start jumping to definition, things are better. Jumping to super.repositories brings you to:

def repositories: Seq[Repository] = Seq(

assemblyClasspath to:

def assemblyClasspath = mill.define.Task.traverse(assemblyProjects)(_.runClasspath)

scalaworker.testArgs to:

def testArgs = Seq(
  "-DMILL_SCALA_WORKER=" + runClasspath().map(_.path).mkString(",")

runClasspath to:

def runClasspath = T{
  upstreamRunClasspath() ++
  Agg(compile().classes) ++
  resources() ++
  unmanagedClasspath() ++
  resolveDeps(T.task{runIvyDeps() ++ scalaLibraryIvyDeps() ++ transitiveIvyDeps()})()

While of these snippets are trivially understandable, each one offers a glimpse into how the returned value is computed, and provides more places you could jump into if you want to dig deeper. You still have to work to understand it, but where in SBT you may hit a wall, in Mill even complex builds can be explored in a straightforward fashion using your IDE.

The reason Mill lets you do this is itself interesting: by following a pure-functional evaluation model, Mill ensures that the source-code-level call-graph of your various Tasks mirrors exactly the runtime dependency graph of the instantiated Tasks, as well as the final data-flow graph of how data flows between them. That is why Mill can provide a useful jump-to-definition while SBT, with it's mutable evaluation model, cannot.

While Mill's builds can get complicated, having a useful jump-to-definition capability means even complex Mill builds are much easier to understand than the SBT equivalent.

Built in capabilities

There are two big things you want a build tool to do:

SBT, surprisingly, does neither of these things by default!

You used to need the SBT Native Packager plugin's stage command to create an entrypoint to run your Scala code without SBT. If you want a fat jar, you need the SBT Assembly plugin. If you want to publish to Sonatype, you need the SBT PGP plugin, and if you want to actually release the code you published you need the SBT Sonatype plugin. Maybe you need the SBT Release plugin to help you do the "run one thing after another" thing.

That's five plugins, and we've barely satisfied what anyone wants a build tool to do by default.

None of the above tasks are particularly complicated; anyone should be able to write a bash script that runs some classfiles, zip things into a jar, or send sonatype some bytes and JSON to make it publish what you want.

These are not hard problems.

Mill comes with far-jar-ing & sonatype publish/releasing by default. It is Barely a few hundred lines of code to implement these capabilities, and it does not make sense to require a fistful of plugins just to do such common tasks.

Anyone using Mill can immediately deploy their code, whether as a fat jar to some webserver or pulically to maven central, without needing to deal with a mess of plugins and configuration. If you want something "custom", like making your project's publishVersion depend on the current git hash, you can trivially implement it yourself using an Input task.

Fixed Overhead

Ammonite$ time sbt ops/compile
real	0m12.861s

Ammonite$ time sbt ops/compile
real	0m6.564s

Ammonite$ time sbt ops/compile
real	0m7.122s
Ammonite$ time mill ops[2.12.4].compile
real	0m15.846s

Ammonite$ time mill ops[2.12.4].compile
real	0m1.910s

Ammonite$ time mill ops[2.12.4].compile
real	0m1.967s

Both SBT and Mill cache output and avoid re-compilation if the inputs don't change.

In both cases above, you can see SBT and Mill taking 10+ seconds for a "cold" compile of the Ammonite-Ops codebase. In both cases, the time drops drastically once the codebase has already been compiled. But Mill's cold no-op compile time is much lower than SBT's no-op compile time.

While Mill's 1.9 second fixed overhead isn't "fast", it is much more bearable than SBT's 6-7+ second fixed overhead for a no-op action. This makes Mill much more pleasant to use from the external command line than SBT is, since you spend much less time waiting for your build tool even before it has to do any real work.

Watch & Re-evaluate

Ammonite$ sbt ~ops/compile |  ts '%H:%M:%.S'
08:19:28.516823 2. Waiting for source changes... (press enter to interrupt)

Ammonite$ echo "" >> ops/src/main/scala/ammonite/ops/FileOps.scala; echo "" | ts '%H:%M:%.S'

08:19:46.209882 [info] Compiling 1 Scala source...

Ammonite$ echo "" >> ops/src/main/scala/ammonite/ops/FileOps.scala; echo "" | ts '%H:%M:%.S'

08:20:06.752445 [info] Compiling 1 Scala source...

Ammonite$ echo "" >> ops/src/main/scala/ammonite/ops/FileOps.scala; echo "" | ts '%H:%M:%.S'

08:20:13.963233 [info] Compiling 1 Scala source...
Ammonite$ mill --watch ops[2.12.4].compile 2>&1 | ts '%H:%M:%.S'
08:23:52.509628 Watching for changes to 10 files... (Ctrl-C to exit)

Ammonite$ echo "" >> ops/src/main/scala/ammonite/ops/FileOps.scala; echo "" | ts '%H:%M:%.S'

08:24:05.346854 [info] Compiling 1 Scala source...

Ammonite$ echo "" >> ops/src/main/scala/ammonite/ops/FileOps.scala; echo "" | ts '%H:%M:%.S'

08:24:23.902898 [info] Compiling 1 Scala source...

Ammonite$ echo "" >> ops/src/main/scala/ammonite/ops/FileOps.scala; echo "" | ts '%H:%M:%.S'

08:24:33.931748 [info] Compiling 1 Scala source...

Mill's --watch feature picks up changes much faster than SBT's ~; in the above trials SBT takes on average 0.91 seconds to pick up the change, while Mill takes 0.13 seconds. That is because Mill knows exactly which files serve as inputs to the tasks you wish to evaluate, and can poll exactly those files at a higher frequency without significant overhead.

Mill's --watch feature is also able to pick up changes in build.sc, re-evaluate your build file, then re-evaluate your selected task automatically. SBT's ~ isn't able to pick up changes in your build file, forcing you to manually interrupt the ~ to reload the latest changes to your build.

Error Messages

Ammonite$ sbt op/compile
[warn] Executing in batch mode.
[warn]   For better performance, hit [ENTER] to switch to interactive mode, or
[warn]   consider launching sbt without any commands, or explicitly passing 'shell'
[info] Loading project definition from /Users/lihaoyi/Dropbox/Github/Ammonite/project
[info] Set current project to ammonite (in build file:/Users/lihaoyi/Dropbox/Github/Ammonite/)
[error] Expected ID character
[error] Not a valid command: op
[error] Expected project ID
[error] Expected configuration
[error] Expected ':' (if selecting a configuration)
[error] Expected key
[error] Not a valid key: op
[error] op/compile
[error]   ^

Ammonite$ sbt ops/compiles
[warn] Executing in batch mode.
[warn]   For better performance, hit [ENTER] to switch to interactive mode, or
[warn]   consider launching sbt without any commands, or explicitly passing 'shell'
[info] Loading project definition from /Users/lihaoyi/Dropbox/Github/Ammonite/project
[info] Set current project to ammonite (in build file:/Users/lihaoyi/Dropbox/Github/Ammonite/)
[error] Expected ':' (if selecting a configuration)
[error] Not a valid key: compiles (similar: compilers, compile, compileInputs)
[error] ops/compiles
[error]             ^
Ammonite$ mill op[2.12.4].compile
Cannot resolve op. Did you mean ops?

Ammonite$ mill ops[2.12.4].compiles
Cannot resolve ops[2.12.4].compiles. Did you mean ops[2.12.4].compile?

Mill provides much better error messages than SBT if you mis-type things at the command line. One reason Mill can do this is that it's internal data model is much simpler than SBTs: Mill has a single hierarchy of tasks organized via foo.bar.baz or foo[cross].baz. That means when something doesn't line up, Mill can more accurately guess what you were trying to do, in contrast with SBT which doesn't know if you mis-typed a project ID, configuration, or key.

Mill also provides better error messages when you make syntax mistakes in your build file:

diff --git a/build.sbt b/build.sbt
 crossScalaVersions := Seq(
-  "2.11.3", "2.11.4", "2.11.5", "2.11.6", "2.11.7", "2.11.8", "2.11.9"
+  "2.11.3", "2.11.4", "2.11.5", "2.11.6", "2.11.7", "2.11.8", "2.11.9",
diff --git a/build.sc b/build.sc
 import mill._, scalalib._
-val binCrossScalaVersions = Seq("2.11.11", "2.12.4")
+val binCrossScalaVersions = Seq("2.11.11", "2.12.4",)
 val fullCrossScalaVersions = Seq(
Ammonite$ sbt ops/compile
[warn] Executing in batch mode.
[warn]   For better performance, hit [ENTER] to switch to interactive mode, or
[warn]   consider launching sbt without any commands, or explicitly passing 'shell'
[info] Loading project definition from /Users/lihaoyi/Dropbox/Github/Ammonite/project
[error] [/Users/lihaoyi/Dropbox/Github/Ammonite/build.sbt]:7: illegal start of simple expression
[error] [/Users/lihaoyi/Dropbox/Github/Ammonite/build.sbt]:374: ')' expected but eof found.
Ammonite$ mill ops[2.12.4].compiles
build.sc:2:52 expected WSChars | Comment | Newline | ")"
val binCrossScalaVersions = Seq("2.11.11", "2.12.4",)

Or runtime errors:

diff --git a/build.sbt b/build.sbt

diff --git a/build.sc b/build.sc
@@ -6,2 +6,5 @@ val fullCrossScalaVersions = Seq(

SBT output:

Ammonite$ sbt ops/compile
[warn] Executing in batch mode.
[warn]   For better performance, hit [ENTER] to switch to interactive mode, or
[warn]   consider launching sbt without any commands, or explicitly passing 'shell'
[info] Loading project definition from /Users/lihaoyi/Dropbox/Github/Ammonite/project
scala.NotImplementedError: an implementation is missing
	at scala.Predef$.$qmark$qmark$qmark(Predef.scala:252)
	at $16a24a9303fcf4f4b34b$.$sbtdef(/Users/lihaoyi/Dropbox/Github/Ammonite/build.sbt:9)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at sbt.compiler.Eval$.getValue(Eval.scala:464)
	at sbt.compiler.Eval$$anonfun$4.apply(Eval.scala:97)
	at sbt.compiler.Eval$$anonfun$4.apply(Eval.scala:97)
	at sbt.EvaluateConfigurations$$anonfun$evaluateDslEntry$1.apply(EvaluateConfigurations.scala:185)
	at sbt.EvaluateConfigurations$$anonfun$evaluateDslEntry$1.apply(EvaluateConfigurations.scala:183)
	at sbt.EvaluateConfigurations$$anonfun$evaluateSbtFile$1$$anonfun$11.apply(EvaluateConfigurations.scala:128)
	at sbt.EvaluateConfigurations$$anonfun$evaluateSbtFile$1$$anonfun$11.apply(EvaluateConfigurations.scala:128)
	at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
	at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
	at scala.collection.immutable.List.foreach(List.scala:318)
	at scala.collection.TraversableLike$class.map(TraversableLike.scala:244)
	at scala.collection.AbstractTraversable.map(Traversable.scala:105)
	at sbt.EvaluateConfigurations$$anonfun$evaluateSbtFile$1.apply(EvaluateConfigurations.scala:128)
	at sbt.EvaluateConfigurations$$anonfun$evaluateSbtFile$1.apply(EvaluateConfigurations.scala:122)
	at sbt.Load$.sbt$Load$$loadSettingsFile$1(Load.scala:775)
	at sbt.Load$$anonfun$sbt$Load$$memoLoadSettingsFile$1$1.apply(Load.scala:781)
	at sbt.Load$$anonfun$sbt$Load$$memoLoadSettingsFile$1$1.apply(Load.scala:780)
	at scala.collection.MapLike$class.getOrElse(MapLike.scala:128)
	at scala.collection.AbstractMap.getOrElse(Map.scala:58)
	at sbt.Load$.sbt$Load$$memoLoadSettingsFile$1(Load.scala:780)
	at sbt.Load$$anonfun$loadFiles$1$2.apply(Load.scala:788)
	at sbt.Load$$anonfun$loadFiles$1$2.apply(Load.scala:788)
	at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
	at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
	at scala.collection.mutable.ResizableArray$class.foreach(ResizableArray.scala:59)
	at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:47)
	at scala.collection.TraversableLike$class.map(TraversableLike.scala:244)
	at scala.collection.AbstractTraversable.map(Traversable.scala:105)
	at sbt.Load$.loadFiles$1(Load.scala:788)
	at sbt.Load$.discoverProjects(Load.scala:799)
	at sbt.Load$.discover$1(Load.scala:585)
	at sbt.Load$.sbt$Load$$loadTransitive(Load.scala:633)
	at sbt.Load$$anonfun$loadUnit$1.sbt$Load$$anonfun$$loadProjects$1(Load.scala:482)
	at sbt.Load$$anonfun$loadUnit$1$$anonfun$40.apply(Load.scala:485)
	at sbt.Load$$anonfun$loadUnit$1$$anonfun$40.apply(Load.scala:485)
	at sbt.Load$.timed(Load.scala:1025)
	at sbt.Load$$anonfun$loadUnit$1.apply(Load.scala:485)
	at sbt.Load$$anonfun$loadUnit$1.apply(Load.scala:459)
	at sbt.Load$.timed(Load.scala:1025)
	at sbt.Load$.loadUnit(Load.scala:459)
	at sbt.Load$$anonfun$25$$anonfun$apply$14.apply(Load.scala:311)
	at sbt.Load$$anonfun$25$$anonfun$apply$14.apply(Load.scala:310)
	at sbt.BuildLoader$$anonfun$componentLoader$1$$anonfun$apply$4$$anonfun$apply$5$$anonfun$apply$6.apply(BuildLoader.scala:91)
	at sbt.BuildLoader$$anonfun$componentLoader$1$$anonfun$apply$4$$anonfun$apply$5$$anonfun$apply$6.apply(BuildLoader.scala:90)
	at sbt.BuildLoader.apply(BuildLoader.scala:140)
	at sbt.Load$.loadAll(Load.scala:365)
	at sbt.Load$.loadURI(Load.scala:320)
	at sbt.Load$.load(Load.scala:316)
	at sbt.Load$.load(Load.scala:305)
	at sbt.Load$$anonfun$4.apply(Load.scala:146)
	at sbt.Load$$anonfun$4.apply(Load.scala:146)
	at sbt.Load$.timed(Load.scala:1025)
	at sbt.Load$.apply(Load.scala:146)
	at sbt.Load$.defaultLoad(Load.scala:39)
	at sbt.BuiltinCommands$.liftedTree1$1(Main.scala:548)
	at sbt.BuiltinCommands$.doLoadProject(Main.scala:548)
	at sbt.BuiltinCommands$$anonfun$loadProjectImpl$2.apply(Main.scala:540)
	at sbt.BuiltinCommands$$anonfun$loadProjectImpl$2.apply(Main.scala:540)
	at sbt.Command$$anonfun$applyEffect$1$$anonfun$apply$2.apply(Command.scala:59)
	at sbt.Command$$anonfun$applyEffect$1$$anonfun$apply$2.apply(Command.scala:59)
	at sbt.Command$$anonfun$applyEffect$2$$anonfun$apply$3.apply(Command.scala:61)
	at sbt.Command$$anonfun$applyEffect$2$$anonfun$apply$3.apply(Command.scala:61)
	at sbt.Command$.process(Command.scala:93)
	at sbt.MainLoop$$anonfun$1$$anonfun$apply$1.apply(MainLoop.scala:96)
	at sbt.MainLoop$$anonfun$1$$anonfun$apply$1.apply(MainLoop.scala:96)
	at sbt.State$$anon$1.doX$1(State.scala:183)
	at sbt.State$$anon$1.process(State.scala:190)
	at sbt.MainLoop$$anonfun$1.apply(MainLoop.scala:96)
	at sbt.MainLoop$$anonfun$1.apply(MainLoop.scala:96)
	at sbt.ErrorHandling$.wideConvert(ErrorHandling.scala:17)
	at sbt.MainLoop$.next(MainLoop.scala:96)
	at sbt.MainLoop$.run(MainLoop.scala:89)
	at sbt.MainLoop$$anonfun$runWithNewLog$1.apply(MainLoop.scala:68)
	at sbt.MainLoop$$anonfun$runWithNewLog$1.apply(MainLoop.scala:63)
	at sbt.Using.apply(Using.scala:24)
	at sbt.MainLoop$.runWithNewLog(MainLoop.scala:63)
	at sbt.MainLoop$.runAndClearLast(MainLoop.scala:46)
	at sbt.MainLoop$.runLoggedLoop(MainLoop.scala:30)
	at sbt.MainLoop$.runLogged(MainLoop.scala:22)
	at sbt.StandardMain$.runManaged(Main.scala:109)
	at sbt.xMain.run(Main.scala:38)
	at xsbt.boot.Launch$$anonfun$run$1.apply(Launch.scala:109)
	at xsbt.boot.Launch$.withContextLoader(Launch.scala:128)
	at xsbt.boot.Launch$.run(Launch.scala:109)
	at xsbt.boot.Launch$$anonfun$apply$1.apply(Launch.scala:35)
	at xsbt.boot.Launch$.launch(Launch.scala:117)
	at xsbt.boot.Launch$.apply(Launch.scala:18)
	at xsbt.boot.Boot$.runImpl(Boot.scala:41)
	at xsbt.boot.Boot$.main(Boot.scala:17)
	at xsbt.boot.Boot.main(Boot.scala)
[error] scala.NotImplementedError: an implementation is missing
[error] Use 'last' for the full log.
Project loading failed: (r)etry, (q)uit, (l)ast, or (i)gnore?

Mill output:

Ammonite$ mill ops[2.12.4].compile
Compiling /Users/lihaoyi/Dropbox/Github/Ammonite/build.sc
scala.NotImplementedError: an implementation is missing

There is no reason why SBT couldn't do the same, but Mill gets this nice error reporting for free since it builds upon Ammonite Scala Scripts

Defining Tasks

lazy val sampleTask = taskKey[Int]("A sample task.")

sampleTask := {
   val sum = 1 + 2
   println("sum: " + sum)

SBT makes it somewhat awkward to define a new task. You first need to define a taskKey, then assign the body of the task to it via :=.

Mill let's you simply def a function with T{...} (for cached Targets) or T.command{...} (for un-cached Commands) and you have a task you can immediately list, inspect and evaluate from the command line:

def sampleTarget = T{ 
  val sum = 1 + 2
  println("sum: " + sum)

def sampleCommand() = T.command{ 
  val sum = 1 + 2
  println("sum: " + sum)

With SBT, your sampleTask now can be called with any subproject, even though you may only want it to apply to one, and when applied to the others is illegal/invalid/meaningless. With Mill, you can choose where sampleTarget or sampleCommand are defined: top-level, within a single module, or in a shared trait so it applies to multiple modules. You have control over exactly what your build looks like, so you can make sure invalid tasks simply do not exist.


SBT requires you to figure out caching of a task yourself, perhaps using FileFunction.cached and hoping to pick a cache folder name that nobody else has picked lest you have cache collisions. This means that most people writing SBT custom tasks do not bother caching as aggressively as they could be (using FileFunction.cached is awkward, picking filesystem names is hard) and portions of your build tend to re-evaluate over and over even if no inputs changed.

Mill caches any Targets (defined with T{...} blocks) automatically, so sampleTarget above only runs once since it has no inputs that can change:

Ammonite$ mill sampleTarget
sum: 3
Ammonite$ mill sampleTarget
Ammonite$ mill sampleTarget

While sampleCommand runs every time:

Ammonite$ mill sampleCommand
sum: 3
Ammonite$ mill sampleCommand
sum: 3
Ammonite$ mill sampleCommand
sum: 3

This means that with Mill, you spend much less time waiting for unnecessary re-evaluations. In SBT you may be used to update triggering spuriously, or compile taking some time to notice nothing has changed, or assembly printing out it's wall-of-text and doing work even when the assembly has already been made. In Mill, all these things are aggressively cached, entirely automatically, and neither the person writing tasks nor the person using them needs to pay any attention to make it work.

Cross Building

val binCrossScalaVersions = Seq("2.11.11", "2.12.4")
val fullCrossScalaVersions = Seq(
  "2.11.3", "2.11.4", "2.11.5", "2.11.6", "2.11.7", "2.11.8", "2.11.9", "2.11.11",
  "2.12.0", "2.12.1", "2.12.2", "2.12.3", "2.12.4"

object runtime extends Cross[RuntimeModule](binCrossScalaVersions:_*)
class RuntimeModule(val crossScalaVersion: String) extends AmmModule{
object interp extends Cross[InterpModule](fullCrossScalaVersions:_*)
class InterpModule(val crossScalaVersion: String) extends AmmModule{
Ammonite$ mill resolve amm.runtime[_]

Ammonite$ mill resolve amm.interp[_]

Unlike SBT, Mill's cross-builds aren't "magic" global settings, but simply a list of modules. Different modules can be cross-built against different sets of crossScalaVersions, which you can select using the mill ops[2.12.4].compile syntax we saw earlier and can list using mill resolve amm.ops[_].

Mill's cross builds aren't limited to scala versions; you can cross build against any list of values. Mill's own build has the various scalajslib.jsbridges, that it uses the integrate with the Scala.js tooling, cross-built against the two sets of Scala.js versions: 0.6.x and 1.0.0-Mx:

object scalajslib extends MillModule {
  object jsbridges extends Cross[JsBridgeModule]("0.6", "1.0")
  class JsBridgeModule(scalajsBinary: String) extends MillModule{
    def moduleDeps = Seq(scalajslib)
    def ivyDeps = scalajsBinary match {
      case "0.6" =>
      case "1.0" =>
mill$ mill resolve scalajslib.jsbridges[_]

Mill's cross-build system allows you to do some things which are entirely impossible using SBT's cross builds, such as depending on multiple different cross-builds of a cross-built modules from a non-cross-built module:

object scalajslib extends MillModule {

  def moduleDeps = Seq(scalalib)

  def testArgs = T{
    val mapping = Map(
      "MILL_SCALAJS_BRIDGE_0_6" -> jsbridges("0.6").compile().classes.path,
      "MILL_SCALAJS_BRIDGE_1_0" -> jsbridges("1.0").compile().classes.path
    scalaworker.testArgs() ++ (for((k, v) <- mapping.toSeq) yield s"-D$k=$v")

Here, I want to use both the 0.6 and 1.0 cross-builds of the jsbridges module from scalajslib, which is itself not cross built. I'm not bundling them up on the classpath, just including them as jar-files to manipulate at runtime. Consider how you might achieve the same thing using SBT, e.g. having a single non-cross-built subproject which uses the 2.10.6, 2.11.11 and 2.12.4 assemblys of another SBT subproject as plain old files.

SBT's cross builds are also exponential in runtime, and trying to run sbt +scalaVersion against a SBT subproject cross-built across:

crossScalaVersions := Seq(
  "2.11.3", "2.11.4", "2.11.5", "2.11.6", "2.11.7", "2.11.8", "2.11.9",
  "2.11.11", "2.12.0", "2.12.1", "2.12.2", "2.12.3", "2.12.4"

Takes forever. Mill's cross-built modules do not suffer this issue.

Exploring your Build

Ammonite$ sbt tasks
[warn] Executing in batch mode.
[warn]   For better performance, hit [ENTER] to switch to interactive mode, or
[warn]   consider launching sbt without any commands, or explicitly passing 'shell'
[info] Loading project definition from /Users/lihaoyi/Dropbox/Github/Ammonite/project
[info] Set current project to ammonite (in build file:/Users/lihaoyi/Dropbox/Github/Ammonite/)

This is a list of tasks defined for the current project.
It does not list the scopes the tasks are defined in; use the 'inspect' command for that.
Tasks produce values.  Use the 'show' command to run the task and print the resulting value.

  checkPgpSignatures   Checks the signatures of artifacts to see if they are trusted.
  clean                Deletes files produced by the build, such as generated sources, compiled classes, and task caches.
  compile              Compiles sources.
  console              Starts the Scala interpreter with the project classes on the classpath.
  consoleProject       Starts the Scala interpreter with the sbt and the build definition on the classpath and useful imports.
  consoleQuick         Starts the Scala interpreter with the project dependencies on the classpath.
  copyResources        Copies resources to the output directory.
  doc                  Generates API documentation.
  package              Produces the main artifact, such as a binary jar.  This is typically an alias for the task that actually does the packaging.
  packageBin           Produces a main artifact, such as a binary jar.
  packageDoc           Produces a documentation artifact, such as a jar containing API documentation.
  packageSrc           Produces a source artifact, such as a jar containing sources and resources.
  publish              Publishes artifacts to a repository.
  publishLocal         Publishes artifacts to the local Ivy repository.
  publishLocalSigned   Publishing all artifacts to a local repository, but SIGNED using PGP.
  publishM2            Publishes artifacts to the local Maven repository.
  publishSigned        Publishing all artifacts, but SIGNED using PGP.
  run                  Runs a main class, passing along arguments provided on the command line.
  runMain              Runs the main class selected by the first argument, passing the remaining arguments to the main method.
  test                 Executes all tests.
  testOnly             Executes the tests provided as arguments or all tests if no arguments are provided.
  testQuick            Executes the tests that either failed before, were not run or whose transitive dependencies changed, among those provided as arguments.
  update               Resolves and optionally retrieves dependencies, producing a report.

SBT's tasks command isn't very useful. It only show a subset of tasks, doesn't show many useful ones (like assembly), shows some that aren't generally useful (like checkPgpSigntures or publishLocalSigned), and generally doesn't show you what tasks are valid where (e.g. not every SBT subproject can be published)

SBT has separate commands to list projects and commands.

Mill on the other hand has a single resolve command that can be used to explore the build hierarchy interactively.

Ammonite$ mill resolve _

Ammonite$ mill resolve ops._

Ammonite$ mill resolve ops[2.12.4]._

Ammonite$ mill resolve __.compile

Ammonite$ mill resolve __[2.12.4].compile

Mill makes it very easy to explore your build and see exactly what you have available. You an also take the tasks resolved by any of these queries and evaluate them simply by ommitting the resolve command:

Ammonite$ mill __[2.12.4].compile

Inspecting Tasks

Ammonite$ sbt "inspect ops/compile"
[warn] Executing in batch mode.
[warn]   For better performance, hit [ENTER] to switch to interactive mode, or
[warn]   consider launching sbt without any commands, or explicitly passing 'shell'
[info] Loading project definition from /Users/lihaoyi/Dropbox/Github/Ammonite/project
[info] Set current project to ammonite (in build file:/Users/lihaoyi/Dropbox/Github/Ammonite/)
[info] Task: sbt.inc.Analysis
[info] Description:
[info]      Compiles sources.
[info] Provided by:
[info]      {file:/Users/lihaoyi/Dropbox/Github/Ammonite/}ops/compile:compile
[info] Defined at:
[info]      (sbt.Defaults) Defaults.scala:286
[info] Dependencies:
[info]      ops/compile:manipulateBytecode
[info]      ops/compile:incCompileSetup
[info] Reverse dependencies:
[info]      ops/compile:products
[info]      ops/compile:discoveredMainClasses
[info]      ops/compile:printWarnings
[info]      ops/compile:discoveredSbtPlugins
[info] Delegates:
[info]      ops/compile:compile
[info]      ops/*:compile
[info]      {.}/compile:compile
[info]      {.}/*:compile
[info]      */compile:compile
[info]      */*:compile
[info] Related:
[info]      amm/compile:compile
[info]      readme/test:compile
[info]      sshd/test:compile
[info]      integration/compile:compile
[info]      ammInterp/test:compile
[info]      singleCrossBuilt/compile:compile
[info]      ammonite/test:compile
[info]      readme/compile:compile
[info]      shell/test:compile
[info]      ammRepl/compile:compile
Ammonite$ mill describe ops[2.12.4].compile
[1/1] describe

Mill lets you describe any arbitary task to find out where it's defined and what it depends on. Unlike SBT it does not need to provide information about Provided by, Delegates or Related: Mill tasks are simple nodes, living in a single build-hierarchy and task-graph, and have no concept of delegation or configurations or four-dimensional scopes. Mill currently doesn't display the Reverse dependencies, but could be made to show them in future.


SBT relies on aggregation to save keystrokes: you make subproject A aggregate subprojects B C and D, so you can run any task (e.g. test) on B C and D simply by running A/test.

This is fine when you have a single root project that aggregates a few others, but when things get more complex, it breaks down. You often want to run different tasks on different sets of subprojects: you may want to publish all your libraries, assembly only your application bundles, but test everything. Due to how SBT Cross Building works, you may need to run one cross-build against one set of modules but a different cross build against another set. When that happens, you end up making fake subprojects that serve no purpose other than aggregation:

lazy val singleCrossBuilt = project
  .aggregate(ops, terminal, ammUtil, ammRuntime)

lazy val fullCrossBuilt = project
  .aggregate(shell, amm, sshd, ammInterp, ammRepl)

lazy val published = project
  .aggregate(fullCrossBuilt, singleCrossBuilt)

Here, we have some SBT subprojects, with their own source folders, test folders, compilation logic, dependency resolution, and many more things. But we are ignoring all of that as invalid, just to use the subprojects as aggregators! That goes entirely against the principle of making invalid states unrepresentable.

With Mill, you can instead use it's query syntax e.g. mill __.compile or mill __.test to run compiles or tests against all relevant modules, or more narrow queries e.g. mill foo.__.compile to compile more limited sets (all modules under foo). Cross-building works by default even if different modules have different sets of cross builds, and if these queries aren't sufficiently flexible you can simply define a Target or Command that forces evaluation of the tasks you want to aggregate:

def compileAll() = T.command{

def testAll() = T.command{

Now you can simply run mill compileAll or mill testAll to achieve what you want, without having a bunch of un-used vestigal subprojects hanging around uselessly.


SBT does a lot of things.

Mill tries to do everything you want to do using SBT, but often lets you do it better: faster, easier, simpler.

There are many things you can do with SBT that you often don't want to do: Mill often leaves these out. Mill's data model and execution model is much simpler than SBT's: rather than N ways of doing things, you often just have 1 way to get what you want.

On the flip side, there are many things that people generally want, that SBT does not provide. Simple things: faster initialization, automatically reloading build files if they change, or packaging for deployment or publishing to Maven Central. Mill provides these things by default

Mill is still a young project, but is already extremely usable. If you think it's time to try something other than SBT, give Mill a try!

About the Author: Haoyi is a software engineer, an early contributor to Scala.js, and the author of many open-source Scala tools such as the Ammonite REPL and FastParse.

If you've enjoyed this blog, or enjoyed using Haoyi's other open source libraries, please chip in (or get your Company to chip in!) via Patreon so he can continue his open-source work

Build Tools as Pure Functional Programs