First pass at splitting out worker-api from mill core. (#504)

This reduces the {scala,scalajs,scalanative}-worker dependency from the entirety of Mill to a much narrower `mill.api` module. This reduces the amount of classpath pollution within these workers, should mean they're much faster to download the first time, and reduces the amount of random junk they would pull in if they were to be used outside of the Mill project.

The interactions between the various *Modules and their *WorkerImpls has been narrowed down to the `*.api` modules, which only depend on other `*.api` modules.

A lot of things have been moved around; user code is unlikely to break, but it's possible some will if it references classes that have been moved around. Forwarders have been left for the few internal classes that Mill uses in it's own `build.sc`, to support bootstrapping. Third-party code which breaks should be a straightforward to fix just by updating imports

The `*.api` modules have minimal dependencies (mostly uPickle and os-lib) and minimal code. There is still a bunch of implementation code in there: some of it defining data-types that are commonly sent across the module/worker interface (`Agg`, `PathRef`, ...), and some of it just general helper functions that are needed both in modules and workers. The latter code isn't strictly API definitions, but for now is small enough it's not worth splitting into it's own module

79 files changed, 878 insertions, 523 deletions

diff --git a/build.sc b/build.sc
index aad309a8..1cbbe992 100755
--- a/build.sc
+++ b/build.sc
@@ -26,8 +26,7 @@ trait MillPublishModule extends PublishModule{
 
   def javacOptions = Seq("-source", "1.8", "-target", "1.8")
 }
-
-trait MillModule extends MillPublishModule with ScalaModule{ outer =>
+trait MillApiModule extends MillPublishModule with ScalaModule{
   def scalaVersion = T{ "2.12.6" }
   def compileIvyDeps = Agg(ivy"com.lihaoyi::acyclic:0.1.7")
   def scalacOptions = Seq("-P:acyclic:force")
@@ -35,6 +34,8 @@ trait MillModule extends MillPublishModule with ScalaModule{ outer =>
   def repositories = super.repositories ++ Seq(
     MavenRepository("https://oss.sonatype.org/content/repositories/releases")
   )
+}
+trait MillModule extends MillApiModule{ outer =>
   def scalacPluginClasspath =
     super.scalacPluginClasspath() ++ Seq(main.moduledefs.jar())
 
@@ -77,9 +78,14 @@ object main extends MillModule {
       Seq(PathRef(shared.generateCoreTestSources(T.ctx().dest)))
     }
   }
-
+  object api extends MillApiModule{
+    def ivyDeps = Agg(
+      ivy"com.lihaoyi::os-lib:0.2.6",
+      ivy"com.lihaoyi::upickle:0.7.1",
+    )
+  }
   object core extends MillModule {
-    def moduleDeps = Seq(moduledefs)
+    def moduleDeps = Seq(moduledefs, api)
 
     def compileIvyDeps = Agg(
       ivy"org.scala-lang:scala-reflect:${scalaVersion()}"
@@ -134,7 +140,7 @@ object main extends MillModule {
 
 
 object scalalib extends MillModule {
-  def moduleDeps = Seq(main)
+  def moduleDeps = Seq(main, scalalib.api)
 
   def ivyDeps = Agg(
     ivy"org.scala-sbt:test-interface:1.0"
@@ -172,23 +178,27 @@ object scalalib extends MillModule {
       )
     }
   }
-  object worker extends MillModule{
-    def moduleDeps = Seq(main, scalalib)
+  object api extends MillApiModule{
+    def moduleDeps = Seq(main.api)
+  }
+  object worker extends MillApiModule{
+
+    def moduleDeps = Seq(scalalib.api)
 
     def ivyDeps = Agg(
       // Keep synchronized with zinc in Versions.scala
       ivy"org.scala-sbt::zinc:1.2.1"
     )
-    def testArgs = Seq(
+    def testArgs = T{Seq(
       "-DMILL_SCALA_WORKER=" + runClasspath().map(_.path).mkString(",")
-    )
+    )}
   }
 }
 
 
 object scalajslib extends MillModule {
 
-  def moduleDeps = Seq(scalalib)
+  def moduleDeps = Seq(scalalib, scalajslib.api)
 
   def testArgs = T{
     val mapping = Map(
@@ -201,9 +211,12 @@ object scalajslib extends MillModule {
     (for((k, v) <- mapping.toSeq) yield s"-D$k=$v")
   }
 
+  object api extends MillApiModule{
+    def moduleDeps = Seq(main.core)
+  }
   object worker extends Cross[WorkerModule]("0.6", "1.0")
-  class WorkerModule(scalajsBinary: String) extends MillModule{
-    def moduleDeps = Seq(scalajslib)
+  class WorkerModule(scalajsBinary: String) extends MillApiModule{
+    def moduleDeps = Seq(scalajslib.api)
     def ivyDeps = scalajsBinary match {
       case "0.6" =>
         Agg(
@@ -257,7 +270,7 @@ object contrib extends MillModule {
 
 
 object scalanativelib extends MillModule {
-  def moduleDeps = Seq(scalalib)
+  def moduleDeps = Seq(scalalib, scalanativelib.api)
 
   def scalacOptions = Seq[String]() // disable -P:acyclic:force
 
@@ -273,11 +286,13 @@ object scalanativelib extends MillModule {
     scalalib.backgroundwrapper.testArgs() ++
     (for((k, v) <- mapping.toSeq) yield s"-D$k=$v")
   }
-
+  object api extends MillApiModule{
+    def moduleDeps = Seq(main.core)
+  }
   object worker extends Cross[WorkerModule]("0.3")
-  class WorkerModule(scalaNativeBinary: String) extends MillModule {
+  class WorkerModule(scalaNativeBinary: String) extends MillApiModule {
     def scalaNativeVersion = T{ "0.3.8" }
-    def moduleDeps = Seq(scalanativelib)
+    def moduleDeps = Seq(scalanativelib.api)
     def ivyDeps = scalaNativeBinary match {
       case "0.3" =>
         Agg(
diff --git a/ci/shared.sc b/ci/shared.sc
index d0fc246f..2f133486 100644
--- a/ci/shared.sc
+++ b/ci/shared.sc
@@ -41,7 +41,7 @@ def generateTarget(dir: Path) = {
     val parameters = lowercases.zip(uppercases).map { case (lower, upper) => s"$lower: TT[$upper]" }.mkString(", ")
     val body       = uppercases.zipWithIndex.map { case (t, i) => s"args[$t]($i)" }.mkString(", ")
 
-    s"def zip[$typeArgs]($parameters) = makeT[($typeArgs)](Seq($zipArgs), (args: mill.util.Ctx) => ($body))"
+    s"def zip[$typeArgs]($parameters) = makeT[($typeArgs)](Seq($zipArgs), (args: mill.api.Ctx) => ($body))"
   }
 
   write(
@@ -50,7 +50,7 @@ def generateTarget(dir: Path) = {
        |import scala.language.higherKinds
        |trait TargetGenerated {
        |  type TT[+X]
-       |  def makeT[X](inputs: Seq[TT[_]], evaluate: mill.util.Ctx => mill.eval.Result[X]): TT[X]
+       |  def makeT[X](inputs: Seq[TT[_]], evaluate: mill.api.Ctx => mill.api.Result[X]): TT[X]
        |  ${(3 to 22).map(generate).mkString("\n")}
        |}""".stripMargin
   )
diff --git a/contrib/buildinfo/src/mill/contrib/BuildInfo.scala b/contrib/buildinfo/src/mill/contrib/BuildInfo.scala
index 9202fd95..1c8d9d9e 100644
--- a/contrib/buildinfo/src/mill/contrib/BuildInfo.scala
+++ b/contrib/buildinfo/src/mill/contrib/BuildInfo.scala
@@ -2,9 +2,9 @@ package mill.contrib
 
 import mill.T
 import mill.define.Target
-import mill.eval.PathRef
+import mill.api.PathRef
 import mill.scalalib.ScalaModule
-import mill.util.Ctx
+import mill.api.Ctx
 
 trait BuildInfo extends ScalaModule {
 
diff --git a/contrib/buildinfo/test/src/mill/contrib/BuildInfoTests.scala b/contrib/buildinfo/test/src/mill/contrib/BuildInfoTests.scala
index 05070985..4293bba7 100644
--- a/contrib/buildinfo/test/src/mill/contrib/BuildInfoTests.scala
+++ b/contrib/buildinfo/test/src/mill/contrib/BuildInfoTests.scala
@@ -3,7 +3,7 @@ package mill.contrib
 import java.util.jar.JarFile
 import mill._
 import mill.define.Target
-import mill.eval.Result._
+import mill.api.Result._
 import mill.eval.{Evaluator, Result}
 import mill.modules.Assembly
 import mill.scalalib.publish.VersionControl
diff --git a/contrib/scalapblib/src/mill/contrib/scalapblib/ScalaPBModule.scala b/contrib/scalapblib/src/mill/contrib/scalapblib/ScalaPBModule.scala
index 9aa5b833..db5c5c8b 100644
--- a/contrib/scalapblib/src/mill/contrib/scalapblib/ScalaPBModule.scala
+++ b/contrib/scalapblib/src/mill/contrib/scalapblib/ScalaPBModule.scala
@@ -4,7 +4,7 @@ package contrib.scalapblib
 import coursier.{Cache, MavenRepository}
 import coursier.core.Version
 import mill.define.Sources
-import mill.eval.PathRef
+import mill.api.PathRef
 import mill.scalalib.Lib.resolveDependencies
 import mill.scalalib._
 import mill.util.Loose
diff --git a/contrib/scalapblib/src/mill/contrib/scalapblib/ScalaPBWorker.scala b/contrib/scalapblib/src/mill/contrib/scalapblib/ScalaPBWorker.scala
index a1b345b4..125cd3fd 100644
--- a/contrib/scalapblib/src/mill/contrib/scalapblib/ScalaPBWorker.scala
+++ b/contrib/scalapblib/src/mill/contrib/scalapblib/ScalaPBWorker.scala
@@ -5,7 +5,7 @@ import java.io.File
 import java.lang.reflect.Method
 import java.net.URLClassLoader
 
-import mill.eval.PathRef
+import mill.api.PathRef
 
 class ScalaPBWorker {
 
@@ -40,7 +40,7 @@ class ScalaPBWorker {
   }
 
   def compile(scalaPBClasspath: Agg[os.Path], scalaPBSources: Seq[os.Path], scalaPBOptions: String, dest: os.Path)
-             (implicit ctx: mill.util.Ctx): mill.eval.Result[PathRef] = {
+             (implicit ctx: mill.api.Ctx): mill.api.Result[PathRef] = {
     val compiler = scalaPB(scalaPBClasspath)
 
     def compileScalaPBDir(inputDir: os.Path) {
@@ -55,7 +55,7 @@ class ScalaPBWorker {
 
     scalaPBSources.foreach(compileScalaPBDir)
 
-    mill.eval.Result.Success(PathRef(dest))
+    mill.api.Result.Success(PathRef(dest))
   }
 }
 
diff --git a/contrib/scalapblib/test/src/mill/contrib/scalapblib/TutorialTests.scala b/contrib/scalapblib/test/src/mill/contrib/scalapblib/TutorialTests.scala
index 65558f72..fe0ce8d5 100644
--- a/contrib/scalapblib/test/src/mill/contrib/scalapblib/TutorialTests.scala
+++ b/contrib/scalapblib/test/src/mill/contrib/scalapblib/TutorialTests.scala
@@ -1,6 +1,6 @@
 package mill.contrib.scalapblib
 
-import mill.eval.Result
+import mill.api.Result
 import mill.util.{TestEvaluator, TestUtil}
 import utest.framework.TestPath
 import utest.{TestSuite, Tests, assert, _}
diff --git a/contrib/tut/test/src/mill/contrib/tut/TutTests.scala b/contrib/tut/test/src/mill/contrib/tut/TutTests.scala
index 8168930f..468654bb 100644
--- a/contrib/tut/test/src/mill/contrib/tut/TutTests.scala
+++ b/contrib/tut/test/src/mill/contrib/tut/TutTests.scala
@@ -2,7 +2,7 @@ package mill.contrib
 package tut
 
 import mill._
-import mill.eval.Result._
+import mill.api.Result._
 import mill.scalalib._
 import mill.util.{TestEvaluator, TestUtil}
 import utest._
diff --git a/contrib/twirllib/src/mill/twirllib/TwirlModule.scala b/contrib/twirllib/src/mill/twirllib/TwirlModule.scala
index 8f735f3a..328afc47 100644
--- a/contrib/twirllib/src/mill/twirllib/TwirlModule.scala
+++ b/contrib/twirllib/src/mill/twirllib/TwirlModule.scala
@@ -3,7 +3,7 @@ package twirllib
 
 import coursier.{Cache, MavenRepository}
 import mill.define.Sources
-import mill.eval.PathRef
+import mill.api.PathRef
 import mill.scalalib.Lib.resolveDependencies
 import mill.scalalib._
 import mill.util.Loose
@@ -41,7 +41,7 @@ trait TwirlModule extends mill.Module {
 
   private def twirlInclusiveDot: Boolean = false
 
-  def compileTwirl: T[CompilationResult] = T.persistent {
+  def compileTwirl: T[mill.scalalib.api.CompilationResult] = T.persistent {
     TwirlWorkerApi.twirlWorker
       .compile(
         twirlClasspath().map(_.path),
diff --git a/contrib/twirllib/src/mill/twirllib/TwirlWorker.scala b/contrib/twirllib/src/mill/twirllib/TwirlWorker.scala
index b8fe9c66..09376a6f 100644
--- a/contrib/twirllib/src/mill/twirllib/TwirlWorker.scala
+++ b/contrib/twirllib/src/mill/twirllib/TwirlWorker.scala
@@ -5,11 +5,10 @@ import java.io.File
 import java.lang.reflect.Method
 import java.net.URLClassLoader
 
-import mill.eval.PathRef
-import mill.scalalib.CompilationResult
+import mill.api.PathRef
+import mill.scalalib.api.CompilationResult
 
 import scala.io.Codec
-
 class TwirlWorker {
 
   private var twirlInstanceCache = Option.empty[(Long, TwirlWorkerApi)]
@@ -103,7 +102,7 @@ class TwirlWorker {
               constructorAnnotations: Seq[String],
               codec: Codec,
               inclusiveDot: Boolean)
-             (implicit ctx: mill.util.Ctx): mill.eval.Result[CompilationResult] = {
+             (implicit ctx: mill.api.Ctx): mill.api.Result[CompilationResult] = {
     val compiler = twirl(twirlClasspath)
 
     def compileTwirlDir(inputDir: os.Path) {
@@ -127,7 +126,7 @@ class TwirlWorker {
     val zincFile = ctx.dest / 'zinc
     val classesDir = ctx.dest
 
-    mill.eval.Result.Success(CompilationResult(zincFile, PathRef(classesDir)))
+    mill.api.Result.Success(CompilationResult(zincFile, PathRef(classesDir)))
   }
 
   private def twirlExtensionFormat(name: String) =
diff --git a/docs/pages/4 - Tasks.md b/docs/pages/4 - Tasks.md
index e69ae662..78a6e58e 100644
--- a/docs/pages/4 - Tasks.md
+++ b/docs/pages/4 - Tasks.md
@@ -176,10 +176,10 @@ There are several APIs available to you within the body of a `T {...}` or
 `T.command {...}` block to help your write the code implementing your Target or
 Command:
 
-### mill.util.Ctx.Dest
+### mill.api.Ctx.Dest
 
 - `T.ctx().dest`
-- `implicitly[mill.util.Ctx.Dest]`
+- `implicitly[mill.api.Ctx.Dest]`
 
 This is the unique `out/classFiles/dest/` path or `out/run/dest/` path that is
 assigned to every Target or Command. It is cleared before your task runs, and
@@ -188,10 +188,10 @@ artifacts. This is guaranteed to be unique for every `Target` or `Command`, so
 you can be sure that you will not collide or interfere with anyone else writing
 to those same paths.
 
-### mill.util.Ctx.Log
+### mill.api.Ctx.Log
 
 - `T.ctx().log`
-- `implicitly[mill.util.Ctx.Log]`
+- `implicitly[mill.api.Ctx.Log]`
 
 This is the default logger provided for every task. While your task is running,
 `System.out` and `System.in` are also redirected to this logger. The logs for a
@@ -202,10 +202,10 @@ specific output is also streamed to a log file on disk, e.g. `out/run/log` or
 Messages logged with `log.debug` appear by default only in the log files.
 You can use the `--debug` option when running mill to show them on the console too.
 
-### mill.util.Ctx.Env
+### mill.api.Ctx.Env
 
 - `T.ctx().env`
-- `implicitly[mill.util.Ctx.Env]`
+- `implicitly[mill.api.Ctx.Env]`
 
 Mill keeps a long-lived JVM server to avoid paying the cost of recurrent 
 classloading. Because of this, running `System.getenv` in a task might not yield
diff --git a/integration/test/resources/ammonite/build.sc b/integration/test/resources/ammonite/build.sc
index 66e67087..86f354b4 100644
--- a/integration/test/resources/ammonite/build.sc
+++ b/integration/test/resources/ammonite/build.sc
@@ -280,7 +280,7 @@ def generateConstantsFile(version: String = buildVersion,
                           unstableCurlUrl: String = "<fill-me-in-in-Constants.scala>",
                           oldCurlUrls: Seq[(String, String)] = Nil,
                           oldUnstableCurlUrls: Seq[(String, String)] = Nil)
-                         (implicit ctx: mill.util.Ctx.Dest)= {
+                         (implicit ctx: mill.api.Ctx.Dest)= {
   val versionTxt = s"""
     package ammonite
     object Constants{
@@ -305,7 +305,7 @@ def generateConstantsFile(version: String = buildVersion,
 def generateDependenciesFile(scalaVersion: String,
                              fileName: String,
                              deps: Seq[coursier.Dependency])
-                            (implicit ctx: mill.util.Ctx.Dest) = {
+                            (implicit ctx: mill.api.Ctx.Dest) = {
 
   val dir = ctx.dest / "extra-resources"
   val dest = dir / fileName
diff --git a/integration/test/resources/jawn/build.sc b/integration/test/resources/jawn/build.sc
index 8b58efc7..144ef814 100644
--- a/integration/test/resources/jawn/build.sc
+++ b/integration/test/resources/jawn/build.sc
@@ -42,7 +42,7 @@ class JawnModule(crossVersion: String) extends mill.Module{
 
     object play extends Support(){
       def ivyDeps = mill.T{
-        Lib.scalaBinaryVersion(scalaVersion()) match{
+        mill.scalalib.api.Util.scalaBinaryVersion(scalaVersion()) match{
           case "2.10" => Agg(ivy"com.typesafe.play::play-json:2.4.11")
           case "2.11" => Agg(ivy"com.typesafe.play::play-json:2.5.15")
           case _ => Agg(ivy"com.typesafe.play::play-json:2.6.0")
diff --git a/integration/test/resources/play-json/build.sc b/integration/test/resources/play-json/build.sc
index 54e63d65..eb51e999 100644
--- a/integration/test/resources/play-json/build.sc
+++ b/integration/test/resources/play-json/build.sc
@@ -36,7 +36,7 @@ trait PlayJsonModule extends BaseModule with PublishModule with MiMa {
 
   trait Tests extends super.Tests with Scalariform with Headers {
     val specs2Core = T {
-      val v = Lib.scalaBinaryVersion(scalaVersion()) match {
+      val v = mill.scalalib.api.Util.scalaBinaryVersion(scalaVersion()) match {
         case "2.10" => "3.9.1"
         case _ => "4.0.2"
       }
diff --git a/integration/test/resources/play-json/headers.sc b/integration/test/resources/play-json/headers.sc
index 9737cb3d..31613fe6 100644
--- a/integration/test/resources/play-json/headers.sc
+++ b/integration/test/resources/play-json/headers.sc
@@ -2,7 +2,7 @@ import $ivy.`com.github.rockjam::license-headers:0.0.1`
 
 
 import mill._, scalalib._
-import mill.eval.PathRef
+import mill.api.PathRef
 import de.heikoseeberger.sbtheader.{CommentStyle, FileType, HeaderCreator, License}
 
 trait Headers extends ScalaModule {
diff --git a/main/api/src/io/github/retronym/java9rtexport/Copy.java b/main/api/src/io/github/retronym/java9rtexport/Copy.java
new file mode 100644
index 00000000..ac3615bb
--- /dev/null
+++ b/main/api/src/io/github/retronym/java9rtexport/Copy.java
@@ -0,0 +1,75 @@
+/*
+Copyright (C) 2012-2014 EPFL
+Copyright (C) 2012-2014 Typesafe, Inc.
+All rights reserved.
+Redistribution and use in source and binary forms, with or without modification,
+are permitted provided that the following conditions are met:
+    * Redistributions of source code must retain the above copyright notice,
+      this list of conditions and the following disclaimer.
+    * Redistributions in binary form must reproduce the above copyright notice,
+      this list of conditions and the following disclaimer in the documentation
+      and/or other materials provided with the distribution.
+    * Neither the name of the EPFL nor the names of its contributors
+      may be used to endorse or promote products derived from this software
+      without specific prior written permission.
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+package io.github.retronym.java9rtexport;
+
+import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.nio.file.*;
+import java.nio.file.attribute.*;
+import java.util.EnumSet;
+
+import static java.nio.file.StandardCopyOption.COPY_ATTRIBUTES;
+import static java.nio.file.StandardCopyOption.REPLACE_EXISTING;
+
+public class Copy {
+    public static void copyDirectory(final Path source, final Path target)
+            throws IOException {
+        Files.walkFileTree(source, EnumSet.of(FileVisitOption.FOLLOW_LINKS),
+                Integer.MAX_VALUE, new FileVisitor<Path>() {
+
+                    @Override
+                    public FileVisitResult preVisitDirectory(Path dir,
+                                                             BasicFileAttributes sourceBasic) throws IOException {
+
+                        String relative = source.relativize(dir).toString();
+                        if (!Files.exists(target.getFileSystem().getPath(relative)))
+                            Files.createDirectory(target.getFileSystem().getPath(relative));
+                        return FileVisitResult.CONTINUE;
+                    }
+
+                    @Override
+                    public FileVisitResult visitFile(Path file,
+                                                     BasicFileAttributes attrs) throws IOException {
+                        String relative = source.relativize(file).toString();
+                        Files.copy(file, target.getFileSystem().getPath(relative), COPY_ATTRIBUTES, REPLACE_EXISTING);
+                        return FileVisitResult.CONTINUE;
+                    }
+
+                    @Override
+                    public FileVisitResult visitFileFailed(Path file, IOException e) throws IOException {
+                        throw e;
+                    }
+
+                    @Override
+                    public FileVisitResult postVisitDirectory(Path dir, IOException e) throws IOException {
+                        if (e != null) throw e;
+                        return FileVisitResult.CONTINUE;
+                    }
+                });
+    }
+
+}
\ No newline at end of file
diff --git a/main/api/src/io/github/retronym/java9rtexport/Export.java b/main/api/src/io/github/retronym/java9rtexport/Export.java
new file mode 100644
index 00000000..b71ca555
--- /dev/null
+++ b/main/api/src/io/github/retronym/java9rtexport/Export.java
@@ -0,0 +1,103 @@
+/*
+Copyright (C) 2012-2014 EPFL
+Copyright (C) 2012-2014 Typesafe, Inc.
+All rights reserved.
+Redistribution and use in source and binary forms, with or without modification,
+are permitted provided that the following conditions are met:
+    * Redistributions of source code must retain the above copyright notice,
+      this list of conditions and the following disclaimer.
+    * Redistributions in binary form must reproduce the above copyright notice,
+      this list of conditions and the following disclaimer in the documentation
+      and/or other materials provided with the distribution.
+    * Neither the name of the EPFL nor the names of its contributors
+      may be used to endorse or promote products derived from this software
+      without specific prior written permission.
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+package io.github.retronym.java9rtexport;
+
+import java.io.File;
+import java.io.IOException;
+import java.net.URI;
+import java.nio.file.*;
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.Map;
+
+public class Export {
+    private final static Object lock = new Object();
+    private static File tempFile = null;
+
+    public static String rtJarName = "rt-" + System.getProperty("java.version") + ".jar";
+
+    public static File rt() {
+        try {
+            synchronized (lock) {
+                if (tempFile == null) {
+                    Path tempPath = Files.createTempFile("rt", ".jar");
+                    tempFile = tempPath.toFile();
+                    tempFile.deleteOnExit();
+                    tempFile.delete();
+                    FileSystem fileSystem = FileSystems.getFileSystem(URI.create("jrt:/"));
+                    Path path = fileSystem.getPath("/modules");
+                    URI uri = URI.create("jar:" + tempPath.toUri());
+                    Map<String, String> env = new HashMap<>();
+                    env.put("create", "true");
+                    try (FileSystem zipfs = FileSystems.newFileSystem(uri, env)) {
+                        Iterator<Path> iterator = Files.list(path).iterator();
+                        while (iterator.hasNext()) {
+                            Path next = iterator.next();
+                            Copy.copyDirectory(next, zipfs.getPath("/"));
+                        }
+                    }
+                }
+            }
+        } catch (IOException e) {
+            e.printStackTrace();
+            System.exit(-1);
+        }
+        return tempFile;
+    }
+
+    public static boolean rtTo(File dest, boolean verbose) {
+        try {
+            if (!dest.exists()) {
+                if (verbose) {
+                    System.out.println("Copying Java " +
+                            System.getProperty("java.version") +
+                            " runtime jar to " +
+                            dest.getParentFile() +
+                            " ...");
+                    System.out.flush();
+                }
+                dest.getParentFile().mkdirs();
+                java.nio.file.Files.copy(rt().toPath(), dest.toPath());
+                return true;
+            }
+        } catch (IOException e) {
+            e.printStackTrace();
+            System.exit(-1);
+        }
+        return false;
+    }
+
+    public static File rtAt(File dir, boolean verbose) {
+        File f = new File(dir, rtJarName);
+        rtTo(f, verbose);
+        return f;
+    }
+
+    public static File rtAt(File dir) {
+        return rtAt(dir, false);
+    }
+}
\ No newline at end of file
diff --git a/main/api/src/mill/api/AggWrapper.scala b/main/api/src/mill/api/AggWrapper.scala
new file mode 100644
index 00000000..98d46d68
--- /dev/null
+++ b/main/api/src/mill/api/AggWrapper.scala
@@ -0,0 +1,120 @@
+package mill.api
+
+
+
+import scala.collection.mutable
+object Strict extends AggWrapper(true)
+object Loose extends AggWrapper(false)
+
+sealed class AggWrapper(strictUniqueness: Boolean){
+  /**
+    * A collection with enforced uniqueness, fast contains and deterministic
+    * ordering. Raises an exception if a duplicate is found; call
+    * `toSeq.distinct` if you explicitly want to make it swallow duplicates
+    */
+  trait Agg[V] extends TraversableOnce[V]{
+    def contains(v: V): Boolean
+    def items: Iterator[V]
+    def indexed: IndexedSeq[V]
+    def flatMap[T](f: V => TraversableOnce[T]): Agg[T]
+    def map[T](f: V => T): Agg[T]
+    def filter(f: V => Boolean): Agg[V]
+    def withFilter(f: V => Boolean): Agg[V]
+    def collect[T](f: PartialFunction[V, T]): Agg[T]
+    def zipWithIndex: Agg[(V, Int)]
+    def reverse: Agg[V]
+    def zip[T](other: Agg[T]): Agg[(V, T)]
+    def ++[T >: V](other: TraversableOnce[T]): Agg[T]
+    def length: Int
+  }
+
+  object Agg{
+    def empty[V]: Agg[V] = new Agg.Mutable[V]
+    implicit def jsonFormat[T: upickle.default.ReadWriter]: upickle.default.ReadWriter[Agg[T]] =
+      upickle.default.readwriter[Seq[T]].bimap[Agg[T]](
+        _.toList,
+        Agg.from(_)
+      )
+
+    def apply[V](items: V*) = from(items)
+
+    implicit def from[V](items: TraversableOnce[V]): Agg[V] = {
+      val set = new Agg.Mutable[V]()
+      items.foreach(set.append)
+      set
+    }
+
+
+    class Mutable[V]() extends Agg[V]{
+
+      private[this] val set0 = mutable.LinkedHashSet.empty[V]
+      def contains(v: V) = set0.contains(v)
+      def append(v: V) = if (!contains(v)){
+        set0.add(v)
+
+      }else if (strictUniqueness){
+        throw new Exception("Duplicated item inserted into OrderedSet: " + v)
+      }
+      def appendAll(vs: Seq[V]) = vs.foreach(append)
+      def items = set0.iterator
+      def indexed: IndexedSeq[V] = items.toIndexedSeq
+      def set: collection.Set[V] = set0
+
+      def map[T](f: V => T): Agg[T] = {
+        val output = new Agg.Mutable[T]
+        for(i <- items) output.append(f(i))
+        output
+      }
+      def flatMap[T](f: V => TraversableOnce[T]): Agg[T] = {
+        val output = new Agg.Mutable[T]
+        for(i <- items) for(i0 <- f(i)) output.append(i0)
+        output
+      }
+      def filter(f: V => Boolean): Agg[V] = {
+        val output = new Agg.Mutable[V]
+        for(i <- items) if (f(i)) output.append(i)
+        output
+      }
+      def withFilter(f: V => Boolean): Agg[V] = filter(f)
+
+      def collect[T](f: PartialFunction[V, T]) = this.filter(f.isDefinedAt).map(x => f(x))
+
+      def zipWithIndex = {
+        var i = 0
+        this.map{ x =>
+          i += 1
+          (x, i-1)
+        }
+      }
+
+      def reverse = Agg.from(indexed.reverseIterator)
+
+      def zip[T](other: Agg[T]) = Agg.from(items.zip(other.items))
+      def ++[T >: V](other: TraversableOnce[T]) = Agg.from(items ++ other)
+      def length: Int = set0.size
+
+      // Members declared in scala.collection.GenTraversableOnce
+      def isTraversableAgain: Boolean = items.isTraversableAgain
+      def toIterator: Iterator[V] = items.toIterator
+      def toStream: Stream[V] = items.toStream
+
+      // Members declared in scala.collection.TraversableOnce
+      def copyToArray[B >: V](xs: Array[B], start: Int,len: Int): Unit = items.copyToArray(xs, start, len)
+      def exists(p: V => Boolean): Boolean = items.exists(p)
+      def find(p: V => Boolean): Option[V] = items.find(p)
+      def forall(p: V => Boolean): Boolean = items.forall(p)
+      def foreach[U](f: V => U): Unit = items.foreach(f)
+      def hasDefiniteSize: Boolean = items.hasDefiniteSize
+      def isEmpty: Boolean = items.isEmpty
+      def seq: scala.collection.TraversableOnce[V] = items
+      def toTraversable: Traversable[V] = items.toTraversable
+
+      override def hashCode() = items.map(_.hashCode()).sum
+      override def equals(other: Any) = other match{
+        case s: Agg[_] => items.sameElements(s.items)
+        case _ => super.equals(other)
+      }
+      override def toString = items.mkString("Agg(", ", ", ")")
+    }
+  }
+}
diff --git a/main/core/src/mill/util/ClassLoader.scala b/main/api/src/mill/api/ClassLoader.scala
index 07ab1ca9..198cbf6c 100644
--- a/main/core/src/mill/util/ClassLoader.scala
+++ b/main/api/src/mill/api/ClassLoader.scala
@@ -1,4 +1,4 @@
-package mill.util
+package mill.api
 
 import java.net.{URL, URLClassLoader}
 
@@ -8,6 +8,7 @@ import io.github.retronym.java9rtexport.Export
 import scala.util.Try
 
 object ClassLoader {
+  def java9OrAbove = !System.getProperty("java.specification.version").startsWith("1.")
   def create(urls: Seq[URL],
              parent: java.lang.ClassLoader)
             (implicit ctx: Ctx.Home): URLClassLoader = {
@@ -38,7 +39,7 @@ object ClassLoader {
     *  mill could be compiled only with jdk 9 or above. We don't want to introduce this restriction now.
     */
   private def refinePlatformParent(parent: java.lang.ClassLoader): ClassLoader = {
-    if (!ammonite.util.Util.java9OrAbove || parent != null) parent
+    if (!java9OrAbove || parent != null) parent
     else {
       // Make sure when `parent == null`, we only delegate java.* classes
       // to the parent getPlatformClassLoader. This is necessary because
@@ -53,7 +54,7 @@ object ClassLoader {
   }
 
   private def makeUrls(urls: Seq[URL])(implicit ctx: Ctx.Home): Seq[URL] = {
-    if (ammonite.util.Util.java9OrAbove) {
+    if (java9OrAbove) {
       urls :+ Export.rtAt(ctx.home.toIO).toURI.toURL
     } else {
       urls
diff --git a/main/core/src/mill/util/Ctx.scala b/main/api/src/mill/api/Ctx.scala
index bbc243b7..567da003 100644
--- a/main/core/src/mill/util/Ctx.scala
+++ b/main/api/src/mill/api/Ctx.scala
@@ -1,8 +1,7 @@
-package mill.util
+package mill.api
 
-import mill.define.Applicative.ImplicitStub
 
-import scala.annotation.compileTimeOnly
+import scala.annotation.{StaticAnnotation, compileTimeOnly}
 import scala.language.implicitConversions
 
 object Ctx{
@@ -32,8 +31,9 @@ object Ctx{
     def args: IndexedSeq[_]
   }
 
-  def defaultHome = ammonite.ops.home / ".mill" / "ammonite"
+  def defaultHome = os.home / ".mill" / "ammonite"
 
+  class ImplicitStub extends StaticAnnotation
 }
 class Ctx(val args: IndexedSeq[_],
           dest0: () => os.Path,
diff --git a/main/api/src/mill/api/JsonFormatters.scala b/main/api/src/mill/api/JsonFormatters.scala
new file mode 100644
index 00000000..918fa693
--- /dev/null
+++ b/main/api/src/mill/api/JsonFormatters.scala
@@ -0,0 +1,44 @@
+package mill.api
+
+import upickle.default.{ReadWriter => RW}
+import scala.util.matching.Regex
+object JsonFormatters extends JsonFormatters
+trait JsonFormatters {
+  implicit val pathReadWrite: RW[os.Path] = upickle.default.readwriter[String]
+    .bimap[os.Path](
+      _.toString,
+      os.Path(_)
+    )
+
+  implicit val regexReadWrite: RW[Regex] = upickle.default.readwriter[String]
+    .bimap[Regex](
+      _.pattern.toString,
+      _.r
+    )
+
+  implicit val bytesReadWrite: RW[os.Bytes] = upickle.default.readwriter[String]
+    .bimap(
+      o => java.util.Base64.getEncoder.encodeToString(o.array),
+      str => new os.Bytes(java.util.Base64.getDecoder.decode(str))
+    )
+
+
+  implicit lazy val crFormat: RW[os.CommandResult] = upickle.default.macroRW
+
+  implicit val stackTraceRW = upickle.default.readwriter[ujson.Obj].bimap[StackTraceElement](
+    ste => ujson.Obj(
+      "declaringClass" -> ujson.Str(ste.getClassName),
+      "methodName" -> ujson.Str(ste.getMethodName),
+      "fileName" -> ujson.Str(ste.getFileName),
+      "lineNumber" -> ujson.Num(ste.getLineNumber)
+    ),
+    {case json: ujson.Obj =>
+      new StackTraceElement(
+        json("declaringClass").str.toString,
+        json("methodName").str.toString,
+        json("fileName").str.toString,
+        json("lineNumber").num.toInt
+      )
+    }
+  )
+}
diff --git a/main/api/src/mill/api/Logger.scala b/main/api/src/mill/api/Logger.scala
new file mode 100644
index 00000000..4ae6e74d
--- /dev/null
+++ b/main/api/src/mill/api/Logger.scala
@@ -0,0 +1,41 @@
+package mill.api
+
+import java.io._
+
+/**
+  * The standard logging interface of the Mill build tool.
+  *
+  * Contains these primary logging methods, in order of increasing importance:
+  *
+  * - `debug` : internal debug messages normally not shown to the user;
+  * mostly useful when debugging issues
+  *
+  * - `ticker`: short-lived logging output where consecutive lines over-write
+  * each other; useful for information which is transient and disposable
+  *
+  * - `info`: miscellaneous logging output which isn't part of the main output
+  * a user is looking for, but useful to provide context on what Mill is doing
+  *
+  * - `error`: logging output which represents problems the user should care
+  * about
+  *
+  *
+  * Also contains the two forwarded stdout and stderr streams, for code executed
+  * by Mill to use directly. Typically these correspond to the stdout and stderr,
+  * but when `show` is used both are forwarded to stderr and stdout is only
+  * used to display the final `show` output for easy piping.
+  */
+trait Logger {
+  def colored: Boolean
+
+  val errorStream: PrintStream
+  val outputStream: PrintStream
+  val inStream: InputStream
+
+  def info(s: String): Unit
+  def error(s: String): Unit
+  def ticker(s: String): Unit
+  def debug(s: String): Unit
+
+  def close(): Unit = ()
+}
diff --git a/main/core/src/mill/eval/PathRef.scala b/main/api/src/mill/api/PathRef.scala
index 92ef8d24..24f3627e 100644
--- a/main/core/src/mill/eval/PathRef.scala
+++ b/main/api/src/mill/api/PathRef.scala
@@ -1,4 +1,4 @@
-package mill.eval
+package mill.api
 
 import java.io.IOException
 import java.nio.file.attribute.BasicFileAttributes
@@ -7,7 +7,6 @@ import java.nio.{file => jnio}
 import java.security.{DigestOutputStream, MessageDigest}
 
 import upickle.default.{ReadWriter => RW}
-import mill.util.{DummyOutputStream, IO, JsonFormatters}
 
 
 /**
@@ -67,3 +66,57 @@ object PathRef{
     }
   )
 }
+
+
+import java.io.{InputStream, OutputStream}
+
+/**
+  * Misc IO utilities, eventually probably should be pushed upstream into
+  * ammonite-ops
+  */
+object IO {
+  def stream(src: InputStream, dest: OutputStream) = {
+    val buffer = new Array[Byte](4096)
+    while ( {
+      src.read(buffer) match {
+        case -1 => false
+        case n =>
+          dest.write(buffer, 0, n)
+          true
+      }
+    }) ()
+  }
+
+
+  def unpackZip(src: os.Path, dest: os.RelPath = "unpacked")
+               (implicit ctx: Ctx.Dest) = {
+
+    val byteStream = os.read.inputStream(src)
+    val zipStream = new java.util.zip.ZipInputStream(byteStream)
+    while({
+      zipStream.getNextEntry match{
+        case null => false
+        case entry =>
+          if (!entry.isDirectory) {
+            val entryDest = ctx.dest / dest / os.RelPath(entry.getName)
+            os.makeDir.all(entryDest / os.up)
+            val fileOut = new java.io.FileOutputStream(entryDest.toString)
+            IO.stream(zipStream, fileOut)
+            fileOut.close()
+          }
+          zipStream.closeEntry()
+          true
+      }
+    })()
+    PathRef(ctx.dest / dest)
+  }
+}
+
+import java.io.{ByteArrayInputStream, OutputStream}
+
+object DummyInputStream extends ByteArrayInputStream(Array())
+object DummyOutputStream extends java.io.OutputStream{
+  override def write(b: Int) = ()
+  override def write(b: Array[Byte]) = ()
+  override def write(b: Array[Byte], off: Int, len: Int) = ()
+}
diff --git a/main/core/src/mill/eval/Result.scala b/main/api/src/mill/api/Result.scala
index d0400599..b4071a99 100644
--- a/main/core/src/mill/eval/Result.scala
+++ b/main/api/src/mill/api/Result.scala
@@ -1,4 +1,4 @@
-package mill.eval
+package mill.api
 
 sealed trait Result[+T]{
   def map[V](f: T => V): Result[V]
diff --git a/main/core/src/mill/define/Applicative.scala b/main/core/src/mill/define/Applicative.scala
index 69c506f7..5e63b1cc 100644
--- a/main/core/src/mill/define/Applicative.scala
+++ b/main/core/src/mill/define/Applicative.scala
@@ -25,7 +25,7 @@ object Applicative {
     def self: M[T]
     def apply()(implicit handler: ApplyHandler[M]): T = handler(self)
   }
-  class ImplicitStub extends StaticAnnotation
+
   type Id[+T] = T
 
   trait Applyer[W[_], T[_], Z[_], Ctx] extends ApplyerGenerated[T, Z, Ctx] {
@@ -81,7 +81,7 @@ object Applicative {
         tempIdent
       case (t, api)
         if t.symbol != null
-        && t.symbol.annotations.exists(_.tree.tpe =:= typeOf[ImplicitStub]) =>
+        && t.symbol.annotations.exists(_.tree.tpe =:= typeOf[mill.api.Ctx.ImplicitStub]) =>
 
         val tempIdent = Ident(ctxSym)
         c.internal.setType(tempIdent, t.tpe)
diff --git a/main/core/src/mill/define/Task.scala b/main/core/src/mill/define/Task.scala
index 07576724..a464bf18 100644
--- a/main/core/src/mill/define/Task.scala
+++ b/main/core/src/mill/define/Task.scala
@@ -27,7 +27,7 @@ abstract class Task[+T] extends Task.Ops[T] with Applyable[Task, T]{
   /**
     * Evaluate this target
     */
-  def evaluate(args: mill.util.Ctx): Result[T]
+  def evaluate(args: mill.api.Ctx): Result[T]
 
   /**
     * Even if this target's inputs did not change, does it need to re-evaluate
@@ -53,7 +53,7 @@ trait Target[+T] extends NamedTask[T]{
   def readWrite: RW[_]
 }
 
-object Target extends TargetGenerated with Applicative.Applyer[Task, Task, Result, mill.util.Ctx] {
+object Target extends TargetGenerated with Applicative.Applyer[Task, Task, Result, mill.api.Ctx] {
 
   implicit def apply[T](t: T)
                        (implicit rw: RW[T],
@@ -64,7 +64,7 @@ object Target extends TargetGenerated with Applicative.Applyer[Task, Task, Resul
                                   (rw: c.Expr[RW[T]],
                                    ctx: c.Expr[mill.define.Ctx]): c.Expr[Target[T]] = {
     import c.universe._
-    val lhs = Applicative.impl0[Task, T, mill.util.Ctx](c)(reify(Result.Success(t.splice)).tree)
+    val lhs = Applicative.impl0[Task, T, mill.api.Ctx](c)(reify(Result.Success(t.splice)).tree)
 
     mill.moduledefs.Cacher.impl0[TargetImpl[T]](c)(
       reify(
@@ -85,7 +85,7 @@ object Target extends TargetGenerated with Applicative.Applyer[Task, Task, Resul
     mill.moduledefs.Cacher.impl0[Target[T]](c)(
       reify(
         new TargetImpl[T](
-          Applicative.impl0[Task, T, mill.util.Ctx](c)(t.tree).splice,
+          Applicative.impl0[Task, T, mill.api.Ctx](c)(t.tree).splice,
           ctx.splice,
           rw.splice
         )
@@ -118,7 +118,7 @@ object Target extends TargetGenerated with Applicative.Applyer[Task, Task, Resul
     import c.universe._
     val wrapped =
       for (value <- values.toList)
-      yield Applicative.impl0[Task, PathRef, mill.util.Ctx](c)(
+      yield Applicative.impl0[Task, PathRef, mill.api.Ctx](c)(
         reify(value.splice.map(PathRef(_))).tree
       ).tree
 
@@ -144,7 +144,7 @@ object Target extends TargetGenerated with Applicative.Applyer[Task, Task, Resul
     mill.moduledefs.Cacher.impl0[Sources](c)(
       reify(
         new Sources(
-          Applicative.impl0[Task, Seq[PathRef], mill.util.Ctx](c)(values.tree).splice,
+          Applicative.impl0[Task, Seq[PathRef], mill.api.Ctx](c)(values.tree).splice,
           ctx.splice
         )
       )
@@ -163,7 +163,7 @@ object Target extends TargetGenerated with Applicative.Applyer[Task, Task, Resul
     mill.moduledefs.Cacher.impl0[Input[T]](c)(
       reify(
         new Input[T](
-          Applicative.impl[Task, T, mill.util.Ctx](c)(value).splice,
+          Applicative.impl[Task, T, mill.api.Ctx](c)(value).splice,
           ctx.splice,
           rw.splice
         )
@@ -194,7 +194,7 @@ object Target extends TargetGenerated with Applicative.Applyer[Task, Task, Resul
     import c.universe._
     reify(
       new Command[T](
-        Applicative.impl[Task, T, mill.util.Ctx](c)(t).splice,
+        Applicative.impl[Task, T, mill.api.Ctx](c)(t).splice,
         ctx.splice,
         w.splice,
         cls.splice.value,
@@ -214,11 +214,11 @@ object Target extends TargetGenerated with Applicative.Applyer[Task, Task, Resul
                                   (ctx: c.Expr[mill.define.Ctx]): c.Expr[Worker[T]] = {
     import c.universe._
     reify(
-      new Worker[T](Applicative.impl[Task, T, mill.util.Ctx](c)(t).splice, ctx.splice)
+      new Worker[T](Applicative.impl[Task, T, mill.api.Ctx](c)(t).splice, ctx.splice)
     )
   }
 
-  def task[T](t: Result[T]): Task[T] = macro Applicative.impl[Task, T, mill.util.Ctx]
+  def task[T](t: Result[T]): Task[T] = macro Applicative.impl[Task, T, mill.api.Ctx]
 
   def persistent[T](t: Result[T])(implicit rw: RW[T],
                                   ctx: mill.define.Ctx): Persistent[T] = macro persistentImpl[T]
@@ -233,7 +233,7 @@ object Target extends TargetGenerated with Applicative.Applyer[Task, Task, Resul
     mill.moduledefs.Cacher.impl0[Persistent[T]](c)(
       reify(
         new Persistent[T](
-          Applicative.impl[Task, T, mill.util.Ctx](c)(t).splice,
+          Applicative.impl[Task, T, mill.api.Ctx](c)(t).splice,
           ctx.splice,
           rw.splice
         )
@@ -242,20 +242,20 @@ object Target extends TargetGenerated with Applicative.Applyer[Task, Task, Resul
   }
 
   type TT[+X] = Task[X]
-  def makeT[X](inputs0: Seq[TT[_]], evaluate0: mill.util.Ctx => Result[X]) = new Task[X] {
+  def makeT[X](inputs0: Seq[TT[_]], evaluate0: mill.api.Ctx => Result[X]) = new Task[X] {
     val inputs = inputs0
-    def evaluate(x: mill.util.Ctx) = evaluate0(x)
+    def evaluate(x: mill.api.Ctx) = evaluate0(x)
   }
 
   def underlying[A](v: Task[A]) = v
-  def mapCtx[A, B](t: Task[A])(f: (A, mill.util.Ctx) => Result[B]) = t.mapDest(f)
+  def mapCtx[A, B](t: Task[A])(f: (A, mill.api.Ctx) => Result[B]) = t.mapDest(f)
   def zip() =  new Task.Task0(())
   def zip[A](a: Task[A]) = a.map(Tuple1(_))
   def zip[A, B](a: Task[A], b: Task[B]) = a.zip(b)
 }
 
 abstract class NamedTaskImpl[+T](ctx0: mill.define.Ctx, t: Task[T]) extends NamedTask[T]{
-  def evaluate(args: mill.util.Ctx) = args[T](0)
+  def evaluate(args: mill.api.Ctx) = args[T](0)
   val ctx = ctx0.copy(segments = ctx0.segments ++ Seq(ctx0.segment))
   val inputs = Seq(t)
 }
@@ -303,12 +303,12 @@ object Task {
   class Task0[T](t: T) extends Task[T]{
     lazy val t0 = t
     val inputs = Nil
-    def evaluate(args: mill.util.Ctx)  = t0
+    def evaluate(args: mill.api.Ctx)  = t0
   }
 
   abstract class Ops[+T]{ this: Task[T] =>
     def map[V](f: T => V) = new Task.Mapped(this, f)
-    def mapDest[V](f: (T, mill.util.Ctx) => Result[V]) = new Task.MappedDest(this, f)
+    def mapDest[V](f: (T, mill.api.Ctx) => Result[V]) = new Task.MappedDest(this, f)
 
     def filter(f: T => Boolean) = this
     def withFilter(f: T => Boolean) = this
@@ -323,22 +323,22 @@ object Task {
 
   class Sequence[+T](inputs0: Seq[Task[T]]) extends Task[Seq[T]]{
     val inputs = inputs0
-    def evaluate(args: mill.util.Ctx) = {
+    def evaluate(args: mill.api.Ctx) = {
       for (i <- 0 until args.length)
       yield args(i).asInstanceOf[T]
     }
 
   }
   class Mapped[+T, +V](source: Task[T], f: T => V) extends Task[V]{
-    def evaluate(args: mill.util.Ctx) = f(args(0))
+    def evaluate(args: mill.api.Ctx) = f(args(0))
     val inputs = List(source)
   }
-  class MappedDest[+T, +V](source: Task[T], f: (T, mill.util.Ctx) => Result[V]) extends Task[V]{
-    def evaluate(args: mill.util.Ctx) = f(args(0), args)
+  class MappedDest[+T, +V](source: Task[T], f: (T, mill.api.Ctx) => Result[V]) extends Task[V]{
+    def evaluate(args: mill.api.Ctx) = f(args(0), args)
     val inputs = List(source)
   }
   class Zipped[+T, +V](source1: Task[T], source2: Task[V]) extends Task[(T, V)]{
-    def evaluate(args: mill.util.Ctx) = (args(0), args(1))
+    def evaluate(args: mill.api.Ctx) = (args(0), args(1))
     val inputs = List(source1, source2)
   }
 }
diff --git a/main/core/src/mill/eval/Evaluator.scala b/main/core/src/mill/eval/Evaluator.scala
index 2aafdb7a..8709064e 100644
--- a/main/core/src/mill/eval/Evaluator.scala
+++ b/main/core/src/mill/eval/Evaluator.scala
@@ -7,7 +7,7 @@ import scala.collection.JavaConverters._
 import mill.util.Router.EntryPoint
 import ammonite.runtime.SpecialClassLoader
 import mill.define.{Ctx => _, _}
-import mill.eval.Result.OuterStack
+import mill.api.Result.OuterStack
 import mill.util
 import mill.util._
 import mill.util.Strict.Agg
diff --git a/main/core/src/mill/eval/package.scala b/main/core/src/mill/eval/package.scala
new file mode 100644
index 00000000..433f9074
--- /dev/null
+++ b/main/core/src/mill/eval/package.scala
@@ -0,0 +1,12 @@
+package mill
+
+package object eval {
+  // Backwards compatibility forwarders
+  val Result = mill.api.Result
+  type Result[+T] = mill.api.Result[T]
+
+  val PathRef = mill.api.PathRef
+  type PathRef = mill.api.PathRef
+
+  type Logger = mill.api.Logger
+}
diff --git a/main/core/src/mill/util/IO.scala b/main/core/src/mill/util/IO.scala
deleted file mode 100644
index 833e52c7..00000000
--- a/main/core/src/mill/util/IO.scala
+++ /dev/null
@@ -1,32 +0,0 @@
-package mill.util
-
-import java.io.{InputStream, OutputStream}
-
-import scala.tools.nsc.interpreter.OutputStream
-
-/**
-  * Misc IO utilities, eventually probably should be pushed upstream into
-  * ammonite-ops
-  */
-object IO {
-  def stream(src: InputStream, dest: OutputStream) = {
-    val buffer = new Array[Byte](4096)
-    while ( {
-      src.read(buffer) match {
-        case -1 => false
-        case n =>
-          dest.write(buffer, 0, n)
-          true
-      }
-    }) ()
-  }
-}
-
-import java.io.{ByteArrayInputStream, OutputStream}
-
-object DummyInputStream extends ByteArrayInputStream(Array())
-object DummyOutputStream extends OutputStream{
-  override def write(b: Int) = ()
-  override def write(b: Array[Byte]) = ()
-  override def write(b: Array[Byte], off: Int, len: Int) = ()
-}
diff --git a/main/core/src/mill/util/JsonFormatters.scala b/main/core/src/mill/util/JsonFormatters.scala
index dba599f7..830782c6 100644
--- a/main/core/src/mill/util/JsonFormatters.scala
+++ b/main/core/src/mill/util/JsonFormatters.scala
@@ -1,49 +1,10 @@
 package mill.util
 
 import upickle.default.{ReadWriter => RW}
-import scala.util.matching.Regex
-object JsonFormatters extends JsonFormatters
-trait JsonFormatters {
-  implicit val pathReadWrite: RW[os.Path] = upickle.default.readwriter[String]
-    .bimap[os.Path](
-      _.toString,
-      os.Path(_)
-    )
-
-  implicit val regexReadWrite: RW[Regex] = upickle.default.readwriter[String]
-    .bimap[Regex](
-      _.pattern.toString,
-      _.r
-    )
-
-  implicit val bytesReadWrite: RW[os.Bytes] = upickle.default.readwriter[String]
-    .bimap(
-      o => java.util.Base64.getEncoder.encodeToString(o.array),
-      str => new os.Bytes(java.util.Base64.getDecoder.decode(str))
-    )
-
-
-  implicit lazy val crFormat: RW[os.CommandResult] = upickle.default.macroRW
 
+trait JsonFormatters extends mill.api.JsonFormatters{
   implicit lazy val modFormat: RW[coursier.Module] = upickle.default.macroRW
   implicit lazy val depFormat: RW[coursier.Dependency]= upickle.default.macroRW
   implicit lazy val attrFormat: RW[coursier.Attributes] = upickle.default.macroRW
-  implicit val stackTraceRW = upickle.default.readwriter[ujson.Obj].bimap[StackTraceElement](
-    ste => ujson.Obj(
-      "declaringClass" -> ujson.Str(ste.getClassName),
-      "methodName" -> ujson.Str(ste.getMethodName),
-      "fileName" -> ujson.Str(ste.getFileName),
-      "lineNumber" -> ujson.Num(ste.getLineNumber)
-    ),
-    {case json: ujson.Obj =>
-      new StackTraceElement(
-        json("declaringClass").str.toString,
-        json("methodName").str.toString,
-        json("fileName").str.toString,
-        json("lineNumber").num.toInt
-      )
-    }
-  )
-
-
 }
+object JsonFormatters extends JsonFormatters
diff --git a/main/core/src/mill/util/Logger.scala b/main/core/src/mill/util/Loggers.scala
index 4857953d..aab1a324 100644
--- a/main/core/src/mill/util/Logger.scala
+++ b/main/core/src/mill/util/Loggers.scala
@@ -1,46 +1,7 @@
 package mill.util
 
 import java.io._
-
-import ammonite.util.Colors
-
-/**
-  * The standard logging interface of the Mill build tool.
-  *
-  * Contains these primary logging methods, in order of increasing importance:
-  *
-  * - `debug` : internal debug messages normally not shown to the user;
-  * mostly useful when debugging issues
-  *
-  * - `ticker`: short-lived logging output where consecutive lines over-write
-  * each other; useful for information which is transient and disposable
-  *
-  * - `info`: miscellaneous logging output which isn't part of the main output
-  * a user is looking for, but useful to provide context on what Mill is doing
-  *
-  * - `error`: logging output which represents problems the user should care
-  * about
-  *
-  *
-  * Also contains the two forwarded stdout and stderr streams, for code executed
-  * by Mill to use directly. Typically these correspond to the stdout and stderr,
-  * but when `show` is used both are forwarded to stderr and stdout is only
-  * used to display the final `show` output for easy piping.
-  */
-trait Logger {
-  def colored: Boolean
-
-  val errorStream: PrintStream
-  val outputStream: PrintStream
-  val inStream: InputStream
-
-  def info(s: String): Unit
-  def error(s: String): Unit
-  def ticker(s: String): Unit
-  def debug(s: String): Unit
-
-  def close(): Unit = ()
-}
+import mill.api.Logger
 
 object DummyLogger extends Logger {
   def colored = false
@@ -160,7 +121,7 @@ case class FileLogger(colored: Boolean, file: os.Path, debugEnabled: Boolean) ex
   def error(s: String) = outputStream.println(s)
   def ticker(s: String) = outputStream.println(s)
   def debug(s: String) = if (debugEnabled) outputStream.println(s)
-  val inStream: InputStream = DummyInputStream
+  val inStream: InputStream = mill.api.DummyInputStream
   override def close() = {
     if (outputStreamUsed)
       outputStream.close()
diff --git a/main/core/src/mill/util/Watched.scala b/main/core/src/mill/util/Watched.scala
index f1ef4fee..29be53c3 100644
--- a/main/core/src/mill/util/Watched.scala
+++ b/main/core/src/mill/util/Watched.scala
@@ -1,6 +1,6 @@
 package mill.util
 
-import mill.eval.PathRef
+import mill.api.PathRef
 
 case class Watched[T](value: T, watched: Seq[PathRef])
 object Watched{
diff --git a/main/core/src/mill/util/package.scala b/main/core/src/mill/util/package.scala
new file mode 100644
index 00000000..ec5d2efc
--- /dev/null
+++ b/main/core/src/mill/util/package.scala
@@ -0,0 +1,7 @@
+package mill
+
+package object util {
+  // Backwards compat stubs
+  val Ctx = mill.api.Ctx
+  type Ctx = mill.api.Ctx
+}
diff --git a/main/src/mill/MillMain.scala b/main/src/mill/MillMain.scala
index e030fdc7..e953e65d 100644
--- a/main/src/mill/MillMain.scala
+++ b/main/src/mill/MillMain.scala
@@ -6,7 +6,7 @@ import scala.collection.JavaConverters._
 import ammonite.main.Cli._
 import io.github.retronym.java9rtexport.Export
 import mill.eval.Evaluator
-import mill.util.DummyInputStream
+import mill.api.DummyInputStream
 
 object MillMain {
 
@@ -38,7 +38,7 @@ object MillMain {
             setIdle: Boolean => Unit): (Boolean, Option[Evaluator.State]) = {
     import ammonite.main.Cli
     
-    val millHome = mill.util.Ctx.defaultHome
+    val millHome = mill.api.Ctx.defaultHome
 
     val removed = Set("predef-code", "no-home-predef")
     var interactive = false
diff --git a/main/src/mill/main/MainScopts.scala b/main/src/mill/main/MainScopts.scala
index 5cc4d7ba..718a30e6 100644
--- a/main/src/mill/main/MainScopts.scala
+++ b/main/src/mill/main/MainScopts.scala
@@ -25,7 +25,7 @@ object Tasks{
 class EvaluatorScopt[T]()
   extends scopt.Read[mill.eval.Evaluator]{
   def arity = 0
-  def reads = s => try{
+  def reads = s => {
     Evaluator.currentEvaluator.get.asInstanceOf[mill.eval.Evaluator]
   }
 }
diff --git a/main/src/mill/main/MillServerMain.scala b/main/src/mill/main/MillServerMain.scala
index 5ced75eb..26ca99e6 100644
--- a/main/src/mill/main/MillServerMain.scala
+++ b/main/src/mill/main/MillServerMain.scala
@@ -9,7 +9,7 @@ import scala.collection.JavaConverters._
 import org.scalasbt.ipcsocket._
 import mill.main.client._
 import mill.eval.Evaluator
-import mill.util.DummyInputStream
+import mill.api.DummyInputStream
 import sun.misc.{Signal, SignalHandler}
 
 trait MillServerMain[T]{
diff --git a/main/src/mill/main/RunScript.scala b/main/src/mill/main/RunScript.scala
index 119ac2aa..47526631 100644
--- a/main/src/mill/main/RunScript.scala
+++ b/main/src/mill/main/RunScript.scala
@@ -9,7 +9,8 @@ import ammonite.util.{Name, Res, Util}
 import mill.define
 import mill.define._
 import mill.eval.{Evaluator, PathRef, Result}
-import mill.util.{EitherOps, Logger, ParseArgs, Watched}
+import mill.util.{EitherOps, ParseArgs, Watched}
+import mill.api.Logger
 import mill.util.Strict.Agg
 
 import scala.collection.mutable
diff --git a/main/src/mill/main/VisualizeModule.scala b/main/src/mill/main/VisualizeModule.scala
index 71b9fc22..e950973f 100644
--- a/main/src/mill/main/VisualizeModule.scala
+++ b/main/src/mill/main/VisualizeModule.scala
@@ -37,7 +37,7 @@ trait VisualizeModule extends mill.define.TaskModule{
     val in = new LinkedBlockingQueue[(Seq[_], Seq[_], os.Path)]()
     val out = new LinkedBlockingQueue[Result[Seq[PathRef]]]()
 
-    val cl = mill.util.ClassLoader.create(
+    val cl = mill.api.ClassLoader.create(
       classpath().map(_.path.toNIO.toUri.toURL).toVector,
       getClass.getClassLoader
     )
diff --git a/main/src/mill/modules/Jvm.scala b/main/src/mill/modules/Jvm.scala
index 8d2c4de4..1a51ed8b 100644
--- a/main/src/mill/modules/Jvm.scala
+++ b/main/src/mill/modules/Jvm.scala
@@ -13,7 +13,8 @@ import coursier.util.{Gather, Task}
 import geny.Generator
 import mill.main.client.InputPumper
 import mill.eval.{PathRef, Result}
-import mill.util.{Ctx, IO}
+import mill.util.Ctx
+import mill.api.IO
 import mill.util.Loose.Agg
 
 import scala.collection.mutable
@@ -165,15 +166,15 @@ object Jvm {
     val urls = classPath.map(_.toIO.toURI.toURL)
     val cl = if (classLoaderOverrideSbtTesting) {
       val outerClassLoader = getClass.getClassLoader
-      mill.util.ClassLoader.create(urls.toVector, null, customFindClass = { name =>
+      mill.api.ClassLoader.create(urls.toVector, null, customFindClass = { name =>
         if (name.startsWith("sbt.testing."))
           Some(outerClassLoader.loadClass(name))
         else None
       })
     } else if (isolated) {
-      mill.util.ClassLoader.create(urls.toVector, null)
+      mill.api.ClassLoader.create(urls.toVector, null)
     } else {
-      mill.util.ClassLoader.create(urls.toVector, getClass.getClassLoader)
+      mill.api.ClassLoader.create(urls.toVector, getClass.getClassLoader)
     }
 
     val oldCl = Thread.currentThread().getContextClassLoader
diff --git a/main/src/mill/modules/Util.scala b/main/src/mill/modules/Util.scala
index 2f57595e..2b98a304 100644
--- a/main/src/mill/modules/Util.scala
+++ b/main/src/mill/modules/Util.scala
@@ -2,8 +2,9 @@ package mill.modules
 
 
 import coursier.Repository
-import mill.eval.PathRef
-import mill.util.{Ctx, IO, Loose}
+import mill.api.{PathRef, IO}
+import mill.util.{Ctx, Loose}
+
 
 object Util {
   def cleanupScaladoc(v: String) = {
@@ -42,31 +43,9 @@ object Util {
 
     val tmpName = if (dest == os.rel / "tmp.zip") "tmp2.zip" else "tmp.zip"
     val downloaded = download(url, tmpName)
-    unpackZip(downloaded.path, dest)
+    IO.unpackZip(downloaded.path, dest)
   }
 
-  def unpackZip(src: os.Path, dest: os.RelPath = "unpacked")
-               (implicit ctx: Ctx.Dest) = {
-
-    val byteStream = os.read.inputStream(src)
-    val zipStream = new java.util.zip.ZipInputStream(byteStream)
-    while({
-      zipStream.getNextEntry match{
-        case null => false
-        case entry =>
-          if (!entry.isDirectory) {
-            val entryDest = ctx.dest / dest / os.RelPath(entry.getName)
-            os.makeDir.all(entryDest / ammonite.ops.up)
-            val fileOut = new java.io.FileOutputStream(entryDest.toString)
-            IO.stream(zipStream, fileOut)
-            fileOut.close()
-          }
-          zipStream.closeEntry()
-          true
-      }
-    })()
-    PathRef(ctx.dest / dest)
-  }
 
   def millProjectModule(key: String,
                         artifact: String,
@@ -75,7 +54,7 @@ object Util {
                         artifactSuffix: String = "_2.12") = {
     val localPath = sys.props(key)
     if (localPath != null) {
-      mill.eval.Result.Success(
+      mill.api.Result.Success(
         Loose.Agg.from(localPath.split(',').map(p => PathRef(os.Path(p), quick = true)))
       )
     } else {
diff --git a/main/src/mill/package.scala b/main/src/mill/package.scala
index 93916c8b..0ccd094f 100644
--- a/main/src/mill/package.scala
+++ b/main/src/mill/package.scala
@@ -3,8 +3,8 @@ import mill.util.JsonFormatters
 package object mill extends JsonFormatters{
   val T = define.Target
   type T[T] = define.Target[T]
-  val PathRef = mill.eval.PathRef
-  type PathRef = mill.eval.PathRef
+  val PathRef = mill.api.PathRef
+  type PathRef = mill.api.PathRef
   type Module = define.Module
   type Cross[T] = define.Cross[T]
   type Agg[T] = util.Loose.Agg[T]
diff --git a/main/test/resources/examples/javac/build.sc b/main/test/resources/examples/javac/build.sc
index c45a6e2d..2ed9f915 100644
--- a/main/test/resources/examples/javac/build.sc
+++ b/main/test/resources/examples/javac/build.sc
@@ -1,6 +1,6 @@
 import mill.T
 import mill.eval.JavaCompileJarTests.compileAll
-import mill.eval.PathRef
+import mill.api.PathRef
 import mill.modules.Jvm
 import mill.util.Loose
 
diff --git a/main/test/src/mill/define/ApplicativeTests.scala b/main/test/src/mill/define/ApplicativeTests.scala
index 72b715bb..9dd2132f 100644
--- a/main/test/src/mill/define/ApplicativeTests.scala
+++ b/main/test/src/mill/define/ApplicativeTests.scala
@@ -1,6 +1,6 @@
 package mill.define
 
-import mill.define.Applicative.ImplicitStub
+import mill.api.Ctx.ImplicitStub
 import utest._
 
 import scala.annotation.compileTimeOnly
diff --git a/main/test/src/mill/define/CacherTests.scala b/main/test/src/mill/define/CacherTests.scala
index 84a8d840..59ebf3f6 100644
--- a/main/test/src/mill/define/CacherTests.scala
+++ b/main/test/src/mill/define/CacherTests.scala
@@ -3,7 +3,7 @@ package mill.define
 import mill.util.{DummyLogger, TestEvaluator, TestUtil}
 import mill.util.Strict.Agg
 import mill.T
-import mill.eval.Result.Success
+import mill.api.Result.Success
 import utest._
 import utest.framework.TestPath
 
diff --git a/main/test/src/mill/eval/FailureTests.scala b/main/test/src/mill/eval/FailureTests.scala
index bf45119c..dcfbcb60 100644
--- a/main/test/src/mill/eval/FailureTests.scala
+++ b/main/test/src/mill/eval/FailureTests.scala
@@ -1,7 +1,7 @@
 package mill.eval
 import mill.T
 import mill.util.{TestEvaluator, TestUtil}
-import mill.eval.Result.OuterStack
+import mill.api.Result.OuterStack
 import utest._
 import utest.framework.TestPath
 
diff --git a/main/test/src/mill/eval/JavaCompileJarTests.scala b/main/test/src/mill/eval/JavaCompileJarTests.scala
index 8e931747..426c6ea6 100644
--- a/main/test/src/mill/eval/JavaCompileJarTests.scala
+++ b/main/test/src/mill/eval/JavaCompileJarTests.scala
@@ -2,7 +2,7 @@ package mill.eval
 
 import mill.define.{Discover, Input, Target, Task}
 import mill.modules.Jvm
-import mill.util.Ctx.Dest
+import mill.api.Ctx.Dest
 import mill.{Module, T}
 import mill.util.{DummyLogger, Loose, TestEvaluator, TestUtil}
 import mill.util.Strict.Agg
diff --git a/main/test/src/mill/util/TestEvaluator.scala b/main/test/src/mill/util/TestEvaluator.scala
index 67ba96dc..9a235679 100644
--- a/main/test/src/mill/util/TestEvaluator.scala
+++ b/main/test/src/mill/util/TestEvaluator.scala
@@ -1,7 +1,7 @@
 package mill.util
 
 import mill.define.{Input, Target, Task}
-import mill.eval.Result.OuterStack
+import mill.api.Result.OuterStack
 import mill.eval.{Evaluator, Result}
 import mill.util.Strict.Agg
 import utest.assert
diff --git a/main/test/src/mill/util/TestUtil.scala b/main/test/src/mill/util/TestUtil.scala
index e5fe906e..baab2992 100644
--- a/main/test/src/mill/util/TestUtil.scala
+++ b/main/test/src/mill/util/TestUtil.scala
@@ -2,8 +2,8 @@ package mill.util
 
 import mill.util.Router.Overrides
 import mill.define._
-import mill.eval.Result
-import mill.eval.Result.OuterStack
+import mill.api.Result
+import mill.api.Result.OuterStack
 import utest.assert
 import mill.util.Strict.Agg
 import utest.framework.TestPath
diff --git a/scalajslib/api/src/mill/scalajslib/api/ScalaJSWorkerApi.scala b/scalajslib/api/src/mill/scalajslib/api/ScalaJSWorkerApi.scala
new file mode 100644
index 00000000..4e44a8df
--- /dev/null
+++ b/scalajslib/api/src/mill/scalajslib/api/ScalaJSWorkerApi.scala
@@ -0,0 +1,41 @@
+package mill.scalajslib.api
+import java.io.File
+import mill.api.Result
+trait ScalaJSWorkerApi {
+  def link(sources: Array[File],
+           libraries: Array[File],
+           dest: File,
+           main: String,
+           fullOpt: Boolean,
+           moduleKind: ModuleKind): Result[File]
+
+  def run(config: NodeJSConfig, linkedFile: File): Unit
+
+  def getFramework(config: NodeJSConfig,
+                   frameworkName: String,
+                   linkedFile: File): (() => Unit, sbt.testing.Framework)
+
+}
+
+
+sealed trait OptimizeMode
+
+object FastOpt extends OptimizeMode
+object FullOpt extends OptimizeMode
+
+sealed trait ModuleKind
+object ModuleKind{
+  object NoModule extends ModuleKind
+  object CommonJSModule extends ModuleKind
+}
+
+
+object NodeJSConfig {
+  import upickle.default.{ReadWriter => RW, macroRW}
+  implicit def rw: RW[NodeJSConfig] = macroRW
+}
+
+final case class NodeJSConfig(executable: String = "node",
+                              args: List[String] = Nil,
+                              env: Map[String, String] = Map.empty,
+                              sourceMap: Boolean = true)
diff --git a/scalajslib/src/mill/scalajslib/NodeJSConfig.scala b/scalajslib/src/mill/scalajslib/NodeJSConfig.scala
deleted file mode 100644
index de39308c..00000000
--- a/scalajslib/src/mill/scalajslib/NodeJSConfig.scala
+++ /dev/null
@@ -1,11 +0,0 @@
-package mill.scalajslib
-
-object NodeJSConfig {
-  import upickle.default.{ReadWriter => RW, macroRW}
-  implicit def rw: RW[NodeJSConfig] = macroRW
-}
-
-final case class NodeJSConfig(executable: String = "node",
-                              args: List[String] = Nil,
-                              env: Map[String, String] = Map.empty,
-                              sourceMap: Boolean = true)
diff --git a/scalajslib/src/mill/scalajslib/ScalaJSModule.scala b/scalajslib/src/mill/scalajslib/ScalaJSModule.scala
index 37537a85..8568c39b 100644
--- a/scalajslib/src/mill/scalajslib/ScalaJSModule.scala
+++ b/scalajslib/src/mill/scalajslib/ScalaJSModule.scala
@@ -4,11 +4,11 @@ package scalajslib
 import coursier.Cache
 import coursier.maven.MavenRepository
 import mill.eval.{PathRef, Result}
-import mill.eval.Result.Success
+import mill.api.Result.Success
 import mill.scalalib.Lib.resolveDependencies
 import mill.scalalib.{DepSyntax, Lib, TestModule, TestRunner}
 import mill.util.{Ctx, Loose}
-
+import mill.scalajslib.api._
 trait ScalaJSModule extends scalalib.ScalaModule { outer =>
 
   def scalaJSVersion: T[String]
@@ -21,7 +21,7 @@ trait ScalaJSModule extends scalalib.ScalaModule { outer =>
     override def moduleDeps = Seq(outer)
   }
 
-  def scalaJSBinaryVersion = T { Lib.scalaBinaryVersion(scalaJSVersion()) }
+  def scalaJSBinaryVersion = T { mill.scalalib.api.Util.scalaBinaryVersion(scalaJSVersion()) }
 
   def scalaJSWorkerVersion = T{ scalaJSVersion().split('.').dropRight(1).mkString(".") }
 
@@ -92,11 +92,11 @@ trait ScalaJSModule extends scalalib.ScalaModule { outer =>
   }
 
   override def runMainLocal(mainClass: String, args: String*) = T.command[Unit] {
-    mill.eval.Result.Failure("runMain is not supported in Scala.js")
+    mill.api.Result.Failure("runMain is not supported in Scala.js")
   }
 
   override def runMain(mainClass: String, args: String*) = T.command[Unit] {
-    mill.eval.Result.Failure("runMain is not supported in Scala.js")
+    mill.api.Result.Failure("runMain is not supported in Scala.js")
   }
 
   def link(worker: ScalaJSWorker,
diff --git a/scalajslib/src/mill/scalajslib/ScalaJSWorkerApi.scala b/scalajslib/src/mill/scalajslib/ScalaJSWorkerApi.scala
index 881a65b9..bcaeb2d3 100644
--- a/scalajslib/src/mill/scalajslib/ScalaJSWorkerApi.scala
+++ b/scalajslib/src/mill/scalajslib/ScalaJSWorkerApi.scala
@@ -4,21 +4,10 @@ import java.io.File
 import java.net.URLClassLoader
 
 import mill.define.Discover
-import mill.eval.Result
-import mill.util.Ctx
+import mill.api.Result
+import mill.api.Ctx
 import mill.{Agg, T}
-
-sealed trait OptimizeMode
-
-object FastOpt extends OptimizeMode
-object FullOpt extends OptimizeMode
-
-sealed trait ModuleKind
-object ModuleKind{
-  object NoModule extends ModuleKind
-  object CommonJSModule extends ModuleKind
-}
-
+import mill.scalajslib.api._
 class ScalaJSWorker {
   private var scalaInstanceCache = Option.empty[(Long, ScalaJSWorkerApi)]
 
@@ -29,7 +18,7 @@ class ScalaJSWorker {
     scalaInstanceCache match {
       case Some((sig, bridge)) if sig == classloaderSig => bridge
       case _ =>
-        val cl = mill.util.ClassLoader.create(
+        val cl = mill.api.ClassLoader.create(
           toolsClasspath.map(_.toIO.toURI.toURL).toVector,
           getClass.getClassLoader
         )
@@ -76,22 +65,6 @@ class ScalaJSWorker {
 
 }
 
-trait ScalaJSWorkerApi {
-  def link(sources: Array[File],
-           libraries: Array[File],
-           dest: File,
-           main: String,
-           fullOpt: Boolean,
-           moduleKind: ModuleKind): Result[File]
-
-  def run(config: NodeJSConfig, linkedFile: File): Unit
-
-  def getFramework(config: NodeJSConfig,
-                   frameworkName: String,
-                   linkedFile: File): (() => Unit, sbt.testing.Framework)
-
-}
-
 object ScalaJSWorkerApi extends mill.define.ExternalModule {
 
   def scalaJSWorker = T.worker { new ScalaJSWorker() }
diff --git a/scalajslib/test/src/mill/scalajslib/HelloJSWorldTests.scala b/scalajslib/test/src/mill/scalajslib/HelloJSWorldTests.scala
index 7b1027e8..1b3e9bb3 100644
--- a/scalajslib/test/src/mill/scalajslib/HelloJSWorldTests.scala
+++ b/scalajslib/test/src/mill/scalajslib/HelloJSWorldTests.scala
@@ -11,7 +11,7 @@ import utest._
 
 
 import scala.collection.JavaConverters._
-
+import mill.scalajslib.api._
 
 object HelloJSWorldTests extends TestSuite {
   val workspacePath =  TestUtil.getOutPathStatic() / "hello-js-world"
diff --git a/scalajslib/test/src/mill/scalajslib/MultiModuleTests.scala b/scalajslib/test/src/mill/scalajslib/MultiModuleTests.scala
index 751c7537..2ffea2cf 100644
--- a/scalajslib/test/src/mill/scalajslib/MultiModuleTests.scala
+++ b/scalajslib/test/src/mill/scalajslib/MultiModuleTests.scala
@@ -6,7 +6,7 @@ import mill.eval.Evaluator
 import mill.util._
 import mill.scalalib._
 import utest._
-
+import mill.scalajslib.api._
 object MultiModuleTests extends TestSuite {
   val workspacePath =  TestUtil.getOutPathStatic() / "multi-module"
   val sourcePath = os.pwd / 'scalajslib / 'test / 'resources / "multi-module"
diff --git a/scalajslib/test/src/mill/scalajslib/NodeJSConfigTests.scala b/scalajslib/test/src/mill/scalajslib/NodeJSConfigTests.scala
index 27ea0a13..d9e6b45b 100644
--- a/scalajslib/test/src/mill/scalajslib/NodeJSConfigTests.scala
+++ b/scalajslib/test/src/mill/scalajslib/NodeJSConfigTests.scala
@@ -6,7 +6,7 @@ import mill.eval.Evaluator
 import mill.scalalib.{CrossScalaModule, DepSyntax}
 import mill.util.{TestEvaluator, TestUtil}
 import utest._
-
+import mill.scalajslib.api._
 
 object NodeJSConfigTests extends TestSuite {
   val workspacePath =  TestUtil.getOutPathStatic() / "hello-js-world"
diff --git a/scalajslib/worker/0.6/src/mill/scalajslib/worker/ScalaJSWorkerImpl.scala b/scalajslib/worker/0.6/src/mill/scalajslib/worker/ScalaJSWorkerImpl.scala
index 7c30ece9..64b668e5 100644
--- a/scalajslib/worker/0.6/src/mill/scalajslib/worker/ScalaJSWorkerImpl.scala
+++ b/scalajslib/worker/0.6/src/mill/scalajslib/worker/ScalaJSWorkerImpl.scala
@@ -4,7 +4,7 @@ package worker
 
 import java.io.File
 
-import mill.eval.Result
+import mill.api.Result
 import org.scalajs.core.tools.io.IRFileCache.IRContainer
 import org.scalajs.core.tools.io._
 import org.scalajs.core.tools.jsdep.ResolvedJSDependency
@@ -13,8 +13,8 @@ import org.scalajs.core.tools.logging.ScalaConsoleLogger
 import org.scalajs.jsenv._
 import org.scalajs.jsenv.nodejs._
 import org.scalajs.testadapter.TestAdapter
-
-class ScalaJSWorkerImpl extends mill.scalajslib.ScalaJSWorkerApi {
+import mill.scalajslib.api.{ModuleKind, NodeJSConfig}
+class ScalaJSWorkerImpl extends mill.scalajslib.api.ScalaJSWorkerApi {
   def link(sources: Array[File],
            libraries: Array[File],
            dest: File,
diff --git a/scalajslib/worker/1.0/src/mill/scalajslib/worker/ScalaJSWorkerImpl.scala b/scalajslib/worker/1.0/src/mill/scalajslib/worker/ScalaJSWorkerImpl.scala
index 45795271..f40f7a6b 100644
--- a/scalajslib/worker/1.0/src/mill/scalajslib/worker/ScalaJSWorkerImpl.scala
+++ b/scalajslib/worker/1.0/src/mill/scalajslib/worker/ScalaJSWorkerImpl.scala
@@ -3,8 +3,8 @@ package scalajslib
 package worker
 
 import java.io.File
-
-import mill.eval.Result
+import mill.scalajslib.api.{ModuleKind, NodeJSConfig}
+import mill.api.Result
 import org.scalajs.core.tools.io._
 import org.scalajs.core.tools.linker.{ModuleInitializer, Semantics, StandardLinker, ModuleKind => ScalaJSModuleKind}
 import org.scalajs.core.tools.logging.ScalaConsoleLogger
@@ -12,7 +12,7 @@ import org.scalajs.jsenv.ConsoleJSConsole
 import org.scalajs.jsenv.nodejs._
 import org.scalajs.testadapter.TestAdapter
 
-class ScalaJSWorkerImpl extends mill.scalajslib.ScalaJSWorkerApi {
+class ScalaJSWorkerImpl extends mill.scalajslib.api.ScalaJSWorkerApi {
   def link(sources: Array[File],
            libraries: Array[File],
            dest: File,
diff --git a/scalalib/api/src/mill/scalalib/api/ZincWorkerApi.scala b/scalalib/api/src/mill/scalalib/api/ZincWorkerApi.scala
new file mode 100644
index 00000000..c5230ec5
--- /dev/null
+++ b/scalalib/api/src/mill/scalalib/api/ZincWorkerApi.scala
@@ -0,0 +1,76 @@
+package mill.scalalib.api
+
+import mill.api.Loose.Agg
+import mill.api.PathRef
+import mill.api.JsonFormatters._
+
+trait ZincWorkerApi {
+  /** Compile a Java-only project */
+  def compileJava(upstreamCompileOutput: Seq[CompilationResult],
+                  sources: Agg[os.Path],
+                  compileClasspath: Agg[os.Path],
+                  javacOptions: Seq[String])
+                 (implicit ctx: mill.api.Ctx): mill.api.Result[CompilationResult]
+
+  /** Compile a mixed Scala/Java or Scala-only project */
+  def compileMixed(upstreamCompileOutput: Seq[CompilationResult],
+                   sources: Agg[os.Path],
+                   compileClasspath: Agg[os.Path],
+                   javacOptions: Seq[String],
+                   scalaVersion: String,
+                   scalacOptions: Seq[String],
+                   compilerBridgeSources: os.Path,
+                   compilerClasspath: Agg[os.Path],
+                   scalacPluginClasspath: Agg[os.Path])
+                  (implicit ctx: mill.api.Ctx): mill.api.Result[CompilationResult]
+
+  def discoverMainClasses(compilationResult: CompilationResult)
+                         (implicit ctx: mill.api.Ctx): Seq[String]
+
+  def docJar(scalaVersion: String,
+             compilerBridgeSources: os.Path,
+             compilerClasspath: Agg[os.Path],
+             scalacPluginClasspath: Agg[os.Path],
+             args: Seq[String])
+            (implicit ctx: mill.api.Ctx): Boolean
+}
+
+
+object CompilationResult {
+  implicit val jsonFormatter: upickle.default.ReadWriter[CompilationResult] = upickle.default.macroRW
+}
+
+// analysisFile is represented by os.Path, so we won't break caches after file changes
+case class CompilationResult(analysisFile: os.Path, classes: PathRef)
+
+object Util{
+  def isDotty(scalaVersion: String) =
+    scalaVersion.startsWith("0.")
+
+
+  def grepJar(classPath: Agg[os.Path], name: String, version: String, sources: Boolean = false) = {
+    val suffix = if (sources) "-sources" else ""
+    val mavenStylePath = s"$name-$version$suffix.jar"
+    val ivyStylePath = {
+      val dir = if (sources) "srcs" else "jars"
+      s"$version/$dir/$name$suffix.jar"
+    }
+
+    classPath
+      .find(p => p.toString.endsWith(mavenStylePath) || p.toString.endsWith(ivyStylePath))
+      .getOrElse(throw new Exception(s"Cannot find $mavenStylePath or $ivyStylePath"))
+  }
+
+  private val ReleaseVersion = raw"""(\d+)\.(\d+)\.(\d+)""".r
+  private val MinorSnapshotVersion = raw"""(\d+)\.(\d+)\.([1-9]\d*)-SNAPSHOT""".r
+  private val DottyVersion = raw"""0\.(\d+)\.(\d+).*""".r
+
+  def scalaBinaryVersion(scalaVersion: String) = {
+    scalaVersion match {
+      case ReleaseVersion(major, minor, _) => s"$major.$minor"
+      case MinorSnapshotVersion(major, minor, _) => s"$major.$minor"
+      case DottyVersion(minor, _) => s"0.$minor"
+      case _ => scalaVersion
+    }
+  }
+}
\ No newline at end of file
diff --git a/scalalib/src/mill/scalalib/Dep.scala b/scalalib/src/mill/scalalib/Dep.scala
index b419462c..714fa21e 100644
--- a/scalalib/src/mill/scalalib/Dep.scala
+++ b/scalalib/src/mill/scalalib/Dep.scala
@@ -5,7 +5,7 @@ import upickle.default.{macroRW, ReadWriter => RW}
 import CrossVersion._
 
 case class Dep(dep: coursier.Dependency, cross: CrossVersion, force: Boolean) {
-  import Dep.isDotty
+  import mill.scalalib.api.Util.isDotty
 
   def artifactName(binaryVersion: String, fullVersion: String, platformSuffix: String) = {
     val suffix = cross.suffixString(binaryVersion, fullVersion, platformSuffix)
@@ -51,9 +51,6 @@ object Dep {
 
   val DefaultConfiguration = "default(compile)"
 
-  def isDotty(scalaVersion: String) =
-    scalaVersion.startsWith("0.")
-
   implicit def parse(signature: String): Dep = {
     val parts = signature.split(';')
     val module = parts.head
diff --git a/scalalib/src/mill/scalalib/GenIdeaImpl.scala b/scalalib/src/mill/scalalib/GenIdeaImpl.scala
index 87b039df..2d76d804 100644
--- a/scalalib/src/mill/scalalib/GenIdeaImpl.scala
+++ b/scalalib/src/mill/scalalib/GenIdeaImpl.scala
@@ -4,7 +4,7 @@ import ammonite.runtime.SpecialClassLoader
 import coursier.{Cache, CoursierPaths, Repository}
 import mill.define._
 import mill.eval.{Evaluator, PathRef, Result}
-import mill.util.Ctx.{Home, Log}
+import mill.api.Ctx.{Home, Log}
 import mill.util.Strict.Agg
 import mill.util.{Loose, Strict}
 import mill.{T, scalalib}
diff --git a/scalalib/src/mill/scalalib/JavaModule.scala b/scalalib/src/mill/scalalib/JavaModule.scala
index 5cd9b600..78be8893 100644
--- a/scalalib/src/mill/scalalib/JavaModule.scala
+++ b/scalalib/src/mill/scalalib/JavaModule.scala
@@ -209,7 +209,7 @@ trait JavaModule extends mill.Module with TaskModule { outer =>
   /**
     * Compiles the current module to generate compiled classfiles/bytecode
     */
-  def compile: T[CompilationResult] = T.persistent{
+  def compile: T[mill.scalalib.api.CompilationResult] = T.persistent{
     zincWorker.worker().compileJava(
       upstreamCompileOutput(),
       allSourceFiles().map(_.path),
diff --git a/scalalib/src/mill/scalalib/Lib.scala b/scalalib/src/mill/scalalib/Lib.scala
index 80723637..b8b253bd 100644
--- a/scalalib/src/mill/scalalib/Lib.scala
+++ b/scalalib/src/mill/scalalib/Lib.scala
@@ -9,49 +9,17 @@ import javax.tools.ToolProvider
 
 import ammonite.util.Util
 import coursier.{Cache, Dependency, Fetch, Repository, Resolution}
-import Dep.isDotty
+import mill.scalalib.api.Util.isDotty
 import mill.Agg
 import mill.eval.{PathRef, Result}
 import mill.modules.Jvm
-import mill.util.Ctx
+import mill.api.Ctx
 import sbt.testing._
 
 import scala.collection.mutable
 
-object CompilationResult {
-  implicit val jsonFormatter: upickle.default.ReadWriter[CompilationResult] = upickle.default.macroRW
-}
-
-// analysisFile is represented by os.Path, so we won't break caches after file changes
-case class CompilationResult(analysisFile: os.Path, classes: PathRef)
 
 object Lib{
-  private val ReleaseVersion = raw"""(\d+)\.(\d+)\.(\d+)""".r
-  private val MinorSnapshotVersion = raw"""(\d+)\.(\d+)\.([1-9]\d*)-SNAPSHOT""".r
-  private val DottyVersion = raw"""0\.(\d+)\.(\d+).*""".r
-
-  def scalaBinaryVersion(scalaVersion: String) = {
-    scalaVersion match {
-      case ReleaseVersion(major, minor, _) => s"$major.$minor"
-      case MinorSnapshotVersion(major, minor, _) => s"$major.$minor"
-      case DottyVersion(minor, _) => s"0.$minor"
-      case _ => scalaVersion
-    }
-  }
-
-  def grepJar(classPath: Agg[os.Path], name: String, version: String, sources: Boolean = false) = {
-    val suffix = if (sources) "-sources" else ""
-    val mavenStylePath = s"$name-$version$suffix.jar"
-    val ivyStylePath = {
-      val dir = if (sources) "srcs" else "jars"
-      s"$version/$dir/$name$suffix.jar"
-    }
-
-    classPath
-      .find(p => p.toString.endsWith(mavenStylePath) || p.toString.endsWith(ivyStylePath))
-      .getOrElse(throw new Exception(s"Cannot find $mavenStylePath or $ivyStylePath"))
-  }
-
   def depToDependencyJava(dep: Dep, platformSuffix: String = ""): Dependency = {
     assert(dep.cross.isConstant, s"Not a Java dependency: $dep")
     depToDependency(dep, "", platformSuffix)
@@ -59,7 +27,7 @@ object Lib{
 
   def depToDependency(dep: Dep, scalaVersion: String, platformSuffix: String = ""): Dependency =
     dep.toDependency(
-      binaryVersion = scalaBinaryVersion(scalaVersion),
+      binaryVersion = mill.scalalib.api.Util.scalaBinaryVersion(scalaVersion),
       fullVersion = scalaVersion,
       platformSuffix = platformSuffix
     )
@@ -98,7 +66,7 @@ object Lib{
     )
   }
   def scalaCompilerIvyDeps(scalaOrganization: String, scalaVersion: String) =
-    if (isDotty(scalaVersion))
+    if (mill.scalalib.api.Util.isDotty(scalaVersion))
       Agg(ivy"$scalaOrganization::dotty-compiler:$scalaVersion".forceVersion())
     else
       Agg(
diff --git a/scalalib/src/mill/scalalib/PublishModule.scala b/scalalib/src/mill/scalalib/PublishModule.scala
index a3d0856f..588781f4 100644
--- a/scalalib/src/mill/scalalib/PublishModule.scala
+++ b/scalalib/src/mill/scalalib/PublishModule.scala
@@ -2,7 +2,7 @@ package mill
 package scalalib
 
 import mill.define.{ExternalModule, Task}
-import mill.eval.PathRef
+import mill.api.PathRef
 import mill.scalalib.publish.{Artifact, SonatypePublisher}
 
 /**
diff --git a/scalalib/src/mill/scalalib/ScalaModule.scala b/scalalib/src/mill/scalalib/ScalaModule.scala
index 38a5a8ea..9d669bf4 100644
--- a/scalalib/src/mill/scalalib/ScalaModule.scala
+++ b/scalalib/src/mill/scalalib/ScalaModule.scala
@@ -6,10 +6,10 @@ import mill.define.{Target, Task, TaskModule}
 import mill.eval.{PathRef, Result}
 import mill.modules.Jvm
 import mill.modules.Jvm.createJar
-import Dep.isDotty
+import mill.scalalib.api.Util.isDotty
 import Lib._
 import mill.util.Loose.Agg
-import mill.util.DummyInputStream
+import mill.api.DummyInputStream
 
 /**
   * Core configuration required to compile a single Scala compilation target
@@ -60,7 +60,7 @@ trait ScalaModule extends JavaModule { outer =>
     publish.Artifact.fromDep(
       _: Dep,
       scalaVersion(),
-      Lib.scalaBinaryVersion(scalaVersion()),
+      mill.scalalib.api.Util.scalaBinaryVersion(scalaVersion()),
       platformSuffix()
     )
   }
@@ -84,7 +84,7 @@ trait ScalaModule extends JavaModule { outer =>
   def scalaCompilerBridgeSources = T {
     val (scalaVersion0, scalaBinaryVersion0) = scalaVersion() match {
       case Milestone213(_, _) => ("2.13.0-M2", "2.13.0-M2")
-      case _ => (scalaVersion(), Lib.scalaBinaryVersion(scalaVersion()))
+      case _ => (scalaVersion(), mill.scalalib.api.Util.scalaBinaryVersion(scalaVersion()))
     }
 
     val (bridgeDep, bridgeName, bridgeVersion) =
@@ -106,7 +106,7 @@ trait ScalaModule extends JavaModule { outer =>
       Seq(bridgeDep),
       sources = true
     ).map(deps =>
-      grepJar(deps.map(_.path), bridgeName, bridgeVersion, sources = true)
+      mill.scalalib.api.Util.grepJar(deps.map(_.path), bridgeName, bridgeVersion, sources = true)
     )
   }
 
@@ -152,7 +152,7 @@ trait ScalaModule extends JavaModule { outer =>
     resolveDeps(T.task{runIvyDeps() ++ scalaLibraryIvyDeps() ++ transitiveIvyDeps()})()
   }
 
-  override def compile: T[CompilationResult] = T.persistent{
+  override def compile: T[mill.scalalib.api.CompilationResult] = T.persistent{
     zincWorker.worker().compileMixed(
       upstreamCompileOutput(),
       allSourceFiles().map(_.path),
@@ -262,7 +262,7 @@ trait ScalaModule extends JavaModule { outer =>
     */
   def artifactScalaVersion: T[String] = T {
     if (crossFullScalaVersion()) scalaVersion()
-    else Lib.scalaBinaryVersion(scalaVersion())
+    else mill.scalalib.api.Util.scalaBinaryVersion(scalaVersion())
   }
 
   /**
diff --git a/scalalib/src/mill/scalalib/ZincWorkerApi.scala b/scalalib/src/mill/scalalib/ZincWorkerApi.scala
deleted file mode 100644
index dbcaa6da..00000000
--- a/scalalib/src/mill/scalalib/ZincWorkerApi.scala
+++ /dev/null
@@ -1,87 +0,0 @@
-package mill.scalalib
-
-import coursier.Cache
-import coursier.maven.MavenRepository
-import mill.Agg
-import mill.T
-import mill.define.{Discover, Worker}
-import mill.scalalib.Lib.resolveDependencies
-import mill.util.Loose
-import mill.util.JsonFormatters._
-
-object ZincWorkerModule extends mill.define.ExternalModule with ZincWorkerModule{
-  lazy val millDiscover = Discover[this.type]
-}
-trait ZincWorkerModule extends mill.Module{
-  def repositories = Seq(
-    Cache.ivy2Local,
-    MavenRepository("https://repo1.maven.org/maven2"),
-    MavenRepository("https://oss.sonatype.org/content/repositories/releases")
-  )
-
-  def classpath = T{
-    mill.modules.Util.millProjectModule("MILL_SCALA_WORKER", "mill-scalalib-worker", repositories)
-  }
-
-  def scalalibClasspath = T{
-    mill.modules.Util.millProjectModule("MILL_SCALA_LIB", "mill-scalalib", repositories)
-  }
-
-  def backgroundWrapperClasspath = T{
-    mill.modules.Util.millProjectModule(
-      "MILL_BACKGROUNDWRAPPER", "mill-scalalib-backgroundwrapper",
-      repositories, artifactSuffix = ""
-    )
-  }
-
-  def worker: Worker[ZincWorkerApi] = T.worker{
-    val cl = mill.util.ClassLoader.create(
-      classpath().map(_.path.toNIO.toUri.toURL).toVector,
-      getClass.getClassLoader
-    )
-    val cls = cl.loadClass("mill.scalalib.worker.ZincWorkerImpl")
-    val instance = cls.getConstructor(classOf[mill.util.Ctx], classOf[Array[String]])
-      .newInstance(T.ctx(), compilerInterfaceClasspath().map(_.path.toString).toArray[String])
-    instance.asInstanceOf[ZincWorkerApi]
-  }
-
-  def compilerInterfaceClasspath = T{
-    resolveDependencies(
-      repositories,
-      Lib.depToDependency(_, "2.12.4", ""),
-      Seq(ivy"org.scala-sbt:compiler-interface:${Versions.zinc}")
-    )
-  }
-
-}
-
-trait ZincWorkerApi {
-  /** Compile a Java-only project */
-  def compileJava(upstreamCompileOutput: Seq[CompilationResult],
-                  sources: Agg[os.Path],
-                  compileClasspath: Agg[os.Path],
-                  javacOptions: Seq[String])
-                 (implicit ctx: mill.util.Ctx): mill.eval.Result[CompilationResult]
-
-  /** Compile a mixed Scala/Java or Scala-only project */
-  def compileMixed(upstreamCompileOutput: Seq[CompilationResult],
-                   sources: Agg[os.Path],
-                   compileClasspath: Agg[os.Path],
-                   javacOptions: Seq[String],
-                   scalaVersion: String,
-                   scalacOptions: Seq[String],
-                   compilerBridgeSources: os.Path,
-                   compilerClasspath: Agg[os.Path],
-                   scalacPluginClasspath: Agg[os.Path])
-                  (implicit ctx: mill.util.Ctx): mill.eval.Result[CompilationResult]
-
-  def discoverMainClasses(compilationResult: CompilationResult)
-                         (implicit ctx: mill.util.Ctx): Seq[String]
-
-  def docJar(scalaVersion: String,
-             compilerBridgeSources: os.Path,
-             compilerClasspath: Agg[os.Path],
-             scalacPluginClasspath: Agg[os.Path],
-             args: Seq[String])
-            (implicit ctx: mill.util.Ctx): Boolean
-}
diff --git a/scalalib/src/mill/scalalib/ZincWorkerModule.scala b/scalalib/src/mill/scalalib/ZincWorkerModule.scala
new file mode 100644
index 00000000..5ca824ce
--- /dev/null
+++ b/scalalib/src/mill/scalalib/ZincWorkerModule.scala
@@ -0,0 +1,56 @@
+package mill.scalalib
+
+import coursier.Cache
+import coursier.maven.MavenRepository
+import mill.Agg
+import mill.T
+import mill.define.{Discover, Worker}
+import mill.scalalib.Lib.resolveDependencies
+import mill.util.Loose
+import mill.util.JsonFormatters._
+
+object ZincWorkerModule extends mill.define.ExternalModule with ZincWorkerModule{
+  lazy val millDiscover = Discover[this.type]
+}
+trait ZincWorkerModule extends mill.Module{
+  def repositories = Seq(
+    Cache.ivy2Local,
+    MavenRepository("https://repo1.maven.org/maven2"),
+    MavenRepository("https://oss.sonatype.org/content/repositories/releases")
+  )
+
+  def classpath = T{
+    mill.modules.Util.millProjectModule("MILL_SCALA_WORKER", "mill-scalalib-worker", repositories)
+  }
+
+  def scalalibClasspath = T{
+    mill.modules.Util.millProjectModule("MILL_SCALA_LIB", "mill-scalalib", repositories)
+  }
+
+  def backgroundWrapperClasspath = T{
+    mill.modules.Util.millProjectModule(
+      "MILL_BACKGROUNDWRAPPER", "mill-scalalib-backgroundwrapper",
+      repositories, artifactSuffix = ""
+    )
+  }
+
+  def worker: Worker[mill.scalalib.api.ZincWorkerApi] = T.worker{
+    val cl = mill.api.ClassLoader.create(
+      classpath().map(_.path.toNIO.toUri.toURL).toVector,
+      getClass.getClassLoader
+    )
+    val cls = cl.loadClass("mill.scalalib.worker.ZincWorkerImpl")
+    val instance = cls.getConstructor(classOf[mill.api.Ctx], classOf[Array[String]])
+      .newInstance(T.ctx(), compilerInterfaceClasspath().map(_.path.toString).toArray[String])
+    instance.asInstanceOf[mill.scalalib.api.ZincWorkerApi]
+  }
+
+  def compilerInterfaceClasspath = T{
+    resolveDependencies(
+      repositories,
+      Lib.depToDependency(_, "2.12.4", ""),
+      Seq(ivy"org.scala-sbt:compiler-interface:${Versions.zinc}")
+    )
+  }
+
+}
diff --git a/scalalib/src/mill/scalalib/dependency/DependencyUpdatesImpl.scala b/scalalib/src/mill/scalalib/dependency/DependencyUpdatesImpl.scala
index dc548c88..3bb94202 100644
--- a/scalalib/src/mill/scalalib/dependency/DependencyUpdatesImpl.scala
+++ b/scalalib/src/mill/scalalib/dependency/DependencyUpdatesImpl.scala
@@ -7,7 +7,7 @@ import mill.scalalib.dependency.updates.{
   UpdatesFinder
 }
 import mill.scalalib.dependency.versions.VersionsFinder
-import mill.util.Ctx.{Home, Log}
+import mill.api.Ctx.{Home, Log}
 
 object DependencyUpdatesImpl {
 
diff --git a/scalalib/src/mill/scalalib/dependency/versions/VersionsFinder.scala b/scalalib/src/mill/scalalib/dependency/versions/VersionsFinder.scala
index b1033688..a831ffc3 100644
--- a/scalalib/src/mill/scalalib/dependency/versions/VersionsFinder.scala
+++ b/scalalib/src/mill/scalalib/dependency/versions/VersionsFinder.scala
@@ -4,7 +4,7 @@ import mill.define.{BaseModule, Task}
 import mill.eval.Evaluator
 import mill.scalalib.dependency.metadata.MetadataLoaderFactory
 import mill.scalalib.{Dep, JavaModule, Lib}
-import mill.util.Ctx.{Home, Log}
+import mill.api.Ctx.{Home, Log}
 import mill.util.{Loose, Strict}
 
 private[dependency] object VersionsFinder {
diff --git a/scalalib/src/mill/scalalib/publish/SonatypePublisher.scala b/scalalib/src/mill/scalalib/publish/SonatypePublisher.scala
index 5e596618..1843943b 100644
--- a/scalalib/src/mill/scalalib/publish/SonatypePublisher.scala
+++ b/scalalib/src/mill/scalalib/publish/SonatypePublisher.scala
@@ -3,7 +3,7 @@ package mill.scalalib.publish
 import java.math.BigInteger
 import java.security.MessageDigest
 
-import mill.util.Logger
+import mill.api.Logger
 
 import scalaj.http.HttpResponse
 
diff --git a/scalalib/src/mill/scalalib/scalafmt/ScalafmtWorker.scala b/scalalib/src/mill/scalalib/scalafmt/ScalafmtWorker.scala
index d6992fc8..47d8375f 100644
--- a/scalalib/src/mill/scalalib/scalafmt/ScalafmtWorker.scala
+++ b/scalalib/src/mill/scalalib/scalafmt/ScalafmtWorker.scala
@@ -3,7 +3,7 @@ package mill.scalalib.scalafmt
 import mill._
 import mill.define.{Discover, ExternalModule, Worker}
 import mill.modules.Jvm
-import mill.util.Ctx
+import mill.api.Ctx
 
 import scala.collection.mutable
 
diff --git a/scalalib/test/src/mill/scalalib/HelloJavaTests.scala b/scalalib/test/src/mill/scalalib/HelloJavaTests.scala
index ff0de5a6..5b7b93b2 100644
--- a/scalalib/test/src/mill/scalalib/HelloJavaTests.scala
+++ b/scalalib/test/src/mill/scalalib/HelloJavaTests.scala
@@ -2,7 +2,7 @@ package mill
 package scalalib
 
 
-import mill.eval.Result
+import mill.api.Result
 import mill.util.{TestEvaluator, TestUtil}
 import utest._
 import utest.framework.TestPath
diff --git a/scalalib/test/src/mill/scalalib/HelloWorldTests.scala b/scalalib/test/src/mill/scalalib/HelloWorldTests.scala
index 67c4b433..da08f056 100644
--- a/scalalib/test/src/mill/scalalib/HelloWorldTests.scala
+++ b/scalalib/test/src/mill/scalalib/HelloWorldTests.scala
@@ -5,7 +5,7 @@ import java.util.jar.JarFile
 
 import mill._
 import mill.define.Target
-import mill.eval.Result.Exception
+import mill.api.Result.Exception
 import mill.eval.{Evaluator, Result}
 import mill.modules.Assembly
 import mill.scalalib.publish._
diff --git a/scalalib/test/src/mill/scalalib/ResolveDepsTests.scala b/scalalib/test/src/mill/scalalib/ResolveDepsTests.scala
index 4240478c..78361625 100644
--- a/scalalib/test/src/mill/scalalib/ResolveDepsTests.scala
+++ b/scalalib/test/src/mill/scalalib/ResolveDepsTests.scala
@@ -2,7 +2,7 @@ package mill.scalalib
 
 import coursier.Cache
 import coursier.maven.MavenRepository
-import mill.eval.Result.{Failure, Success}
+import mill.api.Result.{Failure, Success}
 import mill.eval.{PathRef, Result}
 import mill.util.Loose.Agg
 import utest._
diff --git a/scalalib/worker/src/mill/scalalib/worker/ZincWorkerImpl.scala b/scalalib/worker/src/mill/scalalib/worker/ZincWorkerImpl.scala
index d494760e..705d4682 100644
--- a/scalalib/worker/src/mill/scalalib/worker/ZincWorkerImpl.scala
+++ b/scalalib/worker/src/mill/scalalib/worker/ZincWorkerImpl.scala
@@ -3,18 +3,14 @@ package mill.scalalib.worker
 import java.io.File
 import java.util.Optional
 
-import ammonite.util.Colors
-import mill.Agg
-import mill.eval.PathRef
-import mill.scalalib.{CompilationResult, Lib, TestRunner}
+import mill.api.Loose.Agg
+import mill.api.PathRef
 import xsbti.compile.{CompilerCache => _, FileAnalysisStore => _, ScalaInstance => _, _}
-import mill.scalalib.Dep.isDotty
-import mill.scalalib.Lib.{grepJar, scalaBinaryVersion}
-import mill.util.{Ctx, PrintLogger}
+import mill.scalalib.api.Util.{isDotty, grepJar, scalaBinaryVersion}
 import sbt.internal.inc._
 import sbt.internal.util.{ConsoleOut, MainAppender}
 import sbt.util.LogExchange
-
+import mill.scalalib.api.CompilationResult
 case class MockedLookup(am: File => Optional[CompileAnalysis]) extends PerClasspathEntryLookup {
   override def analysis(classpathEntry: File): Optional[CompileAnalysis] =
     am(classpathEntry)
@@ -23,8 +19,8 @@ case class MockedLookup(am: File => Optional[CompileAnalysis]) extends PerClassp
     Locate.definesClass(classpathEntry)
 }
 
-class ZincWorkerImpl(ctx0: mill.util.Ctx,
-                     compilerBridgeClasspath: Array[String]) extends mill.scalalib.ZincWorkerApi{
+class ZincWorkerImpl(ctx0: mill.api.Ctx,
+                     compilerBridgeClasspath: Array[String]) extends mill.scalalib.api.ZincWorkerApi{
   private val ic = new sbt.internal.inc.IncrementalCompilerImpl()
   val javaOnlyCompilers = {
     // Keep the classpath as written by the user
@@ -53,7 +49,7 @@ class ZincWorkerImpl(ctx0: mill.util.Ctx,
              compilerClasspath: Agg[os.Path],
              scalacPluginClasspath: Agg[os.Path],
              args: Seq[String])
-            (implicit ctx: mill.util.Ctx): Boolean = {
+            (implicit ctx: mill.api.Ctx): Boolean = {
     val compilers: Compilers = prepareCompilers(
       scalaVersion,
       compilerBridgeSources,
@@ -79,8 +75,8 @@ class ZincWorkerImpl(ctx0: mill.util.Ctx,
       os.makeDir.all(workingDir)
       os.makeDir.all(compiledDest)
 
-      val sourceFolder = mill.modules.Util.unpackZip(sourcesJar)(workingDir)
-      val classloader = mill.util.ClassLoader.create(compilerJars.map(_.toURI.toURL), null)(ctx0)
+      val sourceFolder = mill.api.IO.unpackZip(sourcesJar)(workingDir)
+      val classloader = mill.api.ClassLoader.create(compilerJars.map(_.toURI.toURL), null)(ctx0)
       val compilerMain = classloader.loadClass(
         if (isDotty(scalaVersion))
           "dotty.tools.dotc.Main"
@@ -100,7 +96,7 @@ class ZincWorkerImpl(ctx0: mill.util.Ctx,
 
 
 
-  def discoverMainClasses(compilationResult: CompilationResult)(implicit ctx: mill.util.Ctx): Seq[String] = {
+  def discoverMainClasses(compilationResult: CompilationResult)(implicit ctx: mill.api.Ctx): Seq[String] = {
     def toScala[A](o: Optional[A]): Option[A] = if (o.isPresent) Some(o.get) else None
 
     toScala(FileAnalysisStore.binary(compilationResult.analysisFile.toIO).get())
@@ -118,7 +114,7 @@ class ZincWorkerImpl(ctx0: mill.util.Ctx,
                   sources: Agg[os.Path],
                   compileClasspath: Agg[os.Path],
                   javacOptions: Seq[String])
-                 (implicit ctx: mill.util.Ctx): mill.eval.Result[CompilationResult] = {
+                 (implicit ctx: mill.api.Ctx): mill.api.Result[CompilationResult] = {
     compileInternal(
       upstreamCompileOutput,
       sources,
@@ -138,7 +134,7 @@ class ZincWorkerImpl(ctx0: mill.util.Ctx,
                    compilerBridgeSources: os.Path,
                    compilerClasspath: Agg[os.Path],
                    scalacPluginClasspath: Agg[os.Path])
-                  (implicit ctx: mill.util.Ctx): mill.eval.Result[CompilationResult] = {
+                  (implicit ctx: mill.api.Ctx): mill.api.Result[CompilationResult] = {
     val compilers: Compilers = prepareCompilers(
       scalaVersion,
       compilerBridgeSources,
@@ -160,7 +156,7 @@ class ZincWorkerImpl(ctx0: mill.util.Ctx,
                                compilerBridgeSources: os.Path,
                                compilerClasspath: Agg[os.Path],
                                scalacPluginClasspath: Agg[os.Path])
-                              (implicit ctx: mill.util.Ctx)= {
+                              (implicit ctx: mill.api.Ctx)= {
     val combinedCompilerClasspath = compilerClasspath ++ scalacPluginClasspath
     val combinedCompilerJars = combinedCompilerClasspath.toArray.map(_.toIO)
 
@@ -185,7 +181,7 @@ class ZincWorkerImpl(ctx0: mill.util.Ctx,
             "scala-compiler"
         val scalaInstance = new ScalaInstance(
           version = scalaVersion,
-          loader = mill.util.ClassLoader.create(combinedCompilerJars.map(_.toURI.toURL), null),
+          loader = mill.api.ClassLoader.create(combinedCompilerJars.map(_.toURI.toURL), null),
           libraryJar = grepJar(compilerClasspath, "scala-library", scalaVersion).toIO,
           compilerJar = grepJar(compilerClasspath, compilerName, scalaVersion).toIO,
           allJars = combinedCompilerJars,
@@ -209,7 +205,7 @@ class ZincWorkerImpl(ctx0: mill.util.Ctx,
                               javacOptions: Seq[String],
                               scalacOptions: Seq[String],
                               compilers: Compilers)
-                             (implicit ctx: mill.util.Ctx): mill.eval.Result[CompilationResult] = {
+                             (implicit ctx: mill.api.Ctx): mill.api.Result[CompilationResult] = {
     os.makeDir.all(ctx.dest)
 
     val logger = {
@@ -282,7 +278,7 @@ class ZincWorkerImpl(ctx0: mill.util.Ctx,
         )
       )
 
-      mill.eval.Result.Success(CompilationResult(zincFile, PathRef(classesDir)))
-    }catch{case e: CompileFailed => mill.eval.Result.Failure(e.toString)}
+      mill.api.Result.Success(CompilationResult(zincFile, PathRef(classesDir)))
+    }catch{case e: CompileFailed => mill.api.Result.Failure(e.toString)}
   }
 }
diff --git a/scalanativelib/api/src/mill/scalanativelib/api/ScalaNativeWorkerApi.scala b/scalanativelib/api/src/mill/scalanativelib/api/ScalaNativeWorkerApi.scala
new file mode 100644
index 00000000..e1fee0da
--- /dev/null
+++ b/scalanativelib/api/src/mill/scalanativelib/api/ScalaNativeWorkerApi.scala
@@ -0,0 +1,60 @@
+package mill.scalanativelib.api
+
+import upickle.default.{macroRW, ReadWriter => RW}
+import java.io.File
+import sbt.testing.Framework
+
+trait ScalaNativeWorkerApi {
+  def discoverClang: os.Path
+  def discoverClangPP: os.Path
+  def discoverTarget(clang: os.Path, workDir: os.Path): String
+  def discoverCompileOptions: Seq[String]
+  def discoverLinkingOptions: Seq[String]
+
+  def config(nativeLibJar: os.Path,
+             mainClass: String,
+             classpath: Seq[os.Path],
+             nativeWorkdir: os.Path,
+             nativeClang: os.Path,
+             nativeClangPP: os.Path,
+             nativeTarget: String,
+             nativeCompileOptions: Seq[String],
+             nativeLinkingOptions: Seq[String],
+             nativeGC: String,
+             nativeLinkStubs: Boolean,
+             releaseMode: ReleaseMode,
+             logLevel: NativeLogLevel): NativeConfig
+
+  def defaultGarbageCollector: String
+  def nativeLink(nativeConfig: NativeConfig, outPath: os.Path): os.Path
+
+  def newScalaNativeFrameWork(framework: Framework, id: Int, testBinary: File,
+                              logLevel: NativeLogLevel, envVars: Map[String, String]): Framework
+}
+
+
+sealed abstract class NativeLogLevel(val level: Int) extends Ordered[NativeLogLevel] {
+  def compare(that: NativeLogLevel) =  this.level - that.level
+}
+
+object NativeLogLevel {
+  case object Error extends NativeLogLevel(200)
+  case object Warn extends NativeLogLevel(300)
+  case object Info extends NativeLogLevel(400)
+  case object Debug extends NativeLogLevel(500)
+  case object Trace extends NativeLogLevel(600)
+
+  implicit def rw: RW[NativeLogLevel] = macroRW
+}
+
+sealed abstract class ReleaseMode(val name: String)
+
+object ReleaseMode {
+  case object Debug extends ReleaseMode("debug")
+  case object Release extends ReleaseMode("release")
+
+  implicit def rw: RW[ReleaseMode] = macroRW
+}
+
+// result wrapper to preserve some type safety
+case class NativeConfig(config: Any)
\ No newline at end of file
diff --git a/scalanativelib/src/mill/scalanativelib/ScalaNativeModule.scala b/scalanativelib/src/mill/scalanativelib/ScalaNativeModule.scala
index d8a282de..a7a2b96b 100644
--- a/scalanativelib/src/mill/scalanativelib/ScalaNativeModule.scala
+++ b/scalanativelib/src/mill/scalanativelib/ScalaNativeModule.scala
@@ -6,37 +6,14 @@ import java.net.URLClassLoader
 import coursier.Cache
 import coursier.maven.MavenRepository
 import mill.define.{Target, Task}
-import mill.eval.Result
+import mill.api.Result
 import mill.modules.Jvm
 import mill.scalalib.{Dep, DepSyntax, Lib, SbtModule, ScalaModule, TestModule, TestRunner}
 import mill.util.Loose.Agg
 import sbt.testing.{AnnotatedFingerprint, SubclassFingerprint}
 import sbt.testing.Fingerprint
 import upickle.default.{ReadWriter => RW, macroRW}
-
-
-sealed abstract class NativeLogLevel(val level: Int) extends Ordered[NativeLogLevel] {
-  def compare(that: NativeLogLevel) =  this.level - that.level
-}
-
-object NativeLogLevel {
-  case object Error extends NativeLogLevel(200)
-  case object Warn extends NativeLogLevel(300)
-  case object Info extends NativeLogLevel(400)
-  case object Debug extends NativeLogLevel(500)
-  case object Trace extends NativeLogLevel(600)
-
-  implicit def rw: RW[NativeLogLevel] = macroRW
-}
-
-sealed abstract class ReleaseMode(val name: String)
-
-object ReleaseMode {
-  case object Debug extends ReleaseMode("debug")
-  case object Release extends ReleaseMode("release")
-
-  implicit def rw: RW[ReleaseMode] = macroRW
-}
+import mill.scalanativelib.api._
 
 
 trait ScalaNativeModule extends ScalaModule { outer =>
diff --git a/scalanativelib/src/mill/scalanativelib/ScalaNativeWorkerApi.scala b/scalanativelib/src/mill/scalanativelib/ScalaNativeWorkerApi.scala
index be834e89..80325f1e 100644
--- a/scalanativelib/src/mill/scalanativelib/ScalaNativeWorkerApi.scala
+++ b/scalanativelib/src/mill/scalanativelib/ScalaNativeWorkerApi.scala
@@ -5,7 +5,7 @@ import java.net.URLClassLoader
 
 import mill.define.{Discover, Worker}
 import mill.{Agg, T}
-import sbt.testing.Framework
+import mill.scalanativelib.api._
 
 
 class ScalaNativeWorker {
@@ -38,38 +38,6 @@ class ScalaNativeWorker {
   }
 }
 
-
-// result wrapper to preserve some type safety
-case class NativeConfig(config: Any)
-
-trait ScalaNativeWorkerApi {
-  def discoverClang: os.Path
-  def discoverClangPP: os.Path
-  def discoverTarget(clang: os.Path, workDir: os.Path): String
-  def discoverCompileOptions: Seq[String]
-  def discoverLinkingOptions: Seq[String]
-
-  def config(nativeLibJar: os.Path,
-             mainClass: String,
-             classpath: Seq[os.Path],
-             nativeWorkdir: os.Path,
-             nativeClang: os.Path,
-             nativeClangPP: os.Path,
-             nativeTarget: String,
-             nativeCompileOptions: Seq[String],
-             nativeLinkingOptions: Seq[String],
-             nativeGC: String,
-             nativeLinkStubs: Boolean,
-             releaseMode: ReleaseMode,
-             logLevel: NativeLogLevel): NativeConfig
-
-  def defaultGarbageCollector: String
-  def nativeLink(nativeConfig: NativeConfig, outPath: os.Path): os.Path
-
-  def newScalaNativeFrameWork(framework: Framework, id: Int, testBinary: File,
-                              logLevel: NativeLogLevel, envVars: Map[String, String]): Framework
-}
-
 object ScalaNativeWorkerApi extends mill.define.ExternalModule {
   def scalaNativeWorker: Worker[ScalaNativeWorker] = T.worker { new ScalaNativeWorker() }
   lazy val millDiscover = Discover[this.type]
diff --git a/scalanativelib/test/src/mill/scalanativelib/HelloNativeWorldTests.scala b/scalanativelib/test/src/mill/scalanativelib/HelloNativeWorldTests.scala
index 37fbfc80..74d4238a 100644
--- a/scalanativelib/test/src/mill/scalanativelib/HelloNativeWorldTests.scala
+++ b/scalanativelib/test/src/mill/scalanativelib/HelloNativeWorldTests.scala
@@ -12,7 +12,7 @@ import utest._
 
 
 import scala.collection.JavaConverters._
-
+import mill.scalanativelib.api._
 
 object HelloNativeWorldTests extends TestSuite {
   val workspacePath =  TestUtil.getOutPathStatic() / "hello-native-world"
diff --git a/scalanativelib/worker/0.3/src/mill/scalanativelib/worker/ScalaNativeWorkerImpl.scala b/scalanativelib/worker/0.3/src/mill/scalanativelib/worker/ScalaNativeWorkerImpl.scala
index 8c24a39e..b32e84ff 100644
--- a/scalanativelib/worker/0.3/src/mill/scalanativelib/worker/ScalaNativeWorkerImpl.scala
+++ b/scalanativelib/worker/0.3/src/mill/scalanativelib/worker/ScalaNativeWorkerImpl.scala
@@ -4,13 +4,13 @@ import java.io.File
 import java.lang.System.{err, out}
 
 import scala.scalanative.build.{Build, Config, Discover, GC, Logger, Mode}
-import mill.scalanativelib.{NativeConfig, NativeLogLevel, ReleaseMode}
+import mill.scalanativelib.api.{NativeConfig, NativeLogLevel, ReleaseMode}
 import sbt.testing.Framework
 
 import scala.scalanative.testinterface.ScalaNativeFramework
 
 
-class ScalaNativeWorkerImpl extends mill.scalanativelib.ScalaNativeWorkerApi {
+class ScalaNativeWorkerImpl extends mill.scalanativelib.api.ScalaNativeWorkerApi {
   def logger(level: NativeLogLevel) =
     Logger(
       debugFn = msg => if (level >= NativeLogLevel.Debug) out.println(msg),