From 57501fca3b6c2c64d32744e6d534b9de3a6674f6 Mon Sep 17 00:00:00 2001
From: Li Haoyi <haoyi.sg@gmail.com>
Date: Sat, 13 Jan 2018 13:00:21 -0800
Subject: Migrate most classpath-related code onto `Loose.OSet` abstraction, to
 enforce deduplication

---
 build.sc                                           |  32 ++--
 core/src/main/scala/mill/define/Graph.scala        |   3 +-
 core/src/main/scala/mill/discover/Discovered.scala |   2 +-
 core/src/main/scala/mill/eval/Evaluator.scala      |   1 +
 .../main/scala/mill/main/ReplApplyHandler.scala    |   2 +-
 core/src/main/scala/mill/main/RunScript.scala      |   3 +-
 core/src/main/scala/mill/modules/Jvm.scala         |  13 +-
 core/src/main/scala/mill/package.scala             |   2 +
 core/src/main/scala/mill/util/MultiBiMap.scala     |   2 +-
 core/src/main/scala/mill/util/OSet.scala           | 192 +++++++++++----------
 core/src/test/scala/mill/define/CacherTests.scala  |   3 +-
 core/src/test/scala/mill/define/GraphTests.scala   |   4 +-
 .../scala/mill/discover/ConsistencyTests.scala     |   3 +-
 .../src/test/scala/mill/eval/EvaluationTests.scala |   3 +-
 core/src/test/scala/mill/eval/FailureTests.scala   |   3 +-
 .../test/scala/mill/eval/JavaCompileJarTests.scala |   5 +-
 core/src/test/scala/mill/util/TestEvaluator.scala  |   2 +-
 core/src/test/scala/mill/util/TestUtil.scala       |   2 +-
 integration/src/test/resources/acyclic/build.sc    |   9 +-
 .../src/test/resources/better-files/build.sc       |   8 +-
 integration/src/test/resources/jawn/build.sc       |  12 +-
 .../mill/integration/IntegrationTestSuite.scala    |   2 +-
 .../src/main/scala/mill/scalajslib/Lib.scala       |   8 +-
 .../main/scala/mill/scalajslib/ScalaJSModule.scala |  17 +-
 .../src/main/scala/mill/scalalib/GenIdea.scala     |   3 +-
 scalalib/src/main/scala/mill/scalalib/Lib.scala    |  27 +--
 .../src/main/scala/mill/scalalib/MiscModule.scala  |  15 +-
 scalalib/src/main/scala/mill/scalalib/Module.scala |  51 +++---
 .../main/scala/mill/scalalib/PublishModule.scala   |   2 +-
 .../src/main/scala/mill/scalalib/TestRunner.scala  |  13 +-
 .../src/main/scala/mill/scalalib/publish/Ivy.scala |   4 +-
 .../src/main/scala/mill/scalalib/publish/Pom.scala |   4 +-
 .../scala/mill/scalalib/ResolveDepsTests.scala     |  11 +-
 shared.sc                                          |   2 +-
 34 files changed, 250 insertions(+), 215 deletions(-)

diff --git a/build.sc b/build.sc
index 0fa484e9..d6a5448f 100755
--- a/build.sc
+++ b/build.sc
@@ -10,7 +10,7 @@ import mill.modules.Jvm.createAssembly
 
 object moduledefs extends SbtModule{
   def scalaVersion = "2.12.4"
-  def ivyDeps = Seq(
+  def ivyDeps = OSet(
     Dep.Java("org.scala-lang", "scala-compiler", scalaVersion()),
     Dep("com.lihaoyi", "sourcecode", "0.1.4")
   )
@@ -19,9 +19,9 @@ object moduledefs extends SbtModule{
 trait MillModule extends SbtModule{ outer =>
   def scalaVersion = "2.12.4"
 
-  def compileIvyDeps = Seq(Dep("com.lihaoyi", "acyclic", "0.1.7"))
+  def compileIvyDeps = OSet(Dep("com.lihaoyi", "acyclic", "0.1.7"))
   def scalacOptions = Seq("-P:acyclic:force")
-  def scalacPluginIvyDeps = Seq(Dep("com.lihaoyi", "acyclic", "0.1.7"))
+  def scalacPluginIvyDeps = OSet(Dep("com.lihaoyi", "acyclic", "0.1.7"))
 
   def repositories = super.repositories ++ Seq(
     MavenRepository("https://oss.sonatype.org/content/repositories/releases")
@@ -36,7 +36,7 @@ trait MillModule extends SbtModule{ outer =>
     def projectDeps =
       if (this == core.test) Seq(core)
       else Seq(outer, core.test)
-    def ivyDeps = Seq(Dep("com.lihaoyi", "utest", "0.6.0"))
+    def ivyDeps = OSet(Dep("com.lihaoyi", "utest", "0.6.0"))
     def testFramework = "mill.UTestFramework"
     def scalacPluginClasspath = super.scalacPluginClasspath() ++ Seq(moduledefs.jar())
   }
@@ -45,11 +45,11 @@ trait MillModule extends SbtModule{ outer =>
 object core extends MillModule {
   def projectDeps = Seq(moduledefs)
 
-  def compileIvyDeps = Seq(
+  def compileIvyDeps = OSet(
     Dep.Java("org.scala-lang", "scala-reflect", scalaVersion())
   )
 
-  def ivyDeps = Seq(
+  def ivyDeps = OSet(
     Dep("com.lihaoyi", "sourcecode", "0.1.4"),
     Dep("com.lihaoyi", "pprint", "0.5.3"),
     Dep.Point("com.lihaoyi", "ammonite", "1.0.3-21-05b5d32"),
@@ -61,7 +61,7 @@ object core extends MillModule {
   def generatedSources = T {
     mkdir(T.ctx().dest)
     shared.generateCoreSources(T.ctx().dest)
-    Seq(PathRef(T.ctx().dest))
+    OSet(PathRef(T.ctx().dest))
   }
 
   val test = new Tests(implicitly)
@@ -69,7 +69,7 @@ object core extends MillModule {
     def generatedSources = T {
       mkdir(T.ctx().dest)
       shared.generateCoreTestSources(T.ctx().dest)
-      Seq(PathRef(T.ctx().dest))
+      OSet(PathRef(T.ctx().dest))
     }
   }
 }
@@ -83,9 +83,9 @@ class BridgeModule(crossVersion: String) extends PublishModule {
 
   def scalaVersion = crossVersion
   def allSources = T{
-    Seq(PathRef(shared.downloadBridgeSource(T.ctx().dest, crossVersion)))
+    OSet(PathRef(shared.downloadBridgeSource(T.ctx().dest, crossVersion)))
   }
-  def ivyDeps = Seq(
+  def ivyDeps = OSet(
     Dep.Java("org.scala-lang", "scala-compiler", crossVersion),
     Dep.Java("org.scala-sbt", "compiler-interface", "1.0.5")
   )
@@ -125,7 +125,7 @@ class JsBridgeModule(scalajsBinary: String) extends MillModule{
     case "0.6" => "0.6.21"
     case "1.0" => "1.0.0-M2"
   }
-  def ivyDeps = Seq(
+  def ivyDeps = OSet(
     Dep("org.scala-js", "scalajs-tools", scalajsVersion)
   )
 }
@@ -134,8 +134,8 @@ object scalajslib extends MillModule {
 
   def projectDeps = Seq(scalalib)
 
-  def bridgeClasspath(runDepClasspath: Seq[PathRef], classes: PathRef) =
-    (runDepClasspath :+ classes).map(_.path).mkString(File.pathSeparator)
+  def bridgeClasspath(runDepClasspath: OSet[PathRef], classes: PathRef) =
+    (runDepClasspath ++ OSet(classes)).map(_.path).mkString(File.pathSeparator)
   def testArgs = T{
     val mapping = Map(
       "MILL_SCALAJS_BRIDGE_0_6" -> bridgeClasspath(jsbridges("0.6").runDepClasspath(), jsbridges("0.6").compile().classes),
@@ -171,7 +171,7 @@ def publishBridges(credentials: String, gpgPassphrase: String) = T.command {
   mill.define.Task.traverse(bridges.items)(_._2.publish(credentials, gpgPassphrase))
 }
 
-def assemblyBase(classpath: Seq[Path], extraArgs: String)
+def assemblyBase(classpath: OSet[Path], extraArgs: String)
                 (implicit ctx: mill.util.Ctx.DestCtx) = {
   createAssembly(
     classpath,
@@ -182,11 +182,11 @@ def assemblyBase(classpath: Seq[Path], extraArgs: String)
 }
 
 def devAssembly = T{
-  assemblyBase(assemblyClasspath().flatten.map(_.path), (scalalib.testArgs() ++ scalajslib.testArgs()).mkString(" "))
+  assemblyBase(OSet.from(assemblyClasspath().flatten.map(_.path)), (scalalib.testArgs() ++ scalajslib.testArgs()).mkString(" "))
 }
 
 def releaseAssembly = T{
-  assemblyBase(assemblyClasspath().flatten.map(_.path), "")
+  assemblyBase(OSet.from(assemblyClasspath().flatten.map(_.path)), "")
 }
 
 def idea() = T.command{ mill.scalalib.GenIdea() }
\ No newline at end of file
diff --git a/core/src/main/scala/mill/define/Graph.scala b/core/src/main/scala/mill/define/Graph.scala
index e7844691..68fb65ed 100644
--- a/core/src/main/scala/mill/define/Graph.scala
+++ b/core/src/main/scala/mill/define/Graph.scala
@@ -1,7 +1,8 @@
 package mill.define
 
 import mill.eval.Tarjans
-import mill.util.{MultiBiMap, OSet}
+import mill.util.MultiBiMap
+import mill.util.Strict.OSet
 
 object Graph {
   class TopoSorted private[Graph](val values: OSet[Task[_]])
diff --git a/core/src/main/scala/mill/discover/Discovered.scala b/core/src/main/scala/mill/discover/Discovered.scala
index c5ca4843..b7ec2140 100644
--- a/core/src/main/scala/mill/discover/Discovered.scala
+++ b/core/src/main/scala/mill/discover/Discovered.scala
@@ -5,7 +5,7 @@ import ammonite.main.Router
 import ammonite.main.Router.EntryPoint
 import mill.discover.Mirror.TargetPoint
 import mill.util.Ctx.Loader
-import mill.util.OSet
+import mill.util.Strict.OSet
 
 import scala.language.experimental.macros
 import scala.reflect.macros.blackbox.Context
diff --git a/core/src/main/scala/mill/eval/Evaluator.scala b/core/src/main/scala/mill/eval/Evaluator.scala
index efb10d36..2afaec6e 100644
--- a/core/src/main/scala/mill/eval/Evaluator.scala
+++ b/core/src/main/scala/mill/eval/Evaluator.scala
@@ -9,6 +9,7 @@ import mill.discover.{Discovered, Mirror}
 import mill.define.Segment
 import mill.util
 import mill.util._
+import mill.util.Strict.OSet
 
 import scala.collection.mutable
 import scala.util.control.NonFatal
diff --git a/core/src/main/scala/mill/main/ReplApplyHandler.scala b/core/src/main/scala/mill/main/ReplApplyHandler.scala
index d4236487..b83453ee 100644
--- a/core/src/main/scala/mill/main/ReplApplyHandler.scala
+++ b/core/src/main/scala/mill/main/ReplApplyHandler.scala
@@ -5,7 +5,7 @@ import mill.define.Applicative.ApplyHandler
 import mill.define._
 import mill.discover.{Discovered, Mirror}
 import mill.eval.{Evaluator, Result}
-import mill.util.OSet
+import mill.util.Strict.OSet
 
 import scala.collection.mutable
 object ReplApplyHandler{
diff --git a/core/src/main/scala/mill/main/RunScript.scala b/core/src/main/scala/mill/main/RunScript.scala
index b8793b40..9f73fbc8 100644
--- a/core/src/main/scala/mill/main/RunScript.scala
+++ b/core/src/main/scala/mill/main/RunScript.scala
@@ -11,7 +11,8 @@ import mill.define.Task
 import mill.define.Segment
 import mill.discover.Discovered
 import mill.eval.{Evaluator, Result}
-import mill.util.{EitherOps, Logger, OSet}
+import mill.util.{EitherOps, Logger}
+import mill.util.Strict.OSet
 import upickle.Js
 
 /**
diff --git a/core/src/main/scala/mill/modules/Jvm.scala b/core/src/main/scala/mill/modules/Jvm.scala
index e6806923..6e3fe45f 100644
--- a/core/src/main/scala/mill/modules/Jvm.scala
+++ b/core/src/main/scala/mill/modules/Jvm.scala
@@ -9,14 +9,15 @@ import ammonite.ops._
 import mill.define.Task
 import mill.eval.PathRef
 import mill.util.Ctx
+import mill.util.Loose.OSet
 
 import scala.annotation.tailrec
 import scala.collection.mutable
 
 
 object Jvm {
-  def gatherClassloaderJars(): Seq[Path] = {
-    val allJars = collection.mutable.Buffer.empty[Path]
+  def gatherClassloaderJars(): OSet[Path] = {
+    val allJars = new OSet.Mutable[Path]()
     var currentClassloader = Thread.currentThread().getContextClassLoader
     while(currentClassloader != null){
       currentClassloader match{
@@ -29,14 +30,14 @@ object Jvm {
   }
 
   def interactiveSubprocess(mainClass: String,
-                            classPath: Seq[Path],
+                            classPath: OSet[Path],
                             options: Seq[String] = Seq.empty): Unit = {
     import ammonite.ops.ImplicitWd._
     %("java", "-cp", classPath.mkString(":"), mainClass, options)
   }
 
   def subprocess(mainClass: String,
-                 classPath: Seq[Path],
+                 classPath: OSet[Path],
                  jvmOptions: Seq[String] = Seq.empty,
                  options: Seq[String] = Seq.empty,
                  workingDir: Path = null)
@@ -100,7 +101,7 @@ object Jvm {
     m
   }
 
-  def createJar(inputPaths: Seq[Path], mainClass: Option[String] = None)
+  def createJar(inputPaths: OSet[Path], mainClass: Option[String] = None)
                (implicit ctx: Ctx.DestCtx): PathRef = {
     val outputPath = ctx.dest
     rm(outputPath)
@@ -134,7 +135,7 @@ object Jvm {
     PathRef(outputPath)
   }
 
-  def createAssembly(inputPaths: Seq[Path],
+  def createAssembly(inputPaths: OSet[Path],
                      mainClass: Option[String] = None,
                      prependShellScript: String = "")
                     (implicit ctx: Ctx.DestCtx): PathRef = {
diff --git a/core/src/main/scala/mill/package.scala b/core/src/main/scala/mill/package.scala
index 3463b63f..0b4f4873 100644
--- a/core/src/main/scala/mill/package.scala
+++ b/core/src/main/scala/mill/package.scala
@@ -8,4 +8,6 @@ package object mill extends JsonFormatters{
   type Module = define.Module
   val Module = define.Module
   type Cross[T] = define.Cross[T]
+  type OSet[T] = util.Loose.OSet[T]
+  val OSet = util.Loose.OSet
 }
diff --git a/core/src/main/scala/mill/util/MultiBiMap.scala b/core/src/main/scala/mill/util/MultiBiMap.scala
index ef5359bc..abbaa752 100644
--- a/core/src/main/scala/mill/util/MultiBiMap.scala
+++ b/core/src/main/scala/mill/util/MultiBiMap.scala
@@ -1,7 +1,7 @@
 package mill.util
 
 import scala.collection.mutable
-
+import Strict.OSet
 /**
   * A map from keys to collections of values: you can assign multiple values
   * to any particular key. Also allows lookups in both directions: what values
diff --git a/core/src/main/scala/mill/util/OSet.scala b/core/src/main/scala/mill/util/OSet.scala
index 52b47cde..a4fcc406 100644
--- a/core/src/main/scala/mill/util/OSet.scala
+++ b/core/src/main/scala/mill/util/OSet.scala
@@ -3,108 +3,114 @@ package mill.util
 
 
 import scala.collection.mutable
-
-/**
-  * 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 OSet[V] extends TraversableOnce[V]{
-  def contains(v: V): Boolean
-  def items: Iterator[V]
-  def indexed: IndexedSeq[V]
-  def flatMap[T](f: V => TraversableOnce[T]): OSet[T]
-  def map[T](f: V => T): OSet[T]
-  def filter(f: V => Boolean): OSet[V]
-  def withFilter(f: V => Boolean): OSet[V]
-  def collect[T](f: PartialFunction[V, T]): OSet[T]
-  def zipWithIndex: OSet[(V, Int)]
-  def reverse: OSet[V]
-  def zip[T](other: OSet[T]): OSet[(V, T)]
-}
-
-object OSet{
-  implicit def jsonFormat[T: upickle.default.ReadWriter]: upickle.default.ReadWriter[OSet[T]] =
-    upickle.default.ReadWriter[OSet[T]] (
-      oset => upickle.default.writeJs(oset.toList),
-      {case json => OSet.from(upickle.default.readJs[Seq[T]](json))}
-    )
-  def apply[V](items: V*) = from(items)
-
-  def from[V](items: TraversableOnce[V]): OSet[V] = {
-    val set = new OSet.Mutable[V]()
-    items.foreach(set.append)
-    set
+object Strict extends OSetWrapper(true)
+object Loose extends OSetWrapper(false)
+sealed class OSetWrapper(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 OSet[V] extends TraversableOnce[V]{
+    def contains(v: V): Boolean
+    def items: Iterator[V]
+    def indexed: IndexedSeq[V]
+    def flatMap[T](f: V => TraversableOnce[T]): OSet[T]
+    def map[T](f: V => T): OSet[T]
+    def filter(f: V => Boolean): OSet[V]
+    def withFilter(f: V => Boolean): OSet[V]
+    def collect[T](f: PartialFunction[V, T]): OSet[T]
+    def zipWithIndex: OSet[(V, Int)]
+    def reverse: OSet[V]
+    def zip[T](other: OSet[T]): OSet[(V, T)]
+    def ++[T >: V](other: TraversableOnce[T]): OSet[T]
   }
 
+  object OSet{
+    def empty[V]: OSet[V] = new OSet.Mutable[V]
+    implicit def jsonFormat[T: upickle.default.ReadWriter]: upickle.default.ReadWriter[OSet[T]] =
+      upickle.default.ReadWriter[OSet[T]] (
+        oset => upickle.default.writeJs(oset.toList),
+        {case json => OSet.from(upickle.default.readJs[Seq[T]](json))}
+      )
+    def apply[V](items: V*) = from(items)
+
+    def from[V](items: TraversableOnce[V]): OSet[V] = {
+      val set = new OSet.Mutable[V]()
+      items.foreach(set.append)
+      set
+    }
 
-  class Mutable[V]() extends OSet[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)
+    class Mutable[V]() extends OSet[V]{
 
-    }else {
-      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): OSet[T] = {
-      val output = new OSet.Mutable[T]
-      for(i <- items) output.append(f(i))
-      output
-    }
-    def flatMap[T](f: V => TraversableOnce[T]): OSet[T] = {
-      val output = new OSet.Mutable[T]
-      for(i <- items) for(i0 <- f(i)) output.append(i0)
-      output
-    }
-    def filter(f: V => Boolean): OSet[V] = {
-      val output = new OSet.Mutable[V]
-      for(i <- items) if (f(i)) output.append(i)
-      output
-    }
-    def withFilter(f: V => Boolean): OSet[V] = filter(f)
+      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)
 
-    def collect[T](f: PartialFunction[V, T]) = this.filter(f.isDefinedAt).map(x => f(x))
+      }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): OSet[T] = {
+        val output = new OSet.Mutable[T]
+        for(i <- items) output.append(f(i))
+        output
+      }
+      def flatMap[T](f: V => TraversableOnce[T]): OSet[T] = {
+        val output = new OSet.Mutable[T]
+        for(i <- items) for(i0 <- f(i)) output.append(i0)
+        output
+      }
+      def filter(f: V => Boolean): OSet[V] = {
+        val output = new OSet.Mutable[V]
+        for(i <- items) if (f(i)) output.append(i)
+        output
+      }
+      def withFilter(f: V => Boolean): OSet[V] = filter(f)
 
-    def zipWithIndex = {
-      var i = 0
-      this.map{ x =>
-        i += 1
-        (x, i-1)
+      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 = OSet.from(indexed.reverseIterator)
-
-    def zip[T](other: OSet[T]) = OSet.from(items.zip(other.items))
-
-    // 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.hashCode()
-    override def equals(other: Any) = other match{
-      case s: OSet[_] => items.sameElements(s.items)
-      case _ => super.equals(other)
+      def reverse = OSet.from(indexed.reverseIterator)
+
+      def zip[T](other: OSet[T]) = OSet.from(items.zip(other.items))
+      def ++[T >: V](other: TraversableOnce[T]) = OSet.from(items ++ other)
+
+      // 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: OSet[_] => items.sameElements(s.items)
+        case _ => super.equals(other)
+      }
+      override def toString = items.mkString("OSet(", ", ", ")")
     }
-    override def toString = items.mkString("OSet(", ", ", ")")
   }
 }
diff --git a/core/src/test/scala/mill/define/CacherTests.scala b/core/src/test/scala/mill/define/CacherTests.scala
index faf274f2..4127a75b 100644
--- a/core/src/test/scala/mill/define/CacherTests.scala
+++ b/core/src/test/scala/mill/define/CacherTests.scala
@@ -2,7 +2,8 @@ package mill.define
 
 import mill.discover.Discovered
 import mill.eval.Evaluator
-import mill.util.{DummyLogger, OSet, TestUtil}
+import mill.util.{DummyLogger, TestUtil}
+import mill.util.Strict.OSet
 import mill.T
 import mill.eval.Result.Success
 import utest._
diff --git a/core/src/test/scala/mill/define/GraphTests.scala b/core/src/test/scala/mill/define/GraphTests.scala
index 718fd673..7241087a 100644
--- a/core/src/test/scala/mill/define/GraphTests.scala
+++ b/core/src/test/scala/mill/define/GraphTests.scala
@@ -4,9 +4,9 @@ package mill.define
 import mill.discover.Discovered
 import Discovered.mapping
 import mill.eval.Evaluator
-import mill.util.{OSet, TestGraphs, TestUtil}
+import mill.util.{TestGraphs, TestUtil}
 import utest._
-
+import mill.util.Strict.OSet
 object GraphTests extends TestSuite{
 
   val tests = Tests{
diff --git a/core/src/test/scala/mill/discover/ConsistencyTests.scala b/core/src/test/scala/mill/discover/ConsistencyTests.scala
index 190b2cee..4a9ae843 100644
--- a/core/src/test/scala/mill/discover/ConsistencyTests.scala
+++ b/core/src/test/scala/mill/discover/ConsistencyTests.scala
@@ -3,7 +3,8 @@ package mill.discover
 
 import mill.define.Segment.Label
 import mill.define.Segments
-import mill.util.{OSet, TestGraphs}
+import mill.util.{TestGraphs}
+import mill.util.Strict.OSet
 import utest._
 
 object ConsistencyTests extends TestSuite{
diff --git a/core/src/test/scala/mill/eval/EvaluationTests.scala b/core/src/test/scala/mill/eval/EvaluationTests.scala
index 54e184b5..2b9dd102 100644
--- a/core/src/test/scala/mill/eval/EvaluationTests.scala
+++ b/core/src/test/scala/mill/eval/EvaluationTests.scala
@@ -6,7 +6,8 @@ import mill.define.{Graph, Target, Task}
 import mill.{Module, T}
 import mill.discover.Discovered
 import mill.discover.Discovered.mapping
-import mill.util.{DummyLogger, OSet, TestGraphs, TestUtil}
+import mill.util.{DummyLogger, TestGraphs, TestUtil}
+import mill.util.Strict.OSet
 import utest._
 import utest.framework.TestPath
 
diff --git a/core/src/test/scala/mill/eval/FailureTests.scala b/core/src/test/scala/mill/eval/FailureTests.scala
index c2057cd1..ef206740 100644
--- a/core/src/test/scala/mill/eval/FailureTests.scala
+++ b/core/src/test/scala/mill/eval/FailureTests.scala
@@ -3,7 +3,8 @@ package mill.eval
 import mill.define.Target
 import mill.discover.Discovered
 import mill.discover.Discovered.mapping
-import mill.util.{DummyLogger, OSet}
+import mill.util.DummyLogger
+import mill.util.Strict.OSet
 import utest._
 import utest.framework.TestPath
 
diff --git a/core/src/test/scala/mill/eval/JavaCompileJarTests.scala b/core/src/test/scala/mill/eval/JavaCompileJarTests.scala
index c8320fcd..8105f2fa 100644
--- a/core/src/test/scala/mill/eval/JavaCompileJarTests.scala
+++ b/core/src/test/scala/mill/eval/JavaCompileJarTests.scala
@@ -7,7 +7,8 @@ import mill.discover.Discovered
 import mill.modules.Jvm
 import mill.util.Ctx.DestCtx
 import mill.{Module, T}
-import mill.util.{DummyLogger, OSet, TestUtil}
+import mill.util.{DummyLogger, Loose, TestUtil}
+import mill.util.Strict.OSet
 import utest._
 import mill._
 
@@ -40,7 +41,7 @@ object JavaCompileJarTests extends TestSuite{
         def resourceRoot = T.source{ resourceRootPath }
         def allSources = T{ ls.rec(sourceRoot().path).map(PathRef(_)) }
         def classFiles = T{ compileAll(allSources()) }
-        def jar = T{ Jvm.createJar(Seq(resourceRoot().path, classFiles().path)) }
+        def jar = T{ Jvm.createJar(Loose.OSet(resourceRoot().path, classFiles().path)) }
 
         def run(mainClsName: String) = T.command{
           %%('java, "-cp", classFiles().path, mainClsName)
diff --git a/core/src/test/scala/mill/util/TestEvaluator.scala b/core/src/test/scala/mill/util/TestEvaluator.scala
index 646ca418..3f6086ba 100644
--- a/core/src/test/scala/mill/util/TestEvaluator.scala
+++ b/core/src/test/scala/mill/util/TestEvaluator.scala
@@ -4,7 +4,7 @@ import ammonite.ops.Path
 import mill.define.{Input, Target, Task}
 import mill.discover.{Discovered, Mirror}
 import mill.eval.{Evaluator, Result}
-
+import mill.util.Strict.OSet
 class TestEvaluator(mapping: Discovered.Mapping[_],
                     workspacePath: Path,
                     basePath: Path){
diff --git a/core/src/test/scala/mill/util/TestUtil.scala b/core/src/test/scala/mill/util/TestUtil.scala
index ae985665..3cc945db 100644
--- a/core/src/test/scala/mill/util/TestUtil.scala
+++ b/core/src/test/scala/mill/util/TestUtil.scala
@@ -4,7 +4,7 @@ import ammonite.main.Router.Overrides
 import mill.define._
 import mill.eval.Result
 import utest.assert
-
+import mill.util.Strict.OSet
 import scala.collection.mutable
 
 object TestUtil {
diff --git a/integration/src/test/resources/acyclic/build.sc b/integration/src/test/resources/acyclic/build.sc
index 66d64501..094893f5 100644
--- a/integration/src/test/resources/acyclic/build.sc
+++ b/integration/src/test/resources/acyclic/build.sc
@@ -1,9 +1,9 @@
 import mill.Cross
-import mill.scalalib.{SbtModule, PublishModule, Dep}
+import mill.scalalib.{SbtModule, PublishModule, Dep, CrossSbtModule}
 import mill.scalalib.publish.{PomSettings, License, Developer, SCM}
 
 object acyclic extends Cross[AcyclicModule]("2.10.6", "2.11.8", "2.12.3", "2.12.4")
-class AcyclicModule(crossVersion: String) extends SbtModule with PublishModule {
+class AcyclicModule(val crossScalaVersion: String) extends CrossSbtModule with PublishModule {
   def basePath = super.basePath / ammonite.ops.up
   def artifactName = "acyclic"
   def publishVersion = "0.1.7"
@@ -24,13 +24,12 @@ class AcyclicModule(crossVersion: String) extends SbtModule with PublishModule {
     )
   )
 
-  def scalaVersion = crossVersion
-  def ivyDeps = Seq(
+  def ivyDeps = OSet(
     Dep.Java("org.scala-lang", "scala-compiler", scalaVersion())
   )
   object test extends Tests{
     def forkWorkingDir = ammonite.ops.pwd / 'target / 'workspace / 'acyclic
-    def ivyDeps = Seq(
+    def ivyDeps = OSet(
       Dep("com.lihaoyi", "utest", "0.6.0")
     )
     def testFramework = "utest.runner.Framework"
diff --git a/integration/src/test/resources/better-files/build.sc b/integration/src/test/resources/better-files/build.sc
index cd6cac04..fe11f1b8 100644
--- a/integration/src/test/resources/better-files/build.sc
+++ b/integration/src/test/resources/better-files/build.sc
@@ -54,7 +54,7 @@ trait BetterFilesModule extends SbtModule{
     def projectDeps =
       if (this == core.test) super.projectDeps
       else super.projectDeps ++ Seq(core.test)
-    def ivyDeps = Seq(Dep("org.scalatest", "scalatest", "3.0.4"))
+    def ivyDeps = OSet(Dep("org.scalatest", "scalatest", "3.0.4"))
     def testFramework = "org.scalatest.tools.Framework"
   }
 }
@@ -63,17 +63,17 @@ object core extends BetterFilesModule
 
 object akka extends BetterFilesModule{
   def projectDeps = Seq(core)
-  def ivyDeps = Seq(Dep("com.typesafe.akka", "akka-actor", "2.5.6"))
+  def ivyDeps = OSet(Dep("com.typesafe.akka", "akka-actor", "2.5.6"))
 }
 
 object shapeless extends BetterFilesModule{
   def projectDeps = Seq(core)
-  def ivyDeps = Seq(Dep("com.chuusai", "shapeless", "2.3.2"))
+  def ivyDeps = OSet(Dep("com.chuusai", "shapeless", "2.3.2"))
 }
 
 object benchmarks extends BetterFilesModule{
   def projectDeps = Seq(core)
-  def ivyDeps = Seq(
+  def ivyDeps = OSet(
     Dep.Java("commons-io", "commons-io", "2.5")
     // "fastjavaio" % "fastjavaio" % "1.0" from "https://github.com/williamfiset/FastJavaIO/releases/download/v1.0/fastjavaio.jar"
   )
diff --git a/integration/src/test/resources/jawn/build.sc b/integration/src/test/resources/jawn/build.sc
index 8b830f70..a90d93b5 100644
--- a/integration/src/test/resources/jawn/build.sc
+++ b/integration/src/test/resources/jawn/build.sc
@@ -4,7 +4,7 @@ import mill.scalalib.{Dep, TestModule, Module}
 
 object jawn extends Cross[JawnModule]("2.10.6", "2.11.11", "2.12.3")
 class JawnModule(crossVersion: String) extends mill.Module{
-  override def basePath = super.basePath / ammonite.ops.up
+  override def basePath = super.basePath / ammonite.ops.up / ammonite.ops.up
 
   trait JawnModule extends scalalib.SbtModule{
     def scalaVersion = crossVersion
@@ -16,7 +16,7 @@ class JawnModule(crossVersion: String) extends mill.Module{
     def testProjectDeps: Seq[TestModule] = Nil
     object test extends Tests{
       def projectDeps = super.projectDeps ++ testProjectDeps
-      def ivyDeps = Seq(
+      def ivyDeps = OSet(
         Dep("org.scalatest", "scalatest", "3.0.3"),
         Dep("org.scalacheck", "scalacheck", "1.13.5")
       )
@@ -35,7 +35,7 @@ class JawnModule(crossVersion: String) extends mill.Module{
   }
   class Support(ivyDeps0: Dep*)(implicit ctx: mill.Module.Ctx) extends JawnModule{
     def projectDeps = Seq[Module](parser)
-    def ivyDeps = ivyDeps0
+    def ivyDeps = OSet.from(ivyDeps0)
   }
   object support extends mill.Module{
     object argonaut extends Support(Dep("io.argonaut", "argonaut", "6.2"))
@@ -44,9 +44,9 @@ class JawnModule(crossVersion: String) extends mill.Module{
     object play extends Support(){
       def ivyDeps = mill.T{
         scalaBinaryVersion() match{
-          case "2.10" => Seq(Dep("com.typesafe.play", "play-json", "2.4.11"))
-          case "2.11" => Seq(Dep("com.typesafe.play", "play-json", "2.5.15"))
-          case _ => Seq(Dep("com.typesafe.play", "play-json", "2.6.0"))
+          case "2.10" => OSet(Dep("com.typesafe.play", "play-json", "2.4.11"))
+          case "2.11" => OSet(Dep("com.typesafe.play", "play-json", "2.5.15"))
+          case _ => OSet(Dep("com.typesafe.play", "play-json", "2.6.0"))
         }
       }
     }
diff --git a/integration/src/test/scala/mill/integration/IntegrationTestSuite.scala b/integration/src/test/scala/mill/integration/IntegrationTestSuite.scala
index ba4415fa..82f4de72 100644
--- a/integration/src/test/scala/mill/integration/IntegrationTestSuite.scala
+++ b/integration/src/test/scala/mill/integration/IntegrationTestSuite.scala
@@ -8,7 +8,7 @@ import utest._
 abstract class IntegrationTestSuite(repoKey: String, workspaceSlug: String) extends TestSuite{
   val workspacePath = pwd / 'target / 'workspace / workspaceSlug
   val buildFilePath = pwd / 'integration / 'src / 'test / 'resources / workspaceSlug
-  val stdOutErr = new PrintStream(new ByteArrayOutputStream())
+  val stdOutErr = System.out//new PrintStream(new ByteArrayOutputStream())
   val stdIn = new ByteArrayInputStream(Array())
   val runner = new mill.main.MainRunner(
     ammonite.main.Cli.Config(wd = workspacePath), false,
diff --git a/scalajslib/src/main/scala/mill/scalajslib/Lib.scala b/scalajslib/src/main/scala/mill/scalajslib/Lib.scala
index 3de37962..744bb262 100644
--- a/scalajslib/src/main/scala/mill/scalajslib/Lib.scala
+++ b/scalajslib/src/main/scala/mill/scalajslib/Lib.scala
@@ -7,7 +7,7 @@ import java.net.URLClassLoader
 import ammonite.ops.{Path, mkdir, rm, _}
 import mill.eval.PathRef
 import mill.scalalib.Dep
-import mill.util.Ctx
+import mill.util.{Ctx, Loose}
 
 import scala.collection.breakOut
 import scala.language.reflectiveCalls
@@ -21,7 +21,7 @@ object Lib {
   def scalaJSLinkerIvyDep(scalaJSVersion: String): Dep =
     Dep("com.lihaoyi", s"mill-jsbridge_${scalaJSVersion.replace('.', '_')}", "0.1-SNAPSHOT")
 
-  def scalaJSLinkerBridge(classPath: Seq[Path]): ScalaJSLinkerBridge = {
+  def scalaJSLinkerBridge(classPath: Loose.OSet[Path]): ScalaJSLinkerBridge = {
     val classloaderSig = classPath.map(p => p.toString().hashCode + p.mtime.toMillis).sum
     LinkerBridge.scalaInstanceCache match {
       case Some((`classloaderSig`, linker)) => linker
@@ -44,7 +44,7 @@ object Lib {
 
   def link(main: Option[String],
            inputPaths: Seq[Path],
-           libraries: Seq[Path],
+           libraries: Loose.OSet[Path],
            linker: ScalaJSLinkerBridge,
            mode: OptimizeMode)
           (implicit ctx: Ctx.DestCtx): PathRef = {
@@ -53,7 +53,7 @@ object Lib {
     if (inputPaths.nonEmpty) {
       mkdir(outputPath / up)
       val inputFiles: Vector[File] = inputPaths.map(ls).flatMap(_.filter(_.ext == "sjsir")).map(_.toIO)(breakOut)
-      val inputLibraries: Vector[File] = libraries.filter(_.ext == "jar").map(_.toIO)(breakOut)
+      val inputLibraries: Vector[File] = libraries.filter(_.ext == "jar").map(_.toIO).toVector
       linker.link(inputFiles, inputLibraries, outputPath.toIO, main, mode)
     }
     PathRef(outputPath)
diff --git a/scalajslib/src/main/scala/mill/scalajslib/ScalaJSModule.scala b/scalajslib/src/main/scala/mill/scalajslib/ScalaJSModule.scala
index ed0a2c85..03c4e216 100644
--- a/scalajslib/src/main/scala/mill/scalajslib/ScalaJSModule.scala
+++ b/scalajslib/src/main/scala/mill/scalajslib/ScalaJSModule.scala
@@ -7,7 +7,8 @@ import ammonite.ops.Path
 import mill.eval.Result.Success
 import mill.scalajslib.Lib._
 import mill.scalalib.Lib.resolveDependencies
-import mill.scalalib.{Dep, PublishModule, Module, TestModule}
+import mill.scalalib.{Dep, Module, PublishModule, TestModule}
+import mill.util.Loose
 
 trait ScalaJSModule extends scalalib.Module { outer =>
 
@@ -26,10 +27,16 @@ trait ScalaJSModule extends scalalib.Module { outer =>
 
   def scalaJSBridgeVersion = T{ scalaJSVersion().split('.').dropRight(1).mkString(".") }
 
-  def scalaJSLinkerClasspath: T[Seq[PathRef]] = T{
+  def scalaJSLinkerClasspath: T[Loose.OSet[PathRef]] = T{
     val jsBridgeKey = "MILL_SCALAJS_BRIDGE_" + scalaJSBridgeVersion().replace('.', '_')
     val jsBridgePath = sys.props(jsBridgeKey)
-    if (jsBridgePath != null) Success(jsBridgePath.split(File.pathSeparator).map(f => PathRef(Path(f), quick = true)).toVector)
+    if (jsBridgePath != null) {
+      Success(
+        Loose.OSet.from(
+          jsBridgePath.split(File.pathSeparator).map(f => PathRef(Path(f), quick = true))
+        )
+      )
+    }
     else {
       val dep = scalaJSLinkerIvyDep(scalaJSBridgeVersion())
       resolveDependencies(
@@ -51,9 +58,9 @@ trait ScalaJSModule extends scalalib.Module { outer =>
     link(mainClass(), Seq(compile().classes.path), compileDepClasspath().map(_.path), linker, FullOpt)
   }
 
-  override def scalacPluginIvyDeps = T{ Seq(Dep.Point("org.scala-js", "scalajs-compiler", scalaJSVersion())) }
+  override def scalacPluginIvyDeps = T{ Loose.OSet(Dep.Point("org.scala-js", "scalajs-compiler", scalaJSVersion())) }
 
-  override def ivyDeps = T{ Seq(Dep("org.scala-js", "scalajs-library", scalaJSVersion())) }
+  override def ivyDeps = T{ Loose.OSet(Dep("org.scala-js", "scalajs-library", scalaJSVersion())) }
 
   // publish artifact with name "mill_sjs0.6.4_2.12" instead of "mill_sjs0.6_2.12"
   def crossFullScalaJSVersion: T[Boolean] = false
diff --git a/scalalib/src/main/scala/mill/scalalib/GenIdea.scala b/scalalib/src/main/scala/mill/scalalib/GenIdea.scala
index 24daef2c..cd664495 100644
--- a/scalalib/src/main/scala/mill/scalalib/GenIdea.scala
+++ b/scalalib/src/main/scala/mill/scalalib/GenIdea.scala
@@ -5,7 +5,8 @@ import mill.define.{Segment, Segments, Target}
 import mill.discover.{Discovered, Mirror}
 import mill.eval.{Evaluator, PathRef}
 import mill.util.Ctx.{LoaderCtx, LogCtx}
-import mill.util.{OSet, PrintLogger}
+import mill.util.{PrintLogger}
+import mill.util.Strict.OSet
 
 object GenIdea {
 
diff --git a/scalalib/src/main/scala/mill/scalalib/Lib.scala b/scalalib/src/main/scala/mill/scalalib/Lib.scala
index 098da786..54b1a817 100644
--- a/scalalib/src/main/scala/mill/scalalib/Lib.scala
+++ b/scalalib/src/main/scala/mill/scalalib/Lib.scala
@@ -9,7 +9,8 @@ import ammonite.ops._
 import coursier.{Cache, Fetch, MavenRepository, Repository, Resolution, Module => CoursierModule}
 import mill.define.Worker
 import mill.eval.{PathRef, Result}
-import mill.util.Ctx
+import mill.util.{Ctx}
+import mill.util.Loose.OSet
 import sbt.internal.inc._
 import sbt.internal.util.{ConsoleOut, MainAppender}
 import sbt.util.LogExchange
@@ -38,7 +39,7 @@ object Lib{
       Locate.definesClass(classpathEntry)
   }
 
-  def grepJar(classPath: Seq[Path], s: String) = {
+  def grepJar(classPath: OSet[Path], s: String) = {
     classPath
       .find(_.toString.endsWith(s))
       .getOrElse(throw new Exception("Cannot find " + s))
@@ -47,13 +48,13 @@ object Lib{
 
   def compileScala(zincWorker: ZincWorker,
                    scalaVersion: String,
-                   sources: Seq[Path],
-                   compileClasspath: Seq[Path],
-                   compilerClasspath: Seq[Path],
-                   pluginClasspath: Seq[Path],
+                   sources: OSet[Path],
+                   compileClasspath: OSet[Path],
+                   compilerClasspath: OSet[Path],
+                   pluginClasspath: OSet[Path],
                    compilerBridge: Path,
                    scalacOptions: Seq[String],
-                   scalacPluginClasspath: Seq[Path],
+                   scalacPluginClasspath: OSet[Path],
                    javacOptions: Seq[String],
                    upstreamCompileOutput: Seq[CompilationResult])
                   (implicit ctx: Ctx): CompilationResult = {
@@ -171,8 +172,8 @@ object Lib{
   def resolveDependencies(repositories: Seq[Repository],
                           scalaVersion: String,
                           scalaBinaryVersion: String,
-                          deps: Seq[Dep],
-                          sources: Boolean = false): Result[Seq[PathRef]] = {
+                          deps: TraversableOnce[Dep],
+                          sources: Boolean = false): Result[OSet[PathRef]] = {
     val flattened = deps.map{
       case Dep.Java(dep) => dep
       case Dep.Scala(dep) =>
@@ -206,14 +207,16 @@ object Lib{
         .unsafePerformSync
         .flatMap(_.toOption)
 
-      localArtifacts.map(p => PathRef(Path(p), quick = true)).filter(_.path.ext == "jar")
+      OSet.from(
+        localArtifacts.map(p => PathRef(Path(p), quick = true)).filter(_.path.ext == "jar")
+      )
     }
   }
-  def scalaCompilerIvyDeps(scalaVersion: String) = Seq(
+  def scalaCompilerIvyDeps(scalaVersion: String) = OSet[Dep](
     Dep.Java("org.scala-lang", "scala-compiler", scalaVersion),
     Dep.Java("org.scala-lang", "scala-reflect", scalaVersion)
   )
-  def scalaRuntimeIvyDeps(scalaVersion: String) = Seq[Dep](
+  def scalaRuntimeIvyDeps(scalaVersion: String) = OSet[Dep](
     Dep.Java("org.scala-lang", "scala-library", scalaVersion)
   )
   def compilerBridgeIvyDep(scalaVersion: String) =
diff --git a/scalalib/src/main/scala/mill/scalalib/MiscModule.scala b/scalalib/src/main/scala/mill/scalalib/MiscModule.scala
index 60d9d1ce..26609bcd 100644
--- a/scalalib/src/main/scala/mill/scalalib/MiscModule.scala
+++ b/scalalib/src/main/scala/mill/scalalib/MiscModule.scala
@@ -1,18 +1,19 @@
 package mill
 package scalalib
 
+import mill.define.Cross.Resolver
 import mill.define.{Cross, Task}
 import mill.eval.{PathRef, Result}
-import mill.define.Cross.Resolver
+import mill.util.Loose.OSet
 
 
 trait SbtModule extends Module { outer =>
-  override def sources = T.input{ Seq(PathRef(basePath / 'src / 'main / 'scala)) }
-  override def resources = T.input{ Seq(PathRef(basePath / 'src / 'main / 'resources)) }
+  override def sources = T.input{ OSet(PathRef(basePath / 'src / 'main / 'scala)) }
+  override def resources = T.input{ OSet(PathRef(basePath / 'src / 'main / 'resources)) }
   trait Tests extends super.Tests {
     override def basePath = outer.basePath
-    override def sources = T.input{ Seq(PathRef(basePath / 'src / 'test / 'scala)) }
-    override def resources = T.input{ Seq(PathRef(basePath / 'src / 'test / 'resources)) }
+    override def sources = T.input{ OSet(PathRef(basePath / 'src / 'test / 'scala)) }
+    override def resources = T.input{ OSet(PathRef(basePath / 'src / 'test / 'resources)) }
   }
 }
 
@@ -45,7 +46,7 @@ trait CrossSbtModule extends SbtModule { outer =>
     }
 
   }
-  override def resources = T.input{ Seq(PathRef(basePath / 'src / 'main / 'resources)) }
+  override def resources = T.input{ OSet(PathRef(basePath / 'src / 'main / 'resources)) }
   trait Tests extends super.Tests {
     override def basePath = outer.basePath
     override def sources = T.input{
@@ -54,7 +55,7 @@ trait CrossSbtModule extends SbtModule { outer =>
         s => PathRef{ basePath / 'src / 'test / s"scala-$s" }
       }
     }
-    override def resources = T.input{ Seq(PathRef(basePath / 'src / 'test / 'resources)) }
+    override def resources = T.input{ OSet(PathRef(basePath / 'src / 'test / 'resources)) }
   }
 }
 
diff --git a/scalalib/src/main/scala/mill/scalalib/Module.scala b/scalalib/src/main/scala/mill/scalalib/Module.scala
index d0041fb5..2a7d6edb 100644
--- a/scalalib/src/main/scala/mill/scalalib/Module.scala
+++ b/scalalib/src/main/scala/mill/scalalib/Module.scala
@@ -10,6 +10,7 @@ import mill.modules.Jvm
 import mill.modules.Jvm.{createAssembly, createJar, interactiveSubprocess, subprocess}
 import Lib._
 import mill.define.Cross.Resolver
+import mill.util.Loose.OSet
 import sbt.testing.Status
 
 /**
@@ -25,10 +26,10 @@ trait Module extends mill.Module with TaskModule { outer =>
   def mainClass: T[Option[String]] = None
 
   def scalaBinaryVersion = T{ scalaVersion().split('.').dropRight(1).mkString(".") }
-  def ivyDeps = T{ Seq[Dep]() }
-  def compileIvyDeps = T{ Seq[Dep]() }
-  def scalacPluginIvyDeps = T{ Seq[Dep]() }
-  def runIvyDeps = T{ Seq[Dep]() }
+  def ivyDeps = T{ OSet.empty[Dep] }
+  def compileIvyDeps = T{ OSet.empty[Dep] }
+  def scalacPluginIvyDeps = T{ OSet.empty[Dep] }
+  def runIvyDeps = T{ OSet.empty[Dep] }
 
   def scalacOptions = T{ Seq.empty[String] }
   def javacOptions = T{ Seq.empty[String] }
@@ -39,7 +40,7 @@ trait Module extends mill.Module with TaskModule { outer =>
   )
 
   def projectDeps = Seq.empty[Module]
-  def depClasspath = T{ Seq.empty[PathRef] }
+  def depClasspath = T{ OSet.empty[PathRef] }
 
 
   def upstreamRunClasspath = T{
@@ -57,7 +58,7 @@ trait Module extends mill.Module with TaskModule { outer =>
     Task.traverse(projectDeps)(_.compileDepClasspath)().flatten
   }
 
-  def resolveDeps(deps: Task[Seq[Dep]], sources: Boolean = false) = T.task{
+  def resolveDeps(deps: Task[OSet[Dep]], sources: Boolean = false) = T.task{
     resolveDependencies(
       repositories,
       scalaVersion(),
@@ -67,15 +68,15 @@ trait Module extends mill.Module with TaskModule { outer =>
     )
   }
 
-  def externalCompileDepClasspath: T[Seq[PathRef]] = T{
-    Task.traverse(projectDeps)(_.externalCompileDepClasspath)().flatten ++
+  def externalCompileDepClasspath: T[OSet[PathRef]] = T{
+    OSet.from(Task.traverse(projectDeps)(_.externalCompileDepClasspath)().flatten) ++
     resolveDeps(
       T.task{ivyDeps() ++ compileIvyDeps() ++ scalaCompilerIvyDeps(scalaVersion())}
     )()
   }
 
-  def externalCompileDepSources: T[Seq[PathRef]] = T{
-    Task.traverse(projectDeps)(_.externalCompileDepSources)().flatten ++
+  def externalCompileDepSources: T[OSet[PathRef]] = T{
+    OSet.from(Task.traverse(projectDeps)(_.externalCompileDepSources)().flatten) ++
     resolveDeps(
       T.task{ivyDeps() ++ compileIvyDeps() ++ scalaCompilerIvyDeps(scalaVersion())},
       sources = true
@@ -86,7 +87,7 @@ trait Module extends mill.Module with TaskModule { outer =>
     * Things that need to be on the classpath in order for this code to compile;
     * might be less than the runtime classpath
     */
-  def compileDepClasspath: T[Seq[PathRef]] = T{
+  def compileDepClasspath: T[OSet[PathRef]] = T{
     upstreamCompileClasspath() ++
     depClasspath()
   }
@@ -108,7 +109,7 @@ trait Module extends mill.Module with TaskModule { outer =>
       )
       classpath match {
         case Result.Success(resolved) =>
-          resolved.filterNot(_.path.ext == "pom") match {
+          resolved.filter(_.path.ext != "pom").toSeq match {
             case Seq(single) => PathRef(single.path, quick = true)
             case Seq() => throw new Exception(dep + " resolution failed") // TODO: find out, is it possible?
             case _ => throw new Exception(dep + " resolution resulted in more than one file")
@@ -118,7 +119,7 @@ trait Module extends mill.Module with TaskModule { outer =>
     }
   }
 
-  def scalacPluginClasspath: T[Seq[PathRef]] =
+  def scalacPluginClasspath: T[OSet[PathRef]] =
     resolveDeps(
       T.task{scalacPluginIvyDeps()}
     )()
@@ -126,7 +127,7 @@ trait Module extends mill.Module with TaskModule { outer =>
   /**
     * Classpath of the Scala Compiler & any compiler plugins
     */
-  def scalaCompilerClasspath: T[Seq[PathRef]] = T{
+  def scalaCompilerClasspath: T[OSet[PathRef]] = T{
     resolveDeps(
       T.task{scalaCompilerIvyDeps(scalaVersion()) ++ scalaRuntimeIvyDeps(scalaVersion())}
     )()
@@ -135,19 +136,19 @@ trait Module extends mill.Module with TaskModule { outer =>
   /**
     * Things that need to be on the classpath in order for this code to run
     */
-  def runDepClasspath: T[Seq[PathRef]] = T{
-    upstreamRunClasspath().flatten ++
+  def runDepClasspath: T[OSet[PathRef]] = T{
+    OSet.from(upstreamRunClasspath().flatten) ++
     depClasspath() ++
     resolveDeps(
-      T.task{ivyDeps() ++ runIvyDeps() ++ scalaRuntimeIvyDeps(scalaVersion())},
+      T.task{ivyDeps() ++ runIvyDeps() ++ scalaRuntimeIvyDeps(scalaVersion())}
     )()
   }
 
   def prependShellScript: T[String] = T{ "" }
 
-  def sources = T.input{ Seq(PathRef(basePath / 'src)) }
-  def resources = T.input{ Seq(PathRef(basePath / 'resources)) }
-  def generatedSources = T { Seq.empty[PathRef] }
+  def sources = T.input{ OSet(PathRef(basePath / 'src)) }
+  def resources = T.input{ OSet(PathRef(basePath / 'resources)) }
+  def generatedSources = T { OSet.empty[PathRef] }
   def allSources = T{ sources() ++ generatedSources() }
   def compile: T[CompilationResult] = T.persistent{
     compileScala(
@@ -203,11 +204,11 @@ trait Module extends mill.Module with TaskModule { outer =>
 
     subprocess(
       "scala.tools.nsc.ScalaDoc",
-      compileDepClasspath().filterNot(_.path.ext == "pom").map(_.path),
-      options = options
+      compileDepClasspath().filter(_.path.ext != "pom").map(_.path),
+      options = options.toSeq
     )
 
-    createJar(Seq(javadocDir))(outDir / "javadoc.jar")
+    createJar(OSet(javadocDir))(outDir / "javadoc.jar")
   }
 
   def sourcesJar = T {
@@ -283,7 +284,7 @@ trait TestModule extends Module with TaskModule {
       jvmOptions = forkArgs(),
       options = Seq(
         testFramework(),
-        runClasspath().map(_.path).distinct.mkString(" "),
+        runClasspath().map(_.path).mkString(" "),
         Seq(compile().classes.path).mkString(" "),
         args.mkString(" "),
         outputPath.toString,
@@ -301,7 +302,7 @@ trait TestModule extends Module with TaskModule {
     val (doneMsg, results) = TestRunner(
       testFramework(),
       runClasspath().map(_.path),
-      Seq(compile().classes.path),
+      OSet(compile().classes.path),
       args
     )
     TestModule.handleResults(doneMsg, results)
diff --git a/scalalib/src/main/scala/mill/scalalib/PublishModule.scala b/scalalib/src/main/scala/mill/scalalib/PublishModule.scala
index 1b1a0f44..211c94d2 100644
--- a/scalalib/src/main/scala/mill/scalalib/PublishModule.scala
+++ b/scalalib/src/main/scala/mill/scalalib/PublishModule.scala
@@ -3,7 +3,7 @@ package scalalib
 
 import ammonite.ops._
 import mill.eval.{PathRef, Result}
-
+import mill.util.Loose.OSet
 /**
   * Configuration necessary for publishing a Scala module to Maven Central or similar
   */
diff --git a/scalalib/src/main/scala/mill/scalalib/TestRunner.scala b/scalalib/src/main/scala/mill/scalalib/TestRunner.scala
index 0d1768f6..d129e0d4 100644
--- a/scalalib/src/main/scala/mill/scalalib/TestRunner.scala
+++ b/scalalib/src/main/scala/mill/scalalib/TestRunner.scala
@@ -8,7 +8,8 @@ import java.util.zip.ZipInputStream
 import ammonite.ops.{Path, ls, pwd}
 import ammonite.util.Colors
 import mill.util.Ctx.LogCtx
-import mill.util.PrintLogger
+import mill.util.{PrintLogger}
+import mill.util.Loose.OSet
 import sbt.testing._
 import upickle.Js
 import mill.util.JsonFormatters._
@@ -23,7 +24,7 @@ object TestRunner {
       Iterator.continually(zip.getNextEntry).takeWhile(_ != null).map(_.getName).filter(_.endsWith(".class"))
     }
   }
-  def runTests(cl: ClassLoader, framework: Framework, classpath: Seq[Path]) = {
+  def runTests(cl: ClassLoader, framework: Framework, classpath: OSet[Path]) = {
 
 
     val fingerprints = framework.fingerprints()
@@ -48,8 +49,8 @@ object TestRunner {
   def main(args: Array[String]): Unit = {
     val result = apply(
       frameworkName = args(0),
-      entireClasspath = args(1).split(" ").map(Path(_)),
-      testClassfilePath = args(2).split(" ").map(Path(_)),
+      entireClasspath = OSet.from(args(1).split(" ").map(Path(_))),
+      testClassfilePath = OSet.from(args(2).split(" ").map(Path(_))),
       args = args(3) match{ case "" => Nil case x => x.split(" ").toList }
     )(new PrintLogger(
       args(5) == "true",
@@ -69,8 +70,8 @@ object TestRunner {
     System.exit(0)
   }
   def apply(frameworkName: String,
-            entireClasspath: Seq[Path],
-            testClassfilePath: Seq[Path],
+            entireClasspath: OSet[Path],
+            testClassfilePath: OSet[Path],
             args: Seq[String])
            (implicit ctx: LogCtx): (String, Seq[Result]) = {
     val outerClassLoader = getClass.getClassLoader
diff --git a/scalalib/src/main/scala/mill/scalalib/publish/Ivy.scala b/scalalib/src/main/scala/mill/scalalib/publish/Ivy.scala
index ff21c9ac..288b63d0 100644
--- a/scalalib/src/main/scala/mill/scalalib/publish/Ivy.scala
+++ b/scalalib/src/main/scala/mill/scalalib/publish/Ivy.scala
@@ -1,5 +1,7 @@
 package mill.scalalib.publish
 
+import mill.util.Loose.OSet
+
 import scala.xml.PrettyPrinter
 
 object Ivy {
@@ -8,7 +10,7 @@ object Ivy {
 
   def apply(
       artifact: Artifact,
-      dependencies: Seq[Dependency]
+      dependencies: OSet[Dependency]
   ): String = {
     val xml =
       <ivy-module version="2.0" xmlns:e="http://ant.apache.org/ivy/extra">
diff --git a/scalalib/src/main/scala/mill/scalalib/publish/Pom.scala b/scalalib/src/main/scala/mill/scalalib/publish/Pom.scala
index 32ad036a..c1ce3e2a 100644
--- a/scalalib/src/main/scala/mill/scalalib/publish/Pom.scala
+++ b/scalalib/src/main/scala/mill/scalalib/publish/Pom.scala
@@ -1,5 +1,7 @@
 package mill.scalalib.publish
 
+import mill.util.Loose.OSet
+
 import scala.xml.{Elem, NodeSeq, PrettyPrinter}
 
 object Pom {
@@ -8,7 +10,7 @@ object Pom {
 
   //TODO - not only jar packaging support?
   def apply(artifact: Artifact,
-            dependencies: Seq[Dependency],
+            dependencies: OSet[Dependency],
             name: String,
             pomSettings: PomSettings): String = {
     val xml =
diff --git a/scalalib/src/test/scala/mill/scalalib/ResolveDepsTests.scala b/scalalib/src/test/scala/mill/scalalib/ResolveDepsTests.scala
index c5a05d02..c4404fe6 100644
--- a/scalalib/src/test/scala/mill/scalalib/ResolveDepsTests.scala
+++ b/scalalib/src/test/scala/mill/scalalib/ResolveDepsTests.scala
@@ -4,34 +4,35 @@ import coursier.Cache
 import coursier.maven.MavenRepository
 import mill.eval.Result.{Failure, Success}
 import mill.eval.{PathRef, Result}
+import mill.util.Loose.OSet
 import utest._
 
 object ResolveDepsTests extends TestSuite {
   val repos = Seq(Cache.ivy2Local, MavenRepository("https://repo1.maven.org/maven2"))
 
-  def evalDeps(deps: Seq[Dep]): Result[Seq[PathRef]] = Lib.resolveDependencies(repos, "2.12.4", "2.12", deps)
+  def evalDeps(deps: OSet[Dep]): Result[OSet[PathRef]] = Lib.resolveDependencies(repos, "2.12.4", "2.12", deps)
 
   val tests = Tests {
     'resolveValidDeps - {
-      val deps = Seq(Dep("com.lihaoyi", "pprint", "0.5.3"))
+      val deps = OSet(Dep("com.lihaoyi", "pprint", "0.5.3"))
       val Success(paths) = evalDeps(deps)
       assert(paths.nonEmpty)
     }
 
     'errOnInvalidOrgDeps - {
-      val deps = Seq(Dep("xxx.yyy.invalid", "pprint", "0.5.3"))
+      val deps = OSet(Dep("xxx.yyy.invalid", "pprint", "0.5.3"))
       val Failure(errMsg) = evalDeps(deps)
       assert(errMsg.contains("xxx.yyy.invalid"))
     }
 
     'errOnInvalidVersionDeps - {
-      val deps = Seq(Dep("com.lihaoyi", "pprint", "invalid.version.num"))
+      val deps = OSet(Dep("com.lihaoyi", "pprint", "invalid.version.num"))
       val Failure(errMsg) = evalDeps(deps)
       assert(errMsg.contains("invalid.version.num"))
     }
 
     'errOnPartialSuccess - {
-      val deps = Seq(Dep("com.lihaoyi", "pprint", "0.5.3"), Dep("fake", "fake", "fake"))
+      val deps = OSet(Dep("com.lihaoyi", "pprint", "0.5.3"), Dep("fake", "fake", "fake"))
       val Failure(errMsg) = evalDeps(deps)
       assert(errMsg.contains("fake"))
     }
diff --git a/shared.sc b/shared.sc
index ef2f26d3..a475a449 100644
--- a/shared.sc
+++ b/shared.sc
@@ -74,7 +74,7 @@ def generateEval(dir: Path) = {
     s"""package mill.main
        |import mill.eval.Evaluator
        |import mill.define.Task
-       |import mill.util.OSet
+       |import mill.util.Strict.OSet
        |class EvalGenerated(evaluator: Evaluator[_]) {
        |  type TT[+X] = Task[X]
        |  ${(1 to 22).map(generate).mkString("\n")}
-- 
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