Merge pull request #1219 from dotty-staging/fix-strawmans

Fix strawmans

11 files changed, 654 insertions, 46 deletions

diff --git a/src/dotty/tools/dotc/ast/Desugar.scala b/src/dotty/tools/dotc/ast/Desugar.scala
index ac2f8ae73..b6a4dc5e7 100644
--- a/src/dotty/tools/dotc/ast/Desugar.scala
+++ b/src/dotty/tools/dotc/ast/Desugar.scala
@@ -256,7 +256,21 @@ object desugar {
     // prefixed by type or val). `tparams` and `vparamss` are the type parameters that
     // go in `constr`, the constructor after desugaring.
 
+    /** Does `tree' look like a reference to AnyVal? Temporary test before we have inline classes */
+    def isAnyVal(tree: Tree): Boolean = tree match {
+      case Ident(tpnme.AnyVal) => true
+      case Select(qual, tpnme.AnyVal) => isScala(qual)
+      case _ => false
+    }
+    def isScala(tree: Tree): Boolean = tree match {
+      case Ident(nme.scala_) => true
+      case Select(Ident(nme.ROOTPKG), nme.scala_) => true
+      case _ => false
+    }
+
     val isCaseClass = mods.is(Case) && !mods.is(Module)
+    val isValueClass = parents.nonEmpty && isAnyVal(parents.head)
+      // This is not watertight, but `extends AnyVal` will be replaced by `inline` later.
 
     val constrTparams = constr1.tparams map toDefParam
     val constrVparamss =
@@ -398,7 +412,9 @@ object desugar {
         companionDefs(parent, applyMeths ::: unapplyMeth :: defaultGetters)
       }
       else if (defaultGetters.nonEmpty)
-          companionDefs(anyRef, defaultGetters)
+        companionDefs(anyRef, defaultGetters)
+      else if (isValueClass)
+        companionDefs(anyRef, Nil)
       else Nil
 
 
diff --git a/src/dotty/tools/dotc/core/NameOps.scala b/src/dotty/tools/dotc/core/NameOps.scala
index 81240a9fc..72f89aec3 100644
--- a/src/dotty/tools/dotc/core/NameOps.scala
+++ b/src/dotty/tools/dotc/core/NameOps.scala
@@ -116,7 +116,12 @@ object NameOps {
       name.drop(tpnme.hkArgPrefixLength).toString.toInt
 
     def isLambdaTraitName(implicit ctx: Context): Boolean =
-      name.startsWith(tpnme.hkLambdaPrefix)
+      name.isTypeName && name.startsWith(tpnme.hkLambdaPrefix)
+
+    def lambdaTraitVariances(implicit ctx: Context): List[Int] = {
+      val vs = name.drop(tpnme.hkLambdaPrefix.length)
+      vs.toList.map(c => tpnme.varianceSuffixes.indexOf(c) - 1)
+    }
 
     /** If the name ends with $nn where nn are
       * all digits, strip the $ and the digits.
@@ -179,7 +184,13 @@ object NameOps {
      *  an encoded name, e.g. super$$plus$eq. See #765.
      */
     def unexpandedName: N = {
-      val idx = name.lastIndexOfSlice(nme.EXPAND_SEPARATOR)
+      var idx = name.lastIndexOfSlice(nme.EXPAND_SEPARATOR)
+
+      // Hack to make super accessors from traits work. They would otherwise fail because of #765
+      // TODO: drop this once we have more robust name handling
+      if (name.slice(idx - FalseSuperLength, idx) == FalseSuper)
+        idx -= FalseSuper.length
+
       if (idx < 0) name else (name drop (idx + nme.EXPAND_SEPARATOR.length)).asInstanceOf[N]
     }
 
@@ -431,4 +442,7 @@ object NameOps {
       name.dropRight(nme.LAZY_LOCAL.length)
     }
   }
+
+  private final val FalseSuper = "$$super".toTermName
+  private val FalseSuperLength = FalseSuper.length
 }
diff --git a/src/dotty/tools/dotc/core/Names.scala b/src/dotty/tools/dotc/core/Names.scala
index f1e6f7606..10eef16c1 100644
--- a/src/dotty/tools/dotc/core/Names.scala
+++ b/src/dotty/tools/dotc/core/Names.scala
@@ -37,7 +37,7 @@ object Names {
    */
   abstract class Name extends DotClass
     with PreName
-    with Seq[Char]
+    with collection.immutable.Seq[Char]
     with IndexedSeqOptimized[Char, Name] {
 
     /** A type for names of the same kind as this name */
diff --git a/src/dotty/tools/dotc/core/TypeApplications.scala b/src/dotty/tools/dotc/core/TypeApplications.scala
index d5f44e632..3ed1798ed 100644
--- a/src/dotty/tools/dotc/core/TypeApplications.scala
+++ b/src/dotty/tools/dotc/core/TypeApplications.scala
@@ -377,6 +377,14 @@ class TypeApplications(val self: Type) extends AnyVal {
       false
   }
 
+  /** Dealias type if it can be done without forcing anything */
+  def safeDealias(implicit ctx: Context): Type = self match {
+    case self: TypeRef if self.denot.exists && self.symbol.isAliasType =>
+      self.info.bounds.hi.stripTypeVar.safeDealias
+    case _ =>
+      self
+  }
+
   /** Replace references to type parameters with references to hk arguments `this.$hk_i`
    * Care is needed not to cause cyclic reference errors, hence `SafeSubstMap`.
    */
@@ -546,8 +554,8 @@ class TypeApplications(val self: Type) extends AnyVal {
         substHkArgs(body)
       case self: PolyType =>
         self.instantiate(args)
-      case _ =>
-        appliedTo(args, typeParams)
+      case self1 =>
+        self1.safeDealias.appliedTo(args, typeParams)
     }
   }
 
diff --git a/src/dotty/tools/dotc/core/Types.scala b/src/dotty/tools/dotc/core/Types.scala
index fa049815a..913339409 100644
--- a/src/dotty/tools/dotc/core/Types.scala
+++ b/src/dotty/tools/dotc/core/Types.scala
@@ -903,7 +903,9 @@ object Types {
         case pre: RefinedType =>
           object instantiate extends TypeMap {
             var isSafe = true
-            def apply(tp: Type): Type = tp match {
+            def apply(tp: Type): Type =
+              if (!isSafe) tp
+              else tp match {
               case TypeRef(RefinedThis(`pre`), name) if name.isHkArgName =>
                 member(name).info match {
                   case TypeAlias(alias) => alias
@@ -2061,7 +2063,7 @@ object Types {
         false
     }
     override def computeHash = doHash(refinedName, refinedInfo, parent)
-    override def toString = s"RefinedType($parent, $refinedName, $refinedInfo)"
+    override def toString = s"RefinedType($parent, $refinedName, $refinedInfo | $hashCode)"
   }
 
   class CachedRefinedType(parent: Type, refinedName: Name, infoFn: RefinedType => Type) extends RefinedType(parent, refinedName) {
diff --git a/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala b/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala
index ad6ddf7fe..535ddd216 100644
--- a/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala
+++ b/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala
@@ -252,6 +252,8 @@ class TreeUnpickler(reader: TastyReader, tastyName: TastyName.Table) {
           readPackageRef().termRef
         case TYPEREF =>
           val name =  readName().toTypeName
+          if (name.isLambdaTraitName) // Make sure curresponding lambda trait exists
+            defn.LambdaTrait(name.lambdaTraitVariances)
           TypeRef(readType(), name)
         case TERMREF =>
           readNameSplitSig() match {
diff --git a/src/dotty/tools/dotc/transform/ExtensionMethods.scala b/src/dotty/tools/dotc/transform/ExtensionMethods.scala
index a1d2e5c68..8d61cef42 100644
--- a/src/dotty/tools/dotc/transform/ExtensionMethods.scala
+++ b/src/dotty/tools/dotc/transform/ExtensionMethods.scala
@@ -33,7 +33,7 @@ import SymUtils._
  * This is different from the implementation of value classes in Scala 2
  * (see SIP-15) which uses `asInstanceOf` which does not typecheck.
  */
-class ExtensionMethods extends MiniPhaseTransform with DenotTransformer with FullParameterization with NeedsCompanions { thisTransformer =>
+class ExtensionMethods extends MiniPhaseTransform with DenotTransformer with FullParameterization { thisTransformer =>
 
   import tpd._
   import ExtensionMethods._
@@ -45,10 +45,6 @@ class ExtensionMethods extends MiniPhaseTransform with DenotTransformer with Ful
 
   override def runsAfterGroupsOf = Set(classOf[FirstTransform]) // need companion objects to exist
 
-  def isCompanionNeeded(cls: ClassSymbol)(implicit ctx: Context): Boolean = {
-    isDerivedValueClass(cls)
-  }
-
   override def transform(ref: SingleDenotation)(implicit ctx: Context): SingleDenotation = ref match {
     case moduleClassSym: ClassDenotation if moduleClassSym is ModuleClass =>
       moduleClassSym.linkedClass match {
diff --git a/src/dotty/tools/dotc/transform/FirstTransform.scala b/src/dotty/tools/dotc/transform/FirstTransform.scala
index 37ae1d94e..97dd9404f 100644
--- a/src/dotty/tools/dotc/transform/FirstTransform.scala
+++ b/src/dotty/tools/dotc/transform/FirstTransform.scala
@@ -44,7 +44,7 @@ class FirstTransform extends MiniPhaseTransform with IdentityDenotTransformer wi
     this
   }
 
-  def transformInfo(tp: Type, sym: Symbol)(implicit ctx: Context): Type = {
+  def transformInfo(tp: Type, sym: Symbol)(implicit ctx: Context): Type = tp/*{
       tp match {
         //create companions for value classes that are not from currently compiled source file
         case tp@ClassInfo(_, cls, _, decls, _)
@@ -59,7 +59,7 @@ class FirstTransform extends MiniPhaseTransform with IdentityDenotTransformer wi
         case _ => tp
       }
   }
-
+*/
   override def checkPostCondition(tree: Tree)(implicit ctx: Context): Unit = {
     tree match {
       case Select(qual, _) if tree.symbol.exists =>
diff --git a/src/dotty/tools/dotc/transform/SyntheticMethods.scala b/src/dotty/tools/dotc/transform/SyntheticMethods.scala
index a496f80ce..9dfd92fe9 100644
--- a/src/dotty/tools/dotc/transform/SyntheticMethods.scala
+++ b/src/dotty/tools/dotc/transform/SyntheticMethods.scala
@@ -10,6 +10,7 @@ import DenotTransformers._
 import ast.Trees._
 import ast.untpd
 import Decorators._
+import NameOps._
 import ValueClasses.isDerivedValueClass
 import scala.collection.mutable.ListBuffer
 import scala.language.postfixOps
@@ -79,14 +80,17 @@ class SyntheticMethods(thisTransformer: DenotTransformer) {
       def forwardToRuntime(vrefss: List[List[Tree]]): Tree =
         ref(defn.runtimeMethodRef("_" + sym.name.toString)).appliedToArgs(This(clazz) :: vrefss.head)
 
+      def ownName(vrefss: List[List[Tree]]): Tree =
+        Literal(Constant(clazz.name.stripModuleClassSuffix.decode.toString))
+
       def syntheticRHS(implicit ctx: Context): List[List[Tree]] => Tree = synthetic.name match {
         case nme.hashCode_ if isDerivedValueClass(clazz) => vrefss => valueHashCodeBody
         case nme.hashCode_ => vrefss => caseHashCodeBody
-        case nme.toString_ => forwardToRuntime
+        case nme.toString_ => if (clazz.is(ModuleClass)) ownName else forwardToRuntime
         case nme.equals_ => vrefss => equalsBody(vrefss.head.head)
         case nme.canEqual_ => vrefss => canEqualBody(vrefss.head.head)
         case nme.productArity => vrefss => Literal(Constant(accessors.length))
-        case nme.productPrefix => vrefss => Literal(Constant(clazz.name.decode.toString))
+        case nme.productPrefix => ownName
       }
       ctx.log(s"adding $synthetic to $clazz at ${ctx.phase}")
       DefDef(synthetic, syntheticRHS(ctx.withOwner(synthetic)))
diff --git a/src/strawman/collections/CollectionStrawMan4.scala b/src/strawman/collections/CollectionStrawMan4.scala
index 9159b1cfc..874f67a2d 100644
--- a/src/strawman/collections/CollectionStrawMan4.scala
+++ b/src/strawman/collections/CollectionStrawMan4.scala
@@ -2,6 +2,8 @@ package strawman.collections
 
 import Predef.{augmentString => _, wrapString => _, _}
 import scala.reflect.ClassTag
+import annotation.unchecked.uncheckedVariance
+import annotation.tailrec
 
 /** A strawman architecture for new collections. It contains some
  *  example collection classes and methods with the intent to expose
@@ -20,14 +22,7 @@ object CollectionStrawMan4 {
     def iterator: Iterator[A]
   }
 
-  /** Base trait for generic collections */
-  trait Iterable[+A] extends IterableOnce[A] with FromIterable[Iterable] {
-    def iterator: Iterator[A]
-    def view: View[A] = View.fromIterator(iterator)
-    def knownLength: Int = -1
-  }
-
-  /** Base trait for instances that can construct a collection from an iterator */
+  /** Base trait for instances that can construct a collection from an iterable */
   trait FromIterable[+C[X] <: Iterable[X]] {
     def fromIterable[B](v: Iterable[B]): C[B]
   }
@@ -38,16 +33,27 @@ object CollectionStrawMan4 {
     def apply[A](xs: A*): C[A] = fromIterable(View.Elems(xs: _*))
   }
 
+  /** Base trait for generic collections */
+  trait Iterable[+A] extends IterableOnce[A] with FromIterable[Iterable] {
+    def view: View[A] = View.fromIterator(iterator) // view is overridden, cannot be defined in ops
+    def knownLength: Int = -1
+  }
+
   /** Base trait for sequence collections */
   trait Seq[+A] extends Iterable[A] with FromIterable[Seq] {
     def apply(i: Int): A
     def length: Int
   }
 
+  /** Base trait for collection builders */
   trait Builder[-A, +To] {
     def +=(x: A): this.type
-    def ++=(xs: IterableOnce[A]): Unit = xs.iterator.foreach(+=)
     def result: To
+
+    def ++=(xs: IterableOnce[A]): this.type = {
+      xs.iterator.foreach(+=)
+      this
+    }
   }
 
   /* ------------ Operations ----------------------------------- */
@@ -134,17 +140,18 @@ object CollectionStrawMan4 {
       require(!isEmpty)
       if (i == 0) head else tail.apply(i - 1)
     }
-    def :::[B >: A](prefix: List[B]): List[B] =
-      if (prefix.isEmpty) this
-      else Cons(prefix.head, prefix.tail ::: this)
     def length: Int =
       if (isEmpty) 0 else 1 + tail.length
+    def ++:[B >: A](prefix: List[B]): List[B] =
+      if (prefix.isEmpty) this
+      else Cons(prefix.head, prefix.tail ++: this)
   }
 
-  case class Cons[+A](x: A, xs: List[A]) extends List[A] {
+  case class Cons[+A](x: A, private[collections] var next: List[A @uncheckedVariance]) // sound because `next` is used only locally
+  extends List[A] {
     def isEmpty = false
     def head = x
-    def tail = xs
+    def tail = next
   }
 
   case object Nil extends List[Nothing] {
@@ -157,20 +164,64 @@ object CollectionStrawMan4 {
     def fromIterator[B](it: Iterator[B]): List[B] =
       if (it.hasNext) Cons(it.next, fromIterator(it)) else Nil
     def fromIterable[B](c: Iterable[B]): List[B] = c match {
-      case View.Concat(xs, ys: Iterable[B]) =>
-        fromIterable(xs) ::: fromIterable(ys)
+      case View.Concat(xs, ys: List[B]) =>
+        fromIterable(xs) ++: ys
       case View.Drop(xs: List[B], n) =>
-        var i = 0
-        var ys = xs
-        while (i < n && !xs.isEmpty) {
-          ys = ys.tail
-          i += 1
-        }
-        ys
+        @tailrec def loop(xs: List[B], n: Int): List[B] =
+          if (n > 0) loop(xs.tail, n - 1) else xs
+        loop(xs, n)
+      case c: List[B] => c
       case _ => fromIterator(c.iterator)
     }
   }
 
+  /** Concrete collection type: ListBuffer */
+  class ListBuffer[A] extends Seq[A] with FromIterable[ListBuffer] with Builder[A, List[A]] {
+    private var first, last: List[A] = Nil
+    private var aliased = false
+    def iterator = first.iterator
+    def fromIterable[B](coll: Iterable[B]) = ListBuffer.fromIterable(coll)
+    def apply(i: Int) = first.apply(i)
+    def length = first.length
+
+    private def copyElems(): Unit = {
+      val buf = ListBuffer.fromIterable(result)
+      first = buf.first
+      last = buf.last
+      aliased = false
+    }
+    def result = {
+      aliased = true
+      first
+    }
+    def +=(elem: A) = {
+      if (aliased) copyElems()
+      val last1 = Cons(elem, Nil)
+      last match {
+        case last: Cons[A] => last.next = last1
+        case _ => first = last1
+      }
+      last = last1
+      this
+    }
+    override def toString: String =
+      if (first.isEmpty) "ListBuffer()"
+      else {
+        val b = new StringBuilder("ListBuffer(").append(first.head)
+        first.tail.foldLeft(b)(_.append(", ").append(_)).append(")").toString
+      }
+  }
+
+  object ListBuffer extends IterableFactory[ListBuffer] {
+    def fromIterable[B](coll: Iterable[B]): ListBuffer[B] = coll match {
+      case pd @ View.Partitioned(partition: View.Partition[B]) =>
+        partition.distribute(new ListBuffer[B]())
+        pd.forced.get.asInstanceOf[ListBuffer[B]]
+      case _ =>
+        new ListBuffer[B] ++= coll
+    }
+  }
+
   /** Concrete collection type: ArrayBuffer */
   class ArrayBuffer[A] private (initElems: Array[AnyRef], initLength: Int)
   extends Seq[A] with FromIterable[ArrayBuffer] with Builder[A, ArrayBuffer[A]] {
@@ -234,12 +285,6 @@ object CollectionStrawMan4 {
     def apply(n: Int) = elems(start + n).asInstanceOf[A]
   }
 
-  case class StringView(s: String) extends RandomAccessView[Char] {
-    val start = 0
-    val end = s.length
-    def apply(n: Int) = s.charAt(n)
-  }
-
   /** Concrete collection type: String */
   implicit class StringOps(val s: String) extends AnyVal with Ops[Char] {
     def iterator: Iterator[Char] = new StringView(s).iterator
@@ -277,6 +322,12 @@ object CollectionStrawMan4 {
     def ++(xs: String): String = s + xs
   }
 
+  case class StringView(s: String) extends RandomAccessView[Char] {
+    val start = 0
+    val end = s.length
+    def apply(n: Int) = s.charAt(n)
+  }
+
   /* ------------ Views --------------------------------------- */
 
   /** A lazy iterable */
@@ -322,6 +373,8 @@ object CollectionStrawMan4 {
     }
     case class Partition[A](val underlying: Iterable[A], p: A => Boolean) {
       val left, right = Partitioned(this)
+      // `distribute` makes up for the lack of generic push-based functionality.
+      // It forces both halves of the partition with a given builder.
       def distribute(bf: => Builder[A, Iterable[A]]) = {
         val lb, rb = bf
         val it = underlying.iterator
diff --git a/src/strawman/collections/CollectionStrawMan5.scala b/src/strawman/collections/CollectionStrawMan5.scala
new file mode 100644
index 000000000..1a89d9659
--- /dev/null
+++ b/src/strawman/collections/CollectionStrawMan5.scala
@@ -0,0 +1,513 @@
+package strawman.collections
+
+import Predef.{augmentString => _, wrapString => _, _}
+import scala.reflect.ClassTag
+import annotation.unchecked.uncheckedVariance
+import annotation.tailrec
+
+/** A strawman architecture for new collections. It contains some
+ *  example collection classes and methods with the intent to expose
+ *  some key issues. It would be good to compare this to other
+ *  implementations of the same functionality, to get an idea of the
+ *  strengths and weaknesses of different collection architectures.
+ *
+ *  For a test file, see tests/run/CollectionTests.scala.
+ */
+object CollectionStrawMan5 {
+
+  /* ------------ Base Traits -------------------------------- */
+
+  /** Iterator can be used only once */
+  trait IterableOnce[+A] {
+    def iterator: Iterator[A]
+  }
+
+  /** Base trait for instances that can construct a collection from an iterable */
+  trait FromIterable[+C[X] <: Iterable[X]] {
+    def fromIterable[B](it: Iterable[B]): C[B]
+  }
+
+  /** Base trait for companion objects of collections */
+  trait IterableFactory[+C[X] <: Iterable[X]] extends FromIterable[C] {
+    def empty[X]: C[X] = fromIterable(View.Empty)
+    def apply[A](xs: A*): C[A] = fromIterable(View.Elems(xs: _*))
+  }
+
+  /** Base trait for generic collections */
+  trait Iterable[+A] extends IterableOnce[A] with IterableLike[A, Iterable] {
+    protected def coll: Iterable[A] = this
+    def knownLength: Int = -1
+  }
+
+  /** Base trait for sequence collections */
+  trait Seq[+A] extends Iterable[A] with SeqLike[A, Seq] {
+    def apply(i: Int): A
+    def length: Int
+  }
+
+  /** Base trait for strict collections */
+  trait Buildable[+A, +To <: Iterable[A]] extends Iterable[A] {
+    protected[this] def newBuilder: Builder[A, To]
+    override def partition(p: A => Boolean): (To, To) = {
+      val l, r = newBuilder
+      iterator.foreach(x => (if (p(x)) l else r) += x)
+      (l.result, r.result)
+    }
+    // one might also override other transforms here to avoid generating
+    // iterators if it helps efficiency.
+  }
+
+  /** Base trait for collection builders */
+  trait Builder[-A, +To] {
+    def +=(x: A): this.type
+    def result: To
+
+    def ++=(xs: IterableOnce[A]): this.type = {
+      xs.iterator.foreach(+=)
+      this
+    }
+  }
+
+  /* ------------ Operations ----------------------------------- */
+
+  /** Base trait for Iterable operations
+   *
+   *  VarianceNote
+   *  ============
+   *
+   *  We require that for all child classes of Iterable the variance of
+   *  the child class and the variance of the `C` parameter passed to `IterableLike`
+   *  are the same. We cannot express this since we lack variance polymorphism. That's
+   *  why we have to resort at some places to write `C[A @uncheckedVariance]`.
+   *
+   */
+  trait IterableLike[+A, +C[X] <: Iterable[X]]
+    extends FromIterable[C]
+       with IterableOps[A]
+       with IterableMonoTransforms[A, C[A @uncheckedVariance]] // sound bcs of VarianceNote
+       with IterablePolyTransforms[A, C] {
+    protected[this] def fromLikeIterable(coll: Iterable[A]): C[A] = fromIterable(coll)
+  }
+
+  /** Base trait for Seq operations */
+  trait SeqLike[+A, +C[X] <: Seq[X]]
+  extends IterableLike[A, C] with SeqMonoTransforms[A, C[A @uncheckedVariance]] // sound bcs of VarianceNote
+
+  trait IterableOps[+A] extends Any {
+    def iterator: Iterator[A]
+    def foreach(f: A => Unit): Unit = iterator.foreach(f)
+    def foldLeft[B](z: B)(op: (B, A) => B): B = iterator.foldLeft(z)(op)
+    def foldRight[B](z: B)(op: (A, B) => B): B = iterator.foldRight(z)(op)
+    def indexWhere(p: A => Boolean): Int = iterator.indexWhere(p)
+    def isEmpty: Boolean = !iterator.hasNext
+    def head: A = iterator.next
+    def view: View[A] = View.fromIterator(iterator)
+  }
+
+  trait IterableMonoTransforms[+A, +Repr] extends Any {
+    protected def coll: Iterable[A]
+    protected[this] def fromLikeIterable(coll: Iterable[A]): Repr
+    def filter(p: A => Boolean): Repr = fromLikeIterable(View.Filter(coll, p))
+    def partition(p: A => Boolean): (Repr, Repr) = {
+      val pn = View.Partition(coll, p)
+      (fromLikeIterable(pn.left), fromLikeIterable(pn.right))
+    }
+    def drop(n: Int): Repr = fromLikeIterable(View.Drop(coll, n))
+    def to[C[X] <: Iterable[X]](fi: FromIterable[C]): C[A @uncheckedVariance] =
+      // variance seems sound because `to` could just as well have been added
+      // as a decorator. We should investigate this further to be sure.
+      fi.fromIterable(coll)
+  }
+
+  trait IterablePolyTransforms[+A, +C[A]] extends Any {
+    protected def coll: Iterable[A]
+    def fromIterable[B](coll: Iterable[B]): C[B]
+    def map[B](f: A => B): C[B] = fromIterable(View.Map(coll, f))
+    def flatMap[B](f: A => IterableOnce[B]): C[B] = fromIterable(View.FlatMap(coll, f))
+    def ++[B >: A](xs: IterableOnce[B]): C[B] = fromIterable(View.Concat(coll, xs))
+    def zip[B](xs: IterableOnce[B]): C[(A @uncheckedVariance, B)] = fromIterable(View.Zip(coll, xs))
+       // sound bcs of VarianceNote
+  }
+
+  trait SeqMonoTransforms[+A, +Repr] extends Any with IterableMonoTransforms[A, Repr] {
+    def reverse: Repr = {
+      var xs: List[A] = Nil
+      var it = coll.iterator
+      while (it.hasNext) xs = new Cons(it.next, xs)
+      fromLikeIterable(xs)
+    }
+  }
+
+  /* --------- Concrete collection types ------------------------------- */
+
+  /** Concrete collection type: List */
+  sealed trait List[+A] extends Seq[A] with SeqLike[A, List] with Buildable[A, List[A]] { self =>
+    def isEmpty: Boolean
+    def head: A
+    def tail: List[A]
+    def iterator = new Iterator[A] {
+      private[this] var current = self
+      def hasNext = !current.isEmpty
+      def next = { val r = current.head; current = current.tail; r }
+    }
+    def fromIterable[B](c: Iterable[B]): List[B] = List.fromIterable(c)
+    def apply(i: Int): A = {
+      require(!isEmpty)
+      if (i == 0) head else tail.apply(i - 1)
+    }
+    def length: Int =
+      if (isEmpty) 0 else 1 + tail.length
+    protected[this] def newBuilder = new ListBuffer[A]
+    def ++:[B >: A](prefix: List[B]): List[B] =
+      if (prefix.isEmpty) this
+      else Cons(prefix.head, prefix.tail ++: this)
+    override def ++[B >: A](xs: IterableOnce[B]): List[B] = xs match {
+      case xs: List[B] => this ++: xs
+      case _ => super.++(xs)
+    }
+    @tailrec final override def drop(n: Int) =
+      if (n > 0) tail.drop(n - 1) else this
+  }
+
+  case class Cons[+A](x: A, private[collections] var next: List[A @uncheckedVariance]) // sound because `next` is used only locally
+  extends List[A] {
+    override def isEmpty = false
+    override def head = x
+    def tail = next
+  }
+
+  case object Nil extends List[Nothing] {
+    override def isEmpty = true
+    override def head = ???
+    def tail = ???
+  }
+
+  object List extends IterableFactory[List] {
+    def fromIterable[B](coll: Iterable[B]): List[B] = coll match {
+      case coll: List[B] => coll
+      case _ => ListBuffer.fromIterable(coll).result
+    }
+  }
+
+  /** Concrete collection type: ListBuffer */
+  class ListBuffer[A] extends Seq[A] with SeqLike[A, ListBuffer] with Builder[A, List[A]] {
+    private var first, last: List[A] = Nil
+    private var aliased = false
+    def iterator = first.iterator
+    def fromIterable[B](coll: Iterable[B]) = ListBuffer.fromIterable(coll)
+    def apply(i: Int) = first.apply(i)
+    def length = first.length
+
+    private def copyElems(): Unit = {
+      val buf = ListBuffer.fromIterable(result)
+      first = buf.first
+      last = buf.last
+      aliased = false
+    }
+    def result = {
+      aliased = true
+      first
+    }
+    def +=(elem: A) = {
+      if (aliased) copyElems()
+      val last1 = Cons(elem, Nil)
+      last match {
+        case last: Cons[A] => last.next = last1
+        case _ => first = last1
+      }
+      last = last1
+      this
+    }
+    override def toString: String =
+      if (first.isEmpty) "ListBuffer()"
+      else {
+        val b = new StringBuilder("ListBuffer(").append(first.head)
+        first.tail.foldLeft(b)(_.append(", ").append(_)).append(")").toString
+      }
+  }
+
+  object ListBuffer extends IterableFactory[ListBuffer] {
+    def fromIterable[B](coll: Iterable[B]): ListBuffer[B] = new ListBuffer[B] ++= coll
+  }
+
+  /** Concrete collection type: ArrayBuffer */
+  class ArrayBuffer[A] private (initElems: Array[AnyRef], initLength: Int)
+  extends Seq[A] with SeqLike[A, ArrayBuffer] with Builder[A, ArrayBuffer[A]] {
+    def this() = this(new Array[AnyRef](16), 0)
+    private var elems: Array[AnyRef] = initElems
+    private var start = 0
+    private var end = initLength
+    def apply(n: Int) = elems(start + n).asInstanceOf[A]
+    def length = end - start
+    override def knownLength = length
+    override def view = new ArrayBufferView(elems, start, end)
+    def iterator = view.iterator
+    def fromIterable[B](it: Iterable[B]): ArrayBuffer[B] =
+      ArrayBuffer.fromIterable(it)
+    def +=(elem: A): this.type = {
+      if (end == elems.length) {
+        if (start > 0) {
+          Array.copy(elems, start, elems, 0, length)
+          end -= start
+          start = 0
+        }
+        else {
+          val newelems = new Array[AnyRef](end * 2)
+          Array.copy(elems, 0, newelems, 0, end)
+          elems = newelems
+        }
+      }
+      elems(end) = elem.asInstanceOf[AnyRef]
+      end += 1
+      this
+    }
+    def result = this
+    def trimStart(n: Int): Unit = start += (n max 0)
+    override def ++[B >: A](xs: IterableOnce[B]): ArrayBuffer[B] = xs match {
+      case xs: ArrayBuffer[B] =>
+        val elems = new Array[AnyRef](length + xs.length)
+        Array.copy(this.elems, this.start, elems, 0, this.length)
+        Array.copy(xs.elems, xs.start, elems, this.length, xs.length)
+        new ArrayBuffer(elems, elems.length)
+      case _ => super.++(xs)
+    }
+
+    override def toString = s"ArrayBuffer(${elems.slice(start, end).mkString(", ")})"
+  }
+
+  object ArrayBuffer extends IterableFactory[ArrayBuffer] {
+    def fromIterable[B](coll: Iterable[B]): ArrayBuffer[B] =
+      if (coll.knownLength >= 0) {
+        val elems = new Array[AnyRef](coll.knownLength)
+        val it = coll.iterator
+        for (i <- 0 until elems.length) elems(i) = it.next().asInstanceOf[AnyRef]
+        new ArrayBuffer[B](elems, elems.length)
+      }
+      else {
+        val buf = new ArrayBuffer[B]
+        val it = coll.iterator
+        while (it.hasNext) buf += it.next()
+        buf
+      }
+  }
+
+  class ArrayBufferView[A](val elems: Array[AnyRef], val start: Int, val end: Int) extends RandomAccessView[A] {
+    def apply(n: Int) = elems(start + n).asInstanceOf[A]
+  }
+
+  /** Concrete collection type: String */
+  implicit class StringOps(val s: String)
+  extends AnyVal with IterableOps[Char]
+     with SeqMonoTransforms[Char, String]
+     with IterablePolyTransforms[Char, List] {
+    protected def coll = new StringView(s)
+    def iterator = coll.iterator
+    protected def fromLikeIterable(coll: Iterable[Char]): String = {
+      val sb = new StringBuilder
+      for (ch <- coll) sb.append(ch)
+      sb.toString
+    }
+    def fromIterable[B](coll: Iterable[B]): List[B] = List.fromIterable(coll)
+    def map(f: Char => Char): String = {
+      val sb = new StringBuilder
+      for (ch <- s) sb.append(f(ch))
+      sb.toString
+    }
+    def flatMap(f: Char => String): String = {
+      val sb = new StringBuilder
+      for (ch <- s) sb.append(f(ch))
+      sb.toString
+    }
+    def ++(xs: IterableOnce[Char]): String = {
+      val sb = new StringBuilder(s)
+      for (ch <- xs.iterator) sb.append(ch)
+      sb.toString
+    }
+    def ++(xs: String): String = s + xs
+  }
+
+  case class StringView(s: String) extends RandomAccessView[Char] {
+    val start = 0
+    val end = s.length
+    def apply(n: Int) = s.charAt(n)
+  }
+
+/* ---------- Views -------------------------------------------------------*/
+
+  /** Concrete collection type: View */
+  trait View[+A] extends Iterable[A] with IterableLike[A, View] {
+    override def view = this
+    override def fromIterable[B](c: Iterable[B]): View[B] = c match {
+      case c: View[B] => c
+      case _ => View.fromIterator(c.iterator)
+    }
+  }
+
+  /** View defined in terms of indexing a range */
+  trait RandomAccessView[+A] extends View[A] {
+    def start: Int
+    def end: Int
+    def apply(i: Int): A
+    def iterator: Iterator[A] = new Iterator[A] {
+      private var current = start
+      def hasNext = current < end
+      def next: A = {
+        val r = apply(current)
+        current += 1
+        r
+      }
+    }
+    override def knownLength = end - start max 0
+  }
+
+  object View {
+    def fromIterator[A](it: => Iterator[A]): View[A] = new View[A] {
+      def iterator = it
+    }
+    case object Empty extends View[Nothing] {
+      def iterator = Iterator.empty
+      override def knownLength = 0
+    }
+    case class Elems[A](xs: A*) extends View[A] {
+      def iterator = Iterator(xs: _*)
+      override def knownLength = xs.length
+    }
+    case class Filter[A](val underlying: Iterable[A], p: A => Boolean) extends View[A] {
+      def iterator = underlying.iterator.filter(p)
+    }
+    case class Partition[A](val underlying: Iterable[A], p: A => Boolean) {
+      val left = Partitioned(this, true)
+      val right = Partitioned(this, false)
+    }
+    case class Partitioned[A](partition: Partition[A], cond: Boolean) extends View[A] {
+      def iterator = partition.underlying.iterator.filter(x => partition.p(x) == cond)
+    }
+    case class Drop[A](underlying: Iterable[A], n: Int) extends View[A] {
+      def iterator = underlying.iterator.drop(n)
+      override def knownLength =
+        if (underlying.knownLength >= 0) underlying.knownLength - n max 0 else -1
+    }
+    case class Map[A, B](underlying: Iterable[A], f: A => B) extends View[B] {
+      def iterator = underlying.iterator.map(f)
+      override def knownLength = underlying.knownLength
+    }
+    case class FlatMap[A, B](underlying: Iterable[A], f: A => IterableOnce[B]) extends View[B] {
+      def iterator = underlying.iterator.flatMap(f)
+    }
+    case class Concat[A](underlying: Iterable[A], other: IterableOnce[A]) extends View[A] {
+      def iterator = underlying.iterator ++ other
+      override def knownLength = other match {
+        case other: Iterable[_] if underlying.knownLength >= 0 && other.knownLength >= 0 =>
+          underlying.knownLength + other.knownLength
+        case _ =>
+          -1
+      }
+    }
+    case class Zip[A, B](underlying: Iterable[A], other: IterableOnce[B]) extends View[(A, B)] {
+      def iterator = underlying.iterator.zip(other)
+      override def knownLength = other match {
+        case other: Iterable[_] if underlying.knownLength >= 0 && other.knownLength >= 0 =>
+          underlying.knownLength min other.knownLength
+        case _ =>
+          -1
+      }
+    }
+  }
+
+/* ---------- Iterators ---------------------------------------------------*/
+
+  /** A core Iterator class */
+  trait Iterator[+A] extends IterableOnce[A] { self =>
+    def hasNext: Boolean
+    def next(): A
+    def iterator = this
+    def foldLeft[B](z: B)(op: (B, A) => B): B =
+      if (hasNext) foldLeft(op(z, next))(op) else z
+    def foldRight[B](z: B)(op: (A, B) => B): B =
+      if (hasNext) op(next(), foldRight(z)(op)) else z
+    def foreach(f: A => Unit): Unit =
+      while (hasNext) f(next())
+    def indexWhere(p: A => Boolean): Int = {
+      var i = 0
+      while (hasNext) {
+        if (p(next())) return i
+        i += 1
+      }
+      -1
+    }
+    def filter(p: A => Boolean): Iterator[A] = new Iterator[A] {
+      private var hd: A = _
+      private var hdDefined: Boolean = false
+
+      def hasNext: Boolean = hdDefined || {
+        do {
+          if (!self.hasNext) return false
+          hd = self.next()
+        } while (!p(hd))
+        hdDefined = true
+        true
+      }
+
+      def next() =
+        if (hasNext) {
+          hdDefined = false
+          hd
+        }
+        else Iterator.empty.next()
+    }
+
+    def map[B](f: A => B): Iterator[B] = new Iterator[B] {
+      def hasNext = self.hasNext
+      def next() = f(self.next())
+    }
+
+    def flatMap[B](f: A => IterableOnce[B]): Iterator[B] = new Iterator[B] {
+      private var myCurrent: Iterator[B] = Iterator.empty
+      private def current = {
+        while (!myCurrent.hasNext && self.hasNext)
+          myCurrent = f(self.next()).iterator
+        myCurrent
+      }
+      def hasNext = current.hasNext
+      def next() = current.next()
+    }
+    def ++[B >: A](xs: IterableOnce[B]): Iterator[B] = new Iterator[B] {
+      private var myCurrent: Iterator[B] = self
+      private var first = true
+      private def current = {
+        if (!myCurrent.hasNext && first) {
+          myCurrent = xs.iterator
+          first = false
+        }
+        myCurrent
+      }
+      def hasNext = current.hasNext
+      def next() = current.next()
+    }
+    def drop(n: Int): Iterator[A] = {
+      var i = 0
+      while (i < n && hasNext) {
+        next()
+        i += 1
+      }
+      this
+    }
+    def zip[B](that: IterableOnce[B]): Iterator[(A, B)] = new Iterator[(A, B)] {
+      val thatIterator = that.iterator
+      def hasNext = self.hasNext && thatIterator.hasNext
+      def next() = (self.next(), thatIterator.next())
+    }
+  }
+
+  object Iterator {
+    val empty: Iterator[Nothing] = new Iterator[Nothing] {
+      def hasNext = false
+      def next = throw new NoSuchElementException("next on empty iterator")
+    }
+    def apply[A](xs: A*): Iterator[A] = new RandomAccessView[A] {
+      val start = 0
+      val end = xs.length
+      def apply(n: Int) = xs(n)
+    }.iterator
+  }
+}