Add BuildFrom

src/main/scala-2.12/collection/BuildFrom.scala

@@ -0,0 +1,38 @@
+package scala.collection
+
+import scala.collection.generic.CanBuildFrom
+
+/** Builds a collection of type `C` from elements of type `A` when a source collection of type `From` is available.
+  * Implicit instances of `BuildFrom` are available for all collection types.
+  *
+  * @tparam From Type of source collection
+  * @tparam A Type of elements (e.g. `Int`, `Boolean`, etc.)
+  * @tparam C Type of collection (e.g. `List[Int]`, `TreeMap[Int, String]`, etc.)
+  */
+trait BuildFrom[-From, -A, +C] extends Any {
+
+  def fromSpecificIterable(from: From)(it: Iterable[A]): C
+
+  /** Get a Builder for the collection. For non-strict collection types this will use an intermediate buffer.
+    * Building collections with `fromSpecificIterable` is preferred because it can be lazy for lazy collections. */
+  def newBuilder(from: From): mutable.Builder[A, C]
+
+  @deprecated("Use newBuilder() instead of apply()", "2.13.0")
+  @`inline` def apply(from: From): mutable.Builder[A, C] = newBuilder(from)
+
+}
+
+object BuildFrom {
+
+  // Implicit instance derived from an implicit CanBuildFrom instance
+  implicit def fromCanBuildFrom[From, A, C](implicit cbf: CanBuildFrom[From, A, C]): BuildFrom[From, A, C] =
+    new BuildFrom[From, A, C] {
+      def fromSpecificIterable(from: From)(it: Iterable[A]): C = (cbf(from) ++= it).result()
+      def newBuilder(from: From): mutable.Builder[A, C] = cbf(from)
+    }
+
+  // Implicit conversion derived from an implicit conversion to CanBuildFrom
+  implicit def fromCanBuildFromConversion[X, From, A, C](x: X)(implicit toCanBuildFrom: X => CanBuildFrom[From, A, C]): BuildFrom[From, A, C] =
+    fromCanBuildFrom(toCanBuildFrom(x))
+
+}

src/main/scala-2.12/collection/compat/package.scala

@@ -8,19 +8,25 @@ import scala.reflect.ClassTag
 package object compat {
   import scala.collection.compat_impl._
 
-  implicit def genericCompanionToCBF[A, CC[X] <: GenTraversable[X]](fact: GenericCompanion[CC]): CanBuildFrom[Nothing, A, CC[A]] =
+  implicit def genericCompanionToCBF[A, CC[X] <: GenTraversable[X]](fact: GenericCompanion[CC]): CanBuildFrom[Any, A, CC[A]] =
     simpleCBF(fact.newBuilder[A])
 
-  implicit def arrayCompanionToCBF[A : ClassTag](fact: Array.type): CanBuildFrom[Nothing, A, Array[A]] =
+  implicit def sortedSetCompanionToCBF[A : Ordering, CC[X] <: SortedSet[X] with SortedSetLike[X, CC[X]]](fact: SortedSetFactory[CC]): CanBuildFrom[Any, A, CC[A]] =
+    simpleCBF(fact.newBuilder[A])
+
+  implicit def arrayCompanionToCBF[A : ClassTag](fact: Array.type): CanBuildFrom[Any, A, Array[A]] =
     simpleCBF(Array.newBuilder[A])
 
-  implicit def mapFactoryToCBF[K, V, CC[A, B] <: Map[A, B] with MapLike[A, B, CC[A, B]]](fact: MapFactory[CC]): CanBuildFrom[Nothing, (K, V), CC[K, V]] =
+  implicit def mapFactoryToCBF[K, V, CC[A, B] <: Map[A, B] with MapLike[A, B, CC[A, B]]](fact: MapFactory[CC]): CanBuildFrom[Any, (K, V), CC[K, V]] =
+    simpleCBF(fact.newBuilder[K, V])
+
+  implicit def sortedMapFactoryToCBF[K : Ordering, V, CC[A, B] <: SortedMap[A, B] with SortedMapLike[A, B, CC[A, B]]](fact: SortedMapFactory[CC]): CanBuildFrom[Any, (K, V), CC[K, V]] =
     simpleCBF(fact.newBuilder[K, V])
 
-  implicit def immutableBitSetFactoryToCBF(fact: BitSetFactory[immutable.BitSet]): CanBuildFrom[Nothing, Int, ImmutableBitSetCC[Int]] =
+  implicit def immutableBitSetFactoryToCBF(fact: BitSetFactory[immutable.BitSet]): CanBuildFrom[Any, Int, ImmutableBitSetCC[Int]] =
     simpleCBF(fact.newBuilder)
 
-  implicit def mutableBitSetFactoryToCBF(fact: BitSetFactory[mutable.BitSet]): CanBuildFrom[Nothing, Int, MutableBitSetCC[Int]] =
+  implicit def mutableBitSetFactoryToCBF(fact: BitSetFactory[mutable.BitSet]): CanBuildFrom[Any, Int, MutableBitSetCC[Int]] =
     simpleCBF(fact.newBuilder)
 
   implicit class IterableFactoryExtensionMethods[CC[X] <: GenTraversable[X]](private val fact: GenericCompanion[CC]) {

src/main/scala-2.12/collection/compat_impl/package.scala

@@ -5,8 +5,8 @@ import scala.collection.mutable.Builder
 import scala.reflect.ClassTag
 
 package object compat_impl {
-  def simpleCBF[A, C](f: => Builder[A, C]): CanBuildFrom[Nothing, A, C] = new CanBuildFrom[Nothing, A, C] {
-    def apply(from: Nothing): Builder[A, C] = apply()
+  def simpleCBF[A, C](f: => Builder[A, C]): CanBuildFrom[Any, A, C] = new CanBuildFrom[Any, A, C] {
+    def apply(from: Any): Builder[A, C] = apply()
     def apply(): Builder[A, C] = f
   }
 

src/test/scala/collection/BuildFromTest.scala

@@ -0,0 +1,140 @@
+package scala.collection
+
+import org.junit.Test
+
+import scala.collection.mutable.{ArrayBuffer, Builder, ListBuffer}
+import scala.collection.immutable.{HashMap, List, TreeMap, TreeSet}
+import scala.collection.compat._
+
+// Tests copied from the 2.13 scala-library
+class BuildFromTest {
+
+  // Using BuildFrom to abstract over both and also allow building arbitrary collection types
+  def optionSequence2[CC[X] <: Iterable[X], A, To](xs: CC[Option[A]])(implicit bf: BuildFrom[CC[Option[A]], A, To]): Option[To] =
+    xs.foldLeft[Option[Builder[A, To]]](Some(bf.newBuilder(xs))) {
+      case (Some(builder), Some(a)) => Some(builder += a)
+      case _ => None
+    }.map(_.result())
+
+  // Using dependent types:
+  def optionSequence3[A, To](xs: Iterable[Option[A]])(implicit bf: BuildFrom[xs.type, A, To]): Option[To] =
+    xs.foldLeft[Option[Builder[A, To]]](Some(bf.newBuilder(xs))) {
+      case (Some(builder), Some(a)) => Some(builder += a)
+      case _ => None
+    }.map(_.result())
+
+  def eitherSequence[A, B, To](xs: Iterable[Either[A, B]])(implicit bf: BuildFrom[xs.type, B, To]): Either[A, To] =
+    xs.foldLeft[Either[A, Builder[B, To]]](Right(bf.newBuilder(xs))) {
+      case (Right(builder), Right(b)) => Right(builder += b)
+      case (Left(a)       ,        _) => Left(a)
+      case (_             ,  Left(a)) => Left(a)
+    }.right.map(_.result())
+
+  @Test
+  def optionSequence2Test: Unit = {
+    val xs1 = List(Some(1), None, Some(2))
+    val o1 = optionSequence2(xs1)
+    val o1t: Option[List[Int]] = o1
+
+    val xs2 = TreeSet(Some("foo"), Some("bar"), None)
+    val o2 = optionSequence2(xs2)
+    // Not working: the resolved implicit BuildFrom results in a SortedSet instead of a TreeSet
+    // val o2t: Option[TreeSet[String]] = o2
+    val o2t: Option[SortedSet[String]] = o2
+
+    // Breakout-like use case from https://github.com/scala/scala/pull/5233:
+    val xs4 = List[Option[(Int, String)]](Some((1 -> "a")), Some((2 -> "b")))
+    val o4 = optionSequence2(xs4)(TreeMap)
+    val o4t: Option[TreeMap[Int, String]] = o4
+  }
+
+  @Test
+  def optionSequence3Test: Unit = {
+    val xs1 = List(Some(1), None, Some(2))
+    val o1 = optionSequence3(xs1)
+    val o1t: Option[List[Int]] = o1
+
+    val xs2 = TreeSet(Some("foo"), Some("bar"), None)
+    val o2 = optionSequence3(xs2)
+    // Not working: the resolved implicit BuildFrom results in a SortedSet instead of a TreeSet
+    // val o2t: Option[TreeSet[String]] = o2
+    val o2t: Option[SortedSet[String]] = o2
+
+    // Breakout-like use case from https://github.com/scala/scala/pull/5233:
+    val xs4 = List[Option[(Int, String)]](Some((1 -> "a")), Some((2 -> "b")))
+    val o4 = optionSequence3(xs4)(TreeMap) // same syntax as in `.to`
+    val o4t: Option[TreeMap[Int, String]] = o4
+  }
+
+  @Test
+  def eitherSequenceTest: Unit = {
+    val xs3 = ListBuffer(Right("foo"), Left(0), Right("bar"))
+    val e1 = eitherSequence(xs3)
+    val e1t: Either[Int, ListBuffer[String]] = e1
+  }
+
+  // From https://github.com/scala/collection-strawman/issues/44
+  def flatCollect[A, B, To](coll: Iterable[A])(f: PartialFunction[A, IterableOnce[B]])
+                       (implicit bf: BuildFrom[coll.type, B, To]): To = {
+    val builder = bf.newBuilder(coll)
+    for (a <- coll) {
+      if (f.isDefinedAt(a)) builder ++= f(a)
+    }
+    builder.result()
+  }
+
+  def mapSplit[A, B, C, ToL, ToR](coll: Iterable[A])(f: A => Either[B, C])
+              (implicit bfLeft:  BuildFrom[coll.type, B, ToL], bfRight: BuildFrom[coll.type, C, ToR]): (ToL, ToR) = {
+    val left = bfLeft.newBuilder(coll)
+    val right = bfRight.newBuilder(coll)
+    for (a <- coll)
+      f(a).fold(left.+=, right.+=)
+    (left.result(), right.result())
+  }
+
+  @Test
+  def flatCollectTest: Unit = {
+    val xs1 = List(1, 2, 3)
+    val xs2 = flatCollect(xs1) { case 2 => ArrayBuffer("foo", "bar") }
+    val xs3: List[String] = xs2
+
+    val xs4 = TreeMap((1, "1"), (2, "2"))
+    val xs5 = flatCollect(xs4) { case (2, v) => List((v, v)) }
+    val xs6: TreeMap[String, String] = xs5
+
+    val xs7 = HashMap((1, "1"), (2, "2"))
+    val xs8 = flatCollect(xs7) { case (2, v) => List((v, v)) }
+    val xs9: HashMap[String, String] = xs8
+
+    val xs10 = TreeSet(1, 2, 3)
+    val xs11 = flatCollect(xs10) { case 2 => List("foo", "bar") }
+    // Not working: the resolved implicit BuildFrom results in a SortedSet instead of a TreeSet
+    // val xs12: TreeSet[String] = xs11
+    val xs12: SortedSet[String] = xs11
+  }
+
+  @Test
+  def mapSplitTest: Unit = {
+    val xs1 = List(1, 2, 3)
+    val (xs2, xs3) = mapSplit(xs1)(x => if (x % 2 == 0) Left(x) else Right(x.toString))
+    val xs4: List[Int] = xs2
+    val xs5: List[String] = xs3
+
+    val xs6 = TreeMap((1, "1"), (2, "2"))
+    val (xs7, xs8) = mapSplit(xs6) { case (k, v) => Left[(String, Int), (Int, Boolean)]((v, k)) }
+    val xs9: TreeMap[String, Int] = xs7
+    val xs10: TreeMap[Int, Boolean] = xs8
+  }
+
+  implicitly[BuildFrom[String, Char, String]]
+  implicitly[BuildFrom[Array[Int], Char, Array[Char]]]
+  implicitly[BuildFrom[BitSet, Int, BitSet]]
+  implicitly[BuildFrom[immutable.BitSet, Int, immutable.BitSet]]
+  implicitly[BuildFrom[mutable.BitSet, Int, mutable.BitSet]]
+
+  // Check that collection companions can implicitly be converted to a `BuildFrom` instance
+  Iterable: BuildFrom[_, Int, Iterable[Int]]
+  Map: BuildFrom[_, (Int, String), Map[Int, String]]
+  SortedSet: BuildFrom[_, Int, SortedSet[Int]]
+  SortedMap: BuildFrom[_, (Int, String), SortedMap[Int, String]]
+}

src/test/scala/CollectionTest.scala → src/test/scala/collection/CollectionTest.scala

@@ -1,3 +1,5 @@
+package collection
+
 import java.util
 
 import org.junit.Test

src/test/scala/ImmutableArrayTest.scala → src/test/scala/collection/ImmutableArrayTest.scala

@@ -1,5 +1,6 @@
-import org.junit.Assert
-import org.junit.Test
+package collection
+
+import org.junit.{Assert, Test}
 
 import scala.collection.immutable.ImmutableArray