Make BuildFrom capture checked

Author: odersky

tests/pos-special/stdlib/collection/BuildFrom.scala

@@ -16,6 +16,8 @@ import scala.annotation.implicitNotFound
 import scala.collection.mutable.Builder
 import scala.collection.immutable.WrappedString
 import scala.reflect.ClassTag
+import language.experimental.captureChecking
+import caps.unsafe.unsafeAssumePure
 
 /** 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.
@@ -26,7 +28,11 @@ import scala.reflect.ClassTag
   */
 @implicitNotFound(msg = "Cannot construct a collection of type ${C} with elements of type ${A} based on a collection of type ${From}.")
 trait BuildFrom[-From, -A, +C] extends Any { self =>
-  def fromSpecific(from: From)(it: IterableOnce[A]): C
+  def fromSpecific(from: From)(it: IterableOnce[A]^): C
+    // !!! this is wrong, we need two versions of fromSpecific; one mapping
+    // to C^{it} when C is an Iterable, and one mapping to C when C is a Seq, Map, or Set.
+    // But that requires a lareg scale refactoring of BuildFrom. The unsafeAssumePure
+    // calls in this file are needed to sweep that problem under the carpet.
 
   /** Get a Builder for the collection. For non-strict collection types this will use an intermediate buffer.
     * Building collections with `fromSpecific` is preferred because it can be lazy for lazy collections. */
@@ -37,7 +43,7 @@ trait BuildFrom[-From, -A, +C] extends Any { self =>
 
   /** Partially apply a BuildFrom to a Factory */
   def toFactory(from: From): Factory[A, C] = new Factory[A, C] {
-    def fromSpecific(it: IterableOnce[A]): C = self.fromSpecific(from)(it)
+    def fromSpecific(it: IterableOnce[A]^): C = self.fromSpecific(from)(it)
     def newBuilder: Builder[A, C] = self.newBuilder(from)
   }
 }
@@ -48,42 +54,42 @@ object BuildFrom extends BuildFromLowPriority1 {
   implicit def buildFromMapOps[CC[X, Y] <: Map[X, Y] with MapOps[X, Y, CC, _], K0, V0, K, V]: BuildFrom[CC[K0, V0] with Map[K0, V0], (K, V), CC[K, V] with Map[K, V]] = new BuildFrom[CC[K0, V0], (K, V), CC[K, V]] {
     //TODO: Reuse a prototype instance
     def newBuilder(from: CC[K0, V0]): Builder[(K, V), CC[K, V]] = (from: MapOps[K0, V0, CC, _]).mapFactory.newBuilder[K, V]
-    def fromSpecific(from: CC[K0, V0])(it: IterableOnce[(K, V)]): CC[K, V] = (from: MapOps[K0, V0, CC, _]).mapFactory.from(it)
+    def fromSpecific(from: CC[K0, V0])(it: IterableOnce[(K, V)]^): CC[K, V] = (from: MapOps[K0, V0, CC, _]).mapFactory.from(it)
   }
 
   /** Build the source collection type from a SortedMapOps */
   implicit def buildFromSortedMapOps[CC[X, Y] <: SortedMap[X, Y] with SortedMapOps[X, Y, CC, _], K0, V0, K : Ordering, V]: BuildFrom[CC[K0, V0] with SortedMap[K0, V0], (K, V), CC[K, V] with SortedMap[K, V]] = new BuildFrom[CC[K0, V0], (K, V), CC[K, V]] {
     def newBuilder(from: CC[K0, V0]): Builder[(K, V), CC[K, V]] = (from: SortedMapOps[K0, V0, CC, _]).sortedMapFactory.newBuilder[K, V]
-    def fromSpecific(from: CC[K0, V0])(it: IterableOnce[(K, V)]): CC[K, V] = (from: SortedMapOps[K0, V0, CC, _]).sortedMapFactory.from(it)
+    def fromSpecific(from: CC[K0, V0])(it: IterableOnce[(K, V)]^): CC[K, V] = (from: SortedMapOps[K0, V0, CC, _]).sortedMapFactory.from(it)
   }
 
   implicit def buildFromBitSet[C <: BitSet with BitSetOps[C]]: BuildFrom[C, Int, C] =
     new BuildFrom[C, Int, C] {
-      def fromSpecific(from: C)(it: IterableOnce[Int]): C = from.bitSetFactory.fromSpecific(it)
+      def fromSpecific(from: C)(it: IterableOnce[Int]^): C = from.bitSetFactory.fromSpecific(it)
       def newBuilder(from: C): Builder[Int, C] = from.bitSetFactory.newBuilder
     }
 
   implicit val buildFromString: BuildFrom[String, Char, String] =
     new BuildFrom[String, Char, String] {
-      def fromSpecific(from: String)(it: IterableOnce[Char]): String = Factory.stringFactory.fromSpecific(it)
+      def fromSpecific(from: String)(it: IterableOnce[Char]^): String = Factory.stringFactory.fromSpecific(it)
       def newBuilder(from: String): Builder[Char, String] = Factory.stringFactory.newBuilder
     }
 
   implicit val buildFromWrappedString: BuildFrom[WrappedString, Char, WrappedString] =
     new BuildFrom[WrappedString, Char, WrappedString] {
-      def fromSpecific(from: WrappedString)(it: IterableOnce[Char]): WrappedString = WrappedString.fromSpecific(it)
+      def fromSpecific(from: WrappedString)(it: IterableOnce[Char]^): WrappedString = WrappedString.fromSpecific(it)
       def newBuilder(from: WrappedString): mutable.Builder[Char, WrappedString] = WrappedString.newBuilder
     }
 
-  implicit def buildFromArray[A : ClassTag]: BuildFrom[Array[_], A, Array[A]] =
+  implicit def buildFromArray[sealed A : ClassTag]: BuildFrom[Array[_], A, Array[A]] =
     new BuildFrom[Array[_], A, Array[A]] {
-      def fromSpecific(from: Array[_])(it: IterableOnce[A]): Array[A] = Factory.arrayFactory[A].fromSpecific(it)
+      def fromSpecific(from: Array[_])(it: IterableOnce[A]^): Array[A] = Factory.arrayFactory[A].fromSpecific(it)
       def newBuilder(from: Array[_]): Builder[A, Array[A]] = Factory.arrayFactory[A].newBuilder
     }
 
-  implicit def buildFromView[A, B]: BuildFrom[View[A], B, View[B]] =
+  implicit def buildFromView[A, sealed B]: BuildFrom[View[A], B, View[B]] =
     new BuildFrom[View[A], B, View[B]] {
-      def fromSpecific(from: View[A])(it: IterableOnce[B]): View[B] = View.from(it)
+      def fromSpecific(from: View[A])(it: IterableOnce[B]^): View[B] = View.from(it).unsafeAssumePure
       def newBuilder(from: View[A]): Builder[B, View[B]] = View.newBuilder
     }
 
@@ -97,12 +103,12 @@ trait BuildFromLowPriority1 extends BuildFromLowPriority2 {
   // test in test/junit/scala/collection/BuildFromTest.scala and discussion in https://github.com/scala/scala/pull/10209
   implicit def buildFromSortedSetOps[CC[X] <: SortedSet[X] with SortedSetOps[X, CC, _], A0, A : Ordering]: BuildFrom[CC[A0] with SortedSet[A0], A, CC[A] with SortedSet[A]] = new BuildFrom[CC[A0], A, CC[A]] {
     def newBuilder(from: CC[A0]): Builder[A, CC[A]] = (from: SortedSetOps[A0, CC, _]).sortedIterableFactory.newBuilder[A]
-    def fromSpecific(from: CC[A0])(it: IterableOnce[A]): CC[A] = (from: SortedSetOps[A0, CC, _]).sortedIterableFactory.from(it)
+    def fromSpecific(from: CC[A0])(it: IterableOnce[A]^): CC[A] = (from: SortedSetOps[A0, CC, _]).sortedIterableFactory.from(it)
   }
 
   implicit def fallbackStringCanBuildFrom[A]: BuildFrom[String, A, immutable.IndexedSeq[A]] =
     new BuildFrom[String, A, immutable.IndexedSeq[A]] {
-      def fromSpecific(from: String)(it: IterableOnce[A]): immutable.IndexedSeq[A] = immutable.IndexedSeq.from(it)
+      def fromSpecific(from: String)(it: IterableOnce[A]^): immutable.IndexedSeq[A] = immutable.IndexedSeq.from(it)
       def newBuilder(from: String): Builder[A, immutable.IndexedSeq[A]] = immutable.IndexedSeq.newBuilder[A]
   }
 }
@@ -112,11 +118,11 @@ trait BuildFromLowPriority2 {
   implicit def buildFromIterableOps[CC[X] <: Iterable[X] with IterableOps[X, CC, _], A0, A]: BuildFrom[CC[A0], A, CC[A]] = new BuildFrom[CC[A0], A, CC[A]] {
     //TODO: Reuse a prototype instance
     def newBuilder(from: CC[A0]): Builder[A, CC[A]] = (from: IterableOps[A0, CC, _]).iterableFactory.newBuilder[A]
-    def fromSpecific(from: CC[A0])(it: IterableOnce[A]): CC[A] = (from: IterableOps[A0, CC, _]).iterableFactory.from(it)
+    def fromSpecific(from: CC[A0])(it: IterableOnce[A]^): CC[A] = (from: IterableOps[A0, CC, _]).iterableFactory.from(it).unsafeAssumePure
   }
 
   implicit def buildFromIterator[A]: BuildFrom[Iterator[_], A, Iterator[A]] = new BuildFrom[Iterator[_], A, Iterator[A]] {
     def newBuilder(from: Iterator[_]): mutable.Builder[A, Iterator[A]] = Iterator.newBuilder
-    def fromSpecific(from: Iterator[_])(it: IterableOnce[A]): Iterator[A] = Iterator.from(it)
+    def fromSpecific(from: Iterator[_])(it: IterableOnce[A]^): Iterator[A] = Iterator.from(it).unsafeAssumePure
   }
 }

tests/pos-special/stdlib/collection/Factory.scala

@@ -282,7 +282,8 @@ object IterableFactory {
 
   implicit def toBuildFrom[A, CC[_]](factory: IterableFactory[CC]): BuildFrom[Any, A, CC[A]] =
     new BuildFrom[Any, A, CC[A]] {
-      def fromSpecific(from: Any)(it: IterableOnce[A]^): CC[A]^{it} = factory.from(it)
+      def fromSpecific(from: Any)(it: IterableOnce[A]^): CC[A] =
+        factory.from(it).unsafeAssumePure // !!! see remark in BuildFrom why this is necessary
       def newBuilder(from: Any) = factory.newBuilder
     }
 
@@ -535,7 +536,7 @@ object EvidenceIterableFactory {
 
   implicit def toBuildFrom[Ev[_], A: Ev, CC[_]](factory: EvidenceIterableFactory[CC, Ev]): BuildFrom[Any, A, CC[A]] = new EvidenceIterableFactoryToBuildFrom(factory)
   private class EvidenceIterableFactoryToBuildFrom[Ev[_], A: Ev, CC[_]](factory: EvidenceIterableFactory[CC, Ev]) extends BuildFrom[Any, A, CC[A]] {
-    def fromSpecific(from: Any)(it: IterableOnce[A]): CC[A] = factory.from[A](it)
+    def fromSpecific(from: Any)(it: IterableOnce[A]^): CC[A] = factory.from[A](it)
     def newBuilder(from: Any): Builder[A, CC[A]] = factory.newBuilder[A]
   }
 
@@ -783,7 +784,7 @@ object SortedMapFactory {
 
   implicit def toBuildFrom[K : Ordering, V, CC[_, _]](factory: SortedMapFactory[CC]): BuildFrom[Any, (K, V), CC[K, V]] = new SortedMapFactoryToBuildFrom(factory)
   private class SortedMapFactoryToBuildFrom[K : Ordering, V, CC[_, _]](factory: SortedMapFactory[CC]) extends BuildFrom[Any, (K, V), CC[K, V]] {
-    def fromSpecific(from: Any)(it: IterableOnce[(K, V)]) = factory.from(it)
+    def fromSpecific(from: Any)(it: IterableOnce[(K, V)]^) = factory.from(it)
     def newBuilder(from: Any) = factory.newBuilder[K, V]
   }