Deprecate sequence implementations in favor of sequence operations.

libraries/stdlib/src/generated/_Filtering.kt

@@ -579,7 +579,7 @@ public fun <T : Any> Iterable<T?>.filterNotNull(): List<T> {
  * Returns a sequence containing all elements that are not null
  */
 public fun <T : Any> Sequence<T?>.filterNotNull(): Sequence<T> {
-    return FilteringSequence(this, false, { it == null }) as Sequence<T>
+    return filterNot { it == null } as Sequence<T>
 }
 
 
@@ -588,7 +588,7 @@ deprecated("Migrate to using Sequence<T> and respective functions")
  * Returns a stream containing all elements that are not null
  */
 public fun <T : Any> Stream<T?>.filterNotNull(): Stream<T> {
-    return FilteringStream(this, false, { it == null }) as Stream<T>
+    return filterNot { it == null } as Stream<T>
 }
 
 /**

libraries/stdlib/src/generated/_Generators.kt

@@ -705,7 +705,7 @@ public fun <T> Iterable<T>.plus(collection: Iterable<T>): List<T> {
  * Returns a sequence containing all elements of original sequence and then all elements of the given [collection]
  */
 public fun <T> Sequence<T>.plus(collection: Iterable<T>): Sequence<T> {
-    return MultiSequence(sequenceOf(this, collection.sequence()))
+    return sequenceOf(this, collection.sequence()).flatten()
 }
 
 
@@ -714,7 +714,7 @@ deprecated("Migrate to using Sequence<T> and respective functions")
  * Returns a stream containing all elements of original stream and then all elements of the given [collection]
  */
 public fun <T> Stream<T>.plus(collection: Iterable<T>): Stream<T> {
-    return Multistream(streamOf(this, collection.stream()))
+    return streamOf(this, collection.stream()).flatten()
 }
 
 /**
@@ -811,7 +811,7 @@ public fun <T> Iterable<T>.plus(element: T): List<T> {
  * Returns a sequence containing all elements of original sequence and then the given element
  */
 public fun <T> Sequence<T>.plus(element: T): Sequence<T> {
-    return MultiSequence(sequenceOf(this, sequenceOf(element)))
+    return sequenceOf(this, sequenceOf(element)).flatten()
 }
 
 
@@ -820,14 +820,14 @@ deprecated("Migrate to using Sequence<T> and respective functions")
  * Returns a stream containing all elements of original stream and then the given element
  */
 public fun <T> Stream<T>.plus(element: T): Stream<T> {
-    return Multistream(streamOf(this, streamOf(element)))
+    return streamOf(this, streamOf(element)).flatten()
 }
 
 /**
  * Returns a sequence containing all elements of original sequence and then all elements of the given [sequence]
  */
 public fun <T> Sequence<T>.plus(sequence: Sequence<T>): Sequence<T> {
-    return MultiSequence(sequenceOf(this, sequence))
+    return sequenceOf(this, sequence).flatten()
 }
 
 
@@ -836,7 +836,7 @@ deprecated("Migrate to using Sequence<T> and respective functions")
  * Returns a stream containing all elements of original stream and then all elements of the given [stream]
  */
 public fun <T> Stream<T>.plus(stream: Stream<T>): Stream<T> {
-    return Multistream(streamOf(this, stream))
+    return streamOf(this, stream).flatten()
 }
 
 /**

libraries/stdlib/src/generated/_Guards.kt

@@ -49,12 +49,7 @@ public fun <T : Any> List<T?>.requireNoNulls(): List<T> {
  * Returns an original collection containing all the non-*null* elements, throwing an [[IllegalArgumentException]] if there are any null elements
  */
 public fun <T : Any> Sequence<T?>.requireNoNulls(): Sequence<T> {
-    return FilteringSequence(this) {
-        if (it == null) {
-            throw IllegalArgumentException("null element found in $this")
-        }
-        true
-    } as Sequence<T>
+    return map { it ?: throw IllegalArgumentException("null element found in $this") }
 }
 
 
@@ -63,11 +58,6 @@ deprecated("Migrate to using Sequence<T> and respective functions")
  * Returns an original collection containing all the non-*null* elements, throwing an [[IllegalArgumentException]] if there are any null elements
  */
 public fun <T : Any> Stream<T?>.requireNoNulls(): Stream<T> {
-    return FilteringStream(this) {
-        if (it == null) {
-            throw IllegalArgumentException("null element found in $this")
-        }
-        true
-    } as Stream<T>
+    return map { it ?: throw IllegalArgumentException("null element found in $this") }
 }
 

libraries/stdlib/src/generated/_SpecialJVM.kt

@@ -471,7 +471,7 @@ public inline fun <reified R> Iterable<*>.filterIsInstance(): List<R> {
  * Returns a sequence containing all elements that are instances of specified type parameter R
  */
 public inline fun <reified R> Sequence<*>.filterIsInstance(): Sequence<R> {
-    return FilteringSequence(this, true, { it is R }) as Sequence<R>
+    return filter { it is R } as Sequence<R>
 }
 
 
@@ -480,7 +480,7 @@ deprecated("Migrate to using Sequence<T> and respective functions")
  * Returns a stream containing all elements that are instances of specified type parameter R
  */
 public inline fun <reified R> Stream<*>.filterIsInstance(): Stream<R> {
-    return FilteringStream(this, true, { it is R }) as Stream<R>
+    return filter { it is R } as Stream<R>
 }
 
 /**
@@ -501,7 +501,7 @@ public fun <R> Iterable<*>.filterIsInstance(klass: Class<R>): List<R> {
  * Returns a sequence containing all elements that are instances of specified class
  */
 public fun <R> Sequence<*>.filterIsInstance(klass: Class<R>): Sequence<R> {
-    return FilteringSequence(this, true, { klass.isInstance(it) }) as Sequence<R>
+    return filter { klass.isInstance(it) } as Sequence<R>
 }
 
 
@@ -510,7 +510,7 @@ deprecated("Migrate to using Sequence<T> and respective functions")
  * Returns a stream containing all elements that are instances of specified class
  */
 public fun <R> Stream<*>.filterIsInstance(klass: Class<R>): Stream<R> {
-    return FilteringStream(this, true, { klass.isInstance(it) }) as Stream<R>
+    return filter { klass.isInstance(it) } as Stream<R>
 }
 
 /**

libraries/stdlib/src/kotlin/collections/Operations.kt

@@ -18,7 +18,7 @@ public fun <T> Iterable<Iterable<T>>.flatten(): List<T> {
  * Returns a sequence of all elements from all sequences in this sequence.
  */
 public fun <T> Sequence<Sequence<T>>.flatten(): Sequence<T> {
-    return FlatteningSequence(this, { it })
+    return MultiSequence(this)
 }
 
 /**

libraries/stdlib/src/kotlin/collections/Sequence.kt

@@ -63,6 +63,7 @@ public class FilteringStream<T>(stream: Stream<T>, sendWhen: Boolean = true, pre
  * @param sendWhen If `true`, values for which the predicate returns `true` are returned. Otherwise,
 * values for which the predicate returns `false` are returned
  */
+deprecated("This class is an implementation detail and shall be made internal soon. Use sequence.filter() or sequence.filterNot() instead.")
 public class FilteringSequence<T>(private val sequence: Sequence<T>,
                                   private val sendWhen: Boolean = true,
                                   private val predicate: (T) -> Boolean
@@ -112,6 +113,7 @@ public class TransformingStream<T, R>(stream: Stream<T>, transformer: (T) -> R)
  * A sequence which returns the results of applying the given [transformer] function to the values
  * in the underlying [sequence].
  */
+deprecated("This class is an implementation detail and shall be made internal soon. Use sequence.map() instead.")
 public class TransformingSequence<T, R>(private val sequence: Sequence<T>, private val transformer: (T) -> R) : Sequence<R> {
     override fun iterator(): Iterator<R> = object : Iterator<R> {
         val iterator = sequence.iterator()
@@ -134,6 +136,7 @@ public class TransformingIndexedStream<T, R>(stream: Stream<T>, transformer: (In
  * in the underlying [sequence], where the transformer function takes the index of the value in the underlying
  * sequence along with the value itself.
  */
+deprecated("This class is an implementation detail and shall be made internal soon. Use sequence.mapIndexed() instead.")
 public class TransformingIndexedSequence<T, R>(private val sequence: Sequence<T>, private val transformer: (Int, T) -> R) : Sequence<R> {
     override fun iterator(): Iterator<R> = object : Iterator<R> {
         val iterator = sequence.iterator()
@@ -156,6 +159,7 @@ public class IndexingStream<T>(stream: Stream<T>)
  * A sequence which combines values from the underlying [sequence] with their indices and returns them as
  * [IndexedValue] objects.
  */
+deprecated("This class is an implementation detail and shall be made internal soon. Use sequence.withIndex() instead.")
 public class IndexingSequence<T>(private val sequence: Sequence<T>) : Sequence<IndexedValue<T>> {
     override fun iterator(): Iterator<IndexedValue<T>> = object : Iterator<IndexedValue<T>> {
         val iterator = sequence.iterator()
@@ -179,6 +183,7 @@ public class MergingStream<T1, T2, V>(stream1: Stream<T1>, stream2: Stream<T2>,
  * [transform] function and returns the values returned by that function. The sequence stops returning
  * values as soon as one of the underlying sequences stops returning values.
  */
+deprecated("This class is an implementation detail and shall be made internal soon. Use sequence.merge() instead.")
 public class MergingSequence<T1, T2, V>(private val sequence1: Sequence<T1>,
                                         private val sequence2: Sequence<T2>,
                                         private val transform: (T1, T2) -> V
@@ -200,6 +205,7 @@ deprecated("Use FlatteningSequence<T> instead")
 public class FlatteningStream<T, R>(stream: Stream<T>, transformer: (T) -> Stream<R>)
 : Stream<R> by FlatteningSequence(stream.toSequence(), { transformer(it).toSequence() })
 
+deprecated("This class is an implementation detail and shall be made internal soon. Use sequence.flatMap() instead.")
 public class FlatteningSequence<T, R>(private val sequence: Sequence<T>,
                                       private val transformer: (T) -> Sequence<R>
                                      ) : Sequence<R> {
@@ -242,6 +248,7 @@ deprecated("Use MultiSequence<T> instead")
 public class Multistream<T>(stream: Stream<Stream<T>>)
 : Stream<T> by FlatteningSequence(stream.toSequence(), { it.toSequence() })
 
+deprecated("This class is an implementation detail and shall be made internal soon. Use sequence.flatten() instead.")
 public class MultiSequence<T>(private val sequence: Sequence<Sequence<T>>) : Sequence<T> {
     override fun iterator(): Iterator<T> = object : Iterator<T> {
         val iterator = sequence.iterator()
@@ -286,6 +293,7 @@ public class TakeStream<T>(stream: Stream<T>, count: Int)
  * A sequence that returns at most [count] values from the underlying [sequence], and stops returning values
  * as soon as that count is reached.
  */
+deprecated("This class is an implementation detail and shall be made internal soon. Use sequence.take() instead.")
 public class TakeSequence<T>(private val sequence: Sequence<T>,
                              private val count: Int
                             ) : Sequence<T> {
@@ -317,6 +325,7 @@ public class TakeWhileStream<T>(stream: Stream<T>, predicate: (T) -> Boolean) :
  * A sequence that returns values from the underlying [sequence] while the [predicate] function returns
  * `true`, and stops returning values once the function returns `false` for the next element.
  */
+deprecated("This class is an implementation detail and shall be made internal soon. Use sequence.takeWhile() instead.")
 public class TakeWhileSequence<T>(private val sequence: Sequence<T>,
                                   private val predicate: (T) -> Boolean
                                  ) : Sequence<T> {
@@ -366,6 +375,7 @@ public class DropStream<T>(stream: Stream<T>, count: Int)
  * A sequence that skips the specified number of values from the underlying [sequence] and returns
  * all values after that.
  */
+deprecated("This class is an implementation detail and shall be made internal soon. Use sequence.drop() instead.")
 public class DropSequence<T>(private val sequence: Sequence<T>,
                              private val count: Int
                             ) : Sequence<T> {
@@ -404,6 +414,7 @@ public class DropWhileStream<T>(stream: Stream<T>, predicate: (T) -> Boolean) :
  * A sequence that skips the values from the underlying [sequence] while the given [predicate] returns `true` and returns
  * all values after that.
  */
+deprecated("This class is an implementation detail and shall be made internal soon. Use sequence.dropWhile() instead.")
 public class DropWhileSequence<T>(private val sequence: Sequence<T>,
                                   private val predicate: (T) -> Boolean
                                  ) : Sequence<T> {
@@ -450,7 +461,7 @@ public class DropWhileSequence<T>(private val sequence: Sequence<T>,
  * A sequence which repeatedly calls the specified [producer] function and returns its return values, until
  * `null` is returned from [producer].
  */
-deprecated("Implementation detail. Use function sequence(nextFunction: () -> T?) instead.")
+deprecated("This class is an implementation detail and shall be made internal soon. Use function sequence(nextFunction: () -> T?) instead.")
 public class FunctionSequence<T : Any>(private val producer: () -> T?) : Sequence<T> {
     override fun iterator(): Iterator<T> = object : Iterator<T> {
         var nextState: Int = -1 // -1 for unknown, 0 for done, 1 for continue

libraries/tools/kotlin-stdlib-gen/src/templates/Filtering.kt

@@ -351,7 +351,7 @@ fun filtering(): List<GenericFunction> {
         returns(Sequences) { "Sequence<T>" }
         body(Sequences) {
             """
-            return FilteringSequence(this, false, { it == null }) as Sequence<T>
+            return filterNot { it == null } as Sequence<T>
             """
         }
     }

libraries/tools/kotlin-stdlib-gen/src/templates/Generators.kt

@@ -21,7 +21,7 @@ fun generators(): List<GenericFunction> {
         returns(Sequences) { "Sequence<T>" }
         body(Sequences) {
             """
-            return MultiSequence(sequenceOf(this, sequenceOf(element)))
+            return sequenceOf(this, sequenceOf(element)).flatten()
             """
         }
     }
@@ -58,7 +58,7 @@ fun generators(): List<GenericFunction> {
         returns("Sequence<T>")
         body {
             """
-            return MultiSequence(sequenceOf(this, collection.sequence()))
+            return sequenceOf(this, collection.sequence()).flatten()
             """
         }
     }
@@ -69,7 +69,7 @@ fun generators(): List<GenericFunction> {
         returns("Sequence<T>")
         body {
             """
-            return MultiSequence(sequenceOf(this, sequence))
+            return sequenceOf(this, sequence).flatten()
             """
         }
     }

libraries/tools/kotlin-stdlib-gen/src/templates/Guards.kt

@@ -27,12 +27,7 @@ fun guards(): List<GenericFunction> {
         }
         body(Sequences) {
             """
-            return FilteringSequence(this) {
-                if (it == null) {
-                    throw IllegalArgumentException("null element found in $THIS")
-                }
-                true
-            } as Sequence<T>
+            return map { it ?: throw IllegalArgumentException("null element found in $THIS") }
             """
         }
     }

libraries/tools/kotlin-stdlib-gen/src/templates/SpecialJVM.kt

@@ -106,7 +106,7 @@ fun specialJVM(): List<GenericFunction> {
         returns(Sequences) { "Sequence<R>" }
         body(Sequences) {
             """
-            return FilteringSequence(this, true, { klass.isInstance(it) }) as Sequence<R>
+            return filter { klass.isInstance(it) } as Sequence<R>
             """
         }
     }
@@ -146,7 +146,7 @@ fun specialJVM(): List<GenericFunction> {
         receiverAsterisk(true)
         body(Sequences) {
             """
-            return FilteringSequence(this, true, { it is R }) as Sequence<R>
+            return filter { it is R } as Sequence<R>
             """
         }
     }