3.x: Standardize many of the operator fusion interfaces & queues (#7320)

Author: akarnokd

src/main/java/io/reactivex/rxjava3/core/Flowable.java

@@ -25,7 +25,6 @@
 import io.reactivex.rxjava3.flowables.*;
 import io.reactivex.rxjava3.functions.*;
 import io.reactivex.rxjava3.internal.functions.*;
-import io.reactivex.rxjava3.internal.fuseable.ScalarSupplier;
 import io.reactivex.rxjava3.internal.jdk8.*;
 import io.reactivex.rxjava3.internal.operators.flowable.*;
 import io.reactivex.rxjava3.internal.operators.maybe.MaybeToFlowable;
@@ -35,6 +34,7 @@
 import io.reactivex.rxjava3.internal.schedulers.ImmediateThinScheduler;
 import io.reactivex.rxjava3.internal.subscribers.*;
 import io.reactivex.rxjava3.internal.util.*;
+import io.reactivex.rxjava3.operators.ScalarSupplier;
 import io.reactivex.rxjava3.parallel.ParallelFlowable;
 import io.reactivex.rxjava3.plugins.RxJavaPlugins;
 import io.reactivex.rxjava3.schedulers.*;

src/main/java/io/reactivex/rxjava3/core/Observable.java

@@ -24,7 +24,6 @@
 import io.reactivex.rxjava3.exceptions.*;
 import io.reactivex.rxjava3.functions.*;
 import io.reactivex.rxjava3.internal.functions.*;
-import io.reactivex.rxjava3.internal.fuseable.ScalarSupplier;
 import io.reactivex.rxjava3.internal.jdk8.*;
 import io.reactivex.rxjava3.internal.observers.*;
 import io.reactivex.rxjava3.internal.operators.flowable.*;
@@ -35,6 +34,7 @@
 import io.reactivex.rxjava3.internal.util.*;
 import io.reactivex.rxjava3.observables.*;
 import io.reactivex.rxjava3.observers.*;
+import io.reactivex.rxjava3.operators.ScalarSupplier;
 import io.reactivex.rxjava3.plugins.RxJavaPlugins;
 import io.reactivex.rxjava3.schedulers.*;
 

src/main/java/io/reactivex/rxjava3/internal/disposables/EmptyDisposable.java

@@ -15,7 +15,7 @@
 
 import io.reactivex.rxjava3.annotations.Nullable;
 import io.reactivex.rxjava3.core.*;
-import io.reactivex.rxjava3.internal.fuseable.QueueDisposable;
+import io.reactivex.rxjava3.operators.QueueDisposable;
 
 /**
  * Represents a stateless empty Disposable that reports being always

src/main/java/io/reactivex/rxjava3/internal/fuseable/AbstractEmptyQueueFuseable.java

@@ -14,6 +14,9 @@
 package io.reactivex.rxjava3.internal.fuseable;
 
 import io.reactivex.rxjava3.annotations.NonNull;
+import io.reactivex.rxjava3.operators.QueueDisposable;
+import io.reactivex.rxjava3.operators.QueueFuseable;
+import io.reactivex.rxjava3.operators.QueueSubscription;
 
 /**
  * Represents an empty, async-only {@link QueueFuseable} instance.

src/main/java/io/reactivex/rxjava3/internal/fuseable/CancellableQueueFuseable.java

@@ -13,6 +13,8 @@
 
 package io.reactivex.rxjava3.internal.fuseable;
 
+import io.reactivex.rxjava3.operators.QueueFuseable;
+
 /**
  * Represents an empty, async-only {@link QueueFuseable} instance that tracks and exposes a
  * canceled/disposed state.

src/main/java/io/reactivex/rxjava3/internal/jdk8/FlowableFlatMapStream.java

@@ -23,10 +23,12 @@
 import io.reactivex.rxjava3.core.*;
 import io.reactivex.rxjava3.exceptions.*;
 import io.reactivex.rxjava3.functions.*;
-import io.reactivex.rxjava3.internal.fuseable.*;
-import io.reactivex.rxjava3.internal.queue.SpscArrayQueue;
 import io.reactivex.rxjava3.internal.subscriptions.*;
 import io.reactivex.rxjava3.internal.util.*;
+import io.reactivex.rxjava3.operators.QueueFuseable;
+import io.reactivex.rxjava3.operators.QueueSubscription;
+import io.reactivex.rxjava3.operators.SimpleQueue;
+import io.reactivex.rxjava3.operators.SpscArrayQueue;
 import io.reactivex.rxjava3.plugins.RxJavaPlugins;
 
 /**

src/main/java/io/reactivex/rxjava3/internal/jdk8/FlowableFromStream.java

@@ -23,9 +23,10 @@
 import io.reactivex.rxjava3.annotations.*;
 import io.reactivex.rxjava3.core.Flowable;
 import io.reactivex.rxjava3.exceptions.Exceptions;
-import io.reactivex.rxjava3.internal.fuseable.*;
 import io.reactivex.rxjava3.internal.subscriptions.*;
 import io.reactivex.rxjava3.internal.util.BackpressureHelper;
+import io.reactivex.rxjava3.operators.ConditionalSubscriber;
+import io.reactivex.rxjava3.operators.QueueSubscription;
 import io.reactivex.rxjava3.plugins.RxJavaPlugins;
 
 /**

src/main/java/io/reactivex/rxjava3/internal/jdk8/FlowableMapOptional.java

@@ -19,8 +19,8 @@
 
 import io.reactivex.rxjava3.core.Flowable;
 import io.reactivex.rxjava3.functions.Function;
-import io.reactivex.rxjava3.internal.fuseable.ConditionalSubscriber;
 import io.reactivex.rxjava3.internal.subscribers.*;
+import io.reactivex.rxjava3.operators.ConditionalSubscriber;
 
 /**
  * Map the upstream values into an Optional and emit its value if any.

src/main/java/io/reactivex/rxjava3/internal/jdk8/ObservableFromStream.java

@@ -21,7 +21,7 @@
 import io.reactivex.rxjava3.core.Observer;
 import io.reactivex.rxjava3.exceptions.Exceptions;
 import io.reactivex.rxjava3.internal.disposables.EmptyDisposable;
-import io.reactivex.rxjava3.internal.fuseable.QueueDisposable;
+import io.reactivex.rxjava3.operators.QueueDisposable;
 import io.reactivex.rxjava3.plugins.RxJavaPlugins;
 
 /**

src/main/java/io/reactivex/rxjava3/internal/jdk8/ParallelMapOptional.java

@@ -19,8 +19,8 @@
 
 import io.reactivex.rxjava3.exceptions.Exceptions;
 import io.reactivex.rxjava3.functions.Function;
-import io.reactivex.rxjava3.internal.fuseable.ConditionalSubscriber;
 import io.reactivex.rxjava3.internal.subscriptions.SubscriptionHelper;
+import io.reactivex.rxjava3.operators.ConditionalSubscriber;
 import io.reactivex.rxjava3.parallel.ParallelFlowable;
 import io.reactivex.rxjava3.plugins.RxJavaPlugins;
 

src/main/java/io/reactivex/rxjava3/internal/jdk8/ParallelMapTryOptional.java

@@ -19,8 +19,8 @@
 
 import io.reactivex.rxjava3.exceptions.*;
 import io.reactivex.rxjava3.functions.*;
-import io.reactivex.rxjava3.internal.fuseable.ConditionalSubscriber;
 import io.reactivex.rxjava3.internal.subscriptions.SubscriptionHelper;
+import io.reactivex.rxjava3.operators.ConditionalSubscriber;
 import io.reactivex.rxjava3.parallel.*;
 import io.reactivex.rxjava3.plugins.RxJavaPlugins;
 

src/main/java/io/reactivex/rxjava3/internal/observers/BasicFuseableObserver.java

@@ -17,7 +17,7 @@
 import io.reactivex.rxjava3.disposables.Disposable;
 import io.reactivex.rxjava3.exceptions.Exceptions;
 import io.reactivex.rxjava3.internal.disposables.DisposableHelper;
-import io.reactivex.rxjava3.internal.fuseable.QueueDisposable;
+import io.reactivex.rxjava3.operators.QueueDisposable;
 import io.reactivex.rxjava3.plugins.RxJavaPlugins;
 
 /**

src/main/java/io/reactivex/rxjava3/internal/observers/BasicIntQueueDisposable.java

@@ -15,7 +15,7 @@
 
 import java.util.concurrent.atomic.AtomicInteger;
 
-import io.reactivex.rxjava3.internal.fuseable.QueueDisposable;
+import io.reactivex.rxjava3.operators.QueueDisposable;
 
 /**
  * An abstract QueueDisposable implementation, extending an AtomicInteger,

src/main/java/io/reactivex/rxjava3/internal/observers/BasicQueueDisposable.java

@@ -13,7 +13,7 @@
 
 package io.reactivex.rxjava3.internal.observers;
 
-import io.reactivex.rxjava3.internal.fuseable.QueueDisposable;
+import io.reactivex.rxjava3.operators.QueueDisposable;
 
 /**
  * An abstract QueueDisposable implementation that defaults all

src/main/java/io/reactivex/rxjava3/internal/observers/InnerQueuedObserver.java

@@ -18,8 +18,10 @@
 import io.reactivex.rxjava3.core.Observer;
 import io.reactivex.rxjava3.disposables.Disposable;
 import io.reactivex.rxjava3.internal.disposables.DisposableHelper;
-import io.reactivex.rxjava3.internal.fuseable.*;
 import io.reactivex.rxjava3.internal.util.QueueDrainHelper;
+import io.reactivex.rxjava3.operators.QueueDisposable;
+import io.reactivex.rxjava3.operators.QueueSubscription;
+import io.reactivex.rxjava3.operators.SimpleQueue;
 
 /**
  * Subscriber that can fuse with the upstream and calls a support interface

src/main/java/io/reactivex/rxjava3/internal/observers/QueueDrainObserver.java

@@ -17,8 +17,8 @@
 
 import io.reactivex.rxjava3.core.Observer;
 import io.reactivex.rxjava3.disposables.Disposable;
-import io.reactivex.rxjava3.internal.fuseable.SimplePlainQueue;
 import io.reactivex.rxjava3.internal.util.*;
+import io.reactivex.rxjava3.operators.SimplePlainQueue;
 
 /**
  * Abstract base class for subscribers that hold another subscriber, a queue

src/main/java/io/reactivex/rxjava3/internal/operators/completable/CompletableConcat.java

@@ -21,9 +21,8 @@
 import io.reactivex.rxjava3.disposables.Disposable;
 import io.reactivex.rxjava3.exceptions.*;
 import io.reactivex.rxjava3.internal.disposables.DisposableHelper;
-import io.reactivex.rxjava3.internal.fuseable.*;
-import io.reactivex.rxjava3.internal.queue.*;
 import io.reactivex.rxjava3.internal.subscriptions.SubscriptionHelper;
+import io.reactivex.rxjava3.operators.*;
 import io.reactivex.rxjava3.plugins.RxJavaPlugins;
 
 public final class CompletableConcat extends Completable {

src/main/java/io/reactivex/rxjava3/internal/operators/flowable/BlockingFlowableIterable.java

@@ -22,9 +22,9 @@
 import io.reactivex.rxjava3.core.*;
 import io.reactivex.rxjava3.disposables.Disposable;
 import io.reactivex.rxjava3.exceptions.*;
-import io.reactivex.rxjava3.internal.queue.SpscArrayQueue;
 import io.reactivex.rxjava3.internal.subscriptions.SubscriptionHelper;
 import io.reactivex.rxjava3.internal.util.*;
+import io.reactivex.rxjava3.operators.SpscArrayQueue;
 
 public final class BlockingFlowableIterable<T> implements Iterable<T> {
     final Flowable<T> source;

src/main/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableBufferBoundary.java

@@ -22,9 +22,9 @@
 import io.reactivex.rxjava3.disposables.*;
 import io.reactivex.rxjava3.exceptions.Exceptions;
 import io.reactivex.rxjava3.functions.*;
-import io.reactivex.rxjava3.internal.queue.SpscLinkedArrayQueue;
 import io.reactivex.rxjava3.internal.subscriptions.SubscriptionHelper;
 import io.reactivex.rxjava3.internal.util.*;
+import io.reactivex.rxjava3.operators.SpscLinkedArrayQueue;
 import io.reactivex.rxjava3.plugins.RxJavaPlugins;
 
 public final class FlowableBufferBoundary<T, U extends Collection<? super T>, Open, Close>

src/main/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableCombineLatest.java

@@ -23,9 +23,9 @@
 import io.reactivex.rxjava3.exceptions.Exceptions;
 import io.reactivex.rxjava3.functions.Function;
 import io.reactivex.rxjava3.internal.operators.flowable.FlowableMap.MapSubscriber;
-import io.reactivex.rxjava3.internal.queue.SpscLinkedArrayQueue;
 import io.reactivex.rxjava3.internal.subscriptions.*;
 import io.reactivex.rxjava3.internal.util.*;
+import io.reactivex.rxjava3.operators.SpscLinkedArrayQueue;
 import io.reactivex.rxjava3.plugins.RxJavaPlugins;
 
 /**

src/main/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableConcatMap.java

@@ -21,10 +21,11 @@
 import io.reactivex.rxjava3.core.*;
 import io.reactivex.rxjava3.exceptions.Exceptions;
 import io.reactivex.rxjava3.functions.*;
-import io.reactivex.rxjava3.internal.fuseable.*;
-import io.reactivex.rxjava3.internal.queue.SpscArrayQueue;
 import io.reactivex.rxjava3.internal.subscriptions.*;
 import io.reactivex.rxjava3.internal.util.*;
+import io.reactivex.rxjava3.operators.QueueSubscription;
+import io.reactivex.rxjava3.operators.SimpleQueue;
+import io.reactivex.rxjava3.operators.SpscArrayQueue;
 
 public final class FlowableConcatMap<T, R> extends AbstractFlowableWithUpstream<T, R> {
 

src/main/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableConcatMapEager.java

@@ -21,11 +21,11 @@
 import io.reactivex.rxjava3.core.*;
 import io.reactivex.rxjava3.exceptions.*;
 import io.reactivex.rxjava3.functions.Function;
-import io.reactivex.rxjava3.internal.fuseable.SimpleQueue;
-import io.reactivex.rxjava3.internal.queue.SpscLinkedArrayQueue;
 import io.reactivex.rxjava3.internal.subscribers.*;
 import io.reactivex.rxjava3.internal.subscriptions.SubscriptionHelper;
 import io.reactivex.rxjava3.internal.util.*;
+import io.reactivex.rxjava3.operators.SimpleQueue;
+import io.reactivex.rxjava3.operators.SpscLinkedArrayQueue;
 
 public final class FlowableConcatMapEager<T, R> extends AbstractFlowableWithUpstream<T, R> {
 

src/main/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableConcatMapScheduler.java

@@ -21,11 +21,12 @@
 import io.reactivex.rxjava3.core.*;
 import io.reactivex.rxjava3.exceptions.Exceptions;
 import io.reactivex.rxjava3.functions.*;
-import io.reactivex.rxjava3.internal.fuseable.*;
 import io.reactivex.rxjava3.internal.operators.flowable.FlowableConcatMap.*;
-import io.reactivex.rxjava3.internal.queue.SpscArrayQueue;
 import io.reactivex.rxjava3.internal.subscriptions.SubscriptionHelper;
 import io.reactivex.rxjava3.internal.util.*;
+import io.reactivex.rxjava3.operators.QueueSubscription;
+import io.reactivex.rxjava3.operators.SimpleQueue;
+import io.reactivex.rxjava3.operators.SpscArrayQueue;
 
 public final class FlowableConcatMapScheduler<T, R> extends AbstractFlowableWithUpstream<T, R> {
 

src/main/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableCreate.java

@@ -22,10 +22,10 @@
 import io.reactivex.rxjava3.exceptions.*;
 import io.reactivex.rxjava3.functions.Cancellable;
 import io.reactivex.rxjava3.internal.disposables.*;
-import io.reactivex.rxjava3.internal.fuseable.SimplePlainQueue;
-import io.reactivex.rxjava3.internal.queue.SpscLinkedArrayQueue;
 import io.reactivex.rxjava3.internal.subscriptions.SubscriptionHelper;
 import io.reactivex.rxjava3.internal.util.*;
+import io.reactivex.rxjava3.operators.SimplePlainQueue;
+import io.reactivex.rxjava3.operators.SpscLinkedArrayQueue;
 import io.reactivex.rxjava3.plugins.RxJavaPlugins;
 
 public final class FlowableCreate<T> extends Flowable<T> {

src/main/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableDistinct.java

@@ -22,10 +22,10 @@
 import io.reactivex.rxjava3.core.Flowable;
 import io.reactivex.rxjava3.exceptions.Exceptions;
 import io.reactivex.rxjava3.functions.*;
-import io.reactivex.rxjava3.internal.fuseable.QueueFuseable;
 import io.reactivex.rxjava3.internal.subscribers.BasicFuseableSubscriber;
 import io.reactivex.rxjava3.internal.subscriptions.EmptySubscription;
 import io.reactivex.rxjava3.internal.util.ExceptionHelper;
+import io.reactivex.rxjava3.operators.QueueFuseable;
 import io.reactivex.rxjava3.plugins.RxJavaPlugins;
 
 public final class FlowableDistinct<T, K> extends AbstractFlowableWithUpstream<T, T> {

src/main/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableDistinctUntilChanged.java

@@ -18,8 +18,8 @@
 import io.reactivex.rxjava3.annotations.Nullable;
 import io.reactivex.rxjava3.core.Flowable;
 import io.reactivex.rxjava3.functions.*;
-import io.reactivex.rxjava3.internal.fuseable.ConditionalSubscriber;
 import io.reactivex.rxjava3.internal.subscribers.*;
+import io.reactivex.rxjava3.operators.ConditionalSubscriber;
 
 public final class FlowableDistinctUntilChanged<T, K> extends AbstractFlowableWithUpstream<T, T> {
 

src/main/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableDoAfterNext.java

@@ -18,8 +18,8 @@
 import io.reactivex.rxjava3.annotations.Nullable;
 import io.reactivex.rxjava3.core.Flowable;
 import io.reactivex.rxjava3.functions.Consumer;
-import io.reactivex.rxjava3.internal.fuseable.ConditionalSubscriber;
 import io.reactivex.rxjava3.internal.subscribers.*;
+import io.reactivex.rxjava3.operators.ConditionalSubscriber;
 
 /**
  * Calls a consumer after pushing the current item to the downstream.

src/main/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableDoFinally.java

@@ -19,8 +19,9 @@
 import io.reactivex.rxjava3.core.*;
 import io.reactivex.rxjava3.exceptions.Exceptions;
 import io.reactivex.rxjava3.functions.Action;
-import io.reactivex.rxjava3.internal.fuseable.*;
 import io.reactivex.rxjava3.internal.subscriptions.*;
+import io.reactivex.rxjava3.operators.ConditionalSubscriber;
+import io.reactivex.rxjava3.operators.QueueSubscription;
 import io.reactivex.rxjava3.plugins.RxJavaPlugins;
 
 /**

src/main/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableDoOnEach.java

@@ -19,9 +19,9 @@
 import io.reactivex.rxjava3.core.Flowable;
 import io.reactivex.rxjava3.exceptions.*;
 import io.reactivex.rxjava3.functions.*;
-import io.reactivex.rxjava3.internal.fuseable.ConditionalSubscriber;
 import io.reactivex.rxjava3.internal.subscribers.*;
 import io.reactivex.rxjava3.internal.util.ExceptionHelper;
+import io.reactivex.rxjava3.operators.ConditionalSubscriber;
 import io.reactivex.rxjava3.plugins.RxJavaPlugins;
 
 public final class FlowableDoOnEach<T> extends AbstractFlowableWithUpstream<T, T> {

src/main/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableEmpty.java

@@ -16,8 +16,8 @@
 import org.reactivestreams.Subscriber;
 
 import io.reactivex.rxjava3.core.Flowable;
-import io.reactivex.rxjava3.internal.fuseable.ScalarSupplier;
 import io.reactivex.rxjava3.internal.subscriptions.EmptySubscription;
+import io.reactivex.rxjava3.operators.ScalarSupplier;
 
 /**
  * A source Flowable that signals an onSubscribe() + onComplete() only.

src/main/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableFilter.java

@@ -18,8 +18,9 @@
 import io.reactivex.rxjava3.annotations.Nullable;
 import io.reactivex.rxjava3.core.Flowable;
 import io.reactivex.rxjava3.functions.Predicate;
-import io.reactivex.rxjava3.internal.fuseable.*;
 import io.reactivex.rxjava3.internal.subscribers.*;
+import io.reactivex.rxjava3.operators.ConditionalSubscriber;
+import io.reactivex.rxjava3.operators.QueueSubscription;
 
 public final class FlowableFilter<T> extends AbstractFlowableWithUpstream<T, T> {
     final Predicate<? super T> predicate;

src/main/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableFlatMap.java

@@ -22,10 +22,9 @@
 import io.reactivex.rxjava3.disposables.Disposable;
 import io.reactivex.rxjava3.exceptions.*;
 import io.reactivex.rxjava3.functions.*;
-import io.reactivex.rxjava3.internal.fuseable.*;
-import io.reactivex.rxjava3.internal.queue.*;
 import io.reactivex.rxjava3.internal.subscriptions.SubscriptionHelper;
 import io.reactivex.rxjava3.internal.util.*;
+import io.reactivex.rxjava3.operators.*;
 import io.reactivex.rxjava3.plugins.RxJavaPlugins;
 
 public final class FlowableFlatMap<T, U> extends AbstractFlowableWithUpstream<T, U> {

src/main/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableFlatMapMaybe.java

@@ -23,9 +23,9 @@
 import io.reactivex.rxjava3.exceptions.Exceptions;
 import io.reactivex.rxjava3.functions.Function;
 import io.reactivex.rxjava3.internal.disposables.DisposableHelper;
-import io.reactivex.rxjava3.internal.queue.SpscLinkedArrayQueue;
 import io.reactivex.rxjava3.internal.subscriptions.SubscriptionHelper;
 import io.reactivex.rxjava3.internal.util.*;
+import io.reactivex.rxjava3.operators.SpscLinkedArrayQueue;
 
 /**
  * Maps upstream values into MaybeSources and merges their signals into one sequence.

src/main/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableFlatMapSingle.java

@@ -23,9 +23,9 @@
 import io.reactivex.rxjava3.exceptions.Exceptions;
 import io.reactivex.rxjava3.functions.Function;
 import io.reactivex.rxjava3.internal.disposables.DisposableHelper;
-import io.reactivex.rxjava3.internal.queue.SpscLinkedArrayQueue;
 import io.reactivex.rxjava3.internal.subscriptions.SubscriptionHelper;
 import io.reactivex.rxjava3.internal.util.*;
+import io.reactivex.rxjava3.operators.SpscLinkedArrayQueue;
 
 /**
  * Maps upstream values into SingleSources and merges their signals into one sequence.

src/main/java/io/reactivex/rxjava3/internal/operators/flowable/FlowableFlattenIterable.java

@@ -22,10 +22,11 @@
 import io.reactivex.rxjava3.core.*;
 import io.reactivex.rxjava3.exceptions.*;
 import io.reactivex.rxjava3.functions.*;
-import io.reactivex.rxjava3.internal.fuseable.*;
-import io.reactivex.rxjava3.internal.queue.SpscArrayQueue;
 import io.reactivex.rxjava3.internal.subscriptions.*;
 import io.reactivex.rxjava3.internal.util.*;
+import io.reactivex.rxjava3.operators.QueueSubscription;
+import io.reactivex.rxjava3.operators.SimpleQueue;
+import io.reactivex.rxjava3.operators.SpscArrayQueue;
 import io.reactivex.rxjava3.plugins.RxJavaPlugins;
 
 public final class FlowableFlattenIterable<T, R> extends AbstractFlowableWithUpstream<T, R> {