Associative lift operators to fix synchronous sampleOn

ReactiveCocoa/Swift/SignalProducer.swift

@@ -394,6 +394,15 @@ extension SignalProducerType {
 	/// which may not be adviseable for some operators.
 	@warn_unused_result(message="Did you forget to call `start` on the producer?")
 	public func lift<U, F, V, G>(transform: Signal<Value, Error> -> Signal<U, F> -> Signal<V, G>) -> SignalProducer<U, F> -> SignalProducer<V, G> {
+		return liftRight(transform)
+	}
+
+	/// Right-associative lifting of a binary signal operator over producers. That
+	/// is, the argument producer will be started before the receiver. When both
+	/// producers are synchronous this order can be important depending on the operator
+	/// to generate correct results.
+	@warn_unused_result(message="Did you forget to call `start` on the producer?")
+	private func liftRight<U, F, V, G>(transform: Signal<Value, Error> -> Signal<U, F> -> Signal<V, G>) -> SignalProducer<U, F> -> SignalProducer<V, G> {
 		return { otherProducer in
 			return SignalProducer { observer, outerDisposable in
 				self.startWithSignal { signal, disposable in
@@ -409,6 +418,27 @@ extension SignalProducerType {
 		}
 	}
 
+	/// Left-associative lifting of a binary signal operator over producers. That
+	/// is, the receiver will be started before the argument producer. When both
+	/// producers are synchronous this order can be important depending on the operator
+	/// to generate correct results.
+	@warn_unused_result(message="Did you forget to call `start` on the producer?")
+	private func liftLeft<U, F, V, G>(transform: Signal<Value, Error> -> Signal<U, F> -> Signal<V, G>) -> SignalProducer<U, F> -> SignalProducer<V, G> {
+		return { otherProducer in
+			return SignalProducer { observer, outerDisposable in
+				otherProducer.startWithSignal { otherSignal, otherDisposable in
+					outerDisposable.addDisposable(otherDisposable)
+
+					self.startWithSignal { signal, disposable in
+						outerDisposable.addDisposable(disposable)
+
+						transform(signal)(otherSignal).observe(observer)
+					}
+				}
+			}
+		}
+	}
+
 	/// Lifts a binary Signal operator to operate upon a Signal and a SignalProducer instead.
 	///
 	/// In other words, this will create a new SignalProducer which will apply
@@ -473,7 +503,7 @@ extension SignalProducerType {
 	/// will also be interrupted.
 	@warn_unused_result(message="Did you forget to call `start` on the producer?")
 	public func combineLatestWith<U>(otherProducer: SignalProducer<U, Error>) -> SignalProducer<(Value, U), Error> {
-		return lift(Signal.combineLatestWith)(otherProducer)
+		return liftRight(Signal.combineLatestWith)(otherProducer)
 	}
 
 	/// Combines the latest value of the receiver with the latest value from
@@ -527,7 +557,7 @@ extension SignalProducerType {
 	/// completed, or interrupt if either input producer is interrupted.
 	@warn_unused_result(message="Did you forget to call `start` on the producer?")
 	public func sampleOn(sampler: SignalProducer<(), NoError>) -> SignalProducer<Value, Error> {
-		return lift(Signal.sampleOn)(sampler)
+		return liftLeft(Signal.sampleOn)(sampler)
 	}
 
 	/// Forwards the latest value from `self` whenever `sampler` sends a Next
@@ -548,7 +578,7 @@ extension SignalProducerType {
 	/// event, at which point the returned producer will complete.
 	@warn_unused_result(message="Did you forget to call `start` on the producer?")
 	public func takeUntil(trigger: SignalProducer<(), NoError>) -> SignalProducer<Value, Error> {
-		return lift(Signal.takeUntil)(trigger)
+		return liftRight(Signal.takeUntil)(trigger)
 	}
 
 	/// Forwards events from `self` until `trigger` sends a Next or Completed
@@ -606,7 +636,7 @@ extension SignalProducerType {
 	/// already.
 	@warn_unused_result(message="Did you forget to call `start` on the producer?")
 	public func takeUntilReplacement(replacement: SignalProducer<Value, Error>) -> SignalProducer<Value, Error> {
-		return lift(Signal.takeUntilReplacement)(replacement)
+		return liftRight(Signal.takeUntilReplacement)(replacement)
 	}
 
 	/// Forwards events from `self` until `replacement` begins sending events.
@@ -639,7 +669,7 @@ extension SignalProducerType {
 	/// are the Nth elements of the two input producers.
 	@warn_unused_result(message="Did you forget to call `start` on the producer?")
 	public func zipWith<U>(otherProducer: SignalProducer<U, Error>) -> SignalProducer<(Value, U), Error> {
-		return lift(Signal.zipWith)(otherProducer)
+		return liftRight(Signal.zipWith)(otherProducer)
 	}
 
 	/// Zips elements of this producer and a signal into pairs. The elements of 

ReactiveCocoaTests/Swift/SignalProducerLiftingSpec.swift

@@ -831,6 +831,18 @@ class SignalProducerLiftingSpec: QuickSpec {
 				samplerObserver.sendCompleted()
 				expect(completed).to(beTruthy())
 			}
+
+			it("should emit an initial value if the sampler is a synchronous SignalProducer") {
+				let producer = SignalProducer<Int, NoError>(values: [1])
+				let sampler = SignalProducer<(), NoError>(value: ())
+
+				let result = producer.sampleOn(sampler)
+
+				var valueReceived: Int?
+				result.startWithNext { valueReceived = $0 }
+
+				expect(valueReceived) == 1
+			}
 		}
 
 		describe("combineLatestWith") {