Resurrect the original combinePrevious.

Author: andersio

Sources/Signal.swift

@@ -1444,20 +1444,33 @@ extension Signal {
 
 	/// Forward events from `self` with history: values of the returned signal
 	/// are a tuples whose first member is the previous value and whose second member
-	/// is the current value.
-	///
-	/// If an initial value is given, the returned `Signal` would emit tuples as soon as
-	/// the first value is received. If `initial` is nil, the returned `Signal` would not
-	/// emit any tuple until it has received at least two values.
+	/// is the current value. `initial` is supplied as the first member when `self`
+	/// sends its first value.
 	///
 	/// - parameters:
-	///   - initial: An optional initial value.
+	///   - initial: A value that will be combined with the first value sent by
+	///              `self`.
+	///
+	/// - returns: A signal that sends tuples that contain previous and current
+	///            sent values of `self`.
+	public func combinePrevious(_ initial: Value) -> Signal<(Value, Value), Error> {
+		return scan((initial, initial)) { previousCombinedValues, newValue in
+			return (previousCombinedValues.1, newValue)
+		}
+	}
+
+	/// Forward events from `self` with history: values of the returned signal
+	/// are a tuples whose first member is the previous value and whose second member
+	/// is the current value.
+	///
+	/// The returned `Signal` would not emit any tuple until it has received at least two
+	/// values.
 	///
 	/// - returns: A signal that sends tuples that contain previous and current
 	///            sent values of `self`.
-	public func combinePrevious(_ initial: Value? = nil) -> Signal<(Value, Value), Error> {
+	public func combinePrevious() -> Signal<(Value, Value), Error> {
 		return Signal<(Value, Value), Error> { observer in
-			var previous = initial
+			var previous: Value?
 
 			return self.observe { event in
 				switch event {

Sources/SignalProducer.swift

@@ -978,21 +978,32 @@ extension SignalProducer {
 		return liftRight(Signal.skip(until:))(trigger.producer)
 	}
 
+	/// Forward events from `self` with history: values of the returned signal
+	/// are a tuples whose first member is the previous value and whose second member
+	/// is the current value. `initial` is supplied as the first member when `self`
+	/// sends its first value.
+	///
+	/// - parameters:
+	///   - initial: A value that will be combined with the first value sent by
+	///              `self`.
+	///
+	/// - returns: A signal that sends tuples that contain previous and current
+	///            sent values of `self`.
+	public func combinePrevious(_ initial: Value) -> SignalProducer<(Value, Value), Error> {
+		return lift { $0.combinePrevious(initial) }
+	}
+
 	/// Forward events from `self` with history: values of the produced signal
 	/// are a tuples whose first member is the previous value and whose second member
 	/// is the current value.
 	///
-	/// If an initial value is given, the produced `Signal` would emit tuples as soon as
-	/// the first value is received. If `initial` is nil, the produced `Signal` would not
-	/// emit any tuple until it has received at least two values.
-	///
-	/// - parameters:
-	///   - initial: An optional initial value.
+	/// The produced `Signal` would not emit any tuple until it has received at least two
+	/// values.
 	///
 	/// - returns: A producer that sends tuples that contain previous and current
 	///            sent values of `self`.
-	public func combinePrevious(_ initial: Value? = nil) -> SignalProducer<(Value, Value), Error> {
-		return lift { $0.combinePrevious(initial) }
+	public func combinePrevious() -> SignalProducer<(Value, Value), Error> {
+		return lift { $0.combinePrevious() }
 	}
 
 	/// Combine all values from `self`, and forward the final result.

Tests/ReactiveSwiftTests/SignalSpec.swift

@@ -2768,7 +2768,7 @@ class SignalSpec: QuickSpec {
 				(signal, observer) = (baseSignal, baseObserver)
 			}
 
-			it("should forward the latest value with previous value with the given initial value") {
+			it("should forward the latest value with previous value with an initial value") {
 				signal.combinePrevious(initialValue).observeValues { latestValues = $0 }
 
 				expect(latestValues).to(beNil())