RemainingCombineInterface.swift

// This file contains parts of Apple's Combine that remain unimplemented in OpenCombine
// Please remove the corresponding piece from this file if you implement something,
// and complement this file as features are added in Apple's Combine

extension Publishers {

    /// A publisher that receives and combines the latest elements from two publishers.
    public struct CombineLatest<A, B> : Publisher where A : Publisher, B : Publisher, A.Failure == B.Failure {

        /// The kind of values published by this publisher.
        public typealias Output = (A.Output, B.Output)

        /// The kind of errors this publisher might publish.
        ///
        /// Use `Never` if this `Publisher` does not publish errors.
        public typealias Failure = A.Failure

        public let a: A

        public let b: B

        public init(_ a: A, _ b: B)

        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
        ///
        /// - SeeAlso: `subscribe(_:)`
        /// - Parameters:
        ///     - subscriber: The subscriber to attach to this `Publisher`.
        ///                   once attached it can begin to receive values.
        public func receive<S>(subscriber: S) where S : Subscriber, B.Failure == S.Failure, S.Input == (A.Output, B.Output)
    }

    /// A publisher that receives and combines the latest elements from three publishers.
    public struct CombineLatest3<A, B, C> : Publisher where A : Publisher, B : Publisher, C : Publisher, A.Failure == B.Failure, B.Failure == C.Failure {

        /// The kind of values published by this publisher.
        public typealias Output = (A.Output, B.Output, C.Output)

        /// The kind of errors this publisher might publish.
        ///
        /// Use `Never` if this `Publisher` does not publish errors.
        public typealias Failure = A.Failure

        public let a: A

        public let b: B

        public let c: C

        public init(_ a: A, _ b: B, _ c: C)

        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
        ///
        /// - SeeAlso: `subscribe(_:)`
        /// - Parameters:
        ///     - subscriber: The subscriber to attach to this `Publisher`.
        ///                   once attached it can begin to receive values.
        public func receive<S>(subscriber: S) where S : Subscriber, C.Failure == S.Failure, S.Input == (A.Output, B.Output, C.Output)
    }

    /// A publisher that receives and combines the latest elements from four publishers.
    public struct CombineLatest4<A, B, C, D> : Publisher where A : Publisher, B : Publisher, C : Publisher, D : Publisher, A.Failure == B.Failure, B.Failure == C.Failure, C.Failure == D.Failure {

        /// The kind of values published by this publisher.
        public typealias Output = (A.Output, B.Output, C.Output, D.Output)

        /// The kind of errors this publisher might publish.
        ///
        /// Use `Never` if this `Publisher` does not publish errors.
        public typealias Failure = A.Failure

        public let a: A

        public let b: B

        public let c: C

        public let d: D

        public init(_ a: A, _ b: B, _ c: C, _ d: D)

        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
        ///
        /// - SeeAlso: `subscribe(_:)`
        /// - Parameters:
        ///     - subscriber: The subscriber to attach to this `Publisher`.
        ///                   once attached it can begin to receive values.
        public func receive<S>(subscriber: S) where S : Subscriber, D.Failure == S.Failure, S.Input == (A.Output, B.Output, C.Output, D.Output)
    }
}

extension Publisher {

    /// Subscribes to an additional publisher and publishes a tuple upon receiving output from either publisher.
    ///
    /// The combined publisher passes through any requests to *all* upstream publishers. However, it still obeys the demand-fulfilling rule of only sending the request amount downstream. If the demand isn’t `.unlimited`, it drops values from upstream publishers. It implements this by using a buffer size of 1 for each upstream, and holds the most recent value in each buffer.
    /// All upstream publishers need to finish for this publisher to finsh. If an upstream publisher never publishes a value, this publisher never finishes.
    /// If any of the combined publishers terminates with a failure, this publisher also fails.
    /// - Parameters:
    ///   - other: Another publisher to combine with this one.
    /// - Returns: A publisher that receives and combines elements from this and another publisher.
    public func combineLatest<P>(_ other: P) -> Publishers.CombineLatest<Self, P> where P : Publisher, Self.Failure == P.Failure

    /// Subscribes to an additional publisher and invokes a closure upon receiving output from either publisher.
    ///
    /// The combined publisher passes through any requests to *all* upstream publishers. However, it still obeys the demand-fulfilling rule of only sending the request amount downstream. If the demand isn’t `.unlimited`, it drops values from upstream publishers. It implements this by using a buffer size of 1 for each upstream, and holds the most recent value in each buffer.
    /// All upstream publishers need to finish for this publisher to finsh. If an upstream publisher never publishes a value, this publisher never finishes.
    /// If any of the combined publishers terminates with a failure, this publisher also fails.
    /// - Parameters:
    ///   - other: Another publisher to combine with this one.
    ///   - transform: A closure that receives the most recent value from each publisher and returns a new value to publish.
    /// - Returns: A publisher that receives and combines elements from this and another publisher.
    public func combineLatest<P, T>(_ other: P, _ transform: @escaping (Self.Output, P.Output) -> T) -> Publishers.Map<Publishers.CombineLatest<Self, P>, T> where P : Publisher, Self.Failure == P.Failure

    /// Subscribes to two additional publishers and publishes a tuple upon receiving output from any of the publishers.
    ///
    /// The combined publisher passes through any requests to *all* upstream publishers. However, it still obeys the demand-fulfilling rule of only sending the request amount downstream. If the demand isn’t `.unlimited`, it drops values from upstream publishers. It implements this by using a buffer size of 1 for each upstream, and holds the most recent value in each buffer.
    /// All upstream publishers need to finish for this publisher to finish. If an upstream publisher never publishes a value, this publisher never finishes.
    /// If any of the combined publishers terminates with a failure, this publisher also fails.
    /// - Parameters:
    ///   - publisher1: A second publisher to combine with this one.
    ///   - publisher2: A third publisher to combine with this one.
    /// - Returns: A publisher that receives and combines elements from this publisher and two other publishers.
    public func combineLatest<P, Q>(_ publisher1: P, _ publisher2: Q) -> Publishers.CombineLatest3<Self, P, Q> where P : Publisher, Q : Publisher, Self.Failure == P.Failure, P.Failure == Q.Failure

    /// Subscribes to two additional publishers and invokes a closure upon receiving output from any of the publishers.
    ///
    /// The combined publisher passes through any requests to *all* upstream publishers. However, it still obeys the demand-fulfilling rule of only sending the request amount downstream. If the demand isn’t `.unlimited`, it drops values from upstream publishers. It implements this by using a buffer size of 1 for each upstream, and holds the most recent value in each buffer.
    /// All upstream publishers need to finish for this publisher to finish. If an upstream publisher never publishes a value, this publisher never finishes.
    /// If any of the combined publishers terminates with a failure, this publisher also fails.
    /// - Parameters:
    ///   - publisher1: A second publisher to combine with this one.
    ///   - publisher2: A third publisher to combine with this one.
    ///   - transform: A closure that receives the most recent value from each publisher and returns a new value to publish.
    /// - Returns: A publisher that receives and combines elements from this publisher and two other publishers.
    public func combineLatest<P, Q, T>(_ publisher1: P, _ publisher2: Q, _ transform: @escaping (Self.Output, P.Output, Q.Output) -> T) -> Publishers.Map<Publishers.CombineLatest3<Self, P, Q>, T> where P : Publisher, Q : Publisher, Self.Failure == P.Failure, P.Failure == Q.Failure

    /// Subscribes to three additional publishers and publishes a tuple upon receiving output from any of the publishers.
    ///
    /// The combined publisher passes through any requests to *all* upstream publishers. However, it still obeys the demand-fulfilling rule of only sending the request amount downstream. If the demand isn’t `.unlimited`, it drops values from upstream publishers. It implements this by using a buffer size of 1 for each upstream, and holds the most recent value in each buffer.
    /// All upstream publishers need to finish for this publisher to finish. If an upstream publisher never publishes a value, this publisher never finishes.
    /// If any of the combined publishers terminates with a failure, this publisher also fails.
    /// - Parameters:
    ///   - publisher1: A second publisher to combine with this one.
    ///   - publisher2: A third publisher to combine with this one.
    ///   - publisher3: A fourth publisher to combine with this one.
    /// - Returns: A publisher that receives and combines elements from this publisher and three other publishers.
    public func combineLatest<P, Q, R>(_ publisher1: P, _ publisher2: Q, _ publisher3: R) -> Publishers.CombineLatest4<Self, P, Q, R> where P : Publisher, Q : Publisher, R : Publisher, Self.Failure == P.Failure, P.Failure == Q.Failure, Q.Failure == R.Failure

    /// Subscribes to three additional publishers and invokes a closure upon receiving output from any of the publishers.
    ///
    /// The combined publisher passes through any requests to *all* upstream publishers. However, it still obeys the demand-fulfilling rule of only sending the request amount downstream. If the demand isn’t `.unlimited`, it drops values from upstream publishers. It implements this by using a buffer size of 1 for each upstream, and holds the most recent value in each buffer.
    /// All upstream publishers need to finish for this publisher to finish. If an upstream publisher never publishes a value, this publisher never finishes.
    /// If any of the combined publishers terminates with a failure, this publisher also fails.
    /// - Parameters:
    ///   - publisher1: A second publisher to combine with this one.
    ///   - publisher2: A third publisher to combine with this one.
    ///   - publisher3: A fourth publisher to combine with this one.
    ///   - transform: A closure that receives the most recent value from each publisher and returns a new value to publish.
    /// - Returns: A publisher that receives and combines elements from this publisher and three other publishers.
    public func combineLatest<P, Q, R, T>(_ publisher1: P, _ publisher2: Q, _ publisher3: R, _ transform: @escaping (Self.Output, P.Output, Q.Output, R.Output) -> T) -> Publishers.Map<Publishers.CombineLatest4<Self, P, Q, R>, T> where P : Publisher, Q : Publisher, R : Publisher, Self.Failure == P.Failure, P.Failure == Q.Failure, Q.Failure == R.Failure
}

extension Publishers {

    /// A strategy for collecting received elements.
    public enum TimeGroupingStrategy<Context> where Context : Scheduler {

        /// A grouping that collects and periodically publishes items.
        case byTime(Context, Context.SchedulerTimeType.Stride)

        /// A grouping that collects and publishes items periodically or when a buffer reaches a maximum size.
        case byTimeOrCount(Context, Context.SchedulerTimeType.Stride, Int)
    }

    /// A publisher that buffers and periodically publishes its items.
    public struct CollectByTime<Upstream, Context> : Publisher where Upstream : Publisher, Context : Scheduler {

        /// The kind of values published by this publisher.
        public typealias Output = [Upstream.Output]

        /// The kind of errors this publisher might publish.
        ///
        /// Use `Never` if this `Publisher` does not publish errors.
        public typealias Failure = Upstream.Failure

        /// The publisher that this publisher receives elements from.
        public let upstream: Upstream

        /// The strategy with which to collect and publish elements.
        public let strategy: Publishers.TimeGroupingStrategy<Context>

        /// `Scheduler` options to use for the strategy.
        public let options: Context.SchedulerOptions?

        public init(upstream: Upstream, strategy: Publishers.TimeGroupingStrategy<Context>, options: Context.SchedulerOptions?)

        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
        ///
        /// - SeeAlso: `subscribe(_:)`
        /// - Parameters:
        ///     - subscriber: The subscriber to attach to this `Publisher`.
        ///                   once attached it can begin to receive values.
        public func receive<S>(subscriber: S) where S : Subscriber, Upstream.Failure == S.Failure, S.Input == [Upstream.Output]
    }
}

extension Publisher {

    /// Collects elements by a given time-grouping strategy, and emits a single array of
    /// the collection.
    ///
    /// Use `collect(_:options:)` to emit arrays of elements on a schedule specified by
    /// a `Scheduler` and `Stride` that you provide. At the end of each scheduled
    /// interval, the publisher sends an array that contains the items it collected.
    /// If the upstream publisher finishes before filling the buffer, the publisher sends
    /// an array that contains items it received. This may be fewer than the number of
    /// elements specified in the requested `Stride`.
    ///
    /// If the upstream publisher fails with an error, this publisher forwards the error
    /// to the downstream receiver instead of sending its output.
    ///
    /// The example above collects timestamps generated on a one-second `Timer` in groups
    /// (`Stride`) of five.
    ///
    ///     let sub = Timer.publish(every: 1, on: .main, in: .default)
    ///         .autoconnect()
    ///         .collect(.byTime(RunLoop.main, .seconds(5)))
    ///         .sink { print("\($0)", terminator: "\n\n") }
    ///
    ///     // Prints: "[2020-01-24 00:54:46 +0000, 2020-01-24 00:54:47 +0000,
    ///     //          2020-01-24 00:54:48 +0000, 2020-01-24 00:54:49 +0000,
    ///     //          2020-01-24 00:54:50 +0000]"
    ///
    /// > Note: When this publisher receives a request for `.max(n)` elements, it requests
    /// `.max(count * n)` from the upstream publisher.
    ///
    /// - Parameters:
    ///   - strategy: The timing group strategy used by the operator to collect and
    ///     publish elements.
    ///   - options: ``Scheduler`` options to use for the strategy.
    /// - Returns: A publisher that collects elements by a given strategy, and emits
    ///   a single array of the collection.
    public func collect<S>(_ strategy: Publishers.TimeGroupingStrategy<S>, options: S.SchedulerOptions? = nil) -> Publishers.CollectByTime<Self, S> where S : Scheduler
}

extension Publishers {

    public struct PrefixUntilOutput<Upstream, Other> : Publisher where Upstream : Publisher, Other : Publisher {

        /// The kind of values published by this publisher.
        public typealias Output = Upstream.Output

        /// The kind of errors this publisher might publish.
        ///
        /// Use `Never` if this `Publisher` does not publish errors.
        public typealias Failure = Upstream.Failure

        /// The publisher from which this publisher receives elements.
        public let upstream: Upstream

        /// Another publisher, whose first output causes this publisher to finish.
        public let other: Other

        public init(upstream: Upstream, other: Other)

        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
        ///
        /// - SeeAlso: `subscribe(_:)`
        /// - Parameters:
        ///     - subscriber: The subscriber to attach to this `Publisher`.
        ///                   once attached it can begin to receive values.
        public func receive<S>(subscriber: S) where S : Subscriber, Upstream.Failure == S.Failure, Upstream.Output == S.Input
    }
}

extension Publisher {

    /// Republishes elements until another publisher emits an element.
    ///
    /// After the second publisher publishes an element, the publisher returned by this method finishes.
    ///
    /// - Parameter publisher: A second publisher.
    /// - Returns: A publisher that republishes elements until the second publisher publishes an element.
    public func prefix<P>(untilOutputFrom publisher: P) -> Publishers.PrefixUntilOutput<Self, P> where P : Publisher
}

extension Publishers {

    /// A publisher created by applying the merge function to two upstream publishers.
    public struct Merge<A, B> : Publisher where A : Publisher, B : Publisher, A.Failure == B.Failure, A.Output == B.Output {

        /// The kind of values published by this publisher.
        public typealias Output = A.Output

        /// The kind of errors this publisher might publish.
        ///
        /// Use `Never` if this `Publisher` does not publish errors.
        public typealias Failure = A.Failure

        public let a: A

        public let b: B

        public init(_ a: A, _ b: B)

        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
        ///
        /// - SeeAlso: `subscribe(_:)`
        /// - Parameters:
        ///     - subscriber: The subscriber to attach to this `Publisher`.
        ///                   once attached it can begin to receive values.
        public func receive<S>(subscriber: S) where S : Subscriber, B.Failure == S.Failure, B.Output == S.Input

        public func merge<P>(with other: P) -> Publishers.Merge3<A, B, P> where P : Publisher, B.Failure == P.Failure, B.Output == P.Output

        public func merge<Z, Y>(with z: Z, _ y: Y) -> Publishers.Merge4<A, B, Z, Y> where Z : Publisher, Y : Publisher, B.Failure == Z.Failure, B.Output == Z.Output, Z.Failure == Y.Failure, Z.Output == Y.Output

        public func merge<Z, Y, X>(with z: Z, _ y: Y, _ x: X) -> Publishers.Merge5<A, B, Z, Y, X> where Z : Publisher, Y : Publisher, X : Publisher, B.Failure == Z.Failure, B.Output == Z.Output, Z.Failure == Y.Failure, Z.Output == Y.Output, Y.Failure == X.Failure, Y.Output == X.Output

        public func merge<Z, Y, X, W>(with z: Z, _ y: Y, _ x: X, _ w: W) -> Publishers.Merge6<A, B, Z, Y, X, W> where Z : Publisher, Y : Publisher, X : Publisher, W : Publisher, B.Failure == Z.Failure, B.Output == Z.Output, Z.Failure == Y.Failure, Z.Output == Y.Output, Y.Failure == X.Failure, Y.Output == X.Output, X.Failure == W.Failure, X.Output == W.Output

        public func merge<Z, Y, X, W, V>(with z: Z, _ y: Y, _ x: X, _ w: W, _ v: V) -> Publishers.Merge7<A, B, Z, Y, X, W, V> where Z : Publisher, Y : Publisher, X : Publisher, W : Publisher, V : Publisher, B.Failure == Z.Failure, B.Output == Z.Output, Z.Failure == Y.Failure, Z.Output == Y.Output, Y.Failure == X.Failure, Y.Output == X.Output, X.Failure == W.Failure, X.Output == W.Output, W.Failure == V.Failure, W.Output == V.Output

        public func merge<Z, Y, X, W, V, U>(with z: Z, _ y: Y, _ x: X, _ w: W, _ v: V, _ u: U) -> Publishers.Merge8<A, B, Z, Y, X, W, V, U> where Z : Publisher, Y : Publisher, X : Publisher, W : Publisher, V : Publisher, U : Publisher, B.Failure == Z.Failure, B.Output == Z.Output, Z.Failure == Y.Failure, Z.Output == Y.Output, Y.Failure == X.Failure, Y.Output == X.Output, X.Failure == W.Failure, X.Output == W.Output, W.Failure == V.Failure, W.Output == V.Output, V.Failure == U.Failure, V.Output == U.Output
    }

    /// A publisher created by applying the merge function to three upstream publishers.
    public struct Merge3<A, B, C> : Publisher where A : Publisher, B : Publisher, C : Publisher, A.Failure == B.Failure, A.Output == B.Output, B.Failure == C.Failure, B.Output == C.Output {

        /// The kind of values published by this publisher.
        public typealias Output = A.Output

        /// The kind of errors this publisher might publish.
        ///
        /// Use `Never` if this `Publisher` does not publish errors.
        public typealias Failure = A.Failure

        public let a: A

        public let b: B

        public let c: C

        public init(_ a: A, _ b: B, _ c: C)

        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
        ///
        /// - SeeAlso: `subscribe(_:)`
        /// - Parameters:
        ///     - subscriber: The subscriber to attach to this `Publisher`.
        ///                   once attached it can begin to receive values.
        public func receive<S>(subscriber: S) where S : Subscriber, C.Failure == S.Failure, C.Output == S.Input

        public func merge<P>(with other: P) -> Publishers.Merge4<A, B, C, P> where P : Publisher, C.Failure == P.Failure, C.Output == P.Output

        public func merge<Z, Y>(with z: Z, _ y: Y) -> Publishers.Merge5<A, B, C, Z, Y> where Z : Publisher, Y : Publisher, C.Failure == Z.Failure, C.Output == Z.Output, Z.Failure == Y.Failure, Z.Output == Y.Output

        public func merge<Z, Y, X>(with z: Z, _ y: Y, _ x: X) -> Publishers.Merge6<A, B, C, Z, Y, X> where Z : Publisher, Y : Publisher, X : Publisher, C.Failure == Z.Failure, C.Output == Z.Output, Z.Failure == Y.Failure, Z.Output == Y.Output, Y.Failure == X.Failure, Y.Output == X.Output

        public func merge<Z, Y, X, W>(with z: Z, _ y: Y, _ x: X, _ w: W) -> Publishers.Merge7<A, B, C, Z, Y, X, W> where Z : Publisher, Y : Publisher, X : Publisher, W : Publisher, C.Failure == Z.Failure, C.Output == Z.Output, Z.Failure == Y.Failure, Z.Output == Y.Output, Y.Failure == X.Failure, Y.Output == X.Output, X.Failure == W.Failure, X.Output == W.Output

        public func merge<Z, Y, X, W, V>(with z: Z, _ y: Y, _ x: X, _ w: W, _ v: V) -> Publishers.Merge8<A, B, C, Z, Y, X, W, V> where Z : Publisher, Y : Publisher, X : Publisher, W : Publisher, V : Publisher, C.Failure == Z.Failure, C.Output == Z.Output, Z.Failure == Y.Failure, Z.Output == Y.Output, Y.Failure == X.Failure, Y.Output == X.Output, X.Failure == W.Failure, X.Output == W.Output, W.Failure == V.Failure, W.Output == V.Output
    }

    /// A publisher created by applying the merge function to four upstream publishers.
    public struct Merge4<A, B, C, D> : Publisher where A : Publisher, B : Publisher, C : Publisher, D : Publisher, A.Failure == B.Failure, A.Output == B.Output, B.Failure == C.Failure, B.Output == C.Output, C.Failure == D.Failure, C.Output == D.Output {

        /// The kind of values published by this publisher.
        public typealias Output = A.Output

        /// The kind of errors this publisher might publish.
        ///
        /// Use `Never` if this `Publisher` does not publish errors.
        public typealias Failure = A.Failure

        public let a: A

        public let b: B

        public let c: C

        public let d: D

        public init(_ a: A, _ b: B, _ c: C, _ d: D)

        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
        ///
        /// - SeeAlso: `subscribe(_:)`
        /// - Parameters:
        ///     - subscriber: The subscriber to attach to this `Publisher`.
        ///                   once attached it can begin to receive values.
        public func receive<S>(subscriber: S) where S : Subscriber, D.Failure == S.Failure, D.Output == S.Input

        public func merge<P>(with other: P) -> Publishers.Merge5<A, B, C, D, P> where P : Publisher, D.Failure == P.Failure, D.Output == P.Output

        public func merge<Z, Y>(with z: Z, _ y: Y) -> Publishers.Merge6<A, B, C, D, Z, Y> where Z : Publisher, Y : Publisher, D.Failure == Z.Failure, D.Output == Z.Output, Z.Failure == Y.Failure, Z.Output == Y.Output

        public func merge<Z, Y, X>(with z: Z, _ y: Y, _ x: X) -> Publishers.Merge7<A, B, C, D, Z, Y, X> where Z : Publisher, Y : Publisher, X : Publisher, D.Failure == Z.Failure, D.Output == Z.Output, Z.Failure == Y.Failure, Z.Output == Y.Output, Y.Failure == X.Failure, Y.Output == X.Output

        public func merge<Z, Y, X, W>(with z: Z, _ y: Y, _ x: X, _ w: W) -> Publishers.Merge8<A, B, C, D, Z, Y, X, W> where Z : Publisher, Y : Publisher, X : Publisher, W : Publisher, D.Failure == Z.Failure, D.Output == Z.Output, Z.Failure == Y.Failure, Z.Output == Y.Output, Y.Failure == X.Failure, Y.Output == X.Output, X.Failure == W.Failure, X.Output == W.Output
    }

    /// A publisher created by applying the merge function to five upstream publishers.
    public struct Merge5<A, B, C, D, E> : Publisher where A : Publisher, B : Publisher, C : Publisher, D : Publisher, E : Publisher, A.Failure == B.Failure, A.Output == B.Output, B.Failure == C.Failure, B.Output == C.Output, C.Failure == D.Failure, C.Output == D.Output, D.Failure == E.Failure, D.Output == E.Output {

        /// The kind of values published by this publisher.
        public typealias Output = A.Output

        /// The kind of errors this publisher might publish.
        ///
        /// Use `Never` if this `Publisher` does not publish errors.
        public typealias Failure = A.Failure

        public let a: A

        public let b: B

        public let c: C

        public let d: D

        public let e: E

        public init(_ a: A, _ b: B, _ c: C, _ d: D, _ e: E)

        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
        ///
        /// - SeeAlso: `subscribe(_:)`
        /// - Parameters:
        ///     - subscriber: The subscriber to attach to this `Publisher`.
        ///                   once attached it can begin to receive values.
        public func receive<S>(subscriber: S) where S : Subscriber, E.Failure == S.Failure, E.Output == S.Input

        public func merge<P>(with other: P) -> Publishers.Merge6<A, B, C, D, E, P> where P : Publisher, E.Failure == P.Failure, E.Output == P.Output

        public func merge<Z, Y>(with z: Z, _ y: Y) -> Publishers.Merge7<A, B, C, D, E, Z, Y> where Z : Publisher, Y : Publisher, E.Failure == Z.Failure, E.Output == Z.Output, Z.Failure == Y.Failure, Z.Output == Y.Output

        public func merge<Z, Y, X>(with z: Z, _ y: Y, _ x: X) -> Publishers.Merge8<A, B, C, D, E, Z, Y, X> where Z : Publisher, Y : Publisher, X : Publisher, E.Failure == Z.Failure, E.Output == Z.Output, Z.Failure == Y.Failure, Z.Output == Y.Output, Y.Failure == X.Failure, Y.Output == X.Output
    }

    /// A publisher created by applying the merge function to six upstream publishers.
    public struct Merge6<A, B, C, D, E, F> : Publisher where A : Publisher, B : Publisher, C : Publisher, D : Publisher, E : Publisher, F : Publisher, A.Failure == B.Failure, A.Output == B.Output, B.Failure == C.Failure, B.Output == C.Output, C.Failure == D.Failure, C.Output == D.Output, D.Failure == E.Failure, D.Output == E.Output, E.Failure == F.Failure, E.Output == F.Output {

        /// The kind of values published by this publisher.
        public typealias Output = A.Output

        /// The kind of errors this publisher might publish.
        ///
        /// Use `Never` if this `Publisher` does not publish errors.
        public typealias Failure = A.Failure

        public let a: A

        public let b: B

        public let c: C

        public let d: D

        public let e: E

        public let f: F

        public init(_ a: A, _ b: B, _ c: C, _ d: D, _ e: E, _ f: F)

        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
        ///
        /// - SeeAlso: `subscribe(_:)`
        /// - Parameters:
        ///     - subscriber: The subscriber to attach to this `Publisher`.
        ///                   once attached it can begin to receive values.
        public func receive<S>(subscriber: S) where S : Subscriber, F.Failure == S.Failure, F.Output == S.Input

        public func merge<P>(with other: P) -> Publishers.Merge7<A, B, C, D, E, F, P> where P : Publisher, F.Failure == P.Failure, F.Output == P.Output

        public func merge<Z, Y>(with z: Z, _ y: Y) -> Publishers.Merge8<A, B, C, D, E, F, Z, Y> where Z : Publisher, Y : Publisher, F.Failure == Z.Failure, F.Output == Z.Output, Z.Failure == Y.Failure, Z.Output == Y.Output
    }

    /// A publisher created by applying the merge function to seven upstream publishers.
    public struct Merge7<A, B, C, D, E, F, G> : Publisher where A : Publisher, B : Publisher, C : Publisher, D : Publisher, E : Publisher, F : Publisher, G : Publisher, A.Failure == B.Failure, A.Output == B.Output, B.Failure == C.Failure, B.Output == C.Output, C.Failure == D.Failure, C.Output == D.Output, D.Failure == E.Failure, D.Output == E.Output, E.Failure == F.Failure, E.Output == F.Output, F.Failure == G.Failure, F.Output == G.Output {

        /// The kind of values published by this publisher.
        public typealias Output = A.Output

        /// The kind of errors this publisher might publish.
        ///
        /// Use `Never` if this `Publisher` does not publish errors.
        public typealias Failure = A.Failure

        public let a: A

        public let b: B

        public let c: C

        public let d: D

        public let e: E

        public let f: F

        public let g: G

        public init(_ a: A, _ b: B, _ c: C, _ d: D, _ e: E, _ f: F, _ g: G)

        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
        ///
        /// - SeeAlso: `subscribe(_:)`
        /// - Parameters:
        ///     - subscriber: The subscriber to attach to this `Publisher`.
        ///                   once attached it can begin to receive values.
        public func receive<S>(subscriber: S) where S : Subscriber, G.Failure == S.Failure, G.Output == S.Input

        public func merge<P>(with other: P) -> Publishers.Merge8<A, B, C, D, E, F, G, P> where P : Publisher, G.Failure == P.Failure, G.Output == P.Output
    }

    /// A publisher created by applying the merge function to eight upstream publishers.
    public struct Merge8<A, B, C, D, E, F, G, H> : Publisher where A : Publisher, B : Publisher, C : Publisher, D : Publisher, E : Publisher, F : Publisher, G : Publisher, H : Publisher, A.Failure == B.Failure, A.Output == B.Output, B.Failure == C.Failure, B.Output == C.Output, C.Failure == D.Failure, C.Output == D.Output, D.Failure == E.Failure, D.Output == E.Output, E.Failure == F.Failure, E.Output == F.Output, F.Failure == G.Failure, F.Output == G.Output, G.Failure == H.Failure, G.Output == H.Output {

        /// The kind of values published by this publisher.
        public typealias Output = A.Output

        /// The kind of errors this publisher might publish.
        ///
        /// Use `Never` if this `Publisher` does not publish errors.
        public typealias Failure = A.Failure

        public let a: A

        public let b: B

        public let c: C

        public let d: D

        public let e: E

        public let f: F

        public let g: G

        public let h: H

        public init(_ a: A, _ b: B, _ c: C, _ d: D, _ e: E, _ f: F, _ g: G, _ h: H)

        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
        ///
        /// - SeeAlso: `subscribe(_:)`
        /// - Parameters:
        ///     - subscriber: The subscriber to attach to this `Publisher`.
        ///                   once attached it can begin to receive values.
        public func receive<S>(subscriber: S) where S : Subscriber, H.Failure == S.Failure, H.Output == S.Input
    }

    public struct MergeMany<Upstream> : Publisher where Upstream : Publisher {

        /// The kind of values published by this publisher.
        public typealias Output = Upstream.Output

        /// The kind of errors this publisher might publish.
        ///
        /// Use `Never` if this `Publisher` does not publish errors.
        public typealias Failure = Upstream.Failure

        public let publishers: [Upstream]

        public init(_ upstream: Upstream...)

        public init<S>(_ upstream: S) where Upstream == S.Element, S : Sequence

        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
        ///
        /// - SeeAlso: `subscribe(_:)`
        /// - Parameters:
        ///     - subscriber: The subscriber to attach to this `Publisher`.
        ///                   once attached it can begin to receive values.
        public func receive<S>(subscriber: S) where S : Subscriber, Upstream.Failure == S.Failure, Upstream.Output == S.Input

        public func merge(with other: Upstream) -> Publishers.MergeMany<Upstream>
    }
}

extension Publisher {

    /// Combines elements from this publisher with those from another publisher, delivering an interleaved sequence of elements.
    ///
    /// The merged publisher continues to emit elements until all upstream publishers finish. If an upstream publisher produces an error, the merged publisher fails with that error.
    /// - Parameter other: Another publisher.
    /// - Returns: A publisher that emits an event when either upstream publisher emits an event.
    public func merge<P>(with other: P) -> Publishers.Merge<Self, P> where P : Publisher, Self.Failure == P.Failure, Self.Output == P.Output

    /// Combines elements from this publisher with those from two other publishers, delivering an interleaved sequence of elements.
    ///
    /// The merged publisher continues to emit elements until all upstream publishers finish. If an upstream publisher produces an error, the merged publisher fails with that error.
    ///
    /// - Parameters:
    ///   - b: A second publisher.
    ///   - c: A third publisher.
    /// - Returns:  A publisher that emits an event when any upstream publisher emits
    /// an event.
    public func merge<B, C>(with b: B, _ c: C) -> Publishers.Merge3<Self, B, C> where B : Publisher, C : Publisher, Self.Failure == B.Failure, Self.Output == B.Output, B.Failure == C.Failure, B.Output == C.Output

    /// Combines elements from this publisher with those from three other publishers, delivering
    /// an interleaved sequence of elements.
    ///
    /// The merged publisher continues to emit elements until all upstream publishers finish. If an upstream publisher produces an error, the merged publisher fails with that error.
    ///
    /// - Parameters:
    ///   - b: A second publisher.
    ///   - c: A third publisher.
    ///   - d: A fourth publisher.
    /// - Returns: A publisher that emits an event when any upstream publisher emits an event.
    public func merge<B, C, D>(with b: B, _ c: C, _ d: D) -> Publishers.Merge4<Self, B, C, D> where B : Publisher, C : Publisher, D : Publisher, Self.Failure == B.Failure, Self.Output == B.Output, B.Failure == C.Failure, B.Output == C.Output, C.Failure == D.Failure, C.Output == D.Output

    /// Combines elements from this publisher with those from four other publishers, delivering an interleaved sequence of elements.
    ///
    /// The merged publisher continues to emit elements until all upstream publishers finish. If an upstream publisher produces an error, the merged publisher fails with that error.
    ///
    /// - Parameters:
    ///   - b: A second publisher.
    ///   - c: A third publisher.
    ///   - d: A fourth publisher.
    ///   - e: A fifth publisher.
    /// - Returns: A publisher that emits an event when any upstream publisher emits an event.
    public func merge<B, C, D, E>(with b: B, _ c: C, _ d: D, _ e: E) -> Publishers.Merge5<Self, B, C, D, E> where B : Publisher, C : Publisher, D : Publisher, E : Publisher, Self.Failure == B.Failure, Self.Output == B.Output, B.Failure == C.Failure, B.Output == C.Output, C.Failure == D.Failure, C.Output == D.Output, D.Failure == E.Failure, D.Output == E.Output

    /// Combines elements from this publisher with those from five other publishers, delivering an interleaved sequence of elements.
    ///
    /// The merged publisher continues to emit elements until all upstream publishers finish. If an upstream publisher produces an error, the merged publisher fails with that error.
    ///
    /// - Parameters:
    ///   - b: A second publisher.
    ///   - c: A third publisher.
    ///   - d: A fourth publisher.
    ///   - e: A fifth publisher.
    ///   - f: A sixth publisher.
    /// - Returns: A publisher that emits an event when any upstream publisher emits an event.
    public func merge<B, C, D, E, F>(with b: B, _ c: C, _ d: D, _ e: E, _ f: F) -> Publishers.Merge6<Self, B, C, D, E, F> where B : Publisher, C : Publisher, D : Publisher, E : Publisher, F : Publisher, Self.Failure == B.Failure, Self.Output == B.Output, B.Failure == C.Failure, B.Output == C.Output, C.Failure == D.Failure, C.Output == D.Output, D.Failure == E.Failure, D.Output == E.Output, E.Failure == F.Failure, E.Output == F.Output

    /// Combines elements from this publisher with those from six other publishers, delivering an interleaved sequence of elements.
    ///
    /// The merged publisher continues to emit elements until all upstream publishers finish. If an upstream publisher produces an error, the merged publisher fails with that error.
    ///
    /// - Parameters:
    ///   - b: A second publisher.
    ///   - c: A third publisher.
    ///   - d: A fourth publisher.
    ///   - e: A fifth publisher.
    ///   - f: A sixth publisher.
    ///   - g: A seventh publisher.
    /// - Returns: A publisher that emits an event when any upstream publisher emits an event.
    public func merge<B, C, D, E, F, G>(with b: B, _ c: C, _ d: D, _ e: E, _ f: F, _ g: G) -> Publishers.Merge7<Self, B, C, D, E, F, G> where B : Publisher, C : Publisher, D : Publisher, E : Publisher, F : Publisher, G : Publisher, Self.Failure == B.Failure, Self.Output == B.Output, B.Failure == C.Failure, B.Output == C.Output, C.Failure == D.Failure, C.Output == D.Output, D.Failure == E.Failure, D.Output == E.Output, E.Failure == F.Failure, E.Output == F.Output, F.Failure == G.Failure, F.Output == G.Output

    /// Combines elements from this publisher with those from seven other publishers, delivering an interleaved sequence of elements.
    ///
    /// The merged publisher continues to emit elements until all upstream publishers finish. If an upstream publisher produces an error, the merged publisher fails with that error.
    ///
    /// - Parameters:
    ///   - b: A second publisher.
    ///   - c: A third publisher.
    ///   - d: A fourth publisher.
    ///   - e: A fifth publisher.
    ///   - f: A sixth publisher.
    ///   - g: A seventh publisher.
    ///   - h: An eighth publisher.
    /// - Returns: A publisher that emits an event when any upstream publisher emits an event.
    public func merge<B, C, D, E, F, G, H>(with b: B, _ c: C, _ d: D, _ e: E, _ f: F, _ g: G, _ h: H) -> Publishers.Merge8<Self, B, C, D, E, F, G, H> where B : Publisher, C : Publisher, D : Publisher, E : Publisher, F : Publisher, G : Publisher, H : Publisher, Self.Failure == B.Failure, Self.Output == B.Output, B.Failure == C.Failure, B.Output == C.Output, C.Failure == D.Failure, C.Output == D.Output, D.Failure == E.Failure, D.Output == E.Output, E.Failure == F.Failure, E.Output == F.Output, F.Failure == G.Failure, F.Output == G.Output, G.Failure == H.Failure, G.Output == H.Output

    /// Combines elements from this publisher with those from another publisher of the same type, delivering an interleaved sequence of elements.
    ///
    /// - Parameter other: Another publisher of this publisher's type.
    /// - Returns: A publisher that emits an event when either upstream publisher emits
    /// an event.
    public func merge(with other: Self) -> Publishers.MergeMany<Self>
}

extension Publishers {

    /// A publisher that publishes either the most-recent or first element published by the upstream publisher in a specified time interval.
    public struct Throttle<Upstream, Context> : Publisher where Upstream : Publisher, Context : Scheduler {

        /// The kind of values published by this publisher.
        public typealias Output = Upstream.Output

        /// The kind of errors this publisher might publish.
        ///
        /// Use `Never` if this `Publisher` does not publish errors.
        public typealias Failure = Upstream.Failure

        /// The publisher from which this publisher receives elements.
        public let upstream: Upstream

        /// The interval in which to find and emit the most recent element.
        public let interval: Context.SchedulerTimeType.Stride

        /// The scheduler on which to publish elements.
        public let scheduler: Context

        /// A Boolean value indicating whether to publish the most recent element.
        ///
        /// If `false`, the publisher emits the first element received during the interval.
        public let latest: Bool

        public init(upstream: Upstream, interval: Context.SchedulerTimeType.Stride, scheduler: Context, latest: Bool)

        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
        ///
        /// - SeeAlso: `subscribe(_:)`
        /// - Parameters:
        ///     - subscriber: The subscriber to attach to this `Publisher`.
        ///                   once attached it can begin to receive values.
        public func receive<S>(subscriber: S) where S : Subscriber, Upstream.Failure == S.Failure, Upstream.Output == S.Input
    }
}

extension Publisher {

    /// Publishes either the most-recent or first element published by the upstream
    /// publisher in the specified time interval.
    ///
    /// Use `throttle(for:scheduler:latest:`` to selectively republish elements from
    /// an upstream publisher during an interval you specify. Other elements received from
    /// the upstream in the throttling interval aren’t republished.
    ///
    /// In the example below, a `Timer.TimerPublisher` produces elements on 3-second
    /// intervals; the `throttle(for:scheduler:latest:)` operator delivers the first
    /// event, then republishes only the latest event in the following ten second
    /// intervals:
    ///
    ///     cancellable = Timer.publish(every: 3.0, on: .main, in: .default)
    ///         .autoconnect()
    ///         .print("\(Date().description)")
    ///         .throttle(for: 10.0, scheduler: RunLoop.main, latest: true)
    ///         .sink(
    ///             receiveCompletion: { print ("Completion: \($0).") },
    ///             receiveValue: { print("Received Timestamp \($0).") }
    ///          )
    ///
    ///     // Prints:
    ///     //    Publish at: 2020-03-19 18:26:54 +0000: receive value: (2020-03-19 18:26:57 +0000)
    ///     //    Received Timestamp 2020-03-19 18:26:57 +0000.
    ///     //    Publish at: 2020-03-19 18:26:54 +0000: receive value: (2020-03-19 18:27:00 +0000)
    ///     //    Publish at: 2020-03-19 18:26:54 +0000: receive value: (2020-03-19 18:27:03 +0000)
    ///     //    Publish at: 2020-03-19 18:26:54 +0000: receive value: (2020-03-19 18:27:06 +0000)
    ///     //    Publish at: 2020-03-19 18:26:54 +0000: receive value: (2020-03-19 18:27:09 +0000)
    ///     //    Received Timestamp 2020-03-19 18:27:09 +0000.
    ///
    /// - Parameters:
    ///   - interval: The interval at which to find and emit either the most recent or
    ///     the first element, expressed in the time system of the scheduler.
    ///   - scheduler: The scheduler on which to publish elements.
    ///   - latest: A Boolean value that indicates whether to publish the most recent
    ///     element. If `false`, the publisher emits the first element received during
    ///     the interval.
    /// - Returns: A publisher that emits either the most-recent or first element received
    ///   during the specified interval.
    public func throttle<S>(for interval: S.SchedulerTimeType.Stride, scheduler: S, latest: Bool) -> Publishers.Throttle<Self, S> where S : Scheduler
}

extension Publishers {

    /// A publisher created by applying the zip function to two upstream publishers.
    public struct Zip<A, B> : Publisher where A : Publisher, B : Publisher, A.Failure == B.Failure {

        /// The kind of values published by this publisher.
        public typealias Output = (A.Output, B.Output)

        /// The kind of errors this publisher might publish.
        ///
        /// Use `Never` if this `Publisher` does not publish errors.
        public typealias Failure = A.Failure

        public let a: A

        public let b: B

        public init(_ a: A, _ b: B)

        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
        ///
        /// - SeeAlso: `subscribe(_:)`
        /// - Parameters:
        ///     - subscriber: The subscriber to attach to this `Publisher`.
        ///                   once attached it can begin to receive values.
        public func receive<S>(subscriber: S) where S : Subscriber, B.Failure == S.Failure, S.Input == (A.Output, B.Output)
    }

    /// A publisher created by applying the zip function to three upstream publishers.
    public struct Zip3<A, B, C> : Publisher where A : Publisher, B : Publisher, C : Publisher, A.Failure == B.Failure, B.Failure == C.Failure {

        /// The kind of values published by this publisher.
        public typealias Output = (A.Output, B.Output, C.Output)

        /// The kind of errors this publisher might publish.
        ///
        /// Use `Never` if this `Publisher` does not publish errors.
        public typealias Failure = A.Failure

        public let a: A

        public let b: B

        public let c: C

        public init(_ a: A, _ b: B, _ c: C)

        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
        ///
        /// - SeeAlso: `subscribe(_:)`
        /// - Parameters:
        ///     - subscriber: The subscriber to attach to this `Publisher`.
        ///                   once attached it can begin to receive values.
        public func receive<S>(subscriber: S) where S : Subscriber, C.Failure == S.Failure, S.Input == (A.Output, B.Output, C.Output)
    }

    /// A publisher created by applying the zip function to four upstream publishers.
    public struct Zip4<A, B, C, D> : Publisher where A : Publisher, B : Publisher, C : Publisher, D : Publisher, A.Failure == B.Failure, B.Failure == C.Failure, C.Failure == D.Failure {

        /// The kind of values published by this publisher.
        public typealias Output = (A.Output, B.Output, C.Output, D.Output)

        /// The kind of errors this publisher might publish.
        ///
        /// Use `Never` if this `Publisher` does not publish errors.
        public typealias Failure = A.Failure

        public let a: A

        public let b: B

        public let c: C

        public let d: D

        public init(_ a: A, _ b: B, _ c: C, _ d: D)

        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
        ///
        /// - SeeAlso: `subscribe(_:)`
        /// - Parameters:
        ///     - subscriber: The subscriber to attach to this `Publisher`.
        ///                   once attached it can begin to receive values.
        public func receive<S>(subscriber: S) where S : Subscriber, D.Failure == S.Failure, S.Input == (A.Output, B.Output, C.Output, D.Output)
    }
}

extension Publisher {

    /// Combine elements from another publisher and deliver pairs of elements as tuples.
    ///
    /// The returned publisher waits until both publishers have emitted an event, then delivers the oldest unconsumed event from each publisher together as a tuple to the subscriber.
    /// For example, if publisher `P1` emits elements `a` and `b`, and publisher `P2` emits event `c`, the zip publisher emits the tuple `(a, c)`. It won’t emit a tuple with event `b` until `P2` emits another event.
    /// If either upstream publisher finishes successfuly or fails with an error, the zipped publisher does the same.
    ///
    /// - Parameter other: Another publisher.
    /// - Returns: A publisher that emits pairs of elements from the upstream publishers as tuples.
    public func zip<P>(_ other: P) -> Publishers.Zip<Self, P> where P : Publisher, Self.Failure == P.Failure

    /// Combine elements from another publisher and deliver a transformed output.
    ///
    /// The returned publisher waits until both publishers have emitted an event, then delivers the oldest unconsumed event from each publisher together as a tuple to the subscriber.
    /// For example, if publisher `P1` emits elements `a` and `b`, and publisher `P2` emits event `c`, the zip publisher emits the tuple `(a, c)`. It won’t emit a tuple with event `b` until `P2` emits another event.
    /// If either upstream publisher finishes successfuly or fails with an error, the zipped publisher does the same.
    ///
    /// - Parameter other: Another publisher.
    ///   - transform: A closure that receives the most recent value from each publisher and returns a new value to publish.
    /// - Returns: A publisher that emits pairs of elements from the upstream publishers as tuples.
    public func zip<P, T>(_ other: P, _ transform: @escaping (Self.Output, P.Output) -> T) -> Publishers.Map<Publishers.Zip<Self, P>, T> where P : Publisher, Self.Failure == P.Failure

    /// Combine elements from two other publishers and deliver groups of elements as tuples.
    ///
    /// The returned publisher waits until all three publishers have emitted an event, then delivers the oldest unconsumed event from each publisher as a tuple to the subscriber.
    /// For example, if publisher `P1` emits elements `a` and `b`, and publisher `P2` emits elements `c` and `d`, and publisher `P3` emits the event `e`, the zip publisher emits the tuple `(a, c, e)`. It won’t emit a tuple with elements `b` or `d` until `P3` emits another event.
    /// If any upstream publisher finishes successfuly or fails with an error, the zipped publisher does the same.
    ///
    /// - Parameters:
    ///   - publisher1: A second publisher.
    ///   - publisher2: A third publisher.
    /// - Returns: A publisher that emits groups of elements from the upstream publishers as tuples.
    public func zip<P, Q>(_ publisher1: P, _ publisher2: Q) -> Publishers.Zip3<Self, P, Q> where P : Publisher, Q : Publisher, Self.Failure == P.Failure, P.Failure == Q.Failure

    /// Combine elements from two other publishers and deliver a transformed output.
    ///
    /// The returned publisher waits until all three publishers have emitted an event, then delivers the oldest unconsumed event from each publisher as a tuple to the subscriber.
    /// For example, if publisher `P1` emits elements `a` and `b`, and publisher `P2` emits elements `c` and `d`, and publisher `P3` emits the event `e`, the zip publisher emits the tuple `(a, c, e)`. It won’t emit a tuple with elements `b` or `d` until `P3` emits another event.
    /// If any upstream publisher finishes successfuly or fails with an error, the zipped publisher does the same.
    ///
    /// - Parameters:
    ///   - publisher1: A second publisher.
    ///   - publisher2: A third publisher.
    ///   - transform: A closure that receives the most recent value from each publisher and returns a new value to publish.
    /// - Returns: A publisher that emits groups of elements from the upstream publishers as tuples.
    public func zip<P, Q, T>(_ publisher1: P, _ publisher2: Q, _ transform: @escaping (Self.Output, P.Output, Q.Output) -> T) -> Publishers.Map<Publishers.Zip3<Self, P, Q>, T> where P : Publisher, Q : Publisher, Self.Failure == P.Failure, P.Failure == Q.Failure

    /// Combine elements from three other publishers and deliver groups of elements as tuples.
    ///
    /// The returned publisher waits until all four publishers have emitted an event, then delivers the oldest unconsumed event from each publisher as a tuple to the subscriber.
    /// For example, if publisher `P1` emits elements `a` and `b`, and publisher `P2` emits elements `c` and `d`, and publisher `P3` emits the elements `e` and `f`, and publisher `P4` emits the event `g`, the zip publisher emits the tuple `(a, c, e, g)`. It won’t emit a tuple with elements `b`, `d`, or `f` until `P4` emits another event.
    /// If any upstream publisher finishes successfuly or fails with an error, the zipped publisher does the same.
    ///
    /// - Parameters:
    ///   - publisher1: A second publisher.
    ///   - publisher2: A third publisher.
    ///   - publisher3: A fourth publisher.
    /// - Returns: A publisher that emits groups of elements from the upstream publishers as tuples.
    public func zip<P, Q, R>(_ publisher1: P, _ publisher2: Q, _ publisher3: R) -> Publishers.Zip4<Self, P, Q, R> where P : Publisher, Q : Publisher, R : Publisher, Self.Failure == P.Failure, P.Failure == Q.Failure, Q.Failure == R.Failure

    /// Combine elements from three other publishers and deliver a transformed output.
    ///
    /// The returned publisher waits until all four publishers have emitted an event, then delivers the oldest unconsumed event from each publisher as a tuple to the subscriber.
    /// For example, if publisher `P1` emits elements `a` and `b`, and publisher `P2` emits elements `c` and `d`, and publisher `P3` emits the elements `e` and `f`, and publisher `P4` emits the event `g`, the zip publisher emits the tuple `(a, c, e, g)`. It won’t emit a tuple with elements `b`, `d`, or `f` until `P4` emits another event.
    /// If any upstream publisher finishes successfuly or fails with an error, the zipped publisher does the same.
    ///
    /// - Parameters:
    ///   - publisher1: A second publisher.
    ///   - publisher2: A third publisher.
    ///   - publisher3: A fourth publisher.
    ///   - transform: A closure that receives the most recent value from each publisher and returns a new value to publish.
    /// - Returns: A publisher that emits groups of elements from the upstream publishers as tuples.
    public func zip<P, Q, R, T>(_ publisher1: P, _ publisher2: Q, _ publisher3: R, _ transform: @escaping (Self.Output, P.Output, Q.Output, R.Output) -> T) -> Publishers.Map<Publishers.Zip4<Self, P, Q, R>, T> where P : Publisher, Q : Publisher, R : Publisher, Self.Failure == P.Failure, P.Failure == Q.Failure, Q.Failure == R.Failure
}

extension Publishers.CombineLatest : Equatable where A : Equatable, B : Equatable {

    /// Returns a Boolean value that indicates whether two publishers are equivalent.
    ///
    /// - Parameters:
    ///   - lhs: A combineLatest publisher to compare for equality.
    ///   - rhs: Another combineLatest publisher to compare for equality.
    /// - Returns: `true` if the corresponding upstream publishers of each combineLatest publisher are equal, `false` otherwise.
    public static func == (lhs: Publishers.CombineLatest<A, B>, rhs: Publishers.CombineLatest<A, B>) -> Bool
}

extension Publishers.CombineLatest3 : Equatable where A : Equatable, B : Equatable, C : Equatable {

    /// Returns a Boolean value indicating whether two values are equal.
    ///
    /// Equality is the inverse of inequality. For any values `a` and `b`,
    /// `a == b` implies that `a != b` is `false`.
    ///
    /// - Parameters:
    ///   - lhs: A value to compare.
    ///   - rhs: Another value to compare.
    public static func == (lhs: Publishers.CombineLatest3<A, B, C>, rhs: Publishers.CombineLatest3<A, B, C>) -> Bool
}

extension Publishers.CombineLatest4 : Equatable where A : Equatable, B : Equatable, C : Equatable, D : Equatable {

    /// Returns a Boolean value indicating whether two values are equal.
    ///
    /// Equality is the inverse of inequality. For any values `a` and `b`,
    /// `a == b` implies that `a != b` is `false`.
    ///
    /// - Parameters:
    ///   - lhs: A value to compare.
    ///   - rhs: Another value to compare.
    public static func == (lhs: Publishers.CombineLatest4<A, B, C, D>, rhs: Publishers.CombineLatest4<A, B, C, D>) -> Bool
}

extension Publishers.Merge : Equatable where A : Equatable, B : Equatable {

    /// Returns a Boolean value that indicates whether two publishers are equivalent.
    ///
    /// - Parameters:
    ///   - lhs: A merging publisher to compare for equality.
    ///   - rhs: Another merging publisher to compare for equality..
    /// - Returns: `true` if the two merging - rhs: Another merging publisher to compare for equality.
    public static func == (lhs: Publishers.Merge<A, B>, rhs: Publishers.Merge<A, B>) -> Bool
}

extension Publishers.Merge3 : Equatable where A : Equatable, B : Equatable, C : Equatable {

    /// Returns a Boolean value that indicates whether two publishers are equivalent.
    ///
    /// - Parameters:
    ///   - lhs: A merging publisher to compare for equality.
    ///   - rhs: Another merging publisher to compare for equality.
    /// - Returns: `true` if the two merging publishers have equal source publishers, `false` otherwise.
    public static func == (lhs: Publishers.Merge3<A, B, C>, rhs: Publishers.Merge3<A, B, C>) -> Bool
}

extension Publishers.Merge4 : Equatable where A : Equatable, B : Equatable, C : Equatable, D : Equatable {

    /// Returns a Boolean value that indicates whether two publishers are equivalent.
    ///
    /// - Parameters:
    ///   - lhs: A merging publisher to compare for equality.
    ///   - rhs: Another merging publisher to compare for equality.
    /// - Returns: `true` if the two merging publishers have equal source publishers, `false` otherwise.
    public static func == (lhs: Publishers.Merge4<A, B, C, D>, rhs: Publishers.Merge4<A, B, C, D>) -> Bool
}

extension Publishers.Merge5 : Equatable where A : Equatable, B : Equatable, C : Equatable, D : Equatable, E : Equatable {

    /// Returns a Boolean value that indicates whether two publishers are equivalent.
    ///
    /// - Parameters:
    ///   - lhs: A merging publisher to compare for equality.
    ///   - rhs: Another merging publisher to compare for equality.
    /// - Returns: `true` if the two merging publishers have equal source publishers, `false` otherwise.
    public static func == (lhs: Publishers.Merge5<A, B, C, D, E>, rhs: Publishers.Merge5<A, B, C, D, E>) -> Bool
}

extension Publishers.Merge6 : Equatable where A : Equatable, B : Equatable, C : Equatable, D : Equatable, E : Equatable, F : Equatable {

    /// Returns a Boolean value that indicates whether two publishers are equivalent.
    ///
    /// - Parameters:
    ///   - lhs: A merging publisher to compare for equality.
    ///   - rhs: Another merging publisher to compare for equality.
    /// - Returns: `true` if the two merging publishers have equal source publishers, `false` otherwise.
    public static func == (lhs: Publishers.Merge6<A, B, C, D, E, F>, rhs: Publishers.Merge6<A, B, C, D, E, F>) -> Bool
}

extension Publishers.Merge7 : Equatable where A : Equatable, B : Equatable, C : Equatable, D : Equatable, E : Equatable, F : Equatable, G : Equatable {

    /// Returns a Boolean value that indicates whether two publishers are equivalent.
    ///
    /// - Parameters:
    ///   - lhs: A merging publisher to compare for equality.
    ///   - rhs: Another merging publisher to compare for equality.
    /// - Returns: `true` if the two merging publishers have equal source publishers, `false` otherwise.
    public static func == (lhs: Publishers.Merge7<A, B, C, D, E, F, G>, rhs: Publishers.Merge7<A, B, C, D, E, F, G>) -> Bool
}

extension Publishers.Merge8 : Equatable where A : Equatable, B : Equatable, C : Equatable, D : Equatable, E : Equatable, F : Equatable, G : Equatable, H : Equatable {

    /// Returns a Boolean value that indicates whether two publishers are equivalent.
    ///
    /// - Parameters:
    ///   - lhs: A merging publisher to compare for equality.
    ///   - rhs: Another merging publisher to compare for equality.
    /// - Returns: `true` if the two merging publishers have equal source publishers, `false` otherwise.
    public static func == (lhs: Publishers.Merge8<A, B, C, D, E, F, G, H>, rhs: Publishers.Merge8<A, B, C, D, E, F, G, H>) -> Bool
}

extension Publishers.MergeMany : Equatable where Upstream : Equatable {

    /// Returns a Boolean value indicating whether two values are equal.
    ///
    /// Equality is the inverse of inequality. For any values `a` and `b`,
    /// `a == b` implies that `a != b` is `false`.
    ///
    /// - Parameters:
    ///   - lhs: A value to compare.
    ///   - rhs: Another value to compare.
    public static func == (lhs: Publishers.MergeMany<Upstream>, rhs: Publishers.MergeMany<Upstream>) -> Bool
}

extension Publishers.Zip : Equatable where A : Equatable, B : Equatable {

    /// Returns a Boolean value that indicates whether two publishers are equivalent.
    ///
    /// - Parameters:
    ///   - lhs: A zip publisher to compare for equality.
    ///   - rhs: Another zip publisher to compare for equality.
    /// - Returns: `true` if the corresponding upstream publishers of each zip publisher are equal, `false` otherwise.
    public static func == (lhs: Publishers.Zip<A, B>, rhs: Publishers.Zip<A, B>) -> Bool
}

/// Returns a Boolean value that indicates whether two publishers are equivalent.
///
/// - Parameters:
///   - lhs: A zip publisher to compare for equality.
///   - rhs: Another zip publisher to compare for equality.
/// - Returns: `true` if the corresponding upstream publishers of each zip publisher are equal, `false` otherwise.
extension Publishers.Zip3 : Equatable where A : Equatable, B : Equatable, C : Equatable {

    /// Returns a Boolean value indicating whether two values are equal.
    ///
    /// Equality is the inverse of inequality. For any values `a` and `b`,
    /// `a == b` implies that `a != b` is `false`.
    ///
    /// - Parameters:
    ///   - lhs: A value to compare.
    ///   - rhs: Another value to compare.
    public static func == (lhs: Publishers.Zip3<A, B, C>, rhs: Publishers.Zip3<A, B, C>) -> Bool
}

/// Returns a Boolean value that indicates whether two publishers are equivalent.
///
/// - Parameters:
///   - lhs: A zip publisher to compare for equality.
///   - rhs: Another zip publisher to compare for equality.
/// - Returns: `true` if the corresponding upstream publishers of each zip publisher are equal, `false` otherwise.
extension Publishers.Zip4 : Equatable where A : Equatable, B : Equatable, C : Equatable, D : Equatable {

    /// Returns a Boolean value indicating whether two values are equal.
    ///
    /// Equality is the inverse of inequality. For any values `a` and `b`,
    /// `a == b` implies that `a != b` is `false`.
    ///
    /// - Parameters:
    ///   - lhs: A value to compare.
    ///   - rhs: Another value to compare.
    public static func == (lhs: Publishers.Zip4<A, B, C, D>, rhs: Publishers.Zip4<A, B, C, D>) -> Bool
}