Initial implementation of withLatestFrom for Combine

WithLatestFrom.swift

import Combine

var cancellables = [AnyCancellable]()

extension Publisher {
  func withLatestFrom<A, P: Publisher>(
    _ second: P
  ) -> AnyPublisher<A, Failure> where P.Output == A, P.Failure == Failure {
    return second
      .map { result -> AnyPublisher<A, Failure> in
        self.map({ _ in result }).eraseToAnyPublisher()
      }.switchToLatest().eraseToAnyPublisher()
  }
}

let names = PassthroughSubject<String, Never>()
let blips = PassthroughSubject<Void, Never>()

print("Sending \"Alfred\"")
names.send("Alfred")
print("Sending \"Maggie\"")
names.send("Maggie")

blips.withLatestFrom(names)
  .sink { name in
    print("[\(name)]")
  }
  .store(in: &cancellables)

print("*BLIP*")
blips.send(()) // prints nothing
print("Sending \"Ploppo\"")
names.send("Ploppo")
print("*BLIP*")
blips.send(()) // prints "Ploppo"
print("Sending \"Blobbo\"")
names.send("Blobbo")
print("*BLIP*")
blips.send(()) // prints "Blobbo"