Non-erased `Publisher.materialize`

materialize_implemented.swift

extension Publisher {
	func materialize()
		-> Publishers.Catch<
			Publishers.Concatenate<
				Publishers.Map<
					Self,
					Event<Output, Failure>
				>,
				Publishers.Sequence<
					[Event<Output, Failure>],
					Failure
				>
			>,
			Just<Event<Output, Failure>>
		> { /// (1) Before throwing any eggs, there’s reason for returning
		/// the fuller, non-erased type here that I cover in my entry on
		/// [operator fusion](https://jasdev.me/fusion-primer).
		///
		/// If the fusion’s performance benefits still aren’t your jam,
		/// you can erase the above to `AnyPublisher<Event<Output, Failure>, Never>`.
		map(Event.value)
			.append(.completion(.finished))
			.catch { error in Just(.completion(.failure(error))) } /// (2) This line is a bit of an onion to
			/// read—we `catch` any received errors and project down a publisher that’ll emit a single
			/// `Event.completion(.failure(error)` and then finish downstream _without_ error.
	}
}