[Idea] NoValue enum

[ikesyo]

This is similar to NoError enum. With this one can write something like this:

let noValueProducer1 = SignalProducer<NoValue, NoError>.empty
let noValueProducer2 = SignalProducer<NoValue, FooError>(error: FooError())
let noValueProducer3 = SignalProducer<NoValue, FooError>(value: ...) // Cannot be called

typealias Completable<E> = SignalProducer<NoValue, E>

We may replace Signal/Producer<(), Error> with Signal/Producer<NoValue, Error>.

[andersio]

We can use Never.

[ikesyo]

Ah that makes sense.

[andersio]

Hmm, NoValue reads better though. Moreover, Never for now is just intended for return types, and it is not yet a real bottom type.

[mdiep]

I think Never reads fine. If that provides the same guarantee, I'd prefer it.

[andersio]

Never is just NoError (uninhabited type) with a bit compiler sugar AFAIK.

[mdiep]

I believe this is resolved by #392. I don't see any other operators that could use Never.

[andersio]

flatMapError could use Never. 🙂 e.g.

SignalProducer<Never, E1>.flatMapError -> SignalProducer<U, E2>

We can also add an equivalent of promoteError for Never.

[andersio]

Ah, and a new overload of startWith* and observe* that returns a Result<(), Error> (completed or failed).

Just found that the conventional start is enough by asserting event.error.