A variant of combinePrevious that doesn't need an initial value.

[andersio]

Note that combinePrevious() is not implemented using default arguments and optionals, so that existing use cases as concerned by @NachoSoto would not break.

In other words, combinePrevious(nil) and combinePrevious() are not equivalent, given the constraint <U> Value == Optional<U>.

Checklist

• Updated CHANGELOG.md.

[NachoSoto]

👎🏻

This could be backwards incompatible if somebody is passing nil because Value is optional.
Furthermore I don't think changing this API is necessary (I'm generally against parameters that fundamentally change the behavior of a function) since this can be replicated with skip(first: 1)

[andersio]

🤔

Haven't considered optionals. That said we can use this just for combinePrevious(), and keeping the original combinePrevious(initial:) so that combinePrevious(nil) would work as is.

since this can be replicated with skip(first: 1)

Not quite if you consider how many extra hops needed for U to U? to (U?, U?) to (U, U) (probably less with a filterMap, but still).

[sharplet]

For reference, some previous discussion on this general idea: ReactiveCocoa/ReactiveCocoa#2632

[andersio]

@sharplet Thanks for pointing out the previous discussion. My responses to @NachoSoto's arguments:

One of the combinePrevious implementation is not implemented in terms of the other.

Both share the same implementation in this PR.

This operator is redundant: the existing combinePrevious is more general. You can accomplish the same thing by doing something like producer.combinePrevious(nil).skip(1).

From Signal<U, E> to precisely Signal<(U, U), E> with the current combinePrevious, one has to jump through multiple hops:

values // Signal<U, E>
    .optionalize() // Signal<U?, E>
    .combinePrevious(nil) // Signal<(U?, U?), E>
    .filterMap { $0.0 != nil && $0.1 != nil ? ($0.0!, $0.1!) : nil } // Signal<(U, U), E>

which is hardly sane. Moreover, as shown in this PR, combinePrevious() with an optional initial value is actually the most general version, as it backs the original combinePrevious(_:) in a generic context by exploiting the fact that optionality of Value is opaque.

[NachoSoto]

The new implementation is great 👍