SignalProducer.on: restore value as the last parameter

[dmcrodrigues]

Conceptually, value should be defined before failed since that goes in line with the natural order of events, however this reordering can introduce some confusion/inconvenience while using trailing closures specially during the migration from ReactiveCocoa 4.x where value, formerly known as next, was the last parameter and now represents disposed.

This can silently introduce some problems after migration if a project is using .on with a trailing closure to inject side-effects on values without being interested in the value itself.

producer.on { _ in 
    // Inject side-effect, e.g. invalidate context
}

⚠️ Note ⚠️

If we do not go forward with this we need to fix an inconsistency between SignalProducer and Signal where the former was not updated accordingly.

[dmcrodrigues]

@NachoSoto this was discussed on Slack today and I opened a PR to discuss what should we do.

[andersio]

If we prefer the natural order, adding _: Void = () as the last parameter would prevent the use of trailing closure.

[dmcrodrigues]

I thought about that and it's definitely another option but with some quirks.

Closure parameter prior to parameters with default arguments will not be treated as a trailing closure

A warning is raised as expected but can we ignore it?

Despite of having a default value, Xcode will ask us a value for our (unnamed) last parameter which I think is far from desirable and can introduce some confusion about the why and what it really means.

[mdiep]

This seems reasonable to me. Otherwise, I think we need to follow the pattern of startWithFailed, etc.

[andersio]

I don't feel it is frequently used enough to justify a sea of shorthands like how start or observe does. So leaving it as it was in 4.x seems fine.