Un-deprecate left Either projections

[nrinaudo]

Deprecating right projections makes sense - Either is now right-biased, there's no need for them.

Deprecating left projections also seems desirable, but I feel this might have been done a bit quickly, with some unpleasant consequences.

There are existing scenarios where one might want to call .left.map(f) or .left.flatMap(g) - flatMap is particularly useful to recover on errors, for example.

The current state of scala means that we'll get a warning for this (or, for the -Xfatal-warnings crowd, an error) without a clear workaround. Suggestions on scala/contributors were:

e.swap.flatMap(x => f(x).swap).swap

e match {
  case Left(l) => f(l)
  case r@Right(_) => r
}

Both of these work fine, but I doubt anyone who has to replace e.left.flatMap(f) by one of them will feel it's a step forward.

Truth be told, I'd rather see projections disappear and Either get leftMap and leftFlatMap methods, but it's a bit late for that, with the 2.13.0 API being frozen.

[SethTisue]

the previous discussion was at #6682

[hrhino]

I'm okay with this. I'd like that we consider introducing mapLeft or what have you in 2.14, but this combinator has its place IMO.

[martijnhoekstra]

With the amount of support I'm seeing for this, let's go for undeprecating. There is no rush. I think there is space for a nicer, more symmetric design, but let's explore that for 2.14.x

[nrinaudo]

@martijnhoekstra to be clear: I completely agree that projections should disappear. I do feel, however, that they shouldn't take away useful combinators with them.