What's the deal with all the Either cruft?

Question

The Either class seems useful and the ways of using it are pretty obvious. But then I look at the API documentation and I\'m baffled:

def joinLeft [A1 >:          


        

Answer 1

joinLeft and joinRight enable you to "flatten" a nested Either:

scala> val e: Either[Either[String, Int], Int] = Left(Left("foo"))
e: Either[Either[String,Int],Int] = Left(Left(foo))

scala> e.joinLeft
res2: Either[String,Int] = Left(foo)

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Edit: My answer to this question shows one example of how you can use the projections, in this case to fold together a sequence of Eithers without pattern matching or calling isLeft or isRight. If you're familiar with how to use Option without matching or calling isDefined, it's analagous.

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While curiously looking at the current source of Either, I saw that joinLeft and joinRight are implemented with pattern matching. However, I stumbled across this older version of the source and saw that it used to implement the join methods using projections:

def joinLeft[A, B](es: Either[Either[A, B], B]) =
  es.left.flatMap(x => x)