Imported From: https://issues.scala-lang.org/browse/SI-5793?orig=1
Reporter: Lee Mighdoll (mighdoll)

@retronym said:
You are essentially trying to call:

scala> either.right.map(x => x).map(x => x)
<console>:9: error: value map is not a member of Either[Int,Char] with Product with Serializable
              either.right.map(x => x).map(x => x)
                                       ^

But Either#RightProjection#map returns an Either, which doesn't have a map method.

You can enrich Either to have a right-biased set of map/flatMap/foreach methods, along these lines (I'm using Scala 2.10 implicit classes for brevity, but the same is possible in 2.9.x with a class and an implicit conversion):

scala> implicit class RightBiasedEither[A, B](val e: Either[A, B]) extends AnyVal {
     |   def foreach[U](f: B => U): Unit = e.right.foreach(f)
     |   def map[C](f: B => C): Either[A, C] = e.right.map(f)
     |   def flatMap[C](f: B => Either[A, C]) = e.right.flatMap(f)  
     | }
defined class RightBiasedEither

scala> 

scala> def f(c:Char):String = "ha"
f: (c: Char)String

scala> def either:Either[Int,Char] = Right('r')
either: Either[Int,Char]

scala>   for { 
     |     c <- either
     |     s = f(c)
     |   } yield s
res0: Either[Int,String] = Right(ha)

Lee Mighdoll (mighdoll) said:
Thanks Jason. That's very helpful. I understand the flaw in Either now. Your solution is nice both to explain and to try out.

That the stock unbiased Either requires a bit more syntax to use the RightProjection/LeftProjection is apparent from the api, but because of this design flaw it's hard to recommend Either.

Is there an open issue on fixing Either? Or a consensus that it's not fixable? Seems like the standard lib should get some combination of a fix, interim documentation, or a replacement.

@retronym said:
I've started a discussion here:

https://groups.google.com/d/topic/scala-debate/K6wv6KphJK0/discussion

Rob Dickens (robcd) said:
Lee, Jason,

Have just come up with a version of Either which fixes this. Please would you take a look and let me know what you think:

http://robsscala.blogspot.co.uk/2012/05/fixing-scalaeither-leftrightmap-returns.html

Cheers, Rob