Right not working with for comprehension Scala-CodePudding

Background

I am trying to use comprehensions with Scala with the Either type, namely using a Right. However, despite my efforts, I get an error and nothing works.

Code

I am using scala's repl to make a few tests. This is the easiest use case I could come up with:

scala> for {
     |   x <- Right(1)
     |   y <- Right(2)
     |   z <- Right(3)
     | } yield x   y   z

You will see it is basically a copy of this page:

https://www.scala-lang.org/api/2.12.7/scala/util/Either.html

Problem

However, this fails with the following error:

<console>:13: error: value flatMap is not a member of scala.util.Right[Nothing,Int]
         x <- Right(1)
                   ^
<console>:14: error: value flatMap is not a member of scala.util.Right[Nothing,Int]
         y <- Right(2)
                   ^
<console>:15: error: value map is not a member of scala.util.Right[Nothing,Int]
         z <- Right(3)
                   ^

I am using using the following version of scala:

Welcome to Scala 2.11.12 (OpenJDK 64-Bit Server VM, Java 11.0.13).
Type in expressions for evaluation. Or try :help.

I understand some changes were made to Either, so it became Right biased, but I don't see how such changes could affect this example.

What am I missing?

CodePudding user response：

In scala 2.11 Either is not a Monad. Combinators like flatMap and map are missing from it. Instead, you call .right or .left to get a RightProjection or LeftProjection which does have the combinators. You need to project your Either as right. The code below will return Right(6).

  for {
    x <- Right(1).right
    y <- Right(2).right
    z <- Right(3).right
  } yield x   y   z