I can use an = in a scala for-comprehension (as specified in section 6.19 of the SLS) as follows:

Option

Suppose I have some function String => Option[Int]:

scala> def intOpt(s: String) = try { Some(s.toInt) } catch { case _ => None }
intOpt: (s: String)Option[Int]

Then I can use it thus

scala> for {
   |     str <- Option("1")
   |     i <- intOpt(str)
   |     val j = i + 10    //Note use of = in generator
   |   }
   |   yield j
res18: Option[Int] = Some(11)

It was my understanding that this was essentially equivalent to:

scala> Option("1") flatMap { str => intOpt(str) } map { i => i + 10 } map { j => j }
res19: Option[Int] = Some(11)

That is, the embedded generator was a way of injecting a map into a sequence of flatMap calls. So far so good.

Either.RightProjection

What I actually want to do: use a similar for-comprehension as the previous example using the Either monad.

However, if we use it in a similar chain, but this time using the Either.RightProjection monad/functor, it doesn't work:

scala> def intEither(s: String): Either[Throwable, Int] = 
  |      try { Right(s.toInt) } catch { case x => Left(x) }
intEither: (s: String)Either[Throwable,Int]

Then use:

scala> for {
 | str <- Option("1").toRight(new Throwable()).right
 | i <- intEither(str).right //note the "right" projection is used
 | val j = i + 10
 | }
 | yield j
<console>:17: error: value map is not a member of Product with Serializable with Either[java.lang.Throwable,(Int, Int)]
              i <- intEither(str).right
                ^

                    
                    

                        
                        scala
                        
                        for-loop
                        
                        monads
                        
                        either
                        
                    

                    

                        
                        
                        
                        
                        
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