Question

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
                ^

The issue has something to do with the function that a right-projection expects as an argument to its flatMap method (i.e. it expects an R => Either[L, R]). But modifying to not call right on the second generator, it still won’t compile.

scala>  for {
 |        str <- Option("1").toRight(new Throwable()).right
 |        i <- intEither(str) // no "right" projection
 |          val j = i + 10
 |      }
 |      yield j
<console>:17: error: value map is not a member of Either[Throwable,Int]
              i <- intEither(str)
                            ^

Mega-Confusion

But now I get doubly confused. The following works just fine:

scala> for {
 |       x <- Right[Throwable, String]("1").right
 |       y <- Right[Throwable, String](x).right //note the "right" here
 |     } yield y.toInt
res39: Either[Throwable,Int] = Right(1)

But this does not:

scala> Right[Throwable, String]("1").right flatMap { x => Right[Throwable, String](x).right } map { y => y.toInt }
<console>:14: error: type mismatch;
 found   : Either.RightProjection[Throwable,String]
 required: Either[?,?]
              Right[Throwable, String]("1").right flatMap { x => Right[Throwable, String](x).right } map { y => y.toInt }
                                                                                             ^

I thought these were equivalent

• What is going on?
• How can I embed an = generator in a for comprehension across an Either?

Answer 1

The fact that you cannot embed the = in the for-comprehension is related to this issue reported by Jason Zaugg; the solution is to Right-bias Either (or create a new data type isomorphic to it).

For your mega-confusion, you expanded the for sugar incorrectly. The desugaring of

for {
  b <- x(a)
  c <- y(b)
} yield z(c)

is

x(a) flatMap { b =>
 y(b) map { c =>
  z(c) }} 

and not

x(a) flatMap { b => y(b)} map { c => z(c) }

Hence you should have done this:

scala> Right[Throwable, String]("1").right flatMap { x => Right[Throwable, String](x).right map { y => y.toInt } }
res49: Either[Throwable,Int] = Right(1)

More fun about desugaring (the `j = i + 10` issue)

for {
  b <- x(a)
  c <- y(b)
  x1 = f1(b)
  x2 = f2(b, x1)
  ...
  xn = fn(.....)
  d <- z(c, xn)
} yield w(d)

is desugared into

x(a) flatMap { b =>
  y(b) map { c =>
    x1 = ..
    ...
    xn = ..
    (c, x1, .., xn) 
  } flatMap { (_c1, _x1, .., _xn) =>
    z(_c1, _xn) map w }}

So in your case, y(b) has result type Either which doesn’t have map defined.