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 anEither
?