Right projections of Either

[Bruce Eckel]

I have the "Option" version of the following example working, and want to show that you can accomplish the same thing with Either using right projections. Everything works except for the commented portion, where I get an error message:

error: type mismatch;

 found   : Product with Serializable with scala.util.Either[String,Int]

 required: scala.collection.GenTraversableOnce[?]

    j <- f(i).right

      ^

one error found

Thanks for any help...

def cutoff(in:Int, thresh:Int, add:Int) =

  if(in < thresh)

    Left(in + " Below threshold " + thresh)

  else

    Right(in + add)

def A(in:Int) = cutoff(in, 7, 1)

def B(in:Int) = cutoff(in, 8, 2)

def C(in:Int) = cutoff(in, 9, 3)

def f(in:Int) =

  for {

    u <- Right(in).right

    v <- A(u).right

    w <- B(v).right

    x <- C(w).right

    y <- Right(x + 10).right

    z <- Right(y * 2 + 1).right

  } yield z

println(f(7)) // Right(47)

println(f(6)) // Left(6 Below threshold 7)

/*

val result =

  for {

    i <- 1 to 10

    j <- f(i).right

  } yield j

println(result) // Vector(47, 49, 51, 53)

*/

[Vlad Patryshev]

Oh, this is famous. Monads don't compose... :)

Thanks,
-Vlad