Broken Scala For Comprehension

BrokenFor.scala

package vsxmpp

import scalaz.EitherT
import scalaz._
import Scalaz._

case class ABC(s:String)

object ABC {
  
	implicit val m = new Monoid[(ABC, Int)] {
	    def zero: (ABC, Int) = (null, -1)

	    def append(f1: (ABC, Int), f2: => (ABC, Int)): (ABC, Int) = f1
	}
  
    def BrokenMethod(): EitherT[Option, (ABC, Int), (ABC, String)] = {
        EitherT(Some((ABC("abcData"),"Success").right))
    }

    def filterComp() =
      BrokenMethod()
      .filter {
      	case (abc,"Success") => true
      	case _ => false
      }.map {
      	case (abc, "Success") => "yay"
      }

    def forComp() =
        for {
            (a,b) <- BrokenMethod() //erroneous or inaccessible type
        } yield "yay"
    	
    def main(args: Array[String]): Unit = {
    	val a = forComp() 
        println("a = " + a)
    }
}

Here's a closer look at things:

case class ABC(s:String)

object Test {
  def BrokenMethod(): XEitherT[Option, (ABC, Int), (ABC, String)] = ???

  trait XMonoid[F]
  trait XFunctor[F[_]]
  implicit def OptionFunctor: XFunctor[Option] = ???
  implicit val xm: XMonoid[(ABC, Int)] = ???

  trait XEitherT[F[_], A, B] {
    def filter[AA >: A](p: B => Boolean)(implicit M: XMonoid[AA], F: XFunctor[F])
  }

  BrokenMethod().withFilter(_ => true) // okay

  locally {
    import scalaz._
    import Scalaz._

    BrokenMethod().filter(_ => true) // okay
  }

  locally {
    import scalaz._
    import Scalaz._

    BrokenMethod().withFilter(_ => true) // erroneous or inaccessible type.
  }
}

Something imported from Scalaz._ seems to be interfering with the rewriting of withFilter to filter. Scalaz should really define a withFilter method directly; this rewriting is a backward compatibility measure in scalac, as the rewriting of for-comprehensions changed in 2.8. Take a look at the withFilter implementation for, say, scala.Option to see what they should look like.