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    <p>EDIT here is the real solution. I left in the original post because I think its worthwhile seeing the pattern. What works for Klesli works for IterateeT</p> <pre><code>import java.io.{ BufferedReader, File, FileReader } import scalaz._, Scalaz._, effect._, iteratee.{ Iteratee =&gt; I, _ } object IterateeIOExample { type ErrorOr[+A] = EitherT[IO, Throwable, A] def openFile(f: File) = IO(new BufferedReader(new FileReader(f))) def readLine(r: BufferedReader) = IO(Option(r.readLine)) def closeReader(r: BufferedReader) = IO(r.close()) def tryIO[A, B](action: IO[B]) = I.iterateeT[A, ErrorOr, B] { EitherT.fromEither(action.catchLeft).map(r =&gt; I.sdone(r, I.emptyInput)) } def enumBuffered(r: =&gt; BufferedReader) = new EnumeratorT[String, ErrorOr] { lazy val reader = r def apply[A] = (s: StepT[String, ErrorOr, A]) =&gt; s.mapCont(k =&gt; tryIO(readLine(reader)) flatMap { case None =&gt; s.pointI case Some(line) =&gt; k(I.elInput(line)) &gt;&gt;== apply[A] }) } def enumFile(f: File) = new EnumeratorT[String, ErrorOr] { def apply[A] = (s: StepT[String, ErrorOr, A]) =&gt; tryIO(openFile(f)).flatMap(reader =&gt; I.iterateeT[String, ErrorOr, A]( EitherT( enumBuffered(reader).apply(s).value.run.ensuring(closeReader(reader))))) } def main(args: Array[String]) { val action = ( I.consume[String, ErrorOr, List] %= I.filter(a =&gt; a.count(_ == '0') &gt;= 25) &amp;= enumFile(new File(args(0)))).run.run println(action.unsafePerformIO().map(_.size)) } } </code></pre> <p>===== Original Post =====</p> <p>I feel like you need an EitherT in the mix. Without EitherT you are just ending up with a 3 Lefts or Rights. With EitherT it would propergate the left. </p> <p>I think what you really want is</p> <pre><code>type ErrorOr[+A] = EitherT[IO, Throwable, A] I.iterateeT[A, ErrorOr, B] </code></pre> <p>The following code mimics how you are currently composing things. Because IterateeT has no concept of left and right, when you compose it, you just end up with a bunch of IO/Id's.</p> <pre><code>scala&gt; Kleisli((a:Int) =&gt; 4.right[String].point[Id]) res11: scalaz.Kleisli[scalaz.Scalaz.Id,Int,scalaz.\/[String,Int]] = scalaz.KleisliFunctions$$anon$18@73e771ca scala&gt; Kleisli((a:Int) =&gt; "aa".left[Int].point[Id]) res12: scalaz.Kleisli[scalaz.Scalaz.Id,Int,scalaz.\/[String,Int]] = scalaz.KleisliFunctions$$anon$18@be41b41 scala&gt; for { a &lt;- res11; b &lt;- res12 } yield (a,b) res15: scalaz.Kleisli[scalaz.Scalaz.Id,Int,(scalaz.\/[String,Int], scalaz.\/[String,Int])] = scalaz.KleisliFunctions$$anon$18@42fd1445 scala&gt; res15.run(1) res16: (scalaz.\/[String,Int], scalaz.\/[String,Int]) = (\/-(4),-\/(aa)) </code></pre> <p>In the following code, instead of using Id, we use an EitherT. Since EitherT has the same bind behaviour as Either, we end up with what we want.</p> <pre><code>scala&gt; type ErrorOr[+A] = EitherT[Id, String, A] defined type alias ErrorOr scala&gt; Kleisli[ErrorOr, Int, Int]((a:Int) =&gt; EitherT(4.right[String].point[Id])) res22: scalaz.Kleisli[ErrorOr,Int,Int] = scalaz.KleisliFunctions$$anon$18@58b547a0 scala&gt; Kleisli[ErrorOr, Int, Int]((a:Int) =&gt; EitherT("aa".left[Int].point[Id])) res24: scalaz.Kleisli[ErrorOr,Int,Int] = scalaz.KleisliFunctions$$anon$18@342f2ceb scala&gt; for { a &lt;- res22; b &lt;- res24 } yield 2 res25: scalaz.Kleisli[ErrorOr,Int,Int] = scalaz.KleisliFunctions$$anon$18@204eab31 scala&gt; res25.run(2).run res26: scalaz.Scalaz.Id[scalaz.\/[String,Int]] = -\/(aa) </code></pre> <p>You can replace Keisli with IterateeT and Id with IO to get what you need.</p>
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    1. COThanks—I played briefly with using `EitherT` here this morning and couldn't get the types to work out, but I'm trying again now.
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    2. COI'm converting types now too.
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