How to send a process to multiple sinks in scalaz-stream

Question

If I have a simple process which is emitting values of type String and I wish to send these to multiple sinks (i.e. each sink gets sent the String), how do I do this?

For example, running this program:

object Play extends App {

  def prepend(s: String): String => String = s ++ _
  val out1 = io.stdOutLines.map(prepend("1-") andThen _)
  val out2 = io.stdOutLines.map(prepend("2-") andThen _)

  val p = io.stdInLines to (out1 merge out2)
  p.run.run
}

The output looks like:

a     //input
1-a
b     //input
2-b
c     //input
2-c
d     //input
1-d

I want the output to be this:

a     //input
1-a
2-a
b     //input
2-b
1-b
c     //input
2-c
1-c
d     //input
1-d
2-d

---

EDIT

I can achieve this as follows:

implicit class ToBoth[O](p: Process[Task, O]) {
  def toBoth(s1: Sink[Task, O], s2: Sink[Task, O]): Process[Task, Unit] = {
    (for (o <- p; n <- Process.emit(o) ++ Process.emit(o)) yield n) to (s1 interleave s2)
  }
}

That is, I duplicate the input and interleave the output. This can be generalized:

def toAll(sinks: Sink[Task, O] *): Process[Task, Unit] = {
  (for (o <- p; n <- Process.emitAll(sinks.map(_ => o))) yield n) to sinks.reduceLeftOption(_ interleave _).getOrElse(Process.empty)
}

---

EDIT 2

I just realized that the generalization toAll does not work. toBoth does, though

Is there a better (built-in) way?

Answer 1

You could also use observe and to to attach multiple Sinks to a Process:

val p = io.stdInLines.observe(out1).to(out2)

observe is like to but echoes what is passed into the sink. So io.stdInLines.observe(out1) still emits the strings that come from stdin (that means it is of type Process[Task, String]) but also sends them to the sink out1.

---

As Eric pointed out, it is also possible to zip Sinks together. Here is a more elaborate example that sends individual lines of a logfile to different sinks depending on their log level:

sealed trait Loglevel
case object Info extends Loglevel
case object Debug extends Loglevel
case object Warning extends Loglevel

case class Line(level: Loglevel, line: String)

val outInfo = io.stdOutLines.contramap((l: Line) => "I: " + l.line)
val outDebug = io.stdOutLines.contramap((l: Line) => "D: " + l.line)
val outWarning = io.stdOutLines.contramap((l: Line) => "W: " + l.line)

val zipped = outInfo.zip(outDebug).zip(outWarning).map {
  case ((fInfo, fDebug), fWarning) =>
    (l: Line) => l.level match {
      case Info    => fInfo(l)
      case Debug   => fDebug(l)
      case Warning => fWarning(l)
    }
}

val lines = List(
  Line(Info, "Hello"),
  Line(Warning, "Oops"),
  Line(Debug, "ui ui"),
  Line(Info, "World"))

Process.emitAll(lines).liftIO.to(zipped).run.run

Running this will output:

I: Hello
W: Oops
D: ui ui
I: World