Scalaz Writer and traverse / mapM example

exampleWriterTraverse.scala

import scalaz._
import Scalaz._


object ScalazPlay extends App
{
  // having this one-param type is critical for the subsequent type inference!
  type MyWriter[A] = Writer[List[String],A]

  def f1(i:Int) : MyWriter[Int] = for {
    _ <- List("f1 on " + i).tell
  } yield (i+7)
    //Writer(List("f1 on " + i), i+7)

  val t = List(1,2,3).traverse(f1)
  println(t.run)

  val t2 = mapM(List(10,12,13))(f1)
  println(t2.run)

//    List(1,2,3).traverseU(f1)


  // from test test in git:
 // val s: Writer[String, Stream[Int]] =
 //   Stream(1, 2, 3).traverseU(x => Writer(x.toString, x))

  def mapM[A,B,M[_]:Monad](xs:List[A])(f:A=>M[B]) : M[List[B]] =
    xs.map(f).sequence  
}