Scalaz different behaviors when traversing a Stream/Seq with Option/IO and folding left or right.

PragmaticIoSpec.scala

  "traverse left with Stream and IO" >> {
    implicit def traverseImplicit = StreamLeftTraverse

    var result: Seq[Int] = Seq()
    val f = (i: Int) => (result = result :+ i).pure[IO]

    (1 to 3).toStream.map(f).sequence.unsafePerformIO
    result must_== Seq(3, 2, 1)
  }
  "traverse right with Stream and IO" >> {
    var result: Seq[Int] = Seq()
    val f = (i: Int) => (result = result :+ i).pure[IO]

    (1 to 3).toStream.map(f).sequence.unsafePerformIO
    result must_== Seq(1, 2, 3)
  }
  "traverse left with Stream and Option" >> {
    implicit def traverseImplicit = StreamLeftTraverse

    var result: Seq[Int] = Seq()
    val f = (i: Int) => (result = result :+ i).pure[Option]

    (1 to 3).toStream.map(f).sequence.get
    result must_== Seq(1, 2, 3)
  }
  "traverse right with Stream and Option" >> {
    var result: Seq[Int] = Seq()
    val f = (i: Int) => (result = result :+ i).pure[Option]

    (1 to 3).toStream.map(f).sequence.get
    result must_== Seq(1, 2, 3)
  }
  "traverse left with Seq and IO" >> {
    implicit def traverseImplicit = SeqLeftTraverse

    var result: Seq[Int] = Seq()
    val f = (i: Int) => (result = result :+ i).pure[IO]

    (1 to 3).map(f).sequence.unsafePerformIO
    result must_== Seq(1, 2, 3)
  }
  "traverse right with Seq and IO" >> {
    var result: Seq[Int] = Seq()
    val f = (i: Int) => (result = result :+ i).pure[IO]

    (1 to 3).toStream.map(f).sequence.unsafePerformIO
    result must_== Seq(1, 2, 3)
  }