import scalaz.std.option._
import scalaz.syntax.std.option._

// the standard scala way:
for {
  a <- Some(12)
  b <- None: Option[Int]
} yield a + b
// None is of type Option[Nothing] by default,
// so in order to force it to type-check I have to write None: Option[Int]

// the scalaz way:
for {
  a <- 12.some
  b <- none[Int]
} yield a + b
// same goes for none in scalaz,
// but it has none[Int] for convenience