Evaluate long computation only if needed

scala> lazy val longComputation = IO{println("Long Computation ...."); 10.some}
longComputation: scalaz.effect.IO[Option[Int]] = <lazy>

scala> lazy val shortComputation = IO{println("Short Computation ...."); 2.some}
shortComputation: scalaz.effect.IO[Option[Int]] = <lazy>

scala> lazy val shortComputationFail:IO[Option[Int]] = IO{println("Short Computation ...."); none}
shortComputationFail: scalaz.effect.IO[Option[Int]] = <lazy>

scala> (shortComputation >>= {o => o.isDefined? o.pure[IO] | longComputation}).unsafePerformIO
Short Computation ....
res49: Option[Int] = Some(2)

scala> (shortComputationFail >>= {o => o.isDefined? o.pure[IO] | longComputation}).unsafePerformIO
Short Computation ....
Long Computation ....
res51: Option[Int] = Some(10)