179 

   180 

   181 //other example for options...NaN

   182 val squareRoot: PartialFunction[Double, Double] = { 

   183     case d: Double if d > 0 => Math.sqrt(d) 

   184 }

   185 

   186 val list: List[Double] = List(4, 16, 25, -9)

   187 

   188 val result = list.map(Math.sqrt)

   189 // => result: List[Double] = List(2.0, 4.0, 5.0, NaN)

   190 

   191 val result = list.collect(squareRoot)

   192 // => result: List[Double] = List(2.0, 4.0, 5.0)