How to transform Scala collection of Option[X] to collection of X

Question

I'm starting to explore Scala, and one of the things I'm intrigued by is the Option type and the promise of being able to eliminate null related errors.

However I haven't been able to work out how to transform a list (or other collection) of, say, Option[String], to a collection of String (obviously filtering out any values that are None).

In other words, how do I get from this:

List[Option[Int]] = List(Some(1)) 

... to this:

List[Int] = List(1) 

I'm using Scala 2.8 if that has any impact on the answer.

Answer 1

val list1 = List(Some(1), None, Some(2)) val list2 = list1.flatten // will be: List(1,2) 

Answer 2

For educational purposes, you might like some alternatives:

scala> val list1 = List(Some(1), None, Some(2)) list1: List[Option[Int]] = List(Some(1), None, Some(2))  scala> list1 flatten res0: List[Int] = List(1, 2)  // Expanded to show the implicit parameter scala> list1.flatten(Option.option2Iterable) res1: List[Int] = List(1, 2)  scala> list1 flatMap (x => x) res2: List[Int] = List(1, 2)  scala> list1 flatMap Option.option2Iterable res3: List[Int] = List(1, 2)  // collect is a simultaneous map + filter scala> list1 collect { case Some(x) => x } res4: List[Int] = List(1, 2) 

With Scalaz, you can perform a slightly different operation called sequence, that returns Option[List[Int]].

scala> import scalaz._; import Scalaz._ import scalaz._ import Scalaz._  scala> val list1: List[Option[Int]] = List(Some(1), None, Some(2))  list1: List[Option[Int]] = List(Some(1), None, Some(2))  scala> list1.sequence                                               res1: Option[List[Int]] = None  scala> val list2: List[Option[Int]] = List(Some(1), Some(2))          list2: List[Option[Int]] = List(Some(1), Some(2))  scala> list2.sequence res2: Option[List[Int]] = Some(List(1, 2))