How to filter None's out of List[Option]?

Question

If I have a List[Option[A]] in Scala, what is the idiomatic way to filter out the None values?

One way is to use the following:

val someList: List[Option[String]] = List(Some("Hello"), None, Some("Goodbye"))
someList.filter(_ != None)

Is there a more "idiomatic" way? This does seem pretty simple.

Solution 1

If you want to get rid of the options at the same time, you can use flatten:

scala> someList.flatten
res0: List[String] = List(Hello, Goodbye)

Solution 2

someList.filter(_.isDefined) if you want to keep the result type as List[Option[A]]

Solution 3

The cats library also has flattenOption, which turns any F[Option[A]] into an F[A] (where F[_] is a FunctorFilter)

import cats.implicits._

List(Some(1), Some(2), None).flattenOption == List(1, 2)

Comments

[dskrvk]

Related: stackoverflow.com/questions/4730842/…

[Frank]

I would add that flatMap can be used to process the list elements other than Nones, similar to someList.flatten.map, as one often wants to work with these elements and not just flatten the list out of fun.

[Nicolas]

Yes, I hesitated to ask Raph what he's expecting to do with the filtered list. But there are also a lot of scenarios where you just want to flatten.

[Ralph]

Sometimes I want to retrieve the contents of the Option objects, but other times I need them to remain as Option[A]. I am finding more and more uses for Option[A] -- cool feature.

[cdmckay]

Is there an equivalent flatForeach type method?

[Nicolas]

If you're collection is immutable, faltmap will do the trick. If you're collection is mutable, no, there is not.

[matanster]

I find that in some cases this requires the use of type ascription in order to work.

[Will Beason]

@cdmckay Just someList.flatten.foreach

[Mojo]

Why would cats add this when flatten in the std lib does the job? Is it doing something extra?

[crater2150]

@Mojo the cats version is more generic, as it provides this for any type, for which a FunctorFilter typeclass can be defined (e.g. cats provides such an instance for maps, so you can use it on a Map[A, Option[B], which does not have flatten in the standard library)

[Hunger]

@Frank I thinkflatMap behaves like map.flatten rather then flatten.map. As if the map function return None, it will remove from final result.