Scala / Cats: How to unzip an NonEmptyList

Question

The standard library offers the unzip method on List:


scala>val l = List((1, "one"), (2, "two"), (3, "three"), (4, "four"), (5, "five"))

scala> l.unzip
// res13: (List[Int], List[String]) = (
//  List(1, 2, 3, 4, 5),
//  List("one", "two", "three", "four", "five")
//)

Is there a way to achieve the same on NonEmptyList from the cats library:

scala> import cats.data.NonEmptyList

scala> val nel = NonEmptyList.of((1, "one"), (2, "two"), (3, "three"), (4, "four"), (5, "five"))
//res15: NonEmptyList[(Int, String)] = NonEmptyList(
//  (1, "one"),
//  List((2, "two"), (3, "three"), (4, "four"), (5, "five"))
//)

Arnaud Claudel · Answer 1 · 2019-07-18T17:40:25.910

5

You could simply call nel.toList and use the standard l.unzip and then NonEmptyList.fromList(unziped_list) on the result.

Edit: As @Dylan said, you could also use .fromListUnsafe to get rid of the option.

edited Jul 18 '19 at 17:40

answered Jul 18 '19 at 16:56

Arnaud Claudel

3,000
19
25

+1 but I'd recommend using `.fromListUnsafe` at the end, since `.fromList` gets you an option, and since your result list is guaranteed to have come from a non-empty List, the "unsafe" method is safe to use. – Dylan Jul 18 '19 at 17:10
@Dylan: Calling `.fromListUnsafe` is used instead of just calling `.get` on the resulting option? – Ellesedil Jul 18 '19 at 17:28
@Ellesedil basically yes. [See for yourself](https://github.com/typelevel/cats/blob/master/core/src/main/scala/cats/data/NonEmptyList.scala#L458) – Dylan Jul 18 '19 at 17:33

score 2 · Answer 2 · answered Dec 15 '22 at 14:07

The unzip method is available for NonEmptyList if you import cats.syntax.functor._.

scala> import cats.data.NonEmptyList
                                                                             
scala> import cats.syntax.functor._
                                                                             
scala> val nel = NonEmptyList.of((1, "one"), (2, "two"), (3, "three"), (4, "four"))
val nel: cats.data.NonEmptyList[(Int, String)] = NonEmptyList((1,one), (2,two), (3,three), (4,four))
                                                                             
scala> nel.unzip
val res0: (cats.data.NonEmptyList[Int], cats.data.NonEmptyList[String]) = (NonEmptyList(1, 2, 3, 4),NonEmptyList(one, two, three, four))

score 1 · Accepted Answer · answered Jul 28 '19 at 23:57

1

You don't have to do it all in one traversal, and often you don't even want to use one of the parts. I would write it like:

(nel.map(_._1), nel.map(_._2))

This avoids the awkward conversion away from an NEL and back.

answered Jul 28 '19 at 23:57

Karl Bielefeldt

47,314
10
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Scala / Cats: How to unzip an NonEmptyList

3 Answers3