I am learning Scalaz and I have a project that already makes use of Scalaz7. Following this question I would like to use the function
sequence[T](l: List[Option[T]]): Option[List[T]]
(not that it is hard to write it myself). But the aforementioned question mentions Scalaz6.
Where to find the sequence function in Scalaz7?
It's defined in the scalaz.Traverse
type class, where it looks like this:
def sequence[G[_]:Applicative,A](fga: F[G[A]]): G[F[A]] =
traversal[G].run[G[A], A](fga)(ga => ga)
scalaz.syntax.TraverseOps
provides a version that gets pimped onto List
, since List
has a Traverse
instance.
You can either import just what you need:
import scalaz._, std.list._, std.option._, syntax.traverse._
Or everything and the kitchen sink:
import scalaz._, Scalaz._
And then you can use it like this:
scala> val xs: List[Option[Int]] = Some(1) :: Some(2) :: Nil
xs: List[Option[Int]] = List(Some(1), Some(2))
scala> xs.sequence
res0: Option[List[Int]] = Some(List(1, 2))
Or if you want exactly the formulation in your question:
scala> def sequence[T](l: List[Option[T]]): Option[List[T]] = l.sequence
sequence: [T](l: List[Option[T]])Option[List[T]]
scala> sequence(xs)
res1: Option[List[Int]] = Some(List(1, 2))
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