How to convert a `NonEmptyList[Either[Error, User]]` to `Either[Error, NonEmptyList[User]]` with cats?

Question

I'm using cats, wonder how to turn a data with it:

val data = NonEmptyList[Either[Error, User]]

to

val target: Either[Error, NonEmptyList[User]] = howToConvert(data)

Answer 1

Usually when you want to turn the type constructors inside out you are probably looking for sequence. If you have -Ypartial-unification turned on in Scala >= 2.11.9 you can just let the compiler infer everything:

data.sequence

Otherwise:

type EitherError[A] = Either[Error, A]
data.sequence[EitherError, User]

Or if you have the type lambda plugin:

data.sequence[Either[Error, ?], User]

Or if you don't have the plugin, but you dislike type aliases:

data.sequence[({type L[A] = Either[Error, A]})#L, User]

It will do the expected thing of either returning the first error, or all users if there is no error. If we pretend users are ints and errors are strings:

scala> import cats.data.NonEmptyList, cats.implicits._
import cats.data.NonEmptyList
import cats.implicits._

scala> val data: NonEmptyList[Either[Error, User]] = NonEmptyList.of(Right(2), Left("error1"), Right(4))
data: cats.data.NonEmptyList[Either[Error,User]] = NonEmptyList(Right(2), Left(error1), Right(4))

scala> data.sequence
res4: Either[Error,cats.data.NonEmptyList[User]] = Left(error1)