Convert List[ValidationNEL[String, Unit]] to ValidationNEL[String, Unit]

Question

Is there a standard way to flatten the left sides into a single list?

thanks!

Answer 1

I'm not exactly sure what you're trying to do, but the Monoid instance should do the trick:

scala> List(
     |   println("test").successNel[String],
     |   "a".failNel[Unit],
     |   "b".failNel[Unit]
     | ).suml == Failure(NonEmptyList("a", "b"))
test
res0: Boolean = true

Where suml is a method that's pimped onto any collection full of something with a Monoid instance.

Answer 2

Ok, here is some code that does the trick:

scala> type MyValidationNEL[A] = ValidationNEL[String, A]
defined type alias MyValidationNEL

scala> val x: List[MyValidationNEL[Unit]] = 
     | List("a".failNel, "b".failNel)
x: List[MyValidationNEL[Unit]] = List(Failure(NonEmptyList(a)), Failure(NonEmptyList(b)))

scala> x.sequence
res57: MyValidationNEL[List[Unit]] = Failure(NonEmptyList(a, b))

scala> 

I've taken part of the example from this presentation: http://www.scribd.com/doc/93526802/Bjarnason-Scalaz by Rúnar Bjarnason http://scalaz.org