Scalaz: Map of ValidationNel to ValidationNel of Map

Question

How do I convert a

val source: Map[MyKeyType, ValidationNel[MyErrorType, MyValueType]]

to

val target: ValidationNel[MyErrorType, Map[MyKeyType, MyValueType]]

while capturing all validation errors?

Answer 1

You can use sequence to turn a type F[G[A]] inside out (ie, into a G[F[A]] ) if you have two things: an Applicative instance for G and a Traverse instance for F . In this case Scalaz provides both off the shelf, so you can just write source.sequenceU (where the U part indicates that this is a method that uses the Unapply trick to help out Scala's type inference system).

For example:

scala> println(Map("a" -> 1.successNel, "b" -> 2.successNel).sequenceU)
Success(Map(a -> 1, b -> 2))

scala> println(Map("a" -> 1.successNel, "b" -> "BAD".failureNel).sequenceU)
Failure(NonEmptyList(BAD))

And errors will be accumulated as expected.

Answer 2

你想要sequenceU ：

val target = source.sequenceU