Scalaz: Convert ValidationNel of ValidationNel to ValidationNel

Question

How do I convert

val from: ValidationNel[E, ValidationNel[E, T]]

to

val to: ValidationNel[E, T]

while capturing all validation errors?

Answer 1

you can use from.fold(Failure(_), identity) .

In general, flatMap(identity) or .join works for any Monad for turning F[F[A]] into F[A], however, Validation is not a Monad, and the flatMap / join methods for Validation are deprecated.

Answer 2

只是flatMap ， ValidationNel ：

from.flatMap(identity)