Map and reduce/fold over HList of scalaz.Validation

Question

I started out with something like this:

def nonEmpty[A] = (msg: String) => (a: Option[A]) => a.toSuccess(msg)

val postal: Option[String] = request.param(\         


        

Answer 1

Here's what this would look like with shapeless-contrib's traverse:

import scalaz._, Scalaz._
import shapeless._, contrib.scalaz._, syntax.std.tuple._

def nonEmpty[A] = (msg: String) => (a: Option[A]) => a.toSuccess(msg)

val postal: Option[String] = Some("00000")
val country: Option[String] = Some("us")

val params = (
  postal  |> nonEmpty[String]("no postal"),
  country |> nonEmpty[String]("no country")
)

And then:

object ToVNS extends Poly1 {
  implicit def validation[T] = at[Validation[String, T]](_.toValidationNel)
}

val result = traverse(params.productElements)(ToVNS).map(_.tupled)

Now result is a ValidationNel[String, (String, String)], and you can do anything with it that you could do with the awful ApplicativeBuilder thing you'd get from reducing with |@|.