Convert List[Validation[A, B]] to Validation[List[A], List[B]]

Validationz.scala

package com.mypackage

import org.specs.SpecificationWithJUnit
import scalaz._
import Scalaz._

class ValidationTests extends SpecificationWithJUnit {

  "I should be able to convert List[Validation[A, B]] to Validation[List[A], List[B]]" >> {

    def convert[A, B](l: List[Validation[A, B]]): Validation[NonEmptyList[A], List[B]] =
      l.traverse[({type l[a] = ValidationNEL[A, a]})#l, B](_.liftFailNel)

    val someFailures: List[Validation[String, Int]] = List("a".fail, 1.success, "b".fail, 2.success)
    convert(someFailures).fail.map(_.list).validation must be equalTo (List("a", "b").fail)

    val allSuccess: List[Validation[String, Int]] = (0 until 4).map(_.success).toList
    convert(allSuccess) must be equalTo ((0 until 4).toList.success)
  }

}