createUser.scala

import invalid._

sealed abstract class UserValidationError
case object FirstNameEmpty extends UserValidationError
case object LastNameEmpty extends UserValidationError

def createUser(saveUser: User => UserID)(firstName: String, lastName: String) =
  validate(
    firstName.isEmpty -> FirstNameEmpty,
    lastName.isEmpty -> LastNameEmpty
  ) { User(firstName, lastName) } map saveUser

package.scala

import scalaz.{\/, \/-, -\/}

package object invalid {
  def validate[E, R](pairs: (Boolean, E)*)(otherwise: => R): Set[E] \/ R =
    if (pairs.exists(_._1)) -\/(pairs.map(_._2).toSet) else \/-(otherwise)
}