gistfile1.txt

def saveUser(ds: DataSource)(user: User): UserID = {
  // blabla
}

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

def createUser(saveUser: User => UserID)(firstName: String, lastName: String): ValidationNel[UserValidationError, UserID] = {
  def validateFirstName(name: String) = if (name.isEmpty) FirstNameEmpty.fail else name.success
  (validateFirstName ⊛ validateLastName)(User(_, _)) map saveUser
}