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post.scala

case class User(password: String, email: String)

sealed trait UserCreationError
case object PasswordTooShort  extends UserCreationError
case object EmailWrongPattern extends UserCreationError

val emailPattern = "^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,6}$".r

def checkPasswordLength(password: String): ValidatedNel[UserCreationError, String] =
  if (password.length < 6)
    invalidNel(PasswordTooShort)
  else
    valid(password)

def checkEmailPattern(email: String): ValidatedNel[UserCreationError, String] = (emailPattern findFirstIn email) match {
  case Some(_) => valid(email)
  case None    => invalidNel(EmailWrongPattern)
}

def createUser(password: String, email: String): ValidatedNel[UserCreationError, User] =
  (checkPasswordLength(password) |@| checkEmailPattern(email)) map User.apply(_, _)