signup.scala

final case class UserId(id: Long) extends AnyVal
final case class User(email: String, name: String, passwordDigest: String, createdAt: Instant, updatedAt: Instant)
final case class UserAndId(user: User, id: UserId)

sealed abstract class SignupError extends Product with Serializable
case object EmailAlreadyUsed

final case class Authenticated(email: String, token: Token)
final case class Token(get: UUID = UUID.randomUUID())

object UserRepository {
 
  def save(user: User): ConnectionIO[UserId] = ???

  def findByEmail(email: String): ConnectionIO[Option[UserAndId]] = ???
}


object SignupService {
  
  import UserRepository._

  def signup(signupForm: SignupForm): ConnectionIO[Either[SignupError, Authenticated]] = {
    OptionT(findByEmail(signupForm.email)).map { _ =>
      Either.left(EmailAlreadyUsed)
    }.getOrElseF {
      val newUser = createUser(signupForm)
      saveUser(newUser).map { _ =>
        Either.right(Authenticated(user.email, Token()))
      }
    }
  }
                      
  def createUser(signupForm: SignupForm): User = ???
}