Making illegal states unrepresentable the Scala way

no-illegal-states.scala

//> using scala 3.5.2
//> using dep "com.softwaremill.ox::core:0.5.2"

object types:
  opaque type Title = String
  object Title:
    def apply(title: String): Either[Exception, Title] =
      if title.isBlank then Left(Exception("Title must not be blank"))
      else Right(title)

  opaque type NonEmptyList[A] = (A, List[A])
  extension [A](nel: NonEmptyList[A])
    def toList: List[A] = nel match
      case (a, list) => a :: list

  object NonEmptyList:
    def apply[A](a: A, as: A*): NonEmptyList[A] =
      (a, as.toList)

  opaque type ISBN = String
  object ISBN:
    private val isbnRegex = """^(?=(?:\D*\d){10}(?:(?:\D*\d){3})?$)[\d-]+$""".r

    def apply(isbn: String): Either[Exception, ISBN] =
      if isbnRegex.matches(isbn) then Right(isbn)
      else Left(Exception("Not a valid ISBN"))

  opaque type Author = String
  object Author:
    def apply(firstName: String, lastName: String): Either[Exception, Author] =
      if firstName.nonEmpty && lastName.nonEmpty then Right(s"$firstName $lastName")
      else Left(Exception("Author needs to have a first and last name"))

import types.*

case class Book(title: Title, isbn: ISBN, authors: NonEmptyList[Author])

@main def main = println {
  for
    validTitle <- Title("Functional programming in Scala")
    validIsbn  <- ISBN("9781617299582")

    mpilquist  <- Author("Michael", "Pilquist")
    rbjarnason <- Author("Rúnar", "Bjarnason")
    pchiusano  <- Author("Paul", "Chiusano")

    authorsNel = NonEmptyList(mpilquist, rbjarnason, pchiusano)
  yield Book(validTitle, validIsbn, authorsNel)
}

import ox.either, either.ok

@main def mainOx = println {
  either:
    val title = Title("Functional programming in Scala").ok()
    val isbn  = ISBN("9781617299582").ok()

    val authorsNel = NonEmptyList(
      Author("Michael", "Pilquist").ok(),
      Author("Rúnar", "Bjarnason").ok(),
      Author("Paul", "Chiusano").ok()
    )

    Book(title, isbn, authorsNel)
}