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Lawrence Carvalho - Combining Refined Types wit...

Shannon
October 21, 2018

Lawrence Carvalho - Combining Refined Types with Type Class Derivation in Scala

Lawrence Carvalho talk from Scala in the City

Shannon

October 21, 2018
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  1. Lawrence Carvalho Motivation • Deserialize and validate input • Goal

    – correctness – "make illegal states unrepresentable” • Challenge – boilerplate
  2. Lawrence Carvalho Motivation case class Movie(id: String, title: String, credits:

    List[Credit], productionYear: Int, languages: List[String], imageUrl: String) case class Credit(firstName: String, lastName: String, role: Role) UUID Cannot be empty Cannot be empty Greater than or equal to 1889 From predefined set ISO 639-2 URL Cannot be empty
  3. Lawrence Carvalho Ways to “make illegal states unrepresentable” • assert/require

    – Unsafe case class Movie(id: String, title: String, credits: List[Credit], productionYear: Int, languages: List[String], imageUrl: String) { require(id.nonEmpty) } • Smart constructors case class Movie private (id: String, title: String, credits: List[Credit], productionYear: Int, languages: List[String], imageUrl: String) { def copy(id: String = id, title: String = title, credits: List[Credit] = credits, productionYear: Int = productionYear, languages: List[String] = languages, imageUrl: String = imageUrl): Either[Invalid, Movie] = Movie(id, title, credits, productionYear, languages, imageUrl) } object Movie { def apply(id: String, title: String, credits: List[Credit], productionYear: Int, languages: List[String], imageUrl: String): Either[Invalid, Movie] = ??? }
  4. Lawrence Carvalho Manually decoding JSON implicit val stringJsonDecoder: JsonDecoder[String] =

    { case JsonString(str) => Right(str) case _ => Left("Not a string!") } implicit val intJsonDecoder: JsonDecoder[Int] = { case JsonInt(int) => Right(int) case _ => Left("Not an int!") } implicit def listJsonDecoder[T](implicit decoder: JsonDecoder[T]): JsonDecoder[List[T]] = { case JsonArray(jsons) => jsons.traverse(decoder(_)) case _ => Left("Not an array!") } implicit val uuidJsonDecoder: JsonDecoder[UUID] = stringJsonDecoder(_).flatMap(s => Either.catchNonFatal(UUID.fromString(s)).leftMap(_.getMessage))
  5. Lawrence Carvalho Manually decoding JSON implicit val movieJsonDecoder: JsonDecoder[Movie] =

    json => for { id <- json.get[String]("id") title <- json.get[String]("title") credits <- json.get[List[Credit]]("credits") productionYear <- json.get[Int]("productionYear") languages <- json.get[List[String]]("languages") imageUrl <- json.get[String]("imageUrl") } yield Movie(id, title, credits, productionYear, languages, imageUrl) implicit val creditJsonDecoder: JsonDecoder[Credit] = json => for { firstName <- json.get[String]("firstName") lastName <- json.get[String]("lastName") role <- json.get[Role]("role") } yield Credit(firstName, lastName, role) implicit val roleJsonDecoder: JsonDecoder[Role] = json => json.decode[Actor].orElse(json.decode[Writer])
  6. Lawrence Carvalho Refined • Scala library for adding predicates to

    types • Port of the refined Haskell library type NonEmptyStr = String Refined NonEmpty type PosInt = Int Refined Positive type MoreThanTen = Int Refined Greater[W.`10`.T] import eu.timepit.refined.auto._ val r1: NonEmptyStr = "" val e1: PosInt = -1 val f1: MoreThanTen = 9 //compilation fails! val r2: Either[String, Refined[String, NonEmptyStr]] = refineV[NonEmptyStr]("") val e2: Either[String, Refined[Int, PosInt]] = refineV[PosInt](-1) val f2: Either[String, Refined[Int, MoreThanTen]] = refineV[MoreThanTen](9)
  7. Lawrence Carvalho Refined JSON Decoder type `ISO639-2` = String Refined

    MatchesRegex[W.`"^[a-zA-Z]{3}([/][a-zA-Z]{3})?$"`.T] case class Movie(id: UUID, title: String Refined NonEmpty, credits: NonEmptyList[Credit], productionYear: Int Refined GreaterEqual[W.`1889`.T], languages: List[`ISO639-2`], imageUrl: String Refined Url) case class Credit(firstName: String, lastName: String, role: Role) implicit def refinedJsonDecoder[T, P, F[_, _]](implicit jsonDecoder: JsonDecoder[T], validate: Validate[T, P], refType: RefType[F]): JsonDecoder[F[T, P]] = json => for { t <- jsonDecoder(json) refined <- refType.refine[P](t) } yield refined
  8. Lawrence Carvalho Shapeless type class derivation • Heterogeneous lists (HLists)

    • Coproducts • Generic case class Movie(id: UUID, title: String Refined NonEmpty, credits: NonEmptyList[Credit], productionYear: Int Refined GreaterEqual[W.`1889`.T], languages: List[`ISO639-2`], imageUrl: String Refined Url) type GenericMovie = UUID :: Refined[String, NonEmpty] :: NonEmptyList[Credit] :: Refined[Int, GreaterEqual[W.`1889`.T]] :: List[ `ISO639-2`] :: Refined[String, Url] :: HNil sealed trait Role case class Actor(characterName: String Refined NonEmpty) extends Role case class Writer(characters: List[String Refined NonEmpty]) extends Role type GenericRole = Actor :+: Writer :+: CNil
  9. Lawrence Carvalho Shapeless type class derivation for products implicit def

    hlistJsonDecoder[H, T <: HList]( implicit hDecoder: Lazy[JsonDecoder[H]], tDecoder: JsonDecoder[T] ): JsonDecoder[H :: T] = json => for { h <- hDecoder.value(json) t <- tDecoder(json) } yield h +: t implicit val hnilJsonDecoder: JsonDecoder[HNil] = _ => Right(HNil) implicit def genericJsonDecoder[A, R]( implicit gen: Generic.Aux[A, R], hEncoder: Lazy[JsonDecoder[R]] ): JsonDecoder[A] = json => hEncoder.value(json).map(gen.from)
  10. Lawrence Carvalho Shapeless type class derivation for coproducts implicit val

    cnilJsonDecoder: JsonDecoder[CNil] = _ => throw new Exception("cannot happen") implicit def coproductJsonDecoder[H, T <: Coproduct](implicit hJsonDecoder: Lazy[JsonDecoder[H]], tJsonDecoder: JsonDecoder[T]): JsonDecoder[H :+: T] = json => hJsonDecoder .value(json) .map(Inl(_)) .orElse(tJsonDecoder(json).map(Inr(_)))
  11. Lawrence Carvalho Automatically decoding JSON case class Movie(id: UUID, title:

    String Refined NonEmpty, credits: NonEmptyList[Credit], productionYear: Int Refined GreaterEqual[W.`1889`.T], languages: List[`ISO639-2`], imageUrl: String Refined Url) case class Season(id: UUID, title: String Refined NonEmpty, seasonNumber: Int Refined Positive, credits: NonEmptyList[Credit]) case class ContentWatched(contentId: String Refined NonEmpty, userId: UUID) val decodedMovie: Either[String, Movie] = json.decode[Movie] val decodedSeries: Either[String, Series] = json.decode[Series] val decodedContentWatched: Either[String, ContentWatched] = json.decode[ContentWatched]
  12. Lawrence Carvalho Existing implementations • JSON decoding with Circe •

    Typesafe configuration decoding with pureconfig • Command line argument parsing with scopt • Deriving Scalacheck Arbitrary instances
  13. Lawrence Carvalho JSON decoding with Circe import io.circe.parser._ import io.circe.generic.auto._

    case class Movie(id: String Refined Uuid, title: String Refined NonEmpty, credits: NonEmptyList[Credit], productionYear: Int Refined GreaterEqual[W.`1889`.T]) val jsonStr = """ |{ | "id": "5f5e36aa-a4fd-40df-9363-2ce71f2ad262" | "title": "Venom" | "credits": [ | { | "firstName": "Tom" | "lastName": "Hardy" | "role": "actor" | } | ] | "productionYear": 2017 |} """.stripMargin val decoded: Either[circe.Error, Movie] = decode[Movie](jsonStr)
  14. Lawrence Carvalho Typesafe configuration decoding with pureconfig case class AppConfig(port:

    Int Refined Positive, schemaRegistryUrl: String Refined Url) val config = ConfigFactory.parseString( """ |{ | port = 8080, | schema-registry-url = "http://schema-registry.sky.uk" |} """.stripMargin ) val validatedConfig: Either[ConfigReaderFailures, AppConfig] = loadConfig[AppConfig](config)
  15. Lawrence Carvalho Conclusions • Refined – Define constraints (predicates) on

    types. – Make the incorrect impossible – Types cannot lie! • Type class derivation – Let the compiler write your boilerplate! – Evolve domain model – Support deserialization of more data structures • Drawbacks – Fully automatic derivation will create more objects at runtime – Compilation time
  16. Lawrence Carvalho Questions? • Refined: https://github.com/fthomas/refined • Decorate your types

    with refined: https://www.youtube.com/watch?v=zExb9x3fzKs • Shapeless: https://github.com/milessabin/shapeless • Shapeless guide: https://github.com/underscoreio/shapeless-guide • Myself! – https://www.linkedin.com/in/lacarvalho/ – https://github.com/lacarvalho91