Typelevel Mouse to the rescue

We all know that EitherT/OptionT can oftentimes make our code less readable. But is there actually any alternative? Meet the Typelevel Mouse - a tiny library you've probably never heard of that can help you when you want to avoid using EitherT and OptionT

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2. Typelevel Mouse to the rescue by Przemysław Sokół
3. scalaric www.scalaric.com
4. Mouse provides utils for... • Any • Boolean • Double • F[A] for any F, A • F[Either[A, B]] • F[Option[A]] • F[G[A]] • F[G[H[A]]] • F[TupleN] • Int • Long • Map • Option • String • Try • Tuple
6. IO[_]
7. 2 minutes introduction to... IO[_] !
8. IO[_] object Main extends App { val future = Future(println("Hi!")) for { _ <- future _ <- future } yield () // → Hi! }
9. IO[_] object Main extends App { val io = IO(println("Hello!")) val app = for { _ <- io _ <- io } yield () app.unsafeRunSync() // → Hello! // → Hello! }
10. IO[_] object Main extends IOApp { val io = IO(println("Hello!")) def run(args: List[String]): IO[ExitCode] = (for { _ <- io _ <- io } yield ()).as(ExitCode.Success) // → Hello! // → Hello! }
11. referential transparency
12. Benefits? •IO separates code definition and execution •gives us more control •improves code reusability...
13. EitherT OptionT
14. EitherT & OptionT def foo: IO[Either[Left, Right]] = ... Either + IO EitherIO EitherT
15. EitherT & OptionT def foo: IO[Either[Left, Right]] = ... Either + IO EitherIO EitherT[IO, Left, Right]
16. EitherT def parseDoubleAsync(s: String): Future[Either[String, Double]] = ... def divideAsync(a: Double, b: Double): Future[Either[String, Double]] = ... def divisionProgramAsync(inputA: String, inputB: String): Future[Either[String, Double]] = parseDoubleAsync(inputA) flatMap { eitherA => parseDoubleAsync(inputB) flatMap { eitherB => (eitherA, eitherB) match { case (Right(a), Right(b)) => divideAsync(a, b) case (Left(err), _) => Future.successful(Left(err)) case (_, Left(err)) => Future.successful(Left(err)) } } }
17. EitherT def parseDoubleAsync(s: String): Future[Either[String, Double]] = ... def divideAsync(a: Double, b: Double): Future[Either[String, Double]] = ... def divisionProgramAsync(inputA: String, inputB: String): EitherT[Future, String, Double] = for { a <- EitherT(parseDoubleAsync(inputA)) b <- EitherT(parseDoubleAsync(inputB)) result <- EitherT(divideAsync(a, b)) } yield result
18. ...things get a bit more complicated
19. sealed trait Error case object MailNotDelivered extends Error case object MailMissing extends Error case object NoAttachment extends Error case object ... extends Error
20. EitherT def parseUsername(name: String): Either[Error, Username] = ... def getUserId(username: Username): IO[Either[Error, UserId]] = ... def getLastMail(userId: UserId): IO[Option[Mail]] = ... def firstAttachment(mail: Mail): Option[Attachment] = ... (for { username <- EitherT.fromEither[IO](parseUsername("Alice")) userId <- EitherT(getUserId(username)) mail <- EitherT.fromOptionF(getLastMail(userId), ifNone = MailMissing) _ <- EitherT.cond[IO](test = mail.status == Delivered, right = unit, left = MailNotDelivered) result <- EitherT.fromOption[IO](firstAttachment(mail), ifNone = NoAttachment).leftWiden[Error] } yield result).value
21. EitherT.cond // with parameter names: EitherT.cond[IO](test = mail.status == Delivered, right = unit, left = MailNotDelivered) // ...and without: EitherT.cond[IO](mail.status == Delivered, unit, MailNotDelivered) // one could expect this: EitherT.cond[IO](..., LEFT, RIGHT) // ...but it's actually: EitherT.cond[IO](..., RIGHT, LEFT)
22. How about... ...Typelevel Mouse?
23. "org.typelevel" %% "mouse" % "1.1.0"
24. import mouse.all._ import mouse.all.*
25. Mouse def parseUsername(name: String): Either[Error, Username] = ... def getUserId(username: Username): IO[Either[Error, UserId]] = ... def getLastMail(userId: UserId): IO[Option[Mail]] = ... def firstAttachment(mail: Mail): Option[Attachment] = ... parseUsername("Alice") .pure[IO] .flatMapF(getUserId) .flatMapF { userId => getLastMail(userId).toRightIn(left = MailMissing) } .flatMapIn { mail => if (mail.status == Delivered) mail.asRight else MailNotDelivered.asLeft } .flatMapIn(firstAttachment(_).toRight(left = NoAttachment))
26. Mouse F[Either[L, R]] def mapIn[A](f: R => A)(implicit F: Functor[F]): F[Either[L, A]] def flatMapIn[A >: L, B](f: R => Either[A, B])(implicit F: Functor[F]): F[Either[A, B]] def flatMapF[A >: L, B](f: R => F[Either[A, B]])(implicit F: Monad[F]): F[Either[A, B]] def getOrElseF[A >: R](right: => F[A])(implicit F: Monad[F]): F[A] def getOrRaise[E](e: => E)(implicit F: MonadError[F, _ >: E]): F[R] def getOrRaiseMsg(msg: => String)(implicit F: MonadThrow[F]): F[R] (...)
27. Mouse F[Either[L, R]] def mapIn[A](f: R => A)(...): F[Either[L, A]] def flatMapIn[A >: L, B](f: R => Either[A, B])(...): F[Either[A, B]] def flatMapF[A >: L, B](f: R => F[Either[A, B]])(...): F[Either[A, B]] def getOrElseF[A >: R](right: => F[A])(...): F[A] def getOrRaise[E](e: => E)(...): F[R] def getOrRaiseMsg(msg: => String)(...): F[R] (...)
28. Mouse F[Option[A]] def mapIn[B](f: A => B)(...): F[Option[B]] def flatMapIn[B](f: A => Option[B])(...): F[Option[B]] def flatMapF[B](f: A => F[Option[B]])(...): F[Option[B]] def getOrElseF[B >: A](fa: => F[B])(...): F[B] (...)
29. Do we still need EitherT & OptionT...?
30. EitherT def getA: IO[Either[_, Int]] = ... def getB: IO[Either[_, String]] = ... def foo(a: Int, b: String): IO[Either[_, Bar]] = ... for { a <- EitherT(getA) b <- EitherT(getB) result <- EitherT(foo(a, b)) } yield result
31. Mouse def getA: IO[Either[_, Int]] = ... def getB: IO[Either[_, String]] = ... def foo(a: Int, b: String): IO[Either[_, Bar]] = ... getA.flatMapF { a => getB.flatMapF { b => foo(a, b) } }
32. Any alternatives?
33. Monix BIO
34. sealed-monad
35. Mouse is not is not only about EitherT and OptionT
36. ...more from Mouse "1.0".parseFloat // → Right(1.0F) Map(1 -> 2, 3 -> 4).mapKeys(_ * 2) // → Map(2 -> 2, 6 -> 4) List(Option(1), Option(2)).mapNested2(_ * 2) // → List(Some(2), Some(4)) val either: Either[L, R] = ... either.toEitherT[IO] // → EitherT[IO, L, R] (1, 2, 4, 8).last // → 8
38. Thanks! @falconepl @falconepl