Easy and Efficient Data Validation with Cats - Scala Days 2017 Copenhagen

Often when we create a client/server application, we need to validate the requests: can the user associated to the request perform this operation? Can they access or modify the data? Is the input well-formed?

When the data validation component in our application is not well designed, the code can quickly become not expressive enough and probably difficult to maintain. Business rules don't help, adding more and more requirements to add in our validation, making it more and more complex to clearly represent and maintain. At the same time when the validation fails, it should be fairly straight forward to understand why the request was rejected, so that actions can be taken accordingly.

This talk introduces Cats, a Scala library based on category theory, and some of its most interesting components for data validation. In particular we'll discuss some options to achieve efficient and expressive data validation. We will also argue that, compared to other options in the language, Cats is particularly suited for the task thanks to its easy-to-use data types and more approachable syntax.

Throughout the talk, you will see numerous examples on how data validation can be achieved in a clean and robust way, and how we can easily integrate it in our code, without any specific knowledge of category theory.

Transcript

3. HELLOOOOO > ex Java Developer > OOP background > I am not a mathematician !
4. FUNCTIONAL BUZZWORDS FREE ZONE
5. DATA VALIDATION > In almost every application > Can be come complex quite quickly > Needs to be maintained
6. ...NO NEED TO REINVENT THE WHEEL
7. MAP scala> Some("daniela").map(s => "yo " + s) res0: Option[String] = Some(yo daniela) scala> None.map(s => "yo " + s) res1: Option[String] = None
8. MAP + FLATTEN scala> Some("daniela").map(s => Some("yo " + s)) res2: Option[Option[String]] = Some(Some(yo daniela)) scala> Some("daniela").map(s => Some("yo " + s)).flatten res3: Option[String] = Some(yo daniela)
9. FLATMAP map + flatten scala> Some("daniela").flatMap(s => Some("yo " + s)) res4: Option[String] = Some(yo daniela) scala> None.flatMap(s => Some("yo " + s)) res5: Option[String] = None
10. FOR-COMPREHENSION map + flatMap (+ filter) scala > for { a <- Some(1); b <- Some(5) } yield a + b res6: Option[Int] = Some(6) scala > for { a <- Some(1); b <- None } yield a + b res7: Option[Int] = None
11. CASE STUDY
12. SCALA 2.11 NOT THE LATEST VERSION !!!
13. OPTION package scala sealed abstract class Option[+A] final case class Some[+A](x: A) extends Option[A] case object None extends Option[Nothing] def map[B](f: A => B): Option[B] = ??? def flatMap[B](f: A => Option[B]): Option[B] = ???
14. OPTION def myMethod(n: Int): Option[Int] = { if (n > 5) Some(n) else None }
15. OPTION case class Data(email: String, phone: String) def validateEmail(e: String): Option[String] = ??? def validatePhone(p: String): Option[String] = ??? def validateData(d: Data): Option[Data] = for { validEmail <- validateEmail(d.email) validPhone <- validatePhone(d.phone) } yield Data(validEmail, validPhone)
16. OPTION val okEmail = "email@email.com"; val badEmail = "email.com" val okPhone = "+1 1234567890123"; val badPhone = "not-a-valid-phone" > validateData(Data(okEmail, okPhone)) res0: Option[Data] = Some(Data(email@email.com,+1 1234567890123)) > validateData(Data(badEmail, badPhone)) res1: Option[Data] = None > validateData(Data(okEmail, badPhone)) res2: Option[Data] = None > validateData(Data(badEmail, okPhone)) res3: Option[Data] = None
17. Y U NO TELL ME WHICH ONE IS THE WRONG ONE?
18. JUST DO NOT USE OPTION
19. EITHER (2.11) package scala.util sealed abstract class Either[+A, +B] final case class Left[+A, +B](a: A) extends Either[A, B] final case class Right[+A, +B](b: B) extends Either[A, B] // no map // no flatMap
20. EITHER (2.11) package scala.util final case class LeftProjection[+A, +B](e: Either[A, B]) final case class RightProjection[+A, +B](e: Either[A, B]) /** * Right(12).left.map(x => "flower") // Result: Right(12) * Left(12).left.map(x => "flower") // Result: Left("flower") * * Right(12).right.map(x => "flower") // Result: Right("flower") * Left(12).right.map(x => "flower") // Result: Left(12) **/ // same for flatmap!
21. EITHER (2.11) def myMethod(n: Int): Either[String, Int] = { if (n > 5) Right(n) else Left("Not cool bro!") }
22. EITHER (2.11) case class Data(email: String, phone: String) def validateEmail(e: String): Either[List[String], String] = ??? def validatePhone(p: String): Either[List[String], String] = ??? def validateData(d: Data): Either[List[String], Data] = { val validEmail = validateEmail(d.email) val validPhone = validatePhone(d.phone) (validEmail, validPhone) match { case (Right(e), Right(p)) => Right(Data(e, p)) case (Left(errE), Left(errP)) => Left(errE ++ errP) case (Left(errE), _) => Left(errE) case (_, Left(errP)) => Left(errP) } }
23. EITHER (2.11) val okEmail = "email@email.com"; val badEmail = "email.com" val okPhone = "+1 1234567890123"; val badPhone = "not-a-valid-phone" > validateData(Data(okEmail, okPhone)) res0: Either[List[String],Data] = Right(Data(email@email.com,+1 1234567890123)) > validateData(Data(badEmail, badPhone)) res1: Either[List[String],Data] = Left(List("Invalid email format", "Phone number must be numeric")) > validateData(Data(okEmail, badPhone)) res2: Either[List[String],Data] = Left(List("Phone number must be numeric")) > validateData(Data(badEmail, okPhone)) res3: Either[List[String],Data] = Left(List("Invalid email format"))
24. EITHER (2.11) Which one is the error? Which one is the valid value? Either is not biased* *things have changed in Scala 2.12
25. EITHER (2.11) Combine Either instances is not always easy or maintainable
26. SCALA 2.12
27. EITHER (2.12) package scala.util sealed abstract class Either[+A, +B] final case class Left[+A, +B](a: A) extends Either[A, B] final case class Right[+A, +B](b: B) extends Either[A, B] def map[Y](f: B => Y): Either[A, Y] = ??? def flatMap[AA >: A, Y](f: B => Either[AA, Y]): Either[AA, Y] = ???
28. EITHER (2.12) package scala.util final case class LeftProjection[+A, +B](e: Either[A, B]) final case class RightProjection[+A, +B](e: Either[A, B]) // right projection used by default /** * Right(12).left.map(x => "flower") // Result: Right(12) * Left(12).left.map(x => "flower") // Result: Left("flower") * * Right(12).right.map(x => "flower") // Result: Right("flower") * Left(12).right.map(x => "flower") // Result: Left(12) **/ // same for flatmap!
29. EITHER (2.12) def myMethod(n: Int): Either[String, Int] = { if (n > 5) Right(n) else Left("Not cool bro!") }
30. EITHER (2.12) case class Data(email: String, phone: String) def validateEmail(e: String): Either[List[String], String] = ??? def validatePhone(p: String): Either[List[String], String] = ??? def validateData(d: Data): Either[List[String], Data] = for { validEmail <- validateEmail(d.email) validPhone <- validatePhone(d.phone) } yield Data(validEmail, validPhone)
31. EITHER (2.12) val okEmail = "email@email.com"; val badEmail = "email.com" val okPhone = "+1 1234567890123"; val badPhone = "not-a-valid-phone" > validateData(Data(okEmail, okPhone)) res0: Either[List[String],Data] = Right(Data(email@email.com,+1 1234567890123)) > validateData(Data(badEmail, badPhone)) res1: Either[List[String],Data] = Left(List("Invalid email format")) > validateData(Data(okEmail, badPhone)) res2: Either[List[String],Data] = Left(List("Phone number must be numeric")) > validateData(Data(badEmail, okPhone)) res3: Either[List[String],Data] = Left(List("Invalid email format"))
38. EITHER (2.12) > only one validation is performed > ideal only when error accumulation is not needed
39. CATS 0.9.0 GITHUB.COM/TYPELEVEL/CATS
40. BIASED EITHER WITH 2.11 import cats.syntax.either._ * Xor removed from cats 0.8.0
41. VALIDATED package cats.data sealed abstract class Validated[+E, +A] final case class Valid[+A](a: A) extends Validated[Nothing, A] final case class Invalid[+E](e: E) extends Validated[E, Nothing] def map[B](f: A => B): Validated[E,B] // no flatmap //...but we have something else *really* useful!
42. VALIDATED AND APPLY* import cats.Apply import cats.data.Validated import cats.implicits._ def accumulate[E, A1, A2, B](v1: Validated[E, A1], v2: Validated[E, A2]) (f: (A1, A2) => B): Validated[E, B] = (v1 |@| v2).map(f) // same as: Apply[Validated[E, ?]].map2(v1,v2)(f) * More info on Apply at http://typelevel.org/cats/typeclasses/applicative.html
43. VALIDATED import cats.data.Validated import cats.data.Validated._ def myMethod(n: Int): Validated[String, Int] = { if (n > 5) Valid(n) else Invalid("Not cool bro!") } // or alternatively... import cats.data.Validated def myMethod2(n: Int): Validated[String, Int] = { if (n > 5) Validated.valid(n) else Validated.invalid("Not cool bro!") }
44. VALIDATED import cats.implicits._ import cats.data.Validated case class Data(email: String, phone: String) def validateEmail(e: String): Validated[List[String], String] = ??? def validatePhone(p: String): Validated[List[String], String] = ??? def validateData(d: Data): Validated[List[String], Data] = { val validEmail = validateEmail(d.email) val validPhone = validatePhone(d.phone) (validEmail |@| validPhone).map(Data) }
45. VALIDATED val okEmail = "email@email.com"; val badEmail = "email.com" val okPhone = "+1 1234567890123"; val badPhone = "not-a-valid-phone" > validateData(Data(okEmail, okPhone)) res0: cats.data.Validated[List[String],Data] = Valid(Data(email@email.com,+1 1234567890123)) > validateData(Data(badEmail, badPhone)) res1: cats.data.Validated[List[String],Data] = Invalid(List("Invalid email format", "Phone number must be numeric")) > validateData(Data(okEmail, badPhone)) res2: cats.data.Validated[List[String],Data] = Invalid(List("Phone number must be numeric")) > validateData(Data(badEmail, okPhone)) res3: cats.data.Validated[List[String],Data] = Invalid(List("Invalid email format"))
46. VALIDATEDNEL package cats.data type NonEmptyList[A] = OneAnd[List, A] type ValidatedNel[E, A] = Validated[NonEmptyList[E], A]
47. VALIDATEDNEL import cats.data.Validated import cats.data.Validated._ def myMethod(n: Int): ValidatedNel[String, Int] = { if (n > 5) Valid(n) else Invalid(NonEmptyList[String]("Not cool bro!", List.empty)) } // or alternatively... import cats.data.Validated def myMethod2(n: Int): ValidatedNel[String, Int] = { if (n < 5) Validated.valid(n) else Validated.invalidNel("Not cool bro!") }
48. USE AN EXPRESSIVE ERROR TYPE object ErrorCode extends Enumeration { type ErrorCode = Value val InvalidEmailFormat, ..., PhoneMustBeNumeric = Value } import ErrorCode._ case class Err(code: ErrorCode, msg: String)
49. OUR FINAL SOLUTION import cats.data._ import cats.implicits._ case class Data(email: String, phone: String) def validateEmail(e: String): ValidatedNel[Err, String] = ??? def validatePhone(p: String): ValidatedNel[Err, String] = ??? def validateData(d: Data): ValidatedNel[Err, Data] = { val validEmail = validateEmail(d.email) val validPhone = validatePhone(d.phone) (validEmail |@| validPhone).map(Data) }
50. VALIDATEDNEL + ERR val okEmail = "email@email.com"; val badEmail = "email.com" val okPhone = "+1 1234567890123"; val badPhone = "not-a-valid-phone" > validateData(Data(okEmail, okPhone)) res0: cats.data.ValidatedNel[Err,Data] = Valid(Data(email@email.com,+1 1234567890123)) > validateData(Data(badEmail, badPhone)) res1: cats.data.ValidatedNel[Err,Data] = Invalid(NonEmptyList( Err("InvalidEmailFormat","Invalid email format"), Err("PhoneMustBeNumeric","Phone number must be numeric"))) > validateData(Data(okEmail, badPhone)) res2: cats.data.ValidatedNel[Err,Data] = Invalid(NonEmptyList( Err("PhoneMustBeNumeric","Phone number must be numeric"))) > validateData(Data(badEmail, okPhone)) res3: cats.data.ValidatedNel[Err,Data] = Invalid(NonEmptyList( Err("InvalidEmailFormat","Invalid email format")))
51. THE WONDERS OF |@| // sequential computation for { a <- Future(1) b <- Future(2) c <- Future(3) } yield a + b + c // parallel computation import cats.implicits._ (Future(1) |@| Future(2) |@| Future(3)).map(_ + _ + _)
52. HOW TO STRUCTURE VALIDATION WITHIN YOUR APPLICATION?
53. STEP 1 > Pick an error representation > stick to it! case class Err(code: ErrorCode, msg: String)
54. STEP 2 > Use a type alias type Validation[T] = ValidatedNel[Err, T]
55. STEP 3 > Create a companion object > make it simple for your team
56. A CONCRETE EXAMPLE sealed trait Err { val code: String val msg: String val values: Seq[AnyRef] } case class BadRequest(code: String, msg: String) extends Err { val values = Seq.empty } case class NotFound(code: String, msg: String, values: Seq[AnyRef]) extends Err
57. A CONCRETE EXAMPLE type Validation[T] = ValidatedNel[Err, T] import cats.data._ object Validation extends AccumulateArities { def success[T](t: T): Validation[T] = Validated.valid(t) def failure[T](e: Err): Validation[T] = Validated.invalidNel(e) }
58. A CONCRETE EXAMPLE trait AccumulateArities { /** Accumulate function for Validation[T] of arity 2 */ def accumulate[T1,T2,Z](v1: Validation[T1], v2: Validation[T2]) (f: (T1,T2) => Z): Validation[Z] = Apply[Validation].map2(v1,v2)(f) /** Accumulate function for Validation[T] of arity 3 */ def accumulate[T1,T2,T3,Z](v1: Validation[T1], v2: Validation[T2], v3: Validation[T3]) (f: (T1,T2,T3) => Z): Validation[Z] = Apply[Validation].map3(v1,v2,v3)(f) // ...until arity 22! }
59. SUMMARY > Do not reinvent the wheel > Choose an expressive type > Customise the solution to your needs
60. THANK YOU! > Code on github: github.com/DanielaSfregola/data-validation > Twitter: @DanielaSfregola > Blog: danielasfregola.com