Let's refine your Scala Code
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This talk aims to be a gentle introduction to Refinement types in Scala. The talk will also introduce many related topics in the subculture of Typelevel programming in FP like Type theory, Generics (Shapeless), Optics (Monocle) and Law-based testing. Why refinement types? When 'String' just doesn't cut it anymore. By encoding validation into a type at the edge-of-the-system layer of your application, you can build your core domain logic without having to worry much about type-safety. Who is it for? Ajay would be revisiting the necessary foundations at the beginning of the talk, so beginners are welcome. People who have worked on Scala before would be able to appreciate the power of refined types more.
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Let's refine your Scala Code
- 2. Aboutme Sr.SoftwareEngineer@Agoda MyFPjourneyhastakenmethroughBigData,BackEndandFrontEnd LearningFPandScalaforthepast5years Reachouttomeonmyblogscala.ninja ThisPresentationwaswrittenin markdown andcompiledusing marp-cli Somecodeexamplesmayrequireyoutouse Scala 2.13 or Dotty
- 6. AstaticallytypedlanguagelikeScalahastwodomains-atypedomain,andavalue domain. case class Name(firstName: String, lastName: String) val n1 = Name("ajay", "viswanathan") // Name -> Type // n1 -> Value Likeclasses,typescanalsohaveconstructors type IntMap[T] = Map[Int, T]
- 8. DesignPattern#1:PhantomTypes Anabstracttypethatisneverinitialized,hencehasnoeffectonruntime Itisonlyusedtoprovestaticpropertiesusingtypeevidences Thecompilererasestheseonceitcanprovethattheconstraintshold sealed trait Status sealed trait Red extends Status sealed trait Orange extends Status sealed trait Green extends Status class TrafficSignal[T <: Status] { private def to[U <: Status]: TrafficSignal[U] = this.asInstanceOf[TrafficSignal[U]] def stop(implicit ev: T =:= Orange): TrafficSignal[Red] = to[Red] def start(implicit ev: T =:= Red): TrafficSignal[Orange] = to[Orange] def go(implicit ev: T =:= Orange): TrafficSignal[Green] = to[Green] def slow(implicit ev: T =:= Green): TrafficSignal[Orange] = to[Orange] } val signal = new TrafficSignal[Red] signal.stop // Compilation Error: Cannot prove that TrafficLight.Red =:= TrafficLight.Orange signal.start.go // Compilation Successful
- 9. DesignPattern#2:Pathdependenttypes Scalaallowsyoutodefinetypesinsideatype Thisisusefulwhenonlypartialtypeinformationisavailableatcompiletime Theruntimetypecanbeencapsulated case class Cart(user: String, value: Int = 0) { case class Item(name: String, amount: Int) def add(item: this.Item): Cart = copy(value = value + item.amount) } val cart = Cart("suresh") val item: cart.Item = cart.Item("perk", 2) val valid = cart.add(item) // Cart(suresh, 2) val invalid = Cart("ramesh").add(item) // Compilation Error
- 10. DesignPattern#2PathDependentTypesRedux:AUXpattern Wedon'talwayshavetheluxuryofknowingcertaindatatypesatcompiletime Whenwewantafunctiontodependentonaruntimetype,wecanuseTypeclassesto workaroundthisproblem Saythereexistsafunctionthatlooksforatypeclassbasedontheruntimevalue trait Param[T, U] { def convert(obj: T): Option[U] } trait Predicate[U] { def eval(values: List[U], value: U): Boolean } def evaluate[T, U](request: T, values: List[U])(implicit param: Param[T, U], predicate: Predicate[U]): Boolean = { param.convert(request).exists(v => predicate.eval(values, v)) }
- 11. implicit val stringPredicate = new Predicate[String] { def eval(values: List[String], value: String) = values.contains(value) } implicit val intPredicate = new Predicate[Int] { def eval(values: List[Int], value: Int) = values.forall(_ > value) } implicit val stringIntParam = new Param[String, Int] { def convert(obj: String): Option[Int] = Try(obj.toInt).toOption } ... evaluate("3", List(2, 3, 4)) // false evaluate("3", List(4, 5, 6)) // true
- 12. WecouldrewritethesameusingPathdependenttypesas trait Param[T] { type U def convert(obj: T): Option[U] } def evaluate[T, U](request: T, values: List[U])(implicit param: Param[T], predicate: Predicate[param.U]) // Does not compile Weintroducethe Aux patternheretohelpthecompilerreasonaboutthetypeinference object Param { type Aux[T0, U0] = Param[T0] { type U = U0 } } def evaluate[T, U](request: T, values: List[U])(implicit ev: Param.Aux[T, U], predicate: Predicate[U]): Boolean = { ev.convert(request).exists(v => predicate.eval(values, v)) }
- 13. DesignPattern#3:Singletontypes EverySingletonobjectinScalaisitsowntype Singletontypesbridgethegapbetweenthevaluelevelandthetypelevel object Data { val v1: String = "data" } object FakeData { val v1: String = "virus" } def process(obj: Data.type) = println(obj.v1) // data
- 14. DesignPattern#3(Experimental):Literal-BasedSingletonTypes SIP23-ProposaltointegratewithScala3 WillallowLiteralstoappearintype-position val one: 1 = 1 Sampleuse-case:MatrixMultiplication type Dim = Singleton & Int case class Matrix[A <: Dim, B <: Dim](a: A, b: B) { def *[C <: Dim](other: Matrix[B, C]): Matrix[A, C] = Matrix(a, other.b) }
- 16. case class Timestamp(date: String, hour: Int) Somanyquestions: Whatisthedateformatexpected? Areallformatshandled? Day/Month/Year??? Hourin12/24hformat? Houris0-indexed?
- 17. Let'strytofixthis // With Self-checks def verifyDate(date: String, format: String = "yyyyMMdd"): Boolean case class Timestamp(date: String, hour: Int) { require(hour > 0 && hour <= 24) require(verifyDate(date)) } // With Structural Enforcement def apply(year: Int, month: Int, day: Int, hour: Int) = Timestamp(s"$year-$month-$day", hour)
- 18. OpaquevsTransparentDesign ThefirstconsiderationwhiledesigningaDataTypeisthelevelofflexibilityor encapsulationyouwishtoprovide Inotherwords,how opaque or transparent doyouwantyourDataTypetobe? // opaque DataType class ParseError(index: Int, input: String) { def line(): Int def position(): Int def message(): String } // transparent DataType case class ParseError(line: Int, position: Int, message: String)
- 20. Exampleofbaddatadomain case class Request(guid: String)
- 21. Approach#1:Usingtypealias type GUID = String case class Request(guid: GUID) Doesnothingmorethanmakingiteasiertoread
- 22. Approach2:Usinga(value)class case class GUID(value: String) Compilercancheckfortherightinstance,butstillnothingintermsofsafetyofinput Boxingoverhead class GUID(val value: String) extends AnyVal Stilldoesn'tsolvethevalidationproblem Valueclassescanstillgetinitializedifusedincollections,methodsignaturesetc
- 23. Approach3:UsingSmartConstructors case class GUID private (value: String) object GUID { def apply(value: String): Option[GUID] = if (value.startsWith("valid")) Some(new GUID(value)) else None } new GUID("valid") // Does not compile - Good! GUID("valid") // GUID(valid) - Validations work - Great!! GUID("valid").map(_.copy("invalid")) // GUID(invalid) - Copy constructor is vulnerable - Bad!!! Bythistime,we'vealreadyintroducedimmensecomplexitytoasimpleDataType ReturntypeofOptionisabitfrustratingtohandlefordevelopers
- 24. TheNarrow-Widenapproach String => GUID notallstringsarevalid String => Option[GUID] widentheoutputtoaccommodate List[Char] => Option[GUID] NonEmptyList[Char] => Option[GUID] slightrefinementtotheinput NonEmptyList[HexDigit] => Option[GUID] furtherrefininingtheinput ThirtyTwoList[HexDigit] => Option[GUID] => GUID totalrefinementofthe input,plusun-wideningoftheoutput Whenyouwanttonarrowtheinput,youendupWideningtheoutput,butifyounarrow theinputenough,youcanun-widentheoutput
- 25. Approach#4:Refiningtheconstructor Arefinementtypeisbasically Base Type + Predicate Subtypingofrefinements type T + P <: T type T + P <: type T + Q if forall t in T + P, Q(t) is true Eg. A - Set of Int > 10 B - Set of Int > 5 A <: B
- 26. type HexDigit = Char Refined LetterOrDigit type ThirtyTwoList[T] = List[T] Refined Size[32] type GUID = ThirtyTwoList[HexDigit] case class Request(guid: GUID) val r1 = Request("2312k3j123...123dasd") // Compiles val r2 = Request("asdas_asda_21#!@##$@#$...2234") // Does not compile // At runtime refineV[GUID]("asdasd2323...12312asd") // Either[String, GUID] Typeserasedatcompiletimeleavingyouwithonlyprimitives Atruntime,predicatescomputedonvaluesaswouldbethecasenormally
- 27. YoucandomuchmorewithRefined Thelibrarycomeswiththesepredefinedpredicates Boolean True: constant predicate that is always true False: constant predicate that is always false Not[P]: negation of the predicate P And[A, B]: conjunction of the predicates A and B Or[A, B]: disjunction of the predicates A and B Xor[A, B]: exclusive disjunction of the predicates A and B Nand[A, B]: negated conjunction of the predicates A and B Nor[A, B]: negated disjunction of the predicates A and B AllOf[PS]: conjunction of all predicates in PS AnyOf[PS]: disjunction of all predicates in PS OneOf[PS]: exclusive disjunction of all predicates in PS
- 28. Char Digit: checks if a Char is a digit Letter: checks if a Char is a letter LetterOrDigit: checks if a Char is a letter or digit LowerCase: checks if a Char is a lower case character UpperCase: checks if a Char is an upper case character Whitespace: checks if a Char is white space
- 29. Collection Contains[U]: checks if a Traversable contains a value equal to U Count[PA, PC]: counts the number of elements in a Traversable which satisfy the predicate PA and passes the result to the predicate PC Empty: checks if a Traversable is empty NonEmpty: checks if a Traversable is not empty Forall[P]: checks if the predicate P holds for all elements of a Traversable Exists[P]: checks if the predicate P holds for some elements of a Traversable Head[P]: checks if the predicate P holds for the first element of a Traversable Index[N, P]: checks if the predicate P holds for the element at index N of a sequence Init[P]: checks if the predicate P holds for all but the last element of a Traversable Last[P]: checks if the predicate P holds for the last element of a Traversable Tail[P]: checks if the predicate P holds for all but the first element of a Traversable Size[P]: checks if the size of a Traversable satisfies the predicate P MinSize[N]: checks if the size of a Traversable is greater than or equal to N MaxSize[N]: checks if the size of a Traversable is less than or equal to N
- 30. Generic Equal[U]: checks if a value is equal to U
- 31. Numeric Less[N]: checks if a numeric value is less than N LessEqual[N]: checks if a numeric value is less than or equal to N Greater[N]: checks if a numeric value is greater than N GreaterEqual[N]: checks if a numeric value is greater than or equal to N Positive: checks if a numeric value is greater than zero NonPositive: checks if a numeric value is zero or negative Negative: checks if a numeric value is less than zero NonNegative: checks if a numeric value is zero or positive Interval.Open[L, H]: checks if a numeric value is in the interval (L, H) Interval.OpenClosed[L, H]: checks if a numeric value is in the interval (L, H] Interval.ClosedOpen[L, H]: checks if a numeric value is in the interval [L, H) Interval.Closed[L, H]: checks if a numeric value is in the interval [L, H] Modulo[N, O]: checks if an integral value modulo N is O Divisible[N]: checks if an integral value is evenly divisible by N NonDivisible[N]: checks if an integral value is not evenly divisible by N Even: checks if an integral value is evenly divisible by 2 Odd: checks if an integral value is not evenly divisible by 2 NonNaN: checks if a floating-point number is not NaN
- 32. String EndsWith[S]: checks if a String ends with the suffix S IPv4: checks if a String is a valid IPv4 IPv6: checks if a String is a valid IPv6 MatchesRegex[S]: checks if a String matches the regular expression S Regex: checks if a String is a valid regular expression StartsWith[S]: checks if a String starts with the prefix S Uri: checks if a String is a valid URI Url: checks if a String is a valid URL Uuid: checks if a String is a valid UUID ValidByte: checks if a String is a parsable Byte ValidShort: checks if a String is a parsable Short ValidInt: checks if a String is a parsable Int ValidLong: checks if a String is a parsable Long ValidFloat: checks if a String is a parsable Float ValidDouble: checks if a String is a parsable Double ValidBigInt: checks if a String is a parsable BigInt ValidBigDecimal: checks if a String is a parsable BigDecimal Xml: checks if a String is well-formed XML XPath: checks if a String is a valid XPath expression Trimmed: checks if a String has no leading or trailing whitespace HexStringSpec: checks if a String represents a hexadecimal number
- 33. TaggingTypes Letstakeanothercommonexamplewhereaspecifictypeinadomaincanbecritical def getWeight(): Double def getHeight(): Double def bmi(weight: Double, height: Double): Double // Let's assume SI units as input bmi(getWeight(), getHeight()) // OK bmi(getHeight(), getWeight()) // Still OK, but not ideal behavior
- 34. Besidesrefining,taggingtypesofferalow-costalternativetoValueclasses type Tagged[U] = { type Tag = U } type @@[T, U] = T with Tagged[U] implicit class TaggedOps[T](val v: T) extends AnyVal { @inline def taggedWith[U]: T @@ U = v.asInstanceOf[T @@ U] @inline def @@[U]: T @@ U = taggedWith[U] }
- 35. trait Kilogram trait Metres type Weight = Double @@ Kilogram type Height = Double @@ Metres def getWeight(): Weight def getHeight(): Height // Units are already encoded in the type now def bmi(weight: Weight, height: Height): Double bmi(getWeight(), getHeight()) // OK bmi(getHeight(), getWeight()) // Compilation error, just like we wanted Noruntimecostoftypetags MinoroverheadofTaggedOps Coderemainssaneandself-documenting Scalaintroducing NewType and Opaque and Transparent typestoScalasoon enough,whereTaggingandZero-overheadencapsulatedconstructorswouldbecome partofthestandardlibrary
- 36. UseCase:LoadingConfigs Whyconfigs?BecausewesanitizeeverythingelselikeDB,Userinput,butnever configvalues Dangeroustoblindlytrustyourdevelopers.Theyarehumansandmistakescanbe madewhichcanbehardtofindandcostly Validationofconfigshouldbemorethan value is present ,butthat value is usable case class Endpoint(host: String, port: Int) // based on what we've learnt so far, let's rewrite that as type Host = MatchesRegex["^[A-Z0-9-_]{1,255}$"] type Port = Interval.Closed[1024, 65535] case class Endpoint(host: String Refined Host, port: Int Refined Port)
- 37. Experimental:Addingside-effectstoRefinementtypes case class OpenPort() implicit val openPortValidate: Validate.Plain[Int, OpenPort] = Validate.fromPartial(new ServerSocket(_).close(), "OpenPort", OpenPort()) type AvailablePort = Int Refined OpenPort
- 39. LimitationsofRefinementTypes IntelliJsupportsucks AvoidInfixnotations String Refined XXX And YYY String Refined And[XXX, YYY] String Refined (XXX And YYY) Refinedprimitivesarealwaysboxed,asisusualwithnormalScala Validationerrorsnotclearalways Increasedcompiletimes Scalasupportstillnotverymature(Mightchangeafterv3)
- 40. QA