• references
  • https://github.com/optics-dev/Monocle
  • https://www.optics.dev/Monocle/
  • https://www.baeldung.com/scala/monocle-optics
  • https://github.com/optics-dev/Monocle/tree/master/example/src/test/scala-2/monocle/function
  • Monocle 3: A Peek into the Future by Julien Truffaut: Scala in the City Conference
  • Lenses, Prisms and Optics in Scala | Rock the JVM
  • https://blog.rockthejvm.com/lens/
  • Scale By The Bay 2020: Julien Truffaut, Monocle 3: a peek into the future
  • https://medium.com/javascript-scene/lenses-b85976cb0534
  • Mastering Optics in Scala with Monocle. Shimi Bandiel. ScalaUA2017
  • Optics with Monocle - Modeling the Part and the Whole
  • Data Juggling - Part 2 - Homegrown #Lenses (#Monocle - Part 1)
  • Data Juggling - Part 3 - Optics Explained (#Monocle - Part 2)
  • Data Juggling - Part 4 - #Quicklens (#Monocle - Part 3)
  • https://github.com/softwaremill/quicklens
  • https://www.warski.org/blog/2015/02/quicklens-modify-deeply-nested-case-class-fields/
  • https://github.com/DevInsideYou/monocle-vs-quicklens
  • https://kubuszok.com/2018/adt-through-the-looking-glass/
  • https://www.scala-exercises.org/monocle
  • https://medium.com/zyseme-technology/functional-references-lens-and-other-optics-in-scala-e5f7e2fdafe
  • https://circe.github.io/circe/optics.html
  • https://scalac.io/blog/scala-optics-lenses-with-monocle/
  • https://dzone.com/articles/focus-on-your-data-structures-with-scala-lenses
  • https://github.com/falvarezb/blog-bytecode/blob/postLenses/src/test/scala/fjab/LensesTest.scala
  • https://circe.github.io/circe/optics.html
  • https://github.com/softwaremill/quicklens

• goals of this workshop
  • introduction to theoretical basis of optics
  • practice monocle and quicklens libraries
• workshop plan
  • provide implementation of lib.Lens and then use it in VanillaOpticsSpec
    • hint: use lib.Getter and lib.Setter
  • implement tests in OpticsSpec according to its names

• problem: updating nested immutable objects
  • example

    sealed trait PaymentMethod
    object {
        case class PayPal(emai: String) extends PaymentMethod
        case class DebitCard(
            cardNumber: String,
            expirationDate: YearMonth,
            securityCode: Int
        ) extends PaymentMethod
    }
    
    case class User(name: String, address: Address, paymentMethod: PaymentMethod)
    case class Address(streetNumber: Int, postCode: String)
    
    val michal = User("michal", Address(12, "E16 4SR"), PaymentMethod.PayPal("[email protected]"))
    

  • updates of mutable objects (assume that above we have fields defined as vars)

    michal.address.streetNumber = 16
    

  • updated of immutable objects
    • using copy

      michal.copy(address = michal.address.copy(streetNumber = 16)
      

    • but how we will update enum?

      def updateExpiry(user: User, newExpiry: YearMonth): User =
          user.copy(paymentMethod =
              user.paymentMethod match {
                  case card: DebitCard => card.copy(expirationDate = newExpiry)
                  case paypal: PayPal => paypal
              }
      

    • or suppose we have paymentMethods: Map[String, PaymentMethod] instead of single paymentMethod: PaymentMethod

      val michal = User(
          "michal",
          Address(12, "E16 4SR"),
          Map(
              "Personal" -> PayPal("[email protected]"),
              "Business" -> DebitCard("", YearMonth.of(2022, 7), 995)
          )
      )
      
      def updateExpiry(user: User, paymentName: String, newExpiry: YearMonth): User =
          user.copy(paymentMethod =
              user.paymentMethods.get(paymentName) match {
                  case None | Some(_: PayPal) => user.paymentMethods
                  case Some(card: DebitCard) =>
                      val updatedCard = card.copy(expirationDate = newExpiry)
                      user.paymentMethods.updated(paymentName, updatedCard)
              }
      

• it would be nice if we could have something like OOP’s setters for immutable data structures
  • .copy is like that, but it doesn’t compose
  • lenses can be considered the functional representations of getters and setters

    case class Lens[A, B](get: A => B, set: (B, A) => A)
    

    and usage

    val articleTitleLens = Lens[Article, String](get = _.title, set = (t, a) => a.copy(title = t))
    

  • note that this boilerplate could be generated by macros

• working with deeply nested JSON
  • example

    import io.circe._, io.circe.parser._
    
    val json: Json = parse("""
    {
      "order": {
        "customer": {
          "name": "Custy McCustomer",
          "contactDetails": {
            "address": "1 Fake Street, London, England",
            "phone": "0123-456-789"
          }
        },
        "items": [{
          "id": 123,
          "description": "banana",
          "quantity": 1
        }, {
          "id": 456,
          "description": "apple",
          "quantity": 2
        }],
        "total": 123.45
      }
    }
    """).getOrElse(Json.Null)
    

  • migrate one JSON format into the other without having to decode into the ADT

    import io.circe.optics.JsonPath._
    
    val _phoneNum = root.order.customer.contactDetails.phone.string
    // _phoneNum: monocle.package.Optional[Json, String] = monocle.POptional$$anon$1@367b3ec4
    
    val phoneNum: Option[String] = _phoneNum.getOption(json)
    

  • receive events/requests in the older format, check version, run all migrations to the current one
    • keep case classes and case objects only for the current schema
  • modifying json

    val doubleQuantities: Json => Json = root.order.items.each.quantity.int.modify(_ * 2)
    

• working with property based testing
  • introduction to property based testing: https://github.com/mtumilowicz/scala-zio2-test-aspects-property-based-testing-workshop
  • it is often easier to generate general complex object and then in test just change some fields according to needs
  • example

    val genAccount: Gen[Any, Account] = DeriveGen[Account]
    
    test("create locked account test") {
      check(genAccount) { account =>
        val lockedAccount = account.modify(_.details.alerting.locked).setTo(false)
        assertZIO(AccountService.createAccount(lockedAccount))(equalTo(AccountCreated(account.id)))
      }
    }
    

A lens is a first-class reference to a subpart of some data type

School of Haskell

any data accessor for a component of a data structure is ‘function-like’, in the sense that reading ‘consumes’ the component from the data structure and writing ‘produces’ an updated component to put back into the data structure. The type structure of such function-like things — henceforth transformers — is technically known as a profunctor.

Profunctor Optics Modular Data Accessors

• problem: how to implement Lens for a sum type?
  • with current implementation: impossible
    • there is no way to define getter
  • solution: partial lens (prism/optional in monocle)

    case class PartialLens[A, B](get: A => Option[B], set: (B, A) => Option[A])
    

• polymorphic lenses
  • lens construct focuses on a single field
  • what if we have multiple fields?
    • example: setting all the values in certain fields of a json
    • with current implementation: impossible
    • solution: val Laarhoven representation
      • we want to unify all functions

        get: S => A
        set: A => S => S
        modify: (A => A) => (S => S)
        modifyMaybe: (A => Option[A]) => (S => Option[S])
        modifyList: (A => List[A]) => (S => List[S])
        

      • generic representation the can support all features in the same type

        [F: Functor[_], S, T, A, B] // F ~ for example: Option, Collection
        modifyF: (f: A => F[B]): S => F[T]
        

  • when Optic is polymorphic additional two types come into play for "reverse" operation
    • example

      trait PLens[S, T, A, B] {
      	def get(s: S): A
      	def set(b: B)(s: S): T
      }
      

    • B for an argument and T for a result of that operation
  • Traversal is more generic than Optional
    • it composes with everything that Optional composes
    • it makes sense for all optics to have 4 type parameters (for S => T and A => B mappings)
      • this way we can always compose them
• laws
  • identity: if you get and then set back with the same value, the object remains identical

    lens.set(s, lens.get(s)) == s
    

  • retention: if you set with a value and then perform a get, you get the same value back

    lens.get(lens.set(s, a)) == a
    

  • double set — if you set twice in succession and then perform a get, you get back the last set value

    lens.get(lens.set(lens.set(s, a), b)) == b
    

  • example of breaking a law

    michal.focus(_.paymentMethods
        .*
        .as[DebitCard]
        .filter(_.expirationDate.isAfter(YearMonth.of(2020, 5))) // breaks the contract
        .expirationDate
        ).get
    
    val optic = Focus[User](_.paymentMethods
        .*
        .as[DebitCard]
        .filter(_.expirationDate.isAfter(YearMonth.of(2020, 5))) // breaks the contract
        .expirationDate
    
    val updated1 = optic.modify(michal, _.minusYears(2))
    val updated2 = optic.modify(update1, _.plusMonths(3)) // not works
    val updated3 = optic.modify(michal, _.minusYears(2).plusMonths(3)) // not the same result as updated1 + updated2
    

• based on Monocle
  • ISO[S, A]
    • converts elements of type S into elements of type A without loss
    • isomorphism between two types (bijection)
    • example
      • case class <-> tupleN

        case class Person(name: String, age: Int)
        

        • is equivalent to a tuple (String, Int)
        • tuple (String, Int) is equivalent to Person
      • collections: List <-> Vector
    • convenient to lift methods from one type to another
      • example: a String can be seen as a List[Char]
    • you can think of Iso as something that is simultaneously Lens and Prism
      • Lens: navigating from S to A is always successful
      • Prism: navigating from A to S does not need any additional information besides of A value
      • in other words: transformation from S to A is lossless
    • when is useful?
      • anytime when representing essentially the same data in different ways
      • example: physical units

        case class Meter(whole: Int, fraction: Int)
        case class Centimeter(whole: Int)
        
        val centimeterToMeterIso = Iso[Centimeter, Meter] { cm =>
          Meter(cm.whole / 100, cm.whole % 100)
        }{ m =>
          Centimeter(m.whole * 100 + m.fraction)
        }
        
        centimeterToMeterIso.modify(m => m.copy(m.whole + 3))(Centimeter(155))
        

  • Prism[S, A]
    • used for
      • sum type (coproduct)
        • example: sealed trait or Enum
      • when there is no 1-1 correspondence
        • example
          • String - Int
            • not all strings are transformable to Int, all Int transformable to String
              • String <- Int
              • String -> Option[Int]
            • digression
              • notice that "0003".toInt == 3 but it is not reversible
                • you have to put more constraint on String to have that example represents Prism
    • laws
      • if there is an answer, going back must give the source
      • if we go back, there must be an answer, which is the source
    • vs lens
      • sometimes fails to get target (Option) but you can always come back
  • Optional[S, A]
    • you may think of Optional as something more general than both Prism and Lens
      • Prism: the element A we are trying to focus may not exist
      • Lens: we don’t have enough information to go back to S without additional argument
      • summary: worst part of lens and prism - can fail to retrieve and can fail to get back
    • example
      • setup

        sealed trait Error
        
        case class ErrorA(message: String, details: String) extends Error
        case object ErrorB extends Error
        

      • task: Optional[Error, String] to "zoom into" details
        • cannot be Lens[Error, String] as ErrorB does not contain details
      • solution

        val getErrorADetails: Lens[ErrorA, String] = GenLens[ErrorA](_.details)
        val getErrorA: Prism[Error, ErrorA] = GenPrism[Error, ErrorA]
        val getDetails: Optional[Error, String] = getErrorA andThen getErrorADetails
        

    • it’s quite rare to see Optional implemented directly
      • usually: implement Prism and Lens and then compose them together
  • Traversal[S, A]
    • like optional that can focus on 0 to n fields
    • generalisation of an Optional to several targets
    • example: focus into all elements inside of a container (e.g. List, Vector, Option)
  • At[S, I, A]
    • an "index" I, provides a Lens[S, A]
    • built-in instances for map and set
    • Lens
  • Index[S, I, A]
    • like At
    • works on an optional index
    • built-in: list, map, string etc
    • Prism
    • can't delete the value under the key
• hierarchy (based on monocle)
  • diagram meant to be read as UML class hierarchy diagram
    • arrow going from Lens to Optional means that Lens is a special case of Optional
    • what does it mean that both Lens and Prism can be treated as Optional?
      • Lens is an Optional for which getOption always succeeds
      • Prism is an Optional for which we ignores S ("the whole part")
        • A ("the specific part") holds all information to produce new S
  • diagrams

• composing

  val setStreetNumber = GenLens[Address](_.streetNumber)
  val setAddress = GenLens[User](_.address)
  val composed = setAddress andThen setStreetNumber
  
  composed.replace(newStreetNo)(user)
  

• focus
  • https://www.optics.dev/Monocle/docs/focus
    • Focus in scala 2 is much more limited. You can only using to generate lenses for fields inside a case class.
      • optics-dev/Monocle#1329
  • example

    person
      .focus(_.address.street.name).modify(_.toUpperCase)
    

person
  .modify(_.address.street.name).using(_.toUpperCase)
  .modify(_.age).setTo(22)
  .modify(_.weight).setToIfDefined(Some(100))
  .modify(_.payments.each.result).using(_.toUpperCase)
  .modify(_.props.at("Height").value).setTo(178)
  .modify(_.pets.each.when[Dog].age).using(_ + 1)