Scala Dessign Patterns: Lens

Introduction

Immutability Pain.

val moveHarmeet = harmeet.copy(
      company = harmeet.company.copy(
        address = harmeet.company.address.copy(
          city = harmeet.company.address.city.copy(
            country = harmeet.company.address.city.country.copy(
              name = "Canada", code = "CA"
            )
          )
        )
      )
    )

Within complex objects, it is difficult to change inner properties. With Lens, we will see how to use our classes if we want to modify some of their properties.

Agenda

• Implement Lens using Scalaz.
• Implement Lens using Monocle.

Code: By Scalaz.

package WithLensPatternV1 {

  import scalaz.Lens

  case class Country(code: String, name: String)

  case class City(name: String, country: Country)

  case class Address(number: String, street: String, city: City)

  case class Company(name: String, address: Address)

  case class User(name: String, company: Company, address: Address)

  object WithLensPatternV1 extends App {
    val uk = Country("United Kingdom", "UK")
    val london = City("London", uk)
    val buckinghamPalace = Address("1", "Buckingham Place Road", london)
    val company = Company("Knoldus", buckinghamPalace)

    val canada = Country("Canada", "CA")
    val toronto = City("Toronto", canada)
    val mcMurray = Address("2", "610 McMurray Rd", toronto)
    val harmeet = User("Harmeet Singh", company, mcMurray)

    println("************ Before Move ************")
    println(harmeet)

    println("************ Move to Canada ************")
    val userCompany = Lens.lensu[User, Company](
      (u, company) => u.copy(company = company), _.company
    )

    val userAddress = Lens.lensu[User, Address](
      (u, address) => u.copy(address = address), _.address
    )

    val companyAddress = Lens.lensu[Company, Address](
      (c, address) => c.copy(address = address), _.address
    )

    val addressCity = Lens.lensu[Address, City](
      (a, city) => a.copy(city = city), _.city
    )

    val cityCountry = Lens.lensu[City, Country](
      (c, country) => c.copy(country = country), _.country
    )

    val countryName = Lens.lensu[Country, String](
      (c, name) => c.copy(name = name), _.name
    )

    val countryCode = Lens.lensu[Country, String](
      (c, code) => c.copy(code = code), _.code
    )

    val userCompanyCountryCode = userCompany >=> companyAddress >=> addressCity >=> cityCountry >=> countryCode
    val userCompanyCountryName = userCompany >=> companyAddress >=> addressCity >=> cityCountry >=> countryName

    val moveHarmeetCode = userCompanyCountryCode.set(harmeet, "CA")
    val moveHarmeetName = userCompanyCountryName.set(moveHarmeetCode, "Canada")

    println(moveHarmeetName)
  }

}

 

Code: By Monocle.

package WithLensPatternV2 {

  import monocle.Lens
  import monocle.macros.GenLens

  case class Country(code: String, name: String)

  case class City(name: String, country: Country)

  case class Address(number: String, street: String, city: City)

  case class Company(name: String, address: Address)

  case class User(name: String, company: Company, address: Address)

  object WithLensPatternV2 extends App {

    val uk = Country("United Kingdom", "UK")
    val london = City("London", uk)
    val buckinghamPalace = Address("1", "Buckingham Place Road", london)
    val company = Company("Knoldus", buckinghamPalace)

    val canada = Country("Canada", "CA")
    val toronto = City("Toronto", canada)
    val mcMurray = Address("2", "610 McMurray Rd", toronto)
    val harmeet = User("Harmeet Singh", company, mcMurray)

    println("************ Before Move ************")
    println(harmeet)

    println("************ Move to Canada ************")
    val userCompany: Lens[User, Company] = GenLens[User](_.company)
    val userAddress = GenLens[User](_.address)
    val companyAddress = GenLens[Company](_.address)
    val addressCity = GenLens[Address](_.city)
    val cityCountry = GenLens[City](_.country)
    val countryCode = GenLens[Country](_.code)
    val countryName = GenLens[Country](_.name)

    val userCompanyCountryCode = userCompany composeLens companyAddress composeLens addressCity composeLens cityCountry composeLens countryCode
    val userCompanyCountryName = userCompany composeLens companyAddress composeLens addressCity composeLens cityCountry composeLens countryName

    val moveHarmeetCode = userCompanyCountryCode.set("CA")(harmeet)
    val moveHarmeetName = userCompanyCountryName.set("Canada")(moveHarmeetCode)

    println(moveHarmeetName)
  }

}

Download Code from Github Repo

 

References:

Scala Design Patterns By Ivan Nikolov