Traversal

A Traversal is an optic that can see into a structure and set, or modify 0 to N foci. And thus a Traversal is useful when you want to focus into a structure that has 0 to N elements, such as collections etc.

It is a generalization of map. A structure S that has a focus A to which we can apply a function (A) -> B to S and get T. For example, S == List<Int> to which we apply (Int) -> String and we get T == List<String>

A Traversal can simply be created by providing the map function.

import arrow.optics.*

fun main(): Unit {
  //startSample
  val traversal: PTraversal<List<Int>, List<String>, Int, String> =
    PTraversal { s, f -> s.map(f) }
  
  val source = listOf(1, 2, 3, 4)
  val target = traversal.modify(source, Int::toString)
  //endSample
  println(target)
} 

Or by using any of the constructors of Traversal.

Arrow optics also provides a number of predefined Traversal optics.

Usage

data class Street(val number: Int, val name: String)
data class Address(val city: String, val street: Street)
data class Company(val name: String, val address: Address)
data class Employee(val name: String, val company: Company)
data class Employees(val employees: List<Employee>)
val john = Employee("John Doe", Company("Arrow", Address("Functional city", Street(23, "lambda street"))))
val jane = Employee("John Doe", Company("Arrow", Address("Functional city", Street(23, "lambda street"))))
val employees = Employees(listOf(john, jane))

Without lenses, we could use the copy method provided on a data class for dealing with immutable structures.

employees.employees.map { employee ->
  employee.copy(
          company = employee.company.copy(
                  address = employee.company.address.copy(
                          street = employee.company.address.street.copy(
                                  name = employee.company.address.street.name.capitalize()
                          )
                  )
          )
  )
}

Imagine how complex this would look if we would also introduce sealed class into our domain. This is hard to read, does not scale very well, and it draws attention away from the simple operation we wanted to do name.capitalize().

What we actually wanted to do here is the following: focus into every employee, and then focus into the employee’s company, and then focus into the company’s address, and then focus into the street address, and finally, modify the street name by capitalizing it.

val employees: Lens<Employees, List<Employee>> = Lens(
  get = { it.company },
  set = { employee, company -> employee.copy(company = company) }
)

val everyEmployee = Traversal.list<Employee>()

val employeeCompany: Lens<Employee, Company> = Lens(
        get = { it.company },
        set = { employee, company -> employee.copy(company = company) }
)

val companyAddress: Lens<Company, Address> = Lens(
        get = { it.address },
        set = { company, address -> company.copy(address = address) }
)

val addressStrees: Lens<Address, Street> = Lens(
        get = { it.street },
        set = { address, street -> address.copy(street = street) }
)

val streetName: Lens<Street, String> = Lens(
        get = { it.name },
        set = { street, name -> street.copy(name = name) }
)

val employeesStreetName: Lens<Employee, String> = employees compose everyEmployee compose employeeCompany compose companyAddress compose addressStrees compose streetName

employeesStreetName.modify(employee, String::capitalize)

Composition

Composing Traversal can be used for accessing and modifying foci in nested structures.

Traversal can be composed with all optics, and results in the following optics:

  Iso Lens Prism Optional Getter Setter Fold Traversal
Traversal Traversal Traversal Traversal Traversal Fold Setter Fold Traversal

Polymorphic Traversal

When dealing with polymorphic types, we can also have polymorphic Traversals that allow us to morph the type of the foci. Previously, we used a Traversal<List<Int>, Int>; it was able to morph the Int values in the constructed type List<Int>. With a PTraversal<List<Int>, List<String>, Int, String>, we can morph an Int to a String, and thus, also morph the type from List<Int> to List<String>.

Laws

Arrow provides TraversalLaws in the form of test cases for internal verification of lawful instances and third party apps creating their own traversal.

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