Optics DSL
Arrow offers an Optics DSL to compose different Optics while improving ease of use and readability.
To avoid boilerplate, Arrow will generate this property-like DSL using @optics annotation. For Arrow Optics to generate the DSL all the annotated classes should declare a companion object where the optics properties will be projected as extensions.
package com.example.domain
@optics data class Street(val number: Int, val name: String) {
companion object
}
@optics data class Address(val city: String, val street: Street) {
companion object
}
@optics data class Company(val name: String, val address: Address) {
companion object
}
@optics data class Employee(val name: String, val company: Company?) {
companion object
}
The DSL will be generated in the same package as your data class, and can be used on the Companion of your class. In most cases those optics will be lenses, which allow both focus and modification, as shown in the next snippet.
import arrow.optics.dsl.*
import com.example.domain.*
import arrow.optics.Optional
val john = Employee("John Doe", Company("Kategory", Address("Functional city", Street(42, "lambda street"))))
val optional: Optional<Employee, String> = Employee.company.address.street.name
optional.modify(john, String::toUpperCase)
Arrow can also generate DSL for a sealed class, which can help reduce boilerplate code, or improve readability. In that case we speak of optionals, which allow dealing with possibly-missing data, and prisms, which provide construction.
package com.example.domain
@optics sealed class NetworkResult {
companion object
}
@optics data class Success(val content: String): NetworkResult() {
companion object
}
@optics sealed class NetworkError : NetworkResult() {
companion object
}
@optics data class HttpError(val message: String): NetworkError() {
companion object
}
object TimeoutError: NetworkError()
Let’s imagine we have a function f of type (HttpError) -> HttpError, and we want to invoke it on the NetworkResult.
val networkResult: NetworkResult = HttpError("boom!")
val f: (String) -> String = String::toUpperCase
when (networkResult) {
is HttpError -> networkResult.copy(f(networkResult.message))
else -> networkResult
}
We can rewrite this code with our generated DSL.
NetworkResult.networkError.httpError.message.modify(networkResult, f)
There are more kinds of optics, you can read about them in the sidebar to the left. In particular, handling containers of data (lists, collections) becomes easier when using the optics for collections.