Using optics to patch `Gen` instances

GenLenses.scala

import monocle.macros._
import org.scalacheck._
import scalaz.scalacheck.ScalaCheckBinding._

// Deep hierarchy of product types.
case class Document(author: Author)
case class Author(firstName: String, lastName: String, city: City)
case class City(name: String, country: Country)
case class Country(name: String, continent: Continent)
case class Continent(name: String)

object GenLenses {
  // (Stupidly) simple `Gen` instance
  val genDoc: Gen[Document] = for {
    firstName <- Gen.identifier
    lastName  <- Gen.identifier
    city      <- Gen.identifier
    country   <- Gen.identifier
    continent <- Gen.identifier
  } yield Document(Author(firstName, lastName, City(city, Country(country, Continent(continent)))))

  // This is how you modify the way the continent's name is generated.
  val noLens = genDoc.flatMap(doc => doc.copy(
    author = doc.author.copy(
      city = doc.author.city.copy(
        country = doc.author.city.country.copy(
          continent = doc.author.city.country.continent.copy(
            name = doc.author.city.country.continent.name.toUpperCase
          )
        )
      )
    )
  ))

  // Same, but with lenses.
  val continentNameLens = GenLens[Document](_.author.city.country.continent.name)
  val withLens          = genDoc.map(continentNameLens.modify(_.toUpperCase))

  // This is how you replace the `Gen` used for the continent name using lenses.
  val genContinentName = Gen.numStr
  genDoc.flatMap(doc => genContinentName.map(name => continentNameLens.set(name)(doc)))

  // Same thing, slightly terser.
  genDoc.flatMap(continentNameLens.modifyF(_ => genContinentName))
}

If you want to instead "perturb" the continent generator inside, you might write:

genDoc.flatMap( continentNameLens.modifyF( c => Gen.oneOf( "North", "South" ).map( _ + " " + c ) ) )

whereas the modify-restricted approach would be:

genDoc.flatMap( Gen.oneOf( "North", "South" ).map( q => continentNameLens.modify( q + " " + _ ) ) )

... well, I expected that second one to be longer