Zip on Lens

The idea is to compose n Lenses with the same starting type to obtain a single Lens targeting a tuple.

case class User(name: String, address: Address)
case class Address(streetNumber: Int, postCode: String)

(GenLens[User](_.name), GenLens[User](_.address.streetNumber)).zip
// res: Lens[User, (String, Int)] = …

This operation can break the Lens laws if some Lenses intersect, e.g. two Lenses that point to the same field.

val names = (GenLens[User](_.name), GenLens[User](_.name)).zip
// names: Lens[User, (String, String)] = …

val eda = User(“Eda”, Address(12, “ABC”))

val updated = names.replace((“Bob”, “Alice”))(eda)
// updated: User = User(“Alice”, Address(12, “ABC”))

names.get(updated)
// res: (String, String) = (“Alice”, “Alice”)

API questions

• How many overloaded versions with different numbers of parameters should we create? e.g. zip 2 Lenses, zip 3 Lenses, zip 4 Lenses, …
  • Is there a way to generalise this in Scala 3? If yes, should we make this feature Scala 3 only?
• Where should we define zip?
  • On the Lens class: lens1.zip(lens2, lens3)
  • On the Lens companion object: Lens.zip(lens1, lens2, lens3)
  • On an extension method to tuples, (lens1, lens2, lens3).zip. This works like cats but it requires a separate import to get the extension.
• Should we add zipWith that takes an Iso?

case class Pair[A](first: A, second: A)

val tupleToPair: Iso[(String, String), Pair[String]] =  GenIso[Pair[String]].fields.reverse

(
  GenLens[User](_.name), 
  GenLens[User](_.address.postcode)
).zipWith(tupleToPair)
// res: Lens[User, Pair[String]] = ...

No files, tests, or entry points are named. Review the existing Lens and tuple-extension APIs, then investigate the Scala-version constraints and Lens-law implications described in the examples. Done means the API shape, supported arities or generalization, and behavior for intersecting lenses are decided.