Created: 2011-07-15 23:05
Updated: 2018-09-26 08:10



Scala compiler plugin that generates scalaz.Lenses in companion objects for case class fields.


This compiler plugin is in a VERY early stage.





  • Cache lenses in a val and add asInstanceOf casts to the def's body.


git clone https://github.com/gseitz/Lensed.git
cd Lensed
sbt "project usage" console

The last line compiles the projects in this order annotation, plugin (package), testCaseClasses, usage and presents you with a scala REPL ready for playing around:

import lensed.test.Persons._

Now you can play around with some predefined persons.


Because of certain restrictions for compiler plugins (specifically, not being able to run between namer and typer phase), it is not possible to use the created lenses in the same compilation pass (read module, project, ...).

The obvious workaround is to put all case classes for which you want lenses to be generated into a separate project/submodule and have other projects depend on it. The compiler plugin only needs to be active for the project with the case classes.


Because IDEA uses its own parser for scala code, it doesn't know about the generated lenses and will mark the code with errors.


Project A

    case class Person(name: String, address: Address)
    case class Address(city: String)

The following code will be generated:

    object Person {
      def name: Lens[Person, String] = Lens(_.name, (p, n) => p.copy(name = n))
      def address: Lens[Person, Address] = Lens(_.address, (p, a) => p.copy(address = a))

      class PersonW[A](l: Lens[A, Person]) {
        def name: Lens[A, String] = l andThen Person.name
        def address: Lens[A, Address] = l andThen Person.address

      implicit def lens2personW[A](l: Lens[A, Person]): PersonW[A] = new PersonW(l)

    object Address {
      def city: Lens[Address, String] = Lens(_.city, (p, s) => p.copy(city = s))

      class AddressW[A](l: Lens[A, Address]) {
        def city: Lens[A, String] = l andThen Address.city

      implicit def lens2addressW[A](l: Lens[A, Address]): AddressW[A] = new AddressW(l)

Usage Project B

    val yankee = Person("John", Address("NYC"))
    val mounty = Person.address.city.set(yankee, "Montreal")
    Person.address.city.get(mounty) // == "Montreal"
    val cityLens: scalaz.Lens[Person, String] = Person.address.city
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