Lenso

A Swift µframework and code generator for lenses — "functional getters and setters", convenient when changing parts of immutable objects. Featuring dot notation, zero new operators.

struct Person {
    let name: String
    let address: Address
}

struct Address {
    let street: String
}

let author = Person(name: "Maciej Konieczny", address: Address(street: "Sesame Street"))
let author2 = author.throughLens.address.street.set("Baker Street")

// generated lens code "hidden" in another file

To learn about lenses, scroll down to Introduction. For more information and more story, check out my blog post Lenses in Swift, or how to change parts of immutable objects.

Version 0.2.0-dev (following Semantic Versioning)
Developed and tested using Xcode 7.3
Published under the MIT License

Installation

lenso is an executable Python script with no dependencies. Put it somewhere in your $PATH.

In the future I would like to port lenso to Swift and make it installable via Homebrew.

Usage

Lenso is a command line application. It takes as input models described in JSON and returns library code and generated lenses, so that you can keep them in one file:

cat models.json | lenso > lenses.swift

See example playground to see it in action.

Having to specify all model properties in JSON is far from ideal. In the future I would like to be able to inspect source files and generate lenses without extra configuration.

Introduction to lenses

For more information and more story, check out my blog post Lenses in Swift, or how to change parts of immutable objects.

Lenses are "functional getters and setters" for immutable objects. A lens is implemented for a Whole object and its Part. You can "look through" the lens at an object to get its part — the lens acts as a getter. You can also use the lens to change a part of an object. Then it acts like a setter, except we are talking about immutable objects here, so after "setting", the lens returns a new Whole object with the new part swapped in.

struct Lens<Whole, Part> {
    let get: Whole -> Part
    let set: (Part, Whole) -> Whole
}

Given models from the top of this README,

struct Person {
    let name: String
    let address: Address
}

struct Address {
    let street: String
}

let author = Person(name: "Maciej Konieczny", address: Address(street: "Sesame Street"))

a lens for a person's name is implemented and used like this:

let personNameLens = Lens<Person, String>(
    get: { $0.name },
    set: { (newName, person) in
        Person(name: newName, address: person.address)
    }
)

let author2 = personNameLens.set("narf", author)

Lenso generates code for such lenses for each specified model and puts them in a Lenses struct inside a model extension:

let author2 = Person.Lenses.name.set("narf", author)

Composition

Lenses can also be composed: if you have a lens from A to B and a lens from B to C, you can make a new lens out them, from A to C:

let personStreetLens = Person.Lenses.address.compose(Address.Lenses.street)
let author3 = personStreetLens.set("Wisteria Lane", author)

This is much better than manually recreating model hierarchies when you want to change a property value somewhere down the line, but it still requires a lot of typing and feels "indirect". This is where "chain-aware" bound lenses come to help.

Bound lenses

Bound lenses are lenses that are already "used half way". They already have a Whole instance associated with them. This allows for a much nicer, more familiar syntax: author.throughLens.name.get() instead of Person.Lenses.name.get(author). Lenso generates bound lens structs in such a way that they allow access to properties further down the object chain, without having to manually compose lenses:

let author4 = author.throughLens.address.street.set("Baker Street")

I think this is a big win and it makes working with immutable objects easier.