Code Quality Rank: L5
Programming language: Swift
License: MIT License
Tags: Utility    
Latest version: v0.4.0

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Focus is an Optics library for Swift (where Optics includes Lens, Prisms, and Isos) that is inspired by Haskell's Lens library.


Focus exports a number of primitives that make it easy to establish relations between types. Practically, a relation can be thought of as a particular way of viewing and modifying a structure. The most famous of these is a Lens or Functional Reference. While there are an abundance of representations of a Lens (see [van Laarhoven 09], [Kmett et al. 12], [Eidhof et al. 09], we have chosen a data-lens-like implementation using the Indexed Store Comonad. If all of that makes no sense, don't worry! We have hidden all of this behind a simple interface.

Programming With Lenses

The easiest way to explain a lens is with a pair of functions

func get(structure : S) -> A
func set(pair : (self : S, newValue : A)) -> S

This should look quite familiar to you! After all, Swift includes syntax for this very pattern

final class Foo {
    var bar : Qux {
        get { //.. }
        set(newValue) { //.. }

So what a lens actually lets you do is decouple the ability to focus on particular bits and pieces of your data types. Moreover, lenses, like properties, compose freely with other compatible lenses but with normal function composition (denoted ) instead of the usual dot-notation. What sets Lenses apart from straight properties is every part of the process is immutable. A lens performs replacement of the entire structure, a property performs replacement of a mutable value within that structure.

All of these properties, flexibility immutability, and composability, come together to enable a powerful set of operations that allow the programmer to view a structure and its parts at any depth and any angle, not simply those provided by properties.

Practical Lenses

For example, say we have this set of structures for working with a flight tracking app:

import Foundation
import Focus

enum Status {
    case Early
    case OnTime
    case Late

struct Plane {
    let model : String
    let freeSeats : UInt
    let takenSeats : UInt
    let status : Status
    var totalSeats : UInt {
        return self.freeSeats + self.takenSeats

struct Gate {
    let number : UInt
    let letter : Character

struct BoardingPass {
    let plane : Plane
    let gate : Gate
    let departureDate : NSDate
    let arrivalDate : NSDate

Starting with a BoardingPass, getting our flight status is trivial

let plane = Plane(model: "SpaceX Raptor", freeSeats: 4, takenSeats: 0, status: .OnTime)
let gate = Gate(number: 1, letter: "A")
let pass = BoardingPass(plane: plane
                    , gate: gate
                    , departureDate: NSDate.distantFuture()
                    , arrivalDate: NSDate.distantFuture())
let status = pass.plane.status

However, in order to update the status on the boarding pass without lenses, we'd have to go through this rigamarole every time:

let oldPass = BoardingPass(/* */)
// Apparently, we're actually flying Allegiant
let newFlight = Plane(model: oldPass.plane.model
                    , freeSeats: oldPass.plane.freeSeats
                    , takenSeats: oldPass.plane.takenSeats
                    , status: .Late)
let newPass = BoardingPass(plane: newFlight
                        , gate: oldPass.gate
                        , departureDate: oldPass.departureDate
                        , arrivalDate: oldPass.arrivalDate)

After defining a few lenses, this is what we can do instead:

// The composite of two lenses is itself a lens
let newPass = (BoardingPass._plane • Plane._status).set(oldPass, .Late)

Here's the definition of those lenses:

extension BoardingPass {
    static var _plane : SimpleLens<BoardingPass, Plane> {
        return SimpleLens(get: {
            return $0.plane
        }, set: { (oldPass, newP) in
            return BoardingPass(plane: newP
                            , gate: oldPass.gate
                            , departureDate: oldPass.departureDate
                            , arrivalDate: oldPass.arrivalDate)

extension Plane {
    static var _status : SimpleLens<Plane, Status> {
        return SimpleLens(get: {
            return $0.status
        }, set: { (oldP, newS) in
            return Plane( model: oldP.model
                        , freeSeats: oldP.freeSeats
                        , takenSeats: oldP.takenSeats
                        , status: newS)

We've only scratched the surface of the power of Lenses, and we haven't even touched the other members of the family of optics exported by Focus. For more on the Lens Family, check out the additional sources below and the implementation files for each family member.

Further Reading

System Requirements

Focus supports OS X 10.9+ and iOS 8.0+.


Focus is released under the MIT license.

*Note that all licence references and agreements mentioned in the Focus README section above are relevant to that project's source code only.