Lenses and prisms in Swift

Understanding optics

Optics is a sample borrowed from Haskell, that lets you zoom down into objects. In different phrases, you possibly can set or get a property of an object in a useful approach. By useful I imply you possibly can set a property with out inflicting mutation, so as a substitute of altering the unique object, a brand new one will likely be created with the up to date property. Belief me it isn’t that difficult as it’d sounds. 😅

We’ll want only a little bit of Swift code to grasp every part.

struct Deal with {
    let road: String
    let metropolis: String
}

struct Firm {
    let title: String
    let deal with: Deal with
}

struct Individual {
    let title: String
    let firm: Firm
}

As you possibly can see it’s attainable to construct up a hierarchy utilizing these structs. An individual can have an organization and the corporate has an deal with, for instance:

let oneInfiniteLoop = Deal with(road: "One Infinite Loop", metropolis: "Cupertino")
let appleInc = Firm(title: "Apple Inc.", deal with: oneInfiniteLoop)
let steveJobs = Individual(title: "Steve Jobs", firm: appleInc)

Now we could say that the road title of the deal with modifications, how can we alter this one subject and propagate the property change for all the construction? 🤔

struct Deal with {
    var road: String
    let metropolis: String
}

struct Firm {
    let title: String
    var deal with: Deal with
}

struct Individual {
    let title: String
    var firm: Firm
}

var oneInfiniteLoop = Deal with(road: "One Infinite Loop", metropolis: "Cupertino")
var appleInc = Firm(title: "Apple Inc.", deal with: oneInfiniteLoop)
var steveJobs = Individual(title: "Steve Jobs", firm: appleInc)

oneInfiniteLoop.road = "Apple Park Approach"
appleInc.deal with = oneInfiniteLoop
steveJobs.firm = appleInc

print(steveJobs) 

To be able to replace the road property we needed to do numerous work, first we needed to change a number of the properties to variables, and we additionally needed to manually replace all of the references, since structs aren’t reference sorts, however worth sorts, therefore copies are getting used throughout.

This appears to be like actually unhealthy, we have additionally triggered numerous mutation and now others can even change these variable properties, which we do not obligatory need. Is there a greater approach? Nicely…

let newSteveJobs = Individual(title: steveJobs.title,
                      firm: Firm(title: appleInc.title,
                                       deal with: Deal with(road: "Apple Park Approach",
                                                        metropolis: oneInfiniteLoop.metropolis)))

Okay, that is ridiculous, can we truly do one thing higher? 🙄

Lenses

We are able to use a lens to zoom on a property and use that lens to assemble advanced sorts. A lens is a worth representing maps between a fancy sort and one among its property.

Let’s maintain it easy and outline a Lens struct that may rework a complete object to a partial worth utilizing a getter, and set the partial worth on all the object utilizing a setter, then return a brand new “complete object”. That is how the lens definition appears to be like like in Swift.

struct Lens {
    let get: (Complete) -> Half
    let set: (Half, Complete) -> Complete
}

Now we are able to create a lens that zooms on the road property of an deal with and assemble a brand new deal with utilizing an present one.

let oneInfiniteLoop = Deal with(road: "One Infinite Loop", metropolis: "Cupertino")
let appleInc = Firm(title: "Apple Inc.", deal with: oneInfiniteLoop)
let steveJobs = Individual(title: "Steve Jobs", firm: appleInc)

let addressStreetLens = Lens<Deal with, String>(get: { $0.road },
                                              set: { Deal with(road: $0, metropolis: $1.metropolis) })


let newSteveJobs = Individual(title: steveJobs.title,
                          firm: Firm(title: appleInc.title,
                                           deal with: addressStreetLens.set("Apple Park Approach", oneInfiniteLoop)))

Let’s attempt to construct lenses for the opposite properties as properly.

let oneInfiniteLoop = Deal with(road: "One Infinite Loop", metropolis: "Cupertino")
let appleInc = Firm(title: "Apple Inc.", deal with: oneInfiniteLoop)
let steveJobs = Individual(title: "Steve Jobs", firm: appleInc)

let addressStreetLens = Lens<Deal with, String>(get: { $0.road },
                                              set: { Deal with(road: $0, metropolis: $1.metropolis) })

let companyAddressLens = Lens<Firm, Deal with>(get: { $0.deal with },
                                                set: { Firm(title: $1.title, deal with: $0) })

let personCompanyLens = Lens<Individual, Firm>(get: { $0.firm },
                                              set: { Individual(title: $1.title, firm: $0) })

let newAddress = addressStreetLens.set("Apple Park Approach", oneInfiniteLoop)
let newCompany = companyAddressLens.set(newAddress, appleInc)
let newPerson = personCompanyLens.set(newCompany, steveJobs)

print(newPerson)

This may appears to be like a bit unusual at first sight, however we’re simply scratching the floor right here. It’s attainable to compose lenses and create a transition from an object to a different property contained in the hierarchy.

struct Lens {
    let get: (Complete) -> Half
    let set: (Half, Complete) -> Complete
}

extension Lens {
    func transition(_ to: Lens<Half, NewPart>) -> Lens<Complete, NewPart> {
        .init(get: { to.get(get($0)) },
              set: { set(to.set($0, get($1)), $1) })
    }

}



let personStreetLens = personCompanyLens.transition(companyAddressLens)
                                        .transition(addressStreetLens)


let newPerson = personStreetLens.set("Apple Park Approach", steveJobs)

print(newPerson)

So in our case we are able to provide you with a transition technique and create a lens between the particular person and the road property, it will enable us to instantly modify the road utilizing this newly created lens.

Oh, by the best way, we are able to additionally lengthen the unique structs to offer these lenses by default. 👍

extension Deal with {
    struct Lenses {
        static var road: Lens<Deal with, String> {
            .init(get: { $0.road },
                  set: { Deal with(road: $0, metropolis: $1.metropolis) })
        }
    }
}

extension Firm {

    struct Lenses {
        static var deal with: Lens<Firm, Deal with> {
            .init(get: { $0.deal with },
                  set: { Firm(title: $1.title, deal with: $0) })
        }
    }
}

extension Individual {

    struct Lenses {
        static var firm: Lens<Individual, Firm> {
            .init(get: { $0.firm },
                  set: { Individual(title: $1.title, firm: $0) })
        }
        
        static var companyAddressStreet: Lens<Individual, String> {
            Individual.Lenses.firm
                .transition(Firm.Lenses.deal with)
                .transition(Deal with.Lenses.road)
        }
    }

}

let oneInfiniteLoop = Deal with(road: "One Infinite Loop", metropolis: "Cupertino")
let appleInc = Firm(title: "Apple Inc.", deal with: oneInfiniteLoop)
let steveJobs = Individual(title: "Steve Jobs", firm: appleInc)

let newPerson = Individual.Lenses.companyAddressStreet.set("Apple Park Approach", steveJobs)

print(newPerson)

On the decision web site we have been in a position to make use of one single line to replace the road property of an immutable construction, after all we’re creating a brand new copy of all the object, however that is good since we needed to keep away from mutations. After all we’ve got to create numerous lenses to make this magic occur underneath the hood, however generally it’s well worth the effort. ☺️

Prisms

Now that we all know how you can set properties of a struct hierarchy utilizing a lens, let me present you another knowledge sort that we are able to use to change enum values. Prisms are identical to lenses, however they work with sum sorts. Lengthy story quick, enums are sum sorts, structs are product sorts, and the principle distinction is what number of distinctive values are you able to characterize with them.

struct ProductExample {
    let a: Bool 
    let b: Int8 
}



enum SumExample {
    case a(Bool) 
    case b(Int8) 
}

One other distinction is {that a} prism getter can return a zero worth and the setter can “fail”, this implies if it isn’t attainable to set the worth of the property it’s going to return the unique knowledge worth as a substitute.

struct Prism {
    let tryGet: (Complete) -> Half?
    let inject: (Half) -> Complete
}

That is how we are able to implement a prism, we name the getter tryGet, because it returns an non-obligatory worth, the setter is named inject as a result of we attempt to inject a brand new partial worth and return the entire if attainable. Let me present you an instance so it will make extra sense.

enum State {
    case loading
    case prepared(String)
}

extension State {

    enum Prisms {
        static var loading: Prism<State, Void> {
            .init(tryGet: {
                guard case .loading = $0 else {
                    return nil
                }
                return ()
            },
            inject: { .loading })
        }
        
        static var prepared: Prism<State, String> {
            .init(tryGet: {
                guard case let .prepared(message) = $0 else {
                    return nil
                }
                return message
            },
            inject: { .prepared($0) })
        }
    }
}

we have created a easy State enum, plus we have prolonged it and added a brand new Prism namespace as an enum with two static properties. ExactlyOne static prism for each case that we’ve got within the unique State enum. We are able to use these prisms to test if a given state has the proper worth or assemble a brand new state utilizing the inject technique.

let loadingState = State.loading
let readyState = State.prepared("I am prepared.")


let newLoadingState = State.Prisms.loading.inject(())

let newReadyState = State.Prisms.prepared.inject("Hurray!")



let nilMessage = State.Prisms.prepared.tryGet(loadingState)
print(nilMessage)


let message = State.Prisms.prepared.tryGet(readyState)
print(message)

The syntax looks as if a bit unusual on the first sight, however belief me Prisms may be very helpful. You too can apply transformations on prisms, however that is a extra superior matter for an additional day.

Anyway, this time I might prefer to cease right here, since optics are fairly an enormous matter and I merely cannot cowl every part in a single article. Hopefully this little article will provide help to to grasp lenses and prisms only a bit higher utilizing the Swift programming language. 🙂