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 purposeful approach. By purposeful 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 shall be created with the up to date property. Belief me it isn’t that sophisticated as it would sounds. 😅

We will want only a little bit of Swift code to grasp all the things.

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

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

struct Particular person {
    let title: String
    let firm: Firm
}

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

let oneInfiniteLoop = Deal with(avenue: "One Infinite Loop", metropolis: "Cupertino")
let appleInc = Firm(title: "Apple Inc.", tackle: oneInfiniteLoop)
let steveJobs = Particular person(title: "Steve Jobs", firm: appleInc)

Now we could say that the road title of the tackle modifications, how can we alter this one discipline and propagate the property change for the whole construction? 🤔

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

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

struct Particular person {
    let title: String
    var firm: Firm
}

var oneInfiniteLoop = Deal with(avenue: "One Infinite Loop", metropolis: "Cupertino")
var appleInc = Firm(title: "Apple Inc.", tackle: oneInfiniteLoop)
var steveJobs = Particular person(title: "Steve Jobs", firm: appleInc)

oneInfiniteLoop.avenue = "Apple Park Means"
appleInc.tackle = oneInfiniteLoop
steveJobs.firm = appleInc

print(steveJobs) 

To be able to replace the road property we needed to do various work, first we needed to change among the properties to variables, and we additionally needed to manually replace all of the references, since structs are usually not reference varieties, however worth varieties, therefore copies are getting used throughout.

This appears to be like actually unhealthy, we have additionally brought about various mutation and now others can even change these variable properties, which we do not vital need. Is there a greater approach? Effectively…

let newSteveJobs = Particular person(title: steveJobs.title,
                      firm: Firm(title: appleInc.title,
                                       tackle: Deal with(avenue: "Apple Park Means",
                                                        metropolis: oneInfiniteLoop.metropolis)))

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

Lenses

We will use a lens to zoom on a property and use that lens to assemble complicated varieties. A lens is a worth representing maps between a posh sort and one among its property.

Let’s preserve 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 the whole object utilizing a setter, then return a brand new “entire object”. That is how the lens definition appears to be like like in Swift.

struct Lens<Complete, Half> {
    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 tackle and assemble a brand new tackle utilizing an current one.

let oneInfiniteLoop = Deal with(avenue: "One Infinite Loop", metropolis: "Cupertino")
let appleInc = Firm(title: "Apple Inc.", tackle: oneInfiniteLoop)
let steveJobs = Particular person(title: "Steve Jobs", firm: appleInc)

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


let newSteveJobs = Particular person(title: steveJobs.title,
                          firm: Firm(title: appleInc.title,
                                           tackle: addressStreetLens.set("Apple Park Means", oneInfiniteLoop)))

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

let oneInfiniteLoop = Deal with(avenue: "One Infinite Loop", metropolis: "Cupertino")
let appleInc = Firm(title: "Apple Inc.", tackle: oneInfiniteLoop)
let steveJobs = Particular person(title: "Steve Jobs", firm: appleInc)

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

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

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

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

print(newPerson)

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

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

extension Lens {
    func transition<NewPart>(_ 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 Means", steveJobs)

print(newPerson)

So in our case we are able to provide you with a transition methodology and create a lens between the individual 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 supply these lenses by default. 👍

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

extension Firm {

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

extension Particular person {

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

}

let oneInfiniteLoop = Deal with(avenue: "One Infinite Loop", metropolis: "Cupertino")
let appleInc = Firm(title: "Apple Inc.", tackle: oneInfiniteLoop)
let steveJobs = Particular person(title: "Steve Jobs", firm: appleInc)

let newPerson = Particular person.Lenses.companyAddressStreet.set("Apple Park Means", 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, in fact we’re creating a brand new copy of the whole object, however that is good since we needed to keep away from mutations. In fact we’ve to create various lenses to make this magic occur underneath the hood, however typically it’s well worth the effort. ☺️

Prisms

Now that we all know set properties of a struct hierarchy utilizing a lens, let me present you another information sort that we are able to use to change enum values. Prisms are similar to lenses, however they work with sum varieties. Lengthy story quick, enums are sum varieties, structs are product varieties, and the principle distinction is what number of distinctive values are you able to signify 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 0 worth and the setter can “fail”, this implies if it isn’t potential to set the worth of the property it’s going to return the unique information worth as a substitute.

struct Prism<Complete, Half> {
    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 elective worth, the setter is named inject as a result of we attempt to inject a brand new partial worth and return the entire if potential. 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 within the authentic State enum. We will use these prisms to verify if a given state has the correct worth or assemble a brand new state utilizing the inject methodology.

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 could be very helpful. You may also apply transformations on prisms, however that is a extra superior matter for one more day.

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