Lenses and prisms in Swift

Understanding optics

Optics is a sample borrowed from Haskell, that allows you to zoom down into objects. In different phrases, you’ll be able to set or get a property of an object in a useful manner. By useful I imply you’ll be able to set a property with out inflicting mutation, so as an alternative of altering the unique object, a brand new one will probably be created with the up to date property. Belief me it is not that difficult as it would sounds. 😅

We’ll want only a little bit of Swift code to know every little thing.

struct Handle {
    let road: String
    let metropolis: String
}

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

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

As you’ll be able to 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 = Handle(road: "One Infinite Loop", metropolis: "Cupertino")
let appleInc = Firm(title: "Apple Inc.", deal with: oneInfiniteLoop)
let steveJobs = Particular person(title: "Steve Jobs", firm: appleInc)

Now lets say that the road title of the deal with modifications, how will we alter this one area and propagate the property change for the whole construction? 🤔

struct Handle {
    var road: String
    let metropolis: String
}

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

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

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

oneInfiniteLoop.road = "Apple Park Means"
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 should not reference sorts, however worth sorts, therefore copies are getting used throughout.

This seems to be actually unhealthy, we have additionally precipitated numerous mutation and now others may change these variable properties, which we do not crucial need. Is there a greater manner? Effectively…

let newSteveJobs = Particular person(title: steveJobs.title,
                      firm: Firm(title: appleInc.title,
                                       deal with: Handle(road: "Apple Park Means",
                                                        metropolis: oneInfiniteLoop.metropolis)))

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

Lenses

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

Let’s hold it easy and outline a Lens struct that may rework an entire 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 seems to be like in Swift.

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

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

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


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

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

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

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

let companyAddressLens = Lens<Firm, Handle>(get: { $0.deal with },
                                                set: { Firm(title: $1.title, deal with: $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 seems to be 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<Entire, Half> {
    let get: (Entire) -> Half
    let set: (Half, Entire) -> Entire
}

extension Lens {
    func transition<NewPart>(_ to: Lens<Half, NewPart>) -> Lens<Entire, 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 technique and create a lens between the particular person and the road property, this can permit us to instantly modify the road utilizing this newly created lens.

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

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

extension Firm {

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

}

let oneInfiniteLoop = Handle(road: "One Infinite Loop", metropolis: "Cupertino")
let appleInc = Firm(title: "Apple Inc.", deal with: 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 ready 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 the whole object, however that is good since we wished to keep away from mutations. In fact we have now to create numerous lenses to make this magic occur beneath the hood, however generally it’s definitely worth the effort. ☺️

Prisms

Now that we all know the right way to 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 identical to lenses, however they work with sum sorts. Lengthy story brief, enums are sum sorts, structs are product sorts, and the principle distinction is what number of distinctive values are you able to symbolize 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 attainable to set the worth of the property it will return the unique information worth as an alternative.

struct Prism<Entire, Half> {
    let tryGet: (Entire) -> Half?
    let inject: (Half) -> Entire
}

That is how we are able to implement a prism, we name the getter tryGet, because it returns an optionally available worth, the setter is known as 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 have now within the unique State enum. We will use these prisms to test if a given state has the best 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 might be very helpful. You too can apply transformations on prisms, however that is a extra superior matter for one more day.

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