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 may set or get a property of an object in a useful method. By useful I imply you may set a property with out inflicting mutation, so as an alternative of altering the unique object, a brand new one shall be created with the up to date property. Belief me it isn’t that difficult as it’d sounds. 😅

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

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

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

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

As you may 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(road: "One Infinite Loop", metropolis: "Cupertino")
let appleInc = Firm(identify: "Apple Inc.", tackle: oneInfiniteLoop)
let steveJobs = Individual(identify: "Steve Jobs", firm: appleInc)

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

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

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

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

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

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

print(steveJobs) 

With a purpose to replace the road property we needed to do numerous work, first we needed to change a few of the properties to variables, and we additionally needed to manually replace all of the references, since structs will not be reference varieties, however worth varieties, therefore copies are getting used throughout.

This appears to be like actually unhealthy, we have additionally precipitated numerous mutation and now others also can change these variable properties, which we do not mandatory need. Is there a greater method? Effectively…

let newSteveJobs = Individual(identify: steveJobs.identify,
                      firm: Firm(identify: appleInc.identify,
                                       tackle: Deal with(road: "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 price representing maps between a fancy kind and certainly one of its property.

Let’s preserve it easy and outline a Lens struct that may remodel an entire 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 “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 will 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(road: "One Infinite Loop", metropolis: "Cupertino")
let appleInc = Firm(identify: "Apple Inc.", tackle: oneInfiniteLoop)
let steveJobs = Individual(identify: "Steve Jobs", firm: appleInc)

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


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

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

let oneInfiniteLoop = Deal with(road: "One Infinite Loop", metropolis: "Cupertino")
let appleInc = Firm(identify: "Apple Inc.", tackle: oneInfiniteLoop)
let steveJobs = Individual(identify: "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.tackle },
                                                set: { Firm(identify: $1.identify, tackle: $0) })

let personCompanyLens = Lens<Individual, Firm>(get: { $0.firm },
                                              set: { Individual(identify: $1.identify, 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 may 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 will give you a transition technique and create a lens between the individual and the road property, this can permit us to straight modify the road utilizing this newly created lens.

Oh, by the way in which, we will 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 tackle: Lens<Firm, Deal with> {
            .init(get: { $0.tackle },
                  set: { Firm(identify: $1.identify, tackle: $0) })
        }
    }
}

extension Individual {

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

}

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

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

print(newPerson)

On the decision website we had 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 all the 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 underneath the hood, however typically it’s well worth the effort. ☺️

Prisms

Now that we all know easy methods to set properties of a struct hierarchy utilizing a lens, let me present you yet another information kind that we will 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 primary 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 potential to set the worth of the property it’s going to return the unique information worth as an alternative.

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

That is how we will implement a prism, we name the getter tryGet, because it returns an optionally available 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 have now within the authentic State enum. We will use these prisms to verify if a given state has the fitting 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 like a bit unusual on the first sight, however belief me Prisms might be very helpful. You can too apply transformations on prisms, however that is a extra superior subject for an additional day.

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