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 method. By useful I imply you possibly can 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 is not that difficult as it’d sounds. 😅

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

struct Tackle {
    let avenue: String
    let metropolis: String
}

struct Firm {
    let identify: String
    let handle: Tackle
}

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

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

let oneInfiniteLoop = Tackle(avenue: "One Infinite Loop", metropolis: "Cupertino")
let appleInc = Firm(identify: "Apple Inc.", handle: oneInfiniteLoop)
let steveJobs = Particular person(identify: "Steve Jobs", firm: appleInc)

Now lets say that the road identify of the handle adjustments, how can we alter this one discipline and propagate the property change for the whole construction? 🤔

struct Tackle {
    var avenue: String
    let metropolis: String
}

struct Firm {
    let identify: String
    var handle: Tackle
}

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

var oneInfiniteLoop = Tackle(avenue: "One Infinite Loop", metropolis: "Cupertino")
var appleInc = Firm(identify: "Apple Inc.", handle: oneInfiniteLoop)
var steveJobs = Particular person(identify: "Steve Jobs", firm: appleInc)

oneInfiniteLoop.avenue = "Apple Park Approach"
appleInc.handle = oneInfiniteLoop
steveJobs.firm = appleInc

print(steveJobs) 

With the intention 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 sorts, however worth sorts, therefore copies are getting used throughout.

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

let newSteveJobs = Particular person(identify: steveJobs.identify,
                      firm: Firm(identify: appleInc.identify,
                                       handle: Tackle(avenue: "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 complicated sorts. A lens is a price representing maps between a fancy sort and certainly one of 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 “complete object”. That is how the lens definition seems to be 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 handle and assemble a brand new handle utilizing an present one.

let oneInfiniteLoop = Tackle(avenue: "One Infinite Loop", metropolis: "Cupertino")
let appleInc = Firm(identify: "Apple Inc.", handle: oneInfiniteLoop)
let steveJobs = Particular person(identify: "Steve Jobs", firm: appleInc)

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


let newSteveJobs = Particular person(identify: steveJobs.identify,
                          firm: Firm(identify: appleInc.identify,
                                           handle: addressStreetLens.set("Apple Park Approach", oneInfiniteLoop)))

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

let oneInfiniteLoop = Tackle(avenue: "One Infinite Loop", metropolis: "Cupertino")
let appleInc = Firm(identify: "Apple Inc.", handle: oneInfiniteLoop)
let steveJobs = Particular person(identify: "Steve Jobs", firm: appleInc)

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

let companyAddressLens = Lens<Firm, Tackle>(get: { $0.handle },
                                                set: { Firm(identify: $1.identify, handle: $0) })

let personCompanyLens = Lens<Particular person, Firm>(get: { $0.firm },
                                              set: { Particular person(identify: $1.identify, 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 would possibly seems to be a bit unusual at first sight, however we’re simply scratching the floor right here. It’s doable 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 Approach", 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, this may permit us to immediately modify the road utilizing this newly created lens.

Oh, by the way in which, we are able to additionally prolong the unique structs to supply these lenses by default. 👍

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

extension Firm {

    struct Lenses {
        static var handle: Lens<Firm, Tackle> {
            .init(get: { $0.handle },
                  set: { Firm(identify: $1.identify, handle: $0) })
        }
    }
}

extension Particular person {

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

}

let oneInfiniteLoop = Tackle(avenue: "One Infinite Loop", metropolis: "Cupertino")
let appleInc = Firm(identify: "Apple Inc.", handle: oneInfiniteLoop)
let steveJobs = Particular person(identify: "Steve Jobs", firm: appleInc)

let newPerson = Particular person.Lenses.companyAddressStreet.set("Apple Park Approach", 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, 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 numerous lenses to make this magic occur below the hood, however typically it’s definitely worth the effort. ☺️

Prisms

Now that we all know how one can 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 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 doable to set the worth of the property it will return the unique information worth as an alternative.

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 known as inject as a result of we attempt to inject a brand new partial worth and return the entire if doable. 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 are able to use these prisms to test if a given state has the best 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 like a bit unusual on the first sight, however belief me Prisms may be very helpful. You may also apply transformations on prisms, however that is a extra superior subject for one more day.

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