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 a substitute of altering the unique object, a brand new one will likely be created with the up to date property. Belief me it is not that sophisticated as it would sounds. 😅

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

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

struct Firm {
    let identify: String
    let deal with: Tackle
}

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

Now lets say that the road identify of the deal with modifications, how can we alter this one area and propagate the property change for your complete construction? 🤔

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

struct Firm {
    let identify: String
    var deal with: Tackle
}

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

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

oneInfiniteLoop.road = "Apple Park Method"
appleInc.deal with = 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 a few of the properties to variables, and we additionally needed to manually replace all of the references, since structs should not reference varieties, however worth varieties, therefore copies are getting used throughout.

This appears to be like actually unhealthy, we have additionally brought on various mutation and now others may also change these variable properties, which we do not obligatory need. Is there a greater manner? Properly…

let newSteveJobs = Particular person(identify: steveJobs.identify,
                      firm: Firm(identify: appleInc.identify,
                                       deal with: Tackle(road: "Apple Park Method",
                                                        metropolis: oneInfiniteLoop.metropolis)))

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

Lenses

We are able to 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 fancy kind and one in all its property.

Let’s preserve it easy and outline a Lens struct that may remodel a complete object to a partial worth utilizing a getter, and set the partial worth on your complete 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<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 deal with and assemble a brand new deal with utilizing an current one.

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

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


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

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

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

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

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

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

let newAddress = addressStreetLens.set("Apple Park Method", 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<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 Method", steveJobs)

print(newPerson)

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

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

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

extension Firm {

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

}

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

let newPerson = Particular person.Lenses.companyAddressStreet.set("Apple Park Method", steveJobs)

print(newPerson)

On the decision website 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 your complete object, however that is good since we wished to keep away from mutations. After all we now have to create various lenses to make this magic occur underneath the hood, however generally 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 another knowledge 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 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’s not attainable to set the worth of the property it’s going to return the unique knowledge worth as a substitute.

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 non-obligatory 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 now have within the unique State enum. We are able to use these prisms to examine if a given state has the appropriate 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 can too apply transformations on prisms, however that is a extra superior subject for one more day.

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