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 practical means. By practical 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 can be created with the up to date property. Belief me it is not that difficult as it would sounds. ?

We will want only a little bit of Swift code to know the whole lot.

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

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

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

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

Now we could say that the road title of the handle modifications, how will we alter this one area and propagate the property change for your complete construction? ?

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

struct Firm {
    let title: String
    var handle: 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.", handle: oneInfiniteLoop)
var steveJobs = Particular person(title: "Steve Jobs", firm: appleInc)

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

print(steveJobs) 

As a way 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 aren’t reference varieties, however worth varieties, therefore copies are getting used throughout.

This appears actually unhealthy, we have additionally induced numerous mutation and now others may change these variable properties, which we do not needed need. Is there a greater means? Properly…

let newSteveJobs = Particular person(title: steveJobs.title,
                      firm: Firm(title: appleInc.title,
                                       handle: Deal with(avenue: "Apple Park Method",
                                                        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 varieties. A lens is a worth representing maps between a posh sort and one in every of 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 “entire object”. That is how the lens definition appears like in Swift.

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

Now we will create a lens that zooms on the road property of an handle and assemble a brand new handle utilizing an present one.

let oneInfiniteLoop = Deal with(avenue: "One Infinite Loop", metropolis: "Cupertino")
let appleInc = Firm(title: "Apple Inc.", handle: 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,
                                           handle: addressStreetLens.set("Apple Park Method", 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.", handle: 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.handle },
                                                set: { Firm(title: $1.title, handle: $0) })

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

print(newPerson)

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

Oh, by the way in which, we will additionally prolong the unique structs to offer 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 handle: Lens<Firm, Deal with> {
            .init(get: { $0.handle },
                  set: { Firm(title: $1.title, handle: $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.handle)
                .transition(Deal with.Lenses.avenue)
        }
    }

}

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

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

print(newPerson)

On the decision website we had 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 needed to keep away from mutations. In fact we’ve 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 learn how to set properties of a struct hierarchy utilizing a lens, let me present you yet one more information sort that we will use to change enum values. Prisms are identical 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 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 doable to set the worth of the property it’s going to return the unique information worth as an alternative.

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

That is how we will implement a prism, we name the getter tryGet, because it returns an elective 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 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 will use these prisms to test 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 like a bit unusual on the first sight, however belief me Prisms may be very helpful. You can too apply transformations on prisms, however that is a extra superior matter for one more day.

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