Swift Functional Programming - Second Edition

Lens composition

Lens composition will help to simplify our lens; let's examine how:

infix operator >>> : AssociativityRight func >>><A, B, C>(l: Lens<A,B>, r: Lens<B, C>) -> Lens<A, C> {     return Lens(get: {                     r.get(l.get($0))                 },                 set: {                     (c, a) in                     l.set(r.set(c, l.get(a)), a)     }) } 

Let's test this:

let prodProducerAddress = prodProducerLens >>> producerAddressLens let mexBananaProducerAddress = prodProducerAddress.get(mexBananas3) let newProducer = prodProducerAddress.set("Acupulco, Mexico", mexBananas2) print(newProducer) 

The result is going to be as follows:

FunctionalProduct(name: "Banana",                   price: 0.79,                   quantity: 2,                   producer: Producer(name: "QAZ",                                      address: "Acupulco, Mexico")) 

Using lenses and composition, we were able to ...