A Little Respect for AnySequence (http://robnapier.net/erasure)

erasure.swift

import Swift

/*:
    A simple type-erased sequence
*/
let seq = AnySequence([1,2,3])


/*:
    ## Who Needs Types Like That?
*/

/*:
    A not-so-simple, not-type-erased sequence
*/
let xs = [1,2,3]
let ys = ["A","B","C"]
let zs = zip(xs.reverse(), ys.reverse())

func reverseZip<T,U>(xs: [T], _ ys: [U]) -> Zip2Sequence<ReverseRandomAccessCollection<[T]>, ReverseRandomAccessCollection<[U]>> {
    return zip(xs.reverse(), ys.reverse())
}

// Why do we need to change the signature, just to change the implementation?
func reverseZipRefactor<T,U>(xs: [T], _ ys: [U]) -> [(T,U)] {
    return zip(xs, ys).reverse()
}

// This can handle either implementation
func reverseZipAny<T,U>(xs: [T], _ ys: [U]) -> AnySequence<(T,U)> {
    return AnySequence(zip(xs, ys).reverse())
}

/*:
    ## Chains of Association

*/

protocol Animal {
    typealias Food
    func feed(food: Food)
}

// Kinds of Food
struct Grass {}
struct Worm {}

struct Cow: Animal {
    func feed(food: Grass) { print("moo") }
}

struct Goat: Animal {
    func feed(food: Grass) { print("bah") }
}

struct Bird: Animal {
    func feed(food: Worm) { print("chirp") }
}

struct AnyAnimal<Food>: Animal {
    private let _feed: (Food) -> Void
    init<Base: Animal where Food == Base.Food>(_ base: Base) {
        _feed = base.feed
    }
    func feed(food: Food) { _feed(food) }
}

let grassEaters = [AnyAnimal(Cow()), AnyAnimal(Goat())]

let grass = Grass()
for animal in grassEaters {
    animal.feed(grass)
}

Hey Rob! Thanks for the article, it was really helpful to me. I noticed some of the Swift syntax is outdated, so I updated it. You can check it out here: https://gist.github.com/mhmdatallaa/1839f4225553af56d0851d2c963dea8a.

Hope it helps!