Type erasure in Swift: Example using Sequence and AnySequence, which are provided by the Swift Standard Library

type-erasure.swift

//
// DO NOT USE - WORK IN PROGRESS
//

import Foundation

// "A sequence is a list of values that you can step through one at a time. The most common way to iterate over the elements of a sequence is to use a for-in loop.", per the documentation. 

// Let's say one wanted to iterate over the sequence of strings ["apple", "bird", "cake"], such that
// the following code prints each value:
for value in sequence {
 print(value)
}

// Let's attempt to create the sequence: 
let sequenceString: Sequence = Sequence(["apple", "bird", "cake"])
// The line above produces the error: 
//         'Sequence' cannot be constructed because it has no accessible initializers

// Let's then try to specify the type of the sequence elements (which is string): 
let sequenceString: Sequence<String> = Sequence(["apple", "bird", "cake"])
// The line above produces the error: 
//         Cannot specialize non-generic type 'Sequence'

// Why are we getting this errors? 

// Upon inspection of `Sequence`, we see it is a protocol and has three requirements: 
// 1) method makeIterator() -> Self.Iterator (Returns an iterator over the elements of this sequence)
// 2) associatedtype Iterator (A type that provides the sequence’s iteration interface and encapsulates its iteration state)
// 3) associatedtype Element (A type representing the sequence’s elements)

// Of interest if the last requirement. We can see that `Sequence` can handle different element types (via its associatedtype requirement) but -and this is key to understand- one cannot declare an instance of it as `Sequence<String>`, hence the compiler warning "Cannot specialize non-generic type 'Sequence'". 

// So, what can one do? One option (the one that gives birth to the creation of `AnySequence`) is to create a new type which would allow one to implement protocol `Sequence`, including -and this is another key concept to understand- its associated type `Element`. The mechanics of this look like: 

struct AnySequence: Sequence {
 
}

let sequenceString: Sequence<String> = Sequence(["apple", "bird", "cake"])

let anySequenceString: AnySequence<String> = AnySequence(["apple", "bird", "cake"])
let anySequenceInt: AnySequence<Int> = AnySequence([1, 2, 3])

print(anySequenceString)
// AnySequence<String>(_box: Swift._SequenceBox<Swift.Array<Swift.String>>)

print(anySequenceInt)
// AnySequence<Int>(_box: Swift._SequenceBox<Swift.Array<Swift.Int>>)

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