要素に必ずしも実体が必要ないことの説明 (アクセスしたとき値ができる)

SequenceType.swift

struct SampleStruct {}

extension SampleStruct : SequenceType {
    typealias Generator = SampleGenerator
    func generate() -> Generator {
        return Generator()
    }
}

struct SampleGenerator : GeneratorType {
    typealias Element = Int
    
    var index = 0
    mutating func next() -> Element? {
        index++
        if (index > 10) { return nil }
        return Int(arc4random())
    }
}

let sampleStruct = SampleStruct()
sampleStruct.map { $0 }