ScaleSequence.swift

struct Scale {
    let notes = ["C", "D", "E", "F", "G", "A", "B"]
}

struct ScaleIterator: IteratorProtocol {
    private let scale: Scale
    private var index = 0
    
    init(_ scale: Scale) {
        self.scale = scale
    }
    
    mutating func next() -> String? {
        if index == scale.notes.count {
            index = 0
        }
        
        defer {
            index += 1
        }
        
        return scale.notes[index]
    }
}

extension Scale: Sequence {
    func makeIterator() -> ScaleIterator {
        return ScaleIterator(self)
    }
}

let scale = Scale()

for note in scale.prefix(20) {
    print(note)
}