RecurrenceRelation.swift

struct RecurrenceRelation<Element>: SequenceType, GeneratorType {
  
  private var prevs: [Element]
  private let relat: ([Element], Int) -> Element
  private var i: Int = 0
  
  mutating func next() -> Element? {
    guard i == prevs.endIndex else { return prevs[i++] }
    prevs.append(relat(prevs, i))
    prevs.removeAtIndex(0)
    return prevs.last
  }
  
  init(_ base: [Element], relation: ([Element], Int) -> Element) {
    relat = relation
    prevs = base
  }
}

let fibs = RecurrenceRelation([1, 1]) {
  (t, n) in t[n - 1] + t[n - 2]
}

fibs.prefix(10) // [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]