folds.Swift

private extension GeneratorType {
  mutating func foldr<T>(initial: T, @noescape combine: (Element, T) -> T) -> T {
    return next().map { combine($0, foldr(initial, combine: combine) ) } ?? initial
  }
}
public extension SequenceType {
  func foldr<T>(initial: T, @noescape combine: (Generator.Element, T) -> T) -> T {
    var g = self.generate()
    return g.foldr(initial, combine: combine)
  }
  func foldr(@noescape combine: (Generator.Element, Generator.Element) -> Generator.Element) -> Generator.Element? {
    var g = self.generate()
    return g.next().map { g.foldr($0, combine: combine) }
  }
}

[1, 2, 3, 4].foldr(0, combine: +) // 10
[1, 2, 3, 4].foldr(+) // 10