unfoldr.swift

// MARK: unfoldr :: (T, (T)->(U, T)?) -> [U]
// http://hackage.haskell.org/package/base-4.7.0.2/docs/Data-List.html#v:unfoldr
public func unfoldr<T, U>(var seed: T, f: (T)->(U, T)?) -> SequenceOf<U> {
  return SequenceOf {_ -> GeneratorOf<U> in
    return GeneratorOf {
      if let (a, b) = f(seed) {
        seed = b
        return a
      }
      return nil
    }
  }
}

// unfoldr(10, {($0 == 0) ? nil : ($0, $0 - 1)}).array
// => [10,9,8,7,6,5,4,3,2,1]

// unfoldr(1, {($0, 2*$0)}) // sequence of power of two

// fibonacci sequence
// unfoldr((0, 1), {($0.0, ($0.1, $0.0 + $0.1))})
//=> [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, ...]