RecursiveSelectGen.Swift

public class SelectGenerator<T> : GeneratorType {
  
  private let (head, tail): (T, [T])
  private var first: Bool = true
  private var gen: SelectGenerator<T>?
  
  public func next() -> (T, [T])? {
    return first ? {
      first = false
      if !tail.isEmpty { gen = SelectGenerator(vals: tail) }
      return (head, tail)
    }() : gen?.next().map { ($0, [head] + $1) }
  }
  
  public init(var vals: [T]) {
    self.head = vals.removeAtIndex(0)
    self.tail = vals
  }
}

var g = SelectGenerator(vals: [1, 2, 3, 4])

g.next() // (1, [2, 3, 4])
g.next() // (2, [1, 3, 4])
g.next() // (3, [1, 2, 4])
g.next() // (4, [1, 2, 3])
g.next() // nil