switching on pairs of optionals

func optional_pair1<T,U>(t: T?, u: U?)->String {
    switch (t,u) {
    // every case exhaustively covers the possibilities
    case (.None,.None): return "neither"
    case let (.Some(t), .Some(u)): return "both"
    case let (.Some(t),.None): return "left"
    case let (.None,.Some(u)): return "right"
    // so no need for a default clause at all
    // this way, you are totally explicit about
    // which case is being handled how.
    // deleting any of the four cases = compilation error
    }
}

let combos: [(Int?,Int?)] = [
  (  1,   2),
  (nil,   2),
  (  1, nil),
  (nil, nil),
]

for desc in combos.map(optional_pair1) {
    println(desc)
}

func optional_pair2<T,U>(t: T?, u: U?)->String {
    switch (t,u) {
    // you can even leave off the .Some:
    case (nil,nil): return "neither"
    case let (t,nil): return "left"
    case let (nil,u): return "right"
    // but BEWARE!  this last case is
    // secretly a default, because
    // it will also match nil values.
    // make sure it comes last...
    case let (t, u): return "both"
    // (sometimes this behaviour can be useful)
    }
}

for desc in combos.map(optional_pair2) {
    println(desc)
}