chain.swift

// [D std.range.chain]: http://dlang.org/phobos/std_range.html#chain
/// Concatenates several sequences of same kind into a single one.
public func chain<S : SequenceType>(sequences: S...) -> SequenceOf<S.Generator.Element> {
  return SequenceOf {_ -> GeneratorOf<S.Generator.Element> in
    var ss = sequences.generate()
    var g = ss.next()?.generate()
    return GeneratorOf {
      if let x = g?.next() { return x }
      
      // Must be `while let` not `if let` to handle empty sequence in between
      while let s = ss.next() {
        g = s.generate()
        if let x = g?.next() { return x }       
      }

      return nil
    }
  }
}

// extension SequenceOf { var array: [T] { return Array(self) } }
// chain([0,1,2,3], [4,5,6,7], [8,9,10,11,12]).array
//=> [0,1,2,3,4,5,6,7,8,9,10,11,12]
// chain([0]).array
//=> [0]
// chain([0],[],[1]).array
//=> [0,1]
// chain([0,1,2,3].generate(), [4,5,6,7].generate(), [8,9,10,11,12].generate()).array
//=> [0,1,2,3,4,5,6,7,8,9,10,11,12]