Bound a sequence with a start and end marker

Bounded.swift

struct BoundedSequence<Base: Sequence> {
  let _base: Base
}

extension BoundedSequence {
  struct Iterator {
    enum State { case starting, iterating, ended }
    var _state: State
    var _iterator: Base.Iterator
  }
}

extension BoundedSequence.Iterator: IteratorProtocol {
  enum Element {
    case start, end
    case middle(Base.Element)
  }

  mutating func next() -> Element? {
    switch _state {
    case .starting:
      _state = .iterating
      return .start
    case .iterating:
      if let x = _iterator.next() {
        return .middle(x)
      } else {
        _state = .ended
        return .end
      }
    case .ended: return nil
    }
  }
}

extension BoundedSequence: Sequence {
  func makeIterator() -> Iterator {
    return Iterator(_state: .starting, _iterator: _base.makeIterator())
  }
}

extension Sequence {
  func bounded() -> BoundedSequence<Self> {
    return BoundedSequence(_base: self)
  }
}

for bounded in "foo".bounded() {
  switch bounded {
  case .start: print("started")
  case .middle(let x): print(x)
  case .end: print("ended")
  }
}