different_style.swift

public func sequence<T>(first: T, next: (T) -> T?, while test: (T) -> Bool) -> UnfoldSequence<T, (T?, Bool)> {
  return sequence(state: (first, true), next: { (state: inout (T?, Bool)) -> T? in
    switch state {
      case (let value?, true):
        state.1 = false
        if !test(value) { 
          state.0 = nil; return nil 
        }
        return value
      case (let value?, false):
        let nextValue = next(value)
        if let nextValue = nextValue where !test(nextValue) { 
          state.0 = nil; return nil 
        }
        state.0 = nextValue
        return nextValue
      case (nil, _):
          return nil
      }
    })
}