gistfile1.txt

protocol Action { }
protocol State { 
  static var initial: Self { get }
}

typealias Reducer<S: State, A: Action> = (_ state: S, _ action: A) -> S

enum AnAction: Action { }
enum AState: State { 
  case none
  case error(Error)
  static let initial: AState = .none
}

let reducer: Reducer<AState, AnAction> = { (state, action) in switch (state, action) {} }