enum ActionResult<R> {
    case success(R)
    case error(Error)
}

struct Action<Context, Input, Result> {
    let run: (Context, Input) -> ActionResult<Result>
}

func + <Context, Input, ResultA, ResultB>(lhs: Action<Context, Input, ResultA>, rhs: Action<Context, ResultA, ResultB>) -> Action<Context, Input, ResultB> {
    return Action { context, input in 
        switch lhs.run(context, input) {
            case let .success(result): return rhs.run(context, result)
            case let .error(error): return .error(error)
        }
    }
}