正确使用 TypeScript 泛型

Question

我的目标是按操作对请求和响应进行分组，所以如果我有（如下面的代码）两个操作：Play 和 Stay，我将有一对代表请求的请求/响应/response of Play action 和 Stay action 相同。

我想表达一个约束只能耦合同一个动作的请求和响应（see Playground）

type ActionName = "Play" | "Stay";

type RequestD<A extends ActionName, B extends Record<string, unknown>> = {
  _tag: "Request";
  _action: A;
} & B;

type ResponseD<A extends ActionName, B extends Record<string, unknown>> = {
  _tag: "Response";
  _action: A;
} & B;

type PlayRequest = RequestD<"Play", { a: number }>;
type PlayResponse = ResponseD<"Play", { b: number }>;

type StayRequest = RequestD<"Stay", { c: number }>;
type StayResponse = ResponseD<"Stay", { d: number }>;

type ActionOf<X> = X extends RequestD<infer A, any>
  ? A
  : X extends ResponseD<infer A, any>
  ? A
  : never;

// Problem:
// X is "Play" | "Stay"`
// Is there a way to get "Play"?
type X = ActionOf<PlayRequest>;

type TagOf<X> = X extends RequestD<any, any>
  ? "Request"
  : X extends ResponseD<any, any>
  ? "Response"
  : never;

// Problem:
// Y1 is "Request" and that's expected
// Y2 is "Request" as well and that's unexpected, how is this possible? How can I fix it?
type Y1 = TagOf<PlayRequest>;
type Y2 = TagOf<PlayResponse>;

type RoundTrip<
  A extends ActionName,
  Req extends RequestD<A, any>,
  Res extends ResponseD<A, any>
> = {
  _action: A;
  request: Req;
  response: Res;
};

type PlayRT = RoundTrip<"Play", PlayRequest, PlayResponse>;

// Problem:
// The following should be wrong because the first request is related to action "Stay", not "Play"
// Is there a way to enforce that?
type WrongRT = RoundTrip<"Play", StayRequest, PlayResponse>;

我在上面的代码中提出的问题是：

1. 如何将“播放”操作关联到请求/响应以在类型级别使用它？
2. 为什么即使这些类型被适当标记（_tag 字段），我也无法区分请求/响应？
3. 如何将操作限制为RoundTrip 类型的请求/响应中的相同操作？

Answer 1

问题其实很简单，你在Request和Response中使用any作为第二个参数。

如果你看一下你的定义：

type RequestD<A extends ActionName, B extends Record<string, unknown>> = {
  _tag: "Request";
  _action: A;
} & B; // note that you are extending the entire object.

然后当你这样定义一个泛型时：

type TagOf<X> = X extends RequestD<any, any>
  ? "Request"
  : X extends ResponseD<any, any>
  ? "Response"
  : never;

它将失败，因为any 包含一个对象，例如用于响应的{ _tag: "Request" } 或用于请求的{ _tag: "Response" }。

它基本上没有让打字稿工作。要解决此问题，您只需将any 更改为类似空对象{}。

Playground