从reducer函数推断类型

Question

我有一个基于2-3个类型的减速器类型。Store、ReturnStore和Context，默认情况下为void。

如何在Action类型中重用CountReducer类型作为类型参数？

现在，Action需要与CountReducer相同的2-3个类型，这会产生重复的代码，并使应用程序接口变得混乱。

当前代码：

type CountingStore = {
  state: "counting";
  ctx: number;
};

type Store = StartedStore | CountingStore | EndedStore;

type CountReducer = (s: Store, toAdd: number) => CountingStore;

const CountReducer: CountReducer = (s, toAdd) => ({
  state: "counting",
  ctx: s.ctx + toAdd
});

type Action<Store, ReturnStore, Context = void> = {
  act: (() => void) | ((ctx: Context) => void);
  stream: Observable<Context>;
  reducer: Reducer<Store, ReturnStore, Context>;
};

type Actions = {
  count: Action<Store, CountingStore, number>;
  end: Action<Store, EndedStore>;
  restart: Action<Store, StartedStore>;
};

未来可能的API设计：

type Action<MyReducer extends Reducer> = "???";

您可以在以下位置找到可运行项目中的相关源代码：https://github.com/marcusnielsen/rx-machine/blob/master/src/index.test.ts

Answer 1

您可以使用条件类型从reducer函数中提取类型参数：

type ActionFromReducer<T extends (s: any, ctx?: any) => any> =  
  T extends (s: infer TStore, ctx: infer TContext) => infer TReturnStore ? Action<TStore, TReturnStore, TContext> : 
  T extends (s: infer TStore) => infer TReturnStore ? Action<TStore, TReturnStore, > : 
  never;
type Actions = {
  count: ActionFromReducer<CountReducer>;
  end: ActionFromReducer<EndReducer>;
  restart: ActionFromReducer<RestartReducer>;
};