There is no documentation for the Handler and Handlers types:

export type Handler<T = any, State = Record<string, unknown>> = (
  req: Request,
  ctx: FreshContext<State, T>,
) => Response | Promise<Response>;

export type Handlers<T = any, State = Record<string, unknown>> = {
  [K in router.KnownMethod]?: Handler<T, State>;
};

From Custom handlers | Fresh docs:

Handlers can have two forms: a plain function (catchall for all HTTP methods) or a plain object where each property is a function named by the HTTP method it handles.

With this examples:

export const handler: Handlers = {
  async GET(_req, ctx) {
    const resp = await ctx.render();
    resp.headers.set("X-Custom-Header", "Hello");
    return resp;
  },
};

So I take that Handlers is for the object version and Handler is for the function version. Still, I don’t know what their type variables mean. I guess that they are the type of the response, but there is no error yielded to me with this test:

export const handler: Handlers<number> = {
  GET() {
    return new Response('a string')
  }
}

So how to use them correctly?