weak typing for yield results on `callActivity`

[Crisfole]

Describe the bug

The choice to use Generators for Durable Functions in Azure was deliberate as I understand it, but it has made strongly typing the APIs for Orchestrations and Activities very challenging. With custom Promise implementations the typing is pretty straightforward, but the replay-ability is probably harder?

I don't think Typescript supports stronger typing of the types on a per-yield result basis (https://github.com/microsoft/TypeScript/issues/32523). Is there a plan for this that will make this process way less painful? I'm usually pretty fantastic at hacking around stuff like this with typescript, but I can't figure out how in this scenario (specifically for activity return types).

Investigative information

• Durable Functions extension version: ? Installed automagically by tooling. No clue. ?,
• durable-functions npm module version: `"durable-functions": "^3.0.0-alpha.5"
• Language (JavaScript/TypeScript) and version: Typescript ^5.1.3
• Node.js version: node v18.12.1

If deployed to Azure App Service

Local only at this time.

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To Reproduce

Check out the ActivityCaller type below. VS Code complains (with good reason) that the Output parameter is unused:

import type { FunctionInput, FunctionOutput, InvocationContext } from "@azure/functions";
import type { OrchestrationContext, ActivityHandler, RetryOptions } from "durable-functions";

import { app } from "durable-functions";

type ActivityExtraOptions = Partial<{
  extraInputs: FunctionInput[];
  extraOutputs: FunctionOutput[];
}>;

type CallOptions = Partial<{
  retry: RetryOptions;
}>;

type Handler<Input, Output> = (input: Input, ctx: InvocationContext) => Promise<Output>;

// LOOK HERE: I cannot type this in any meaningful way that results in a type I can manipulate into
// const out: Output = yield callThisFunction(ctx, "input");
type ActivityCaller<Input, Output> = (ctx: OrchestrationContext, input: Input, { retry }?: CallOptions) => Task;

export function createActivity<Input, Output>(name: string, handler: Handler<Input, Output>): ActivityCaller<Input, Output>;
export function createActivity<Input, Output>(name: string, options: ActivityExtraOptions, handler: Handler<Input, Output>): ActivityCaller<Input, Output>;
export function createActivity<Input, Output>(
  name: string,
  handlerOrOptions: ActivityHandler | ActivityExtraOptions,
  maybeHandler?: ActivityHandler,
): ActivityCaller<Input, Output>  {
  let options = typeof handlerOrOptions === "function" ? {} : handlerOrOptions;
  let handler = typeof handlerOrOptions === "function" ? handlerOrOptions : maybeHandler;
  if (handler == null) {
    throw new Error("Options were passed, but no handler was passed?");
  }

  app.activity(name, {
    ...options,
    handler,
  });

  return Object.defineProperty(
    function (ctx: OrchestrationContext, input: Input, { retry }: CallOptions = {}) {
      if (retry) {
        return ctx.df.callActivityWithRetry(name, retry, input) as Output;
      } else {
        return ctx.df.callActivity(name, input) as Output;
      }
    },
    "name",
    { value: name }
  );
}

Expected behavior

There is some way other than as or type assertions in the orchestrator to bind the return type of a called function to the Task result type.

Actual behavior

There is not such a technique that I know of.

Screenshots

N/A

Known workarounds

as or manually specifying this stuff.

Additional context

N/A

[Crisfole]

OK, so I have to take back the "I Can't do any better'. I've added:

type TypedTask<Output> = Task & { result: Output };
export type Yielded<T extends ActivityCallerF<any> | ActivityCallerFx<any, any>> = ReturnType<T> extends TypedTask<infer O> ? O : never;

And I've used as to return those typed tasks from the caller functions:

return ctx.df.callActivityWithRetry(name, retry, input) as TypedTask<Output>;

And in the calling orchestrator I can use them like so:

import { createActivityWithInput, type Yielded } from "../createActivity.js";
import { createOrchestrator } from "../createOrchestrator.js";

export const load = createActivityWithInput<number, string>("demo", async (input, ctx) => {
    return input.toString();
});

export const sync = createOrchestrator("sync-orchestrator", function* (ctx) {
  ctx.log("Syncing");
  const x: Yielded<typeof load> = yield load(ctx, 10);
  ctx.log("Yielded " + JSON.stringify(x) + " as " + typeof x);
});

It behaves as expected, it is still way less than ideal.