core[patch]: Add generics to RunnablePassthrough.assign

[zirkelc]

Fixes #3827

This PR adds the generics RunInput and RunOutput to RunnablePasstrough.assign(). It infers run input type from the given RunnableMap and constructs the output type as merged type of run input and output.

Here's an example:

  type InferRunnableOutput<R extends Runnable<any, any>> = R extends Runnable<any, infer O>
    ? O
    : never;

  const inputMap = RunnableMap.from({
    context: (input: { context: string; question: string }) => input.context,
    question: (input: { context: string; question: string }) => input.question,
  });

  const chat = new FakeChatModel({});

  const assignChat = RunnablePassthrough.assign({
    chat: (input: InferRunnableOutput<typeof inputMap>) => chat,
  });

  const llm = new FakeStreamingLLM({ sleep: 0 });

  const assignLLM = RunnablePassthrough.assign({
    llm: (input: InferRunnableOutput<typeof assignChat>) => llm,
  });

  type Output = InferRunnableOutput<typeof assignLLM>;
  //   ^^^^^^
  // type Output = {
  //     context: string;
  //     question: string;
  // } & {
  //     chat: FakeChatModel;
  // } & {
  //     llm: FakeStreamingLLM;
  // }

The final output type is being inferred as the merged type from inputMap, assignChat and assignLLM.

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[bracesproul]

One small question, besides that, this looks like a good improvement!

[bracesproul]

Any reason you switched Record<string, unknown> to Record<string, any>? If possible I'd prefer to keep unknown over any, but if that was giving issues it's not too big a deal.

[zirkelc]

No particular reason, I just wanted to align it with the other runnables that use Record<string, any>. I switched it back to Record<string, unknown>, it shouldn't cause any issues.

[bracesproul]

Also looks like lint is failing, if you could run yarn format from root and push up the changes that would be great

[zirkelc]

Hey @bracesproul I applied the formatting and switched back to Record<string, unknown>. Let me know if there's something else to do! :-)

[bracesproul]

lgtm, cc @jacoblee93 for a second look.

Thank you for adding this!

[jacoblee93]

This is great, thank you!