Create a ReadableStream from a OpenAI GPT3/4 LangChain chain for Edge streaming

route.ts

import { OpenAI } from "langchain/llms/openai";
import { PromptTemplate } from "langchain/prompts";
import { LLMChain } from "langchain/chains";
import { streamCall } from "./stream-call"

export const config = {
  runtime: "edge",
};

export async function GET(request: Request) {
  const model = new OpenAI({
    streaming: true,
  });

  const template = "What is a good name for a company that makes {product}?";
  const prompt = new PromptTemplate({ template, inputVariables: ["product"] });
  const chain = new LLMChain({ llm: model, prompt });

  return new Response(streamCall(chain, { product: "colorful socks" }));
}

stream-call.ts

function streamCall(chain: BaseChain, inputs: ChainValues): ReadableStream {
  const encoder = new TextEncoder();

  return new ReadableStream({
    async start(controller) {
      await chain.call(inputs, [
        {
          handleLLMNewToken(token) {
            controller.enqueue(encoder.encode(token));
          },

          handleChainError(err) {
            controller.error(err);
          },
        },
      ]);

      controller.close();
    },
  });
}