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Sequential

The next step after calling a language model is make a series of calls to a language model. This is particularly useful when you want to take the output from one call and use it as the input to another.

In this notebook we will walk through some examples for how to do this, using sequential chains. Sequential chains allow you to connect multiple chains and compose them into pipelines that execute some specific scenario. There are two types of sequential chains:

  • SimpleSequentialChain: The simplest form of sequential chains, where each step has a singular input/output, and the output of one step is the input to the next.
  • SequentialChain: A more general form of sequential chains, allowing for multiple inputs/outputs.

SimpleSequentialChain​

Let's start with the simplest possible case which is SimpleSequentialChain.

An SimpleSequentialChain is a chain that allows you to join multiple single-input/single-output chains into one chain.

The example below shows a sample usecase. In the first step, given a title, a synopsis of a play is generated. In the second step, based on the generated synopsis, a review of the play is generated.

import { SimpleSequentialChain, LLMChain } from "langchain/chains";
import { OpenAI } from "langchain/llms/openai";
import { PromptTemplate } from "langchain/prompts";

// This is an LLMChain to write a synopsis given a title of a play.
const llm = new OpenAI({ temperature: 0 });
const template = `You are a playwright. Given the title of play, it is your job to write a synopsis for that title.
 
  Title: {title}
  Playwright: This is a synopsis for the above play:`;
const promptTemplate = new PromptTemplate({
  template,
  inputVariables: ["title"],
});
const synopsisChain = new LLMChain({ llm, prompt: promptTemplate });

// This is an LLMChain to write a review of a play given a synopsis.
const reviewLLM = new OpenAI({ temperature: 0 });
const reviewTemplate = `You are a play critic from the New York Times. Given the synopsis of play, it is your job to write a review for that play.
 
  Play Synopsis:
  {synopsis}
  Review from a New York Times play critic of the above play:`;
const reviewPromptTemplate = new PromptTemplate({
  template: reviewTemplate,
  inputVariables: ["synopsis"],
});
const reviewChain = new LLMChain({
  llm: reviewLLM,
  prompt: reviewPromptTemplate,
});

const overallChain = new SimpleSequentialChain({
  chains: [synopsisChain, reviewChain],
  verbose: true,
});
const review = await overallChain.run("Tragedy at sunset on the beach");
console.log(review);
/*
    variable review contains the generated play review based on the input title and synopsis generated in the first step:

    "Tragedy at Sunset on the Beach is a powerful and moving story of love, loss, and redemption. The play follows the story of two young lovers, Jack and Jill, whose plans for a future together are tragically cut short when Jack is killed in a car accident. The play follows Jill as she struggles to cope with her grief and eventually finds solace in the arms of another man. 
    The play is beautifully written and the performances are outstanding. The actors bring the characters to life with their heartfelt performances, and the audience is taken on an emotional journey as Jill is forced to confront her grief and make a difficult decision between her past and her future. The play culminates in a powerful climax that will leave the audience in tears. 
    Overall, Tragedy at Sunset on the Beach is a powerful and moving story that will stay with you long after the curtain falls. It is a must-see for anyone looking for an emotionally charged and thought-provoking experience."
*/

API Reference:

SequentialChain​

More advanced scenario useful when you have multiple chains that have more than one input or ouput keys.

Unlike for SimpleSequentialChain, outputs from all previous chains will be available to the next chain.

import { SequentialChain, LLMChain } from "langchain/chains";
import { OpenAI } from "langchain/llms/openai";
import { PromptTemplate } from "langchain/prompts";

// This is an LLMChain to write a synopsis given a title of a play and the era it is set in.
const llm = new OpenAI({ temperature: 0 });
const template = `You are a playwright. Given the title of play and the era it is set in, it is your job to write a synopsis for that title.

Title: {title}
Era: {era}
Playwright: This is a synopsis for the above play:`;
const promptTemplate = new PromptTemplate({
  template,
  inputVariables: ["title", "era"],
});
const synopsisChain = new LLMChain({
  llm,
  prompt: promptTemplate,
  outputKey: "synopsis",
});

// This is an LLMChain to write a review of a play given a synopsis.
const reviewLLM = new OpenAI({ temperature: 0 });
const reviewTemplate = `You are a play critic from the New York Times. Given the synopsis of play, it is your job to write a review for that play.
  
   Play Synopsis:
   {synopsis}
   Review from a New York Times play critic of the above play:`;
const reviewPromptTemplate = new PromptTemplate({
  template: reviewTemplate,
  inputVariables: ["synopsis"],
});
const reviewChain = new LLMChain({
  llm: reviewLLM,
  prompt: reviewPromptTemplate,
  outputKey: "review",
});

const overallChain = new SequentialChain({
  chains: [synopsisChain, reviewChain],
  inputVariables: ["era", "title"],
  // Here we return multiple variables
  outputVariables: ["synopsis", "review"],
  verbose: true,
});
const chainExecutionResult = await overallChain.call({
  title: "Tragedy at sunset on the beach",
  era: "Victorian England",
});
console.log(chainExecutionResult);
/*
    variable chainExecutionResult contains final review and intermediate synopsis (as specified by outputVariables). The data is generated based on the input title and era:

    "{
      "review": "

    Tragedy at Sunset on the Beach is a captivating and heartbreaking story of love and loss. Set in Victorian England, the play follows Emily, a young woman struggling to make ends meet in a small coastal town. Emily's dreams of a better life are dashed when she discovers her employer's scandalous affair, and her plans are further thwarted when she meets a handsome stranger on the beach.

    The play is a powerful exploration of the human condition, as Emily must grapple with the truth and make a difficult decision that will change her life forever. The performances are outstanding, with the actors bringing a depth of emotion to their characters that is both heartbreaking and inspiring.

    Overall, Tragedy at Sunset on the Beach is a beautiful and moving play that will leave audiences in tears. It is a must-see for anyone looking for a powerful and thought-provoking story.",
      "synopsis": "

    Tragedy at Sunset on the Beach is a play set in Victorian England. It tells the story of a young woman, Emily, who is struggling to make ends meet in a small coastal town. She works as a maid for a wealthy family, but her dreams of a better life are dashed when she discovers that her employer is involved in a scandalous affair.

    Emily is determined to make a better life for herself, but her plans are thwarted when she meets a handsome stranger on the beach one evening. The two quickly fall in love, but their happiness is short-lived when Emily discovers that the stranger is actually a member of the wealthy family she works for.

    The play follows Emily as she struggles to come to terms with the truth and make sense of her life. As the sun sets on the beach, Emily must decide whether to stay with the man she loves or to leave him and pursue her dreams. In the end, Emily must make a heartbreaking decision that will change her life forever.",
    }"
*/

API Reference: