Apply IDE/compile-time 'exhaustive checks' (TS) in places where new/removed cases might go unnoticed

[kostasx]

Checked

• I searched existing ideas and did not find a similar one
• I added a very descriptive title
• I've clearly described the feature request and motivation for it

Feature request

There are multiple places in the codebase where conditional branching does not include compile-time exhaustive checks.

The TypeScript pattern allows for early warnings directly through the IDE when a new conditional case is added or modified and is not addressed accordingly in a long switch statement or if/else/if branching.

For example:

• langchainjs/langchain/src/util/axios-fetch-adapter.js

  Line 284 in d303e90

  case "arraybuffer":

• langchainjs/langchain/src/chains/base.ts

  Line 275 in d303e90

  case "llm_chain": {

• langchainjs/langchain/src/evaluation/loader.ts

  Line 61 in d303e90

  case "criteria":

• More...

Motivation

I believe this to be a really useful pattern for new or modified cases in branching conditionals that might remain unhandled, especially in a codebase like this which will definitely introduce additional cases in multiple places in the future. It's an extra layer of stability and maintainability since it catches such problems before runtime has a chance to produce an error and a tester/user notices it.

For example, in langchain/src/chains/base.ts in the deserialize function, through applying exhaustive checks, we immediately see that 3 cases are not handled (probably something intentional, but nevertheless nice to see):

TypeScript complains: Type '"chat-vector-db" | "analyze_document_chain" | "constitutional_chain"' is not assignable to type 'never'

Proposal (If applicable)

Here's an example:

// Path: langchain/src/retrievers/hyde.ts
export type PromptKey =
  | "websearch"
  | "scifact"
  | "arguana"
  | "trec-covid"
  | "fiqa"
  | "dbpedia-entity"
  | "trec-news"
  | "new_type" // Let's add a new case and forget to address it below...
  | "mr-tydi";

  /**
 * Returns a BasePromptTemplate instance based on a given PromptKey.
 */
export function getPromptTemplateFromKey(key: PromptKey): BasePromptTemplate {
  let template: string;

  switch (key) {
    case "websearch":
      template = `Please write a passage to answer the question
Question: {question}
Passage:`;
      break;
    case "scifact":
      template = `Please write a scientific paper passage to support/refute the claim
Claim: {question}
Passage:`;
      break;
    case "arguana":
      template = `Please write a counter argument for the passage
Passage: {question}
Counter Argument:`;
      break;
    case "trec-covid":
      template = `Please write a scientific paper passage to answer the question
Question: {question}
Passage:`;
      break;
    case "fiqa":
      template = `Please write a financial article passage to answer the question
Question: {question}
Passage:`;
      break;
    case "dbpedia-entity":
      template = `Please write a passage to answer the question.
Question: {question}
Passage:`;
      break;
    case "trec-news":
      template = `Please write a news passage about the topic.
Topic: {question}
Passage:`;
      break;
// FORGET TO ADDRESS A CASE:
//     case "mr-tydi":
//       template = `Please write a passage in Swahili/Korean/Japanese/Bengali to answer the question in detail.
// Question: {question}
// Passage:`;
//       break;
    default:
      // ERROR: Type '"new_type" | "mr-tydi"' is not assignable to type 'never'.
      const _exhaustive_check: never = key; // <= Exhaustive check will give us an early warning immediately in our IDE
      throw new Error(`Invalid prompt key: ${key}`);
  }

  return PromptTemplate.fromTemplate(template);
}