langchain-core-workflow-a

# LangChain Core Workflow A: Chains & Prompts ## Overview Build production chains using LCEL (LangChain Expression Language). Covers prompt templates, output parsers, RunnableSequence, RunnableParallel, RunnableBranch, RunnablePassthrough, and chain composition patterns. ## Prerequisites - `langchain-install-auth` completed - `@langchain/core` and at least one provider installed ## Prompt Templates ### ChatPromptTemplate ```typescript import { ChatPromptTemplate, MessagesPlaceholder } from "@langchain/core/prompts"; // Simple template const simple = ChatPromptTemplate.fromTemplate( "Translate '{text}' to {language}" ); // Multi-message template with chat history slot const chat = ChatPromptTemplate.fromMessages([ ["system", "You are a {role}. Respond in {style} style."], new MessagesPlaceholder("history"), // dynamic message injection ["human", "{input}"], ]); // Inspect required variables console.log(chat.inputVariables); // ["role", "style", "history", "input"] ``` ### Partial Templates ```typescript // Pre-fill some variables, leave others for later const partial = await chat.partial({ role: "senior engineer", style: "concise", }); // Now only needs: history, input const result = await partial.invoke({ history: [], input: "Explain LCEL", }); ``` ## Output Parsers ```typescript import { StringOutputParser } from "@langchain/core/output_parsers"; import { JsonOutputParser } from "@langchain/core/output_parsers"; import { StructuredOutputParser...