LCEL
LangChain Expression Language (LCEL) is a declarative composition language for connecting LangChain components into executable chains and workflows.
AI Agents and Applications
LCEL is LangChain's composition language for building modular and expressive large-language-model pipelines.
- How it is used
- Connects reusable components into summarization, web-research, and retrieval-chatbot workflows.
- How deep it goes
- Works through pipe composition, input mapping, parsing, sub-chains, and parallel execution; introduces fallbacks and tracing without detailing their implementation.
- Examples
- Uses a question-to-instructions chain, text-chunking function, parallel web-research chains, and a chatbot with memory.
- What you can do after
- Build it from scratch with LCEL by composing and testing modular chains, then combining them into a research summarization workflow.
Build AI-Enhanced Web Apps
LangChain Expression Language (LCEL) is a declarative syntax for composing LangChain functions into executable chains and workflows.
- How it is used
- Orchestrates readable, reusable linear AI workflows from existing LangChain Runnables.
- How deep it goes
- Defines core primitives and execution behavior, then stops short of loops, conditional logic, and stateful agent architecture.
- Examples
- Demonstrates uppercase transformation, vowel counting, prompt formatting, ChatOpenAI invocation, and string output parsing.
- What you can do after
- Build linear chains with JavaScript functions, prompts, language models, parsers, batching, streaming, and Runnable composition.
Related concepts
- pipelineused by · A pipeline is an ordered or coordinated sequence of connected processing stages or tasks that passes data, work, or results toward an outcome.
- LangGraphalternative to · LangGraph is a framework for building stateful, graph-structured workflows and applications, including those involving language models and agents.
- Runnable interfaceuses · The Runnable interface is a LangChain programming abstraction for components that accept input, perform processing, produce output, and support composition in workflows.
- pipe operatoruses · A pipe operator is language-specific syntax that forwards a value or command output to another operation as input, often enabling left-to-right composition.
- LangChainused by · LangChain is an open-source software framework for building applications that compose language models with prompts, data sources, tools, and other components.
- directed acyclic graphuses · A directed acyclic graph is a graph with directed edges and no directed cycles.
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