LangChain Expression Language (LCEL)

LangChain Expression Language or LCEL is a declarative way to easily compose chains together. There are several benefits to writing chains in this manner (as opposed to writing normal code):

Async, Batch, and Streaming Support Any chain constructed this way will automatically have full sync, async, batch, and streaming support. This makes it easy to prototype a chain in a Jupyter notebook using the sync interface, and then expose it as an async streaming interface.

Fallbacks The non-determinism of LLMs makes it important to be able to handle errors gracefully. With LCEL you can easily attach fallbacks to any chain.

Parallelism Since LLM applications involve (sometimes long) API calls, it often becomes important to run things in parallel. With LCEL syntax, any components that can be run in parallel automatically are.

Seamless LangSmith Tracing Integration As your chains get more and more complex, it becomes increasingly important to understand what exactly is happening at every step. With LCEL, all steps are automatically logged to LangSmith for maximal observability and debuggability.

Interface

The base interface shared by all LCEL objects

How to

How to use core features of LCEL

Cookbook

Examples of common LCEL usage patterns