Module runnables 

Runnable module - LangChain Expression Language (LCEL) core

The Runnable trait is the foundation of LangChain’s composability. Every component (LLM, Prompt, Tool, etc.) implements Runnable, enabling them to be chained together seamlessly.

LCEL Composition

Use pipe() to chain runnables:

let chain = prompt.pipe(llm).pipe(parser);
let result = chain.invoke("What is Rust?".to_string(), None).await?;

Core Types

• Runnable<I, O>: Base execution trait
• RunnableExt: Extension providing pipe() composition
• RunnableSequence<I, O>: Pipeline of chained runnables
• RunnableLambda<I, O>: Closure wrapper
• RunnablePassthrough<I>: Identity pass-through
• RunnableParallel<I>: Fan-out/fan-in
• RunnableBranch<I, O>: Conditional routing
• RunnableBinding<I, O>: Config/kwargs binding
• LcelError: Unified error type for pipelines

Structs

CancellationToken

A token that can be used to signal cancellation of a long-running operation.

RetryConfig

Configuration for retry behavior.

RunnableAnyWrapper

Wrapper that implements RunnableAny for any Runnable<I, O>.

RunnableAssign

A Runnable that adds new key-value pairs to a HashMap<String, Value>.

RunnableBinding

A Runnable that binds runtime parameters and config to an inner runnable.

RunnableBranch

A Runnable that routes input to different branches based on conditions.

RunnableConfig

Runnable execution configuration.

RunnableConfigurable

Routes between a default runnable and named alternatives at invoke time.

RunnableConfigurableFields

Overrides recognized config fields at invoke time from config.configurable (Python’s Runnable.configurable_fields).

RunnableLambda

A Runnable that wraps a closure.

RunnableParallel

A Runnable that runs multiple steps in parallel on the same input.

RunnablePassthrough

A Runnable that passes its input through unchanged.

RunnableRetry

A Runnable wrapper that retries the inner Runnable on failure.

RunnableSequence

A sequence of Runnable steps composed into a pipeline.

RunnableWithFallbacks

A Runnable that tries fallbacks when the primary fails.

Enums

LcelError

Unified error type for LCEL pipelines.

RetryOn

Determines which errors should trigger a retry.

Constants

RUN_META_PARENT_RUN_ID

Reserved metadata key (UUID string): id of the parent run.

RUN_META_TRACE_ID

Reserved metadata key (UUID string): id of the trace root run.

Traits

Runnable

Base trait for all LangChain components.

RunnableAny

Type-erased Runnable with unified LcelError.

RunnableExt

Extension trait that provides LCEL composition methods for Runnable.

RunnablePick

Extracts keys / values from a Runnable whose output is a HashMap<String, Value>.

Functions

into_runnable_any

Helper function to convert any Runnable into Box<dyn RunnableAny>.

run_tree_from_config

Builds the RunTree a component dispatches its lifecycle callbacks against, applying the config every provider is expected to honor.