Prelude Cheat Sheet

Quick-reference table of common types, constructors, operators, and functions in Langchain.Prelude.

Core Types & Functions

Function / Type	Type Signature	Purpose
`invoke`	`ChatModel m => m -> [Message] -> IO Message`	Invoke model with messages
`streamModel`	`ChatModel m => m -> [Message] -> (StreamEvent -> IO ()) -> IO ()`	Stream response events
`userMessage`	`Text -> Message`	Construct user message
`systemMessage`	`Text -> Message`	Construct system message
`assistantMessage`	`Text -> Message`	Construct assistant message
`extractMessageText`	`Message -> Text`	Extract plain text from message

Runnable AST Operators

Operator	Type	Meaning
`(\\|>>)`	`RunnableTree m i a -> RunnableTree m a o -> RunnableTree m i o`	Sequential pipeline composition
`(&>&)`	`RunnableTree m i a -> RunnableTree m i b -> RunnableTree m i (a, b)`	Parallel branch execution
`interpret`	`RunnableTree m i o -> i -> m o`	Evaluate runnable AST
`runLambda`	`(i -> o) -> RunnableTree m i o`	Pure function node
`runPrim`	`(i -> m o) -> RunnableTree m i o`	Monadic effect node

StateGraph & LangGraph

Function	Type Signature	Purpose
`emptyStateGraph`	`StateReducer s -> StateGraph s m`	Create empty state graph
`addNode`	`NodeId -> Node s m -> StateGraph s m -> StateGraph s m`	Add workflow node
`addEdge`	`NodeId -> NodeId -> StateGraph s m -> StateGraph s m`	Add directed edge
`addConditionalEdge`	`NodeId -> (s -> NodeId) -> [(Text, NodeId)] -> StateGraph s m -> StateGraph s m`	Add branching edge
`compileGraph`	`StateGraph s m -> Maybe (Checkpointer s m) -> CompiledGraph s m`	Compile state graph
`runGraph`	`CompiledGraph s m -> s -> m s`	Execute compiled workflow
`toDot`	`StateGraph s m -> String`	Export Graphviz DOT representation

Model Context Protocol (MCP)

Function	Type Signature	Purpose
`newStdioMcpClient`	`FilePath -> [String] -> IO McpClient`	Connect to stdio MCP server
`newHttpMcpClient`	`Text -> IO McpClient`	Connect to HTTP SSE MCP server
`listMcpTools`	`McpClient -> IO [McpToolInfo]`	Discover available MCP tools
`mcpToolToLangchainTool`	`McpToolInfo -> Tool m`	Convert to native Langchain Tool

Observability & Resilience

Function	Type Signature	Purpose
`newOTelTracer`	`Maybe Text -> IO OTelTracer`	Create OpenTelemetry tracer
`withSpan`	`OTelTracer -> Text -> Maybe SpanId -> SpanKind -> Map Text Text -> m a -> m a`	Trace block in OpenTelemetry span
`exportSpansJson`	`OTelTracer -> IO Text`	Export traces to JSON
`newCircuitBreaker`	`CircuitBreakerConfig -> IO CircuitBreaker`	Create 3-state Circuit Breaker
`withCircuitBreaker`	`CircuitBreaker -> m a -> m a`	Protect action with breaker