feat(langgraph): functional API

[benjamincburns]

feat(langgraph): Add Functional API

This PR introduces a new functional API to langgraphjs that provides a simpler, more intuitive way to create and compose tasks in a graph. The changes focus on improving developer experience while maintaining the power and flexibility of the existing graph-based system.

Key Changes

1. New Functional API Components

• Added new task and entrypoint functions for creating graph nodes
• Introduced experimental TaskOptions and EntrypointOptions interfaces
• Added support for retry policies at the task level

2. Configuration and Constants Updates

• Added new configuration keys for the functional API (CONFIG_KEY_CALL)
• Introduced new control flow constants (NO_WRITES, RETURN)
• Enhanced type safety for the SendInterface validation

3. Pregel Engine Improvements

• Modified task handling to support functional API calls
• Enhanced error handling and null value checking in task writes
• Updated the IGNORE set to include new control flow signals

4. Type System Enhancements

• Added new types for task paths and calls
• Improved type safety throughout the codebase

Implementation Details

The implementation focuses on maintaining backward compatibility while introducing the new functional API. Key architectural decisions include:

1. Using the Pregel engine as the underlying execution system
2. Maintaining the existing channel-based communication system
3. Adding proper TypeScript types for improved developer experience
4. Implementing proper error handling and validation

Example Usage

import { task, entrypoint } from "@langchain/langgraph";

// Define a task with optional retry policy
const processData = task(
  "process",
  async (data) => {
    // Process the data
    return result;
  },
  { retry: { maxAttempts: 3 } }
);

// Create an entrypoint
const workflow = entrypoint({ name: "main" }, async (input) => {
  const result = await processData(input);
  return result;
});

Breaking Changes

None. This is a purely additive change that maintains compatibility with existing code.

Testing

Added new test files and cases to cover the functional API functionality:

• func.test.ts for testing the new API
• Additional test coverage in existing test files

[jacoblee93]

Do we actually want this warning in? Seems harsh

[nfcampos]

Yea this is not the copy I'd want, I think better to say something like

!!! warning "Beta"
This is a new API which might evolve over the next few weeks.
We plan to mark it as stable by X day [next 3 weeks]

[benjamincburns]

I lifted it directly from the python lib. The @experimental and @beta tsdoc tags should ideally convey the same meaning, although per the spec they may be removed by tooling on non-beta releases. I haven't checked the docs build yet to see what actually happens, and whether we can configure this, so I left it in for now as a sort of reminder to myself.

[benjamincburns]

@nfcampos @eyurtsev is the current copy in the python lib what we're happy to go with here?

for reference:

    !!! warning "Beta"
        The Functional API is currently in beta and is subject to change.

[benjamincburns]

aligned order w/ python code so it's easier to check

[benjamincburns]

these are actually unused now, but I left them in, in case they're handy.

[jacoblee93]

Overall looks good to me, will leave for @nfcampos for final review