to/fromMap serialization for ChatMessage, PromptValue & in_memory ChatHistory

[cyberpwnn]

Hey! I’ve been using LangChain for a few projects now, and whenever I need serialization—be it Firebase, local storage, or something wacky like Protobuf—I always end up basically rebuilding the changing model structure of LangChain. After searching for toJson or toMap in the issue tracker and finding nothing, I figured I’d do something about it.

It looks like the main goal is parity with the Python version of LangChain. My reasoning is that many other languages have reflection or don’t require such detailed typing, so a built-in serialization might not be as big of a deal there. But in Dart, something like this is straightforward and easy to iterate on, so it makes sense.

This PR is super simple and doesn’t interfere with any plans to expand storage features in the framework. It’s just a step toward providing some built-in serialization. From here, you can drop these maps directly into Firestore (or wherever else). I only added toMap/fromMap for what I consider the most-used models: ChatMessage, PromptValue, and ChatHistory (plus all relevant subclasses and field types).

I also tossed in a unit test (a basic encode-reencode-compare test) to confirm the toMap/fromMap flow works as expected. I tried to handle missing required fields gracefully, but generally it shouldn’t be an issue if new fields are added in the future.

The only extra thing to note is that if new fields or new serializable subclasses pop up, they’ll need their own toMap/fromMap implementations. That’s pretty normal, but just calling it out so everything stays in sync.

[cyberpwnn]

Any eta on when / if this will land?

[davidmigloz]

hey @cyberpwnn!

Thanks for the PR, it makes sense. I'm aiming to issue a new release next week, I'll try to get this in.