Accept a function to determine how to save the previous message in WithPrevious #344

What's needed?

WithPrevious is still a bit limited as how the previous message is saved is hardcoded (it only saves the new message unconditionally).

Proposed solution

Accept an extra argument with a function to determine what to save as the last message instead of always saving the new message.

def __init__(self, ..., get_previous: Callable[[T, T], T], ...):

Where the first argument is the previous message, the second argument is the new message and the return is what's saved as the previous message.

Use cases

• Implement a filter that ensures the received messages are monotonically increasing: get_previous=lambda prev, new: max(prev, new).

Alternatives and workarounds

Write a custom filter predicate from scratch.

Additional context

This would make the API a bit more complicated as we actually need to expose the sentinel, because the previous message could be not received yet, or make the feature less flexible and always saving the first message as the previous message.

With a sentinel it would look like Callable[[T | MISSING, T], T].