Related Issues

• fixes Add a ListJoiner Component to Merge Multiple Lists into a Single List #8714

Proposed Changes:

This PR adds a new ListJoiner component that allows merging multiple lists into a single flat list while preserving the order of inputs. This is particularly useful for scenarios where multiple List[ChatMessage] need to be consolidated into one unified list.

Changes
Add ListJoiner component in haystack/components/joiners/list_joiner.py
Add comprehensive test suite in test/components/joiners/test_list_joiner.py
Key Features
Generic implementation that works with any list type (e.g., List[ChatMessage], List[int], etc.)
Preserves input order (earlier inputs appear first in the output)
Simple interface that works with Haystack's Pipeline
Type-safe implementation using Python's generics
Example usage with ChatMessages:
from haystack import Pipeline
from haystack.dataclasses import ChatMessage
from haystack.components.joiners import ListJoiner

messages1 = [ChatMessage.from_user("Hello"), ChatMessage.from_assistant("Hi there")]
messages2 = [ChatMessage.from_user("How are you?")]

p = Pipeline()
p.add_component("joiner", ListJoiner())
result = p.run(data={"lists": [messages1, messages2]})

result.messages will be [messages1[0], messages1[1], messages2[0]]

How did you test it?

The test suite checks:

Empty list handling
Single list input
Multiple list inputs
ChatMessage list handling
Pipeline integration

Notes for the reviewer

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test: and added ! in case the PR includes breaking changes.
• I documented my code
• I ran pre-commit hooks and fixed any issue