ListJoiner

A component that joins multiple lists into a single flat list.

Basic Information

• Type: haystack_integrations.joiners.list_joiner.ListJoiner

Inputs

Parameter	Type	Default	Description
values	Variadic[List[Any]]		The list to be joined.

Outputs

Parameter	Type	Default	Description

Overview

Work in Progress

Bear with us while we're working on adding pipeline examples and most common components connections.

A component that joins multiple lists into a single flat list.

The ListJoiner receives multiple lists of the same type and concatenates them into a single flat list. The output order respects the pipeline's execution sequence, with earlier inputs being added first.

Usage example:

from haystack.components.builders import ChatPromptBuilder
from haystack.components.generators.chat import OpenAIChatGenerator
from haystack.dataclasses import ChatMessage
from haystack import Pipeline
from haystack.components.joiners import ListJoiner
from typing import List

user_message = [ChatMessage.from_user("Give a brief answer the following question: {{query}}")]

feedback_prompt = """
    You are given a question and an answer.
    Your task is to provide a score and a brief feedback on the answer.
    Question: {{query}}
    Answer: {{response}}
    """
feedback_message = [ChatMessage.from_system(feedback_prompt)]

prompt_builder = ChatPromptBuilder(template=user_message)
feedback_prompt_builder = ChatPromptBuilder(template=feedback_message)
llm = OpenAIChatGenerator(model="gpt-4o-mini")
feedback_llm = OpenAIChatGenerator(model="gpt-4o-mini")

pipe = Pipeline()
pipe.add_component("prompt_builder", prompt_builder)
pipe.add_component("llm", llm)
pipe.add_component("feedback_prompt_builder", feedback_prompt_builder)
pipe.add_component("feedback_llm", feedback_llm)
pipe.add_component("list_joiner", ListJoiner(List[ChatMessage]))

pipe.connect("prompt_builder.prompt", "llm.messages")
pipe.connect("prompt_builder.prompt", "list_joiner")
pipe.connect("llm.replies", "list_joiner")
pipe.connect("llm.replies", "feedback_prompt_builder.response")
pipe.connect("feedback_prompt_builder.prompt", "feedback_llm.messages")
pipe.connect("feedback_llm.replies", "list_joiner")

query = "What is nuclear physics?"
ans = pipe.run(data={"prompt_builder": {"template_variables":{"query": query}},
    "feedback_prompt_builder": {"template_variables":{"query": query}}})

print(ans["list_joiner"]["values"])

Usage Example

components:
  ListJoiner:
    type: components.joiners.list_joiner.ListJoiner
    init_parameters:

Parameters

Init Parameters

These are the parameters you can configure in Pipeline Builder:

Parameter	Type	Default	Description
list_type_	Optional[Type]	None	The expected type of the lists this component will join (e.g., List[ChatMessage]). If specified, all input lists must conform to this type. If None, the component defaults to handling lists of any type including mixed types.

Run Method Parameters

These are the parameters you can configure for the component's run() method. This means you can pass these parameters at query time through the API, in Playground, or when running a job. For details, see Modify Pipeline Parameters at Query Time.

Parameter	Type	Default	Description
values	Variadic[List[Any]]		The list to be joined.