---
title: "ListJoiner"
id: listjoiner
slug: "/listjoiner"
description: "A component that joins multiple lists into a single flat list."
---

# ListJoiner

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

|                                        |                                                                                                                                                                             |
| -------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| **Most common position in a pipeline** | In indexing and query pipelines, after components that return lists of documents such as multiple [Retrievers](../retrievers.mdx) or multiple [Converters](../converters.mdx) |
| **Mandatory run variables**            | “values”: The dictionary of lists to be joined                                                                                                                              |
| **Output variables**                   | “values”: A dictionary with a `values` key containing the joined list                                                                                                       |
| **API reference**                      | [Joiners](/reference/joiners-api)                                                                                                                                                  |
| **GitHub link**                        | https://github.com/deepset-ai/haystack/blob/main/haystack/components/joiners/list_joiner.py                                                                               |

## Overview

The `ListJoiner` component combines multiple lists into one list. It is useful for combining multiple lists from different pipeline components, merging LLM responses, handling multi-step data processing, and gathering data from different sources into one list.

The items stay in order based on when each input list was processed in a pipeline.

You can optionally specify a `list_type_` parameter to set the expected type of the lists being joined (for example, `List[ChatMessage]`). If not set, `ListJoiner` will accept lists containing mixed data types.

## Usage

### On its own

```python
from haystack.components.joiners import ListJoiner

list1 = ["Hello", "world"]
list2 = ["This", "is", "Haystack"]
list3 = ["ListJoiner", "Example"]

joiner = ListJoiner()

result = joiner.run(values=[list1, list2, list3])

print(result["values"])
```

### In a pipeline

```python
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"])
```
