ListJoiner

Join multiple lists into a single flat list.

Basic Information

• Type: haystack.components.joiners.list_joiner.ListJoiner
• Components it can connect with:
  • Any component that outputs lists, such as generators that output List[ChatMessage] or other list-producing components.

Inputs

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

Outputs

Parameter	Type	Default	Description

Overview

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.

This component is useful when you need to combine list outputs from multiple components, such as:

• Joining documents from multiple file converters in an indexing pipeline
• Collecting chat messages from different stages of a conversation
• Merging LLM responses from parallel processing

Usage Example

This example shows an index that converts different file types (text, markdown, PDF) and joins all documents into a single list for processing.

components:
  TextFileToDocument:
    type: haystack.components.converters.txt.TextFileToDocument
    init_parameters:
      encoding: utf-8
      store_full_path: false
  MarkdownToDocument:
    type: haystack.components.converters.markdown.MarkdownToDocument
    init_parameters:
      store_full_path: false
  PDFMinerToDocument:
    type: haystack.components.converters.pdfminer.PDFMinerToDocument
    init_parameters:
      store_full_path: false
  ListJoiner:
    type: haystack.components.joiners.list_joiner.ListJoiner
    init_parameters:
      list_type_: list[haystack.dataclasses.document.Document]
  DocumentSplitter:
    type: haystack.components.preprocessors.document_splitter.DocumentSplitter
    init_parameters:
      split_by: sentence
      split_length: 100
      split_overlap: 0
      split_threshold: 0
      splitting_function:
  DocumentWriter:
    type: haystack.components.writers.document_writer.DocumentWriter
    init_parameters:
      policy: OVERWRITE
      document_store:
        type: haystack.document_stores.in_memory.document_store.InMemoryDocumentStore
        init_parameters:
          bm25_tokenization_regex: (?u)\b\w\w+\b
          bm25_algorithm: BM25L
          bm25_parameters:
          embedding_similarity_function: dot_product
          index: 'default'
          async_executor:
          return_embedding: true

  FileTypeRouter:
    type: haystack.components.routers.file_type_router.FileTypeRouter
    init_parameters:
      mime_types:
      - application/pdf
      - text/markdown
      - text/plain
      additional_mimetypes:
      raise_on_failure: false

connections:
- sender: TextFileToDocument.documents
  receiver: ListJoiner.values
- sender: MarkdownToDocument.documents
  receiver: ListJoiner.values
- sender: PDFMinerToDocument.documents
  receiver: ListJoiner.values
- sender: ListJoiner.values
  receiver: DocumentSplitter.documents
- sender: DocumentSplitter.documents
  receiver: DocumentWriter.documents

- sender: FileTypeRouter.application/pdf
  receiver: PDFMinerToDocument.sources
- sender: FileTypeRouter.text/markdown
  receiver: MarkdownToDocument.sources
- sender: FileTypeRouter.text/plain
  receiver: TextFileToDocument.sources

max_runs_per_component: 100

metadata: {}

inputs:
  files:
  - FileTypeRouter.sources

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.