ListJoiner (Legacy)

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

Legacy Component

With smart connections, components can accept connections from multiple lists of the same type, making ListJoiner no longer necessary. It's no longer available in Builder's component library but you can still use and configure it through YAML. To learn how to remove it from your pipelines, see Simplify Your Pipelines with Smart Connections.

Key Features

• Concatenates multiple input lists of the same type into a single flat list.
• Preserves order based on the pipeline's execution sequence.
• Accepts any list type, including List[Document] and List[ChatMessage].
• Optionally enforces type safety by specifying the expected list type.
• Useful for combining outputs from parallel converters or processors.

Configuration

1. Drag the ListJoiner component onto the canvas from the Component Library.
2. Click the component to open the configuration panel.
3. Configure the parameters as needed.

Connections

ListJoiner accepts multiple values inputs — any component that outputs a list of the same type can connect to it. It outputs values, a single concatenated list. Connect its output to DocumentSplitter, DocumentWriter, or any other component that accepts a list input.

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

Inputs

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

Outputs

Parameter	Type	Default	Description

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 (for example, 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.