.. _message_composition:

Message Composition and Decomposition
=====================================

If there are multiple command handlers in different modules for the same Command, all handlers for that
command will be executed (decomposition), and the results of command handlers will be merged into single response
(composition).

.. note::
    Each module can only register **one** handler per Command or Query. Attempting to register a second handler
    for the same Command or Query within the same module will raise :class:`~lato.DuplicateHandlerError`.
    Events are not subject to this restriction.

.. literalinclude:: ../../examples/example5.py

By default, handler responses of type ``dict`` will be recursively merged into a single dict, and responses of type 
``list`` will be merged a into single list.


You can use ``Application.compose`` decorator to declare a custom composer. A custom composer will receive kwargs
with names of the modules handling the response.

.. literalinclude:: ../../examples/example6.py