Transformation ============== .. _transformations-with-pools: Suppose, we want to apply certain transforming functions to the ``immer`` containers inside a large document type. The most straightforward way would be to simply create new containers with the new data, running the transforming function over each element. However, this approach has some disadvantages: - All new containers will be independent, no structural sharing will be preserved, leading to the same data being stored multiple times. - The transformation would be applied more times than necessary when some of the data is shared. Example: one vector is built by appending elements to the other vector. Transforming shared elements multiple times could be unnecessary. Basic example ~~~~~~~~~~~~~ Let's consider a simple case using the document from the :ref:`first-example`. The desired transformation would be to multiply each element of the ``immer::vector`` by 10. First, the document value would be created in the same way: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: intro/start-prepare-value :end-before: intro/end-prepare-value The next component we need is the :ref:`pools` of all the containers from the value: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-get_output_pools :end-before: end-get_output_pools The ``get_output_pools`` function returns the output pools of all ``immer`` containers that would be serialized using pools, as controlled by the policy. Here we use the default policy ``hana_struct_auto_policy`` which will use pools for all ``immer`` containers inside the document type which must be a ``hana::Struct``. The other required component is the ``conversion_map``: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-conversion_map :end-before: end-conversion_map This is a ``hana::map`` that describes the desired transformations to be applied. The key of the map is an ``immer`` container and the value is the function to be applied to each element of the corresponding container type. In this case, it will apply ``[](int val) { return val * 10; }`` to each ``int`` of the ``vector_one`` type, we have two of those in the ``document``. Having these two parts, we can create new pools with the transformations: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-transformed_pools :end-before: end-transformed_pools At this point, we can start converting the ``immer`` containers and create the transformed document value with them, ``new_value``: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-convert-containers :end-before: end-convert-containers In order to confirm that the structural sharing has been preserved after applying the transformations, let's serialize the ``new_value`` and inspect the JSON: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-save-new_value :end-before: end-save-new_value And indeed, we can see in the JSON that the node ``[2, [10, 20]]`` is reused in both vectors. Converting types ~~~~~~~~~~~~~~~~ The transforming function can even return a different type. In the following example, ``vector`` is transformed into ``vector``. The first two steps are the same as in the previous example: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: intro/start-prepare-value :end-before: intro/end-prepare-value .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-get_output_pools :end-before: end-get_output_pools Only this time the transforming function will convert an integer into a string: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-conversion_map-string :end-before: end-conversion_map-string Then we convert the two vectors the same way as before: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-convert-vectors-of-strings :end-before: end-convert-vectors-of-strings And in order to confirm that the structural sharing has been preserved, we can introduce a new document type with the two vectors being ``vector``. .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-document_str :end-before: end-document_str And serialize it with pools: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-save-new_value-str :end-before: end-save-new_value-str In the resulting JSON we can confirm that the node ``[2, ["_1_", "_2_"]]`` is reused for both vectors. .. _transforming-hash-based-containers: Hash-based containers ~~~~~~~~~ As it was shown, converting ``vectors`` is conceptually simple: the transforming function is applied to each element of each node, producing a new node with the transformed elements. When it comes to the hash-based containers, that is `set `_, `map `_ and `table `_, their structure is defined by the used hash function, so defining the transformation may become a bit more verbose. In the following example, we'll start with a simple case of transforming a map. For a map, only the hash of the key matters and we will not modify the key yet. We will focus on transformations here and not on the structural sharing within the document, so we will use the ``immer`` container itself as the document. Let's define the following policy to indicate that we want to use pools only for our container: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-direct_container_policy :end-before: end-direct_container_policy By default, ``immer`` uses ``std::hash`` for the hash-based containers. While this hash is sufficient for runtime use, it can't be used for persistence, as noted in the `C++ reference `_: .. note:: Hash functions are only required to produce the same result for the same input within a single execution of a program We will use `xxHash `_ as the hash for this example. Let's create a small map like this: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-prepare-int-map :end-before: end-prepare-int-map Our goal is to convert the value from ``int`` to ``std::string``. Let's create the ``conversion_map`` like this: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-prepare-conversion_map :end-before: end-prepare-conversion_map A few important details to note: - For maps, the transforming function accepts a pair of key and value, ``std::pair``. - The transforming function must also be able to handle an argument of type ``immer::persist::target_container_type_request``. This is achieved by using ``hana::overload`` to combine 2 lambdas into one callable value. When called with that argument, it should return an empty container of the type we're transforming to. This explicit approach is necessary because there is no reliable way to automatically determine the hash algorithm for the new container. Even though in this case the type of the key doesn't change (and so the hash remains the same), in other scenarios it might. Once the ``conversion_map`` is defined, the actual conversion is done as before: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-transform-map :end-before: end-transform-map And we can see that the original map's values have been transformed into strings. Transforming table's ID ------------------------ For this example, we'll transform the type of the ID of the table element while keeping the hash of it the same. This can occur, for instance, if the member that serves as the ID gets wrapped in a wrapper type. To begin, let's define an item type for a table: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-old_item :end-before: end-old_item We can create a table value with some data and get the pools for it like this: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-prepare-table-value :end-before: end-prepare-table-value In this example, we want to change the type of the ``old_item's`` ID, which is ``std::string``, while keeping its hash the same. Let's define a wrapper for ``std::string`` and a ``new_item`` type like this: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-new-table-types :end-before: end-new-table-types We're also changing the type for ``data`` from ``int`` to ``std::string`` but this change doesn't affect the structure of the table. We define the ``xx_hash_value`` function for the ``new_id_t`` type to make it compatible with the ``immer::persist::xx_hash`` hash. Then, we can define the target ``new_table_t`` type and the ``conversion_map`` that describes how to convert ``old_item`` into a ``new_item``. .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-prepare-new_table_t-type :end-before: end-prepare-new_table_t-type .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-prepare-new_table_t-conversion_map :end-before: end-prepare-new_table_t-conversion_map Finally, to convert the ``value`` using the defined ``conversion_map`` we prepare the converted pools with ``transform_output_pool`` and use ``convert_container`` to convert the ``value`` table. .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-new_table_t-transformation :end-before: end-new_table_t-transformation We can see that the ``new_value`` table contains the transformed data from the original ``value`` table. .. _modifying-the-hash-of-the-id: Modifying the hash of the ID ---------------------------- If the key of a map, the ID of a table item or an element of a set changes its hash due to a transformation, the transformed hash-based container can no longer keep its shape and it can't be efficiently transformed by simply applying transformations to its nodes. ``immer::persist`` validates every container it creates from a pool. If such a hash modification occurs, a runtime exception will be thrown because it is not possible to detect this issue during compile-time. Let's modify the previous example to also change the data of the ID: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-prepare-new_table_t-broken-conversion_map :end-before: end-prepare-new_table_t-broken-conversion_map Now, if we attempt to convert the original table, a ``immer::persist::champ::hash_validation_failed_exception`` will be thrown: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-new_table_t-broken-transformation :end-before: end-new_table_t-broken-transformation Even though such transformation can't be performed efficiently, on a node level, we can still request these transformations to be applied. This will run for each value of the original container, creating a new independent container that doesn't use structural sharing: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-prepare-new_table_t-new-hash-conversion_map :end-before: end-prepare-new_table_t-new-hash-conversion_map We can request for such container-level (as opposed to per-node level) transformation to be performed by wrapping the desired new container type ``new_table_t`` in a ``immer::persist::incompatible_hash_wrapper`` as the result of the ``immer::persist::target_container_type_request`` call. .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-new_table_t-new-hash-transformation :end-before: end-new_table_t-new-hash-transformation We can see that the transformation has been applied, the keys have the ``_key`` suffix. .. note:: While different transformed containers will not have structural sharing, transforming the same container multiple times will reuse previously transformed data. In other words, transformation will be cached on the container level but not on the nodes level. .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-returned-transformed-container-is-the-same :end-before: end-returned-transformed-container-is-the-same .. _transforming-nested-containers: Transforming nested containers ------------------------------ Let's consider a scenario where a transforming function works on an item within an ``immer`` container and also needs to transform another ``immer`` container. We define the types as follows: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-define-nested-types :end-before: end-define-nested-types The important property here is that we have a ``vector`` where ``nested_t`` contains ``vector``, so we can say a ``vector`` is nested inside another ``vector``. We can prepare a value with some structural sharing and then serialize it: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-prepare-nested-value :end-before: end-prepare-nested-value The resulting JSON looks like: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-nested-value-json :end-before: end-nested-value-json Looking at the JSON we can confirm that the node ``[2, [1, 2]]`` is reused. Let's define a ``conversion_map`` like this: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-nested-conversion_map :end-before: end-nested-conversion_map The transforming function for ``vector_one`` is simple as it transforms an ``int`` into a ``std::string``. However, the function for the ``vector`` is more involved. When we attempt to transform one item of that vector, ``nested_t``, we realize that inside that function we have a ``vector`` to deal with. This brings us back to the problems described in the beginning of the :ref:`transformations-with-pools` section. To solve this issue, ``immer::persist`` provides an optional second argument to the transforming function, a function called ``convert_container``. This function can be called with two arguments: the desired container type and the ``immer`` container to convert. This allows us to access the ``conversion_map`` we're defining. This transformation will be performed using pools and will preserve structural sharing as expected. Having defined the ``conversion_map``, we apply it in the usual way and get the ``new_value``: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-apply-nested-transformations :end-before: end-apply-nested-transformations We can verify that the ``new_value`` has the expected content: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-verify-nested-value :end-before: end-verify-nested-value And we can serialize it again to confirm that the structural sharing of the nested vectors has been preserved: .. literalinclude:: ../test/extra/persist/test_for_docs.cpp :language: c++ :start-after: start-verify-structural-sharing-of-nested :end-before: end-verify-structural-sharing-of-nested We can see that the ``[2, ["_1_", "_2_"]]`` node is still being reused in the two vectors.