###### 2.1.4.3.5 Generalization

Specialized content can be generalized to any ancestor type. The generalization process can preserve information about the former level of specialization to allow round-tripping between specialized and unspecialized forms of the same content.

Among the purposes of generalization:

- Migration of content (for example, when retiring an unsuccessful specialization),
- Temporary round-tripping (for example, when moving content through a process that is not specialization aware and has only been enabled for instances of the base structural type),
- Reuse of specialized content in an environment that does not support one or more of its specializations (which may be thought of as a special case of round-tripping).

When generalizing for migration, the @class attribute and @domains attribute should be absent from the generalized instance document so that the default values in the DITA document type shell will be used. When generalizing for round-tripping, the @class attribute and @domains attribute should retain the original specialized values in the generalized instance document.

All DITA documents contain a mix of markup from at least one structural type and zero or more domains. When generalizing the document, the generalizer may choose to leave a structural type or domain as-is, or may choose to generalize that type or domain to any of its ancestors.

The generalizer can supply the source and target modules for each generalization, for example, "generalize from reference to topic". The generalizer can specify multiple target modules, for example, "generalize from reference to topic and from ui-d to topic". When the source and target modules are not supplied, generalization is assumed to be from all structural types to the base (topic or map), and no generalization is performed for domains.

The generalizer can also supply the target DITA document type shell. When the target document type is not supplied, the generalized document will not contain a reference to a DITA document-type shell. With the exception of topic nesting constraints, it is possible to generate a document type shell based on the @class and @domains attributes in the specialized documents. If the@ domains attribute includes all structural, domain, and constraint modules used, the @domains attribute alone is sufficient to enable generation of a document type shell.

A generalization process should be able to handle cases where it is given:

- Just source modules for generalization (in which case the designated source types are generalized to topic or map),
- Just target modules for generalization (in which case all descendants of the target are generalized to that target), or
- Both (in which case only the specified descendants of the target are generalized to that target).

For each structural type instance, the generalization process checks whether the structural type instance is a candidate for generalization, or whether it has domains that are candidates for generalization. It is important to be selective about which structural type instances to process; if the process simply generalizes every element based on its @class attribute values, an instruction to generalize "reference" to "topic" could leave an APIReference topic with an invalid content model, since any elements it reuses from "reference" would have been renamed to topic-level equivalents.

The @class attribute for the root element of the structural type is checked before generalizing structural types:

| Source module unspecified | Source module specified
Target module unspecified | Generalize this structural type to its base ancestor | Check whether the root element of the topic type matches a specified source module; generalize to its base ancestor if it does, otherwise ignore the structural type instance unless it has domains to generalize.
Target module specified | Check whether the @class attribute contains the target module. If it does contain the target, rename the element to the value associated with the target module. Otherwise, ignore the element. | It is an error if the root element matches a specified source but its @class attribute does not contain the target. If the root element matches a specified source module and its @class attribute does contain the target module, generalize to the target module. Otherwise, ignore the structural type instance unless it has domains to generalize.

The @domains attribute for the root element of the structural type is checked before generalizing domains:

| Source module unspecified | Source module specified
Target module unspecified | Do not generalize domain specializations in this structural type. | Check whether the @domains attribute lists the specified domain; proceed with generalization if it does, otherwise ignore the structural type instance unless it is itself a candidate for generalization.
Target module specified | Check whether the @domains attribute contains the target module. If it does, generalize to the target module. Otherwise, skip the structural type instance unless it is itself a candidate for generalization. | It is an error if the @domains attribute matches a specified source but the domain value string does not contain the target. If the @domains attribute matches a specified source module and the domain value string does contain the target module, generalize to the target module. Otherwise, ignore the structural type instance unless it is itself a candidate for generalization.

For each element in a candidate structural type instance:

| Source module unspecified | Source module specified
Target module unspecified | If the @class attribute starts with "-" (part of a structural type), rename the element to its base ancestor equivalent. Otherwise ignore it. | Check whether the last value of the @class attribute matches a specified source; generalize to its base ancestor if it does, otherwise ignore the element.
Target module specified | Check whether the @class attribute contains the target module; rename the element to the value associated with the target module if it does contain the target, otherwise ignore the element. | It is an error if the last value in the @class attribute matches a specified source but the previous values do not include the target. If the last value in the @class attribute matches a specified source module and the previous values do include the target module, rename the element to the value associated with the target module. Otherwise, ignore the element.

preserve the values of all attributes except the @class and @domains attribute, both of which should be supplied by the target document type.

Previous Topic: 2.1.4.3.4 Domain usage declaration (the @domains attribute)

Next Topic: 2.1.4.3.6 Attribute generalization

Parent Topic: 2.1.4.3 Specialization

Sibling Topics:

2.1.4.3.1 Vocabulary modules

2.1.4.3.2 Requirements for specialized element types and attributes

2.1.4.3.3 Element type specialization hierarchy declaration (the @class attribute)

2.1.4.3.4 Domain usage declaration (the @domains attribute)

2.1.4.3.6 Attribute generalization

2.1.4.3.7 Specializing foreign or unknown content

2.1.4.3.8 Specialization module coding requirements