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How validation and well-formedness modes are implemented

Validation mainly means to check whether the XML tree fulfills a condition, but this is not all. Validation also performs some normalizations, e.g.:

  • Whitespace characters may be removed from the tree where the DTD allows it
  • Default values of attributes are added when the XML text omits them
If there were no modifications of the XML tree, validation could be completely implemented as a property of the DTD object, which is the logical instance for checking such a constraint. However, this is not true, so many checks and normalizations have been implemented within the document tree, and are triggered there. Nevertheless, these checks and normalizations are finally controlled by the DTD, which remains the final controlling instance. This means that validation can be turned off by setting a flag in the DTD object.

An example: If the DTD does not find the declaration of an element, attribute, or notation, it is free to react in two ways. It could immediately signal a validation error. It can also indicate that the requested object is not found, but that the caller should accept that. The caller is here the document tree which tries to trigger this validation check by going to the DTD. If the result of this check is "accept", the tree simply skips all further validation checks. This is how well-formedness mode is implemented.

Now the validation case: If the declaration is found, the document tree takes it, and calls the corresponding validation check routine (often implemented as private methods of the tree nodes). The declaration is often not passed back from the DTD to the node tree in the form as originally parsed, but preprocessed, so that the validation check can run quicker. For elements, the preprocessed declaration is the validation_record, defined in pxp_dtd.ml.

When default values of attributes have to be complemented, the validation_record contains a preprocessed list of attributes. Actually, the node tree takes this list, and looks whether the XML text overrides any of these, and makes the resulting list to the official attribute list of the element node. This kind of dealing with default values is optimized for the case that the are many default values, and overriding occurs only seldom.

The basic well-formedness checks (like proper nesting of tags) are already implemented in the recursive-descent parser module. Neither the document tree nor the DTD has to check any of these.

The mixed mode

Because well-formedness mode is achieved by turning off certain validation checks, it is also possible to run PXP in a mixed mode between both standard modes. Especially, it is possible to check existing declarations, but also to accept missing declarations.

There are two special processing instructions one can include into the DTD part of a document:

  • <?pxp:dtd optional-element-and-notation-declarations?>: This instruction allows to use elements and notation in the XML text without declaration. These elements and notations are then handled as in well-formedness mode. Existing declarations have to be obeyed, however.
  • <?pxp:dtd optional-attribute-declarations elements="e1 e2 ..."?> This instruction allows to use attributes of the mentioned elements e1, e2, etc. without declaration. These attributes are then handled as in well-formedness mode. Existing declarations have to be obeyed, however. Also, attributes of elements not mentioned still need to be declared.
Programmatically, the same effects can also be achieved by setting the allow_arbitrary flags of declaration objects.

Irregular nodes: namespace nodes and attribute nodes

These node types primarily exist because XML standards require them. For example, in XPath it is possible to include attributes into sets of nodes. Of course, this requires that attributes have the same type as other nodes. In order to support these standards better, the node types T_attribute and T_namespace have been added to the tree definition.

Note that these node types are meant "read-only": They provide an alternate view of the properties of the node tree. It does not make sense to modify these nodes, because they are only derived from some original values that would remain unmodified.

In order to get the attribute nodes, just call attributes_as_node (link: Pxp_document.node.attributes_as_nodes). This method takes a snapshot of the current attribute list, turns it into the form of a node list, and returns it. Note that when the original attribute list is modified, the attribute nodes are not notified by this, and remain unchanged.

See Pxp_document.node.attributes_as_nodes for details how the attribute nodes work (e.g. how their value can be retrieved).

Attribute nodes are irregular nodes. They are only returned by this special method, but do not appear in the regular list of children of the element. Note that this corresponds to how XML standards like XPath define attribute nodes.

Namespace nodes are very similar to attribute nodes. They provide an alternate view of the namespace_scope objects, and there is the method namespaces_as_nodes (link: Pxp_document.node.namespaces_as_nodes) that returns these nodes. As attribute nodes, namespace nodes are irregular, and once created, they are not automatically updated when the orginal namespace scope objects are changed.

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