Creating an evaluator

Call creation procedure make (digits, on_or_off) from class XM_XPATH_EVALUATOR. Digits is the precision for decimal arithmetic. It should be at least 18 (or 0, for infinite precision). On_or_off indicates wheter or not line numbering should be turned on for error messages.

XM_XPATH_EVALUATOR inherits from XM_STRING_MODE, so you should then call one of the string-mode setting procedures from that class on the evaluator. It will in turn call the same procedure on the Eiffel XML parser.

Building a static context

Before calling evaluate, it is first necessary to create a static context. Two features are provided to do this:

• build_static_context (a_source_uri: STRING, xpath_one_compatibility: BOOLEAN, warnings: BOOLEAN, use_tiny_tree_model: BOOLEAN, compact_tree: BOOLEAN)

  The first argument is the URI of an XML source, against which the XPath expression will be evaluated.

  The second argument specifies if we should run in XPath 1.0 compatibility mode.

  The third argument requests warnings of differences between XPath 1.0 and XPath 1.0 compatibility mode. It is only meaningful if the second argument is True.

  The fourth argument specifies the tree model to use. If performace and/or memory usage are important to your application, then you should make measurements to see which tree model suits you best.

  The fifth argument specifies whether or not to compact the tree after parsing. it is not implemented at the time of writing.

  This feature parses a_source_uri as XML, and builds an instance of XM_XPATH_DOCUMENT. It sets was_build_error to true if any error was found, in which case error_message will contain the text of the parse error.

  If no error was detected, it then creates a static context, using the document node as the context item.

• set_static_context (a_static_context: XM_XPATH_STAND_ALONE_CONTEXT)

  This feature enables you to supply a tailored static context. You can also use it if you wish to perform an XPath evaluation without referencing an XML document (this is of rather limited usefulness, but you might want to use it for evaluating environment variables as booleans, for instance).

  Having built your static context, you may then declare variables, if you wish.

Performing the evaluation

The sole argument to evaluate (an_expression_text: STRING) is the text of the XPath expression which you wish to evaluate.

Checking for evaluation errors

After calling evaluate, check is_error to see if an error occurred. if it did, then feature error_value contains the details of the error. See XM_XPATH_ERROR_VALUE.

Retrieving the results of an evaluation

If the evaluation was successful, evaluated_items: DS_LINKED_LIST [XM_XPATH_ITEM] contains a list of results. These are all of type XM_XPATH_ITEM, representing an XPath item. An item may either be a node (XM_XPATH_NODE), or an atomic value (XM_XPATH_ATOMIC_VALUE).