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Ideas on list/array access/modification issue

From: Gary Brown <gary@enigmatec.net>
Date: Wed, 9 Mar 2005 13:32:14 -0000
Message-ID: <006b01c524ac$6bd87590$0300a8c0@LATTITUDEGary>
To: <public-ws-chor@w3.org>
Following on from Tony's ideas:

List/Array Access/Update issue

Date: 9th March 2005



Accessing a list or array

This can be achieved using existing XPath expression support. The CDL 'getVariable' function will return the value associated with a variable and optionally a component within the XML document represented by the variable. It would then be possible for the XPath expression to perform further evaluation on the returned result.

Alternatively the list accessor could be placed in the XPath expression supplied as a parameter to the 'getVariable' function.

For example, if we have a variable called 'doc' which contains the XML data from Tony's previous example, then

variable 'doc' will contain:

    <listA>
        <listItem>1</listItem>
        <listItem>2</listItem>
        <listItem>4</listItem>
        <listItem>8</listItem>
    </listA>

If we also have a variable 'index' that is acting as an index value, so is of type xs:integer, then the following Xpath expression would be used to access the list element associated with the current index value,

cdl:getVariable('doc', '', "/listA/listItem[cdl:getVariable('index','','','')]", '')

Alternatively, we could move the list accessing expression outside the function:

cdl:getVariable('doc','')/listA/listItem[cdl:getVariable('index','','')]

This may be a better approach than embedding the XPath expression inside the 'getVariable' function, which avoids nested functions. It may be even better to be able to reference CDL variables natively within the XPath expression, although this would not cater for multi-part messages - and other 'isVariableAvailable' function would still be required.



Modifying a list or array

I don't think it is appropriate to add functions to XPath that modify the document being queried, as this may have unknown consequences on the remainder of the xpath evaluation.

My preference would be to use XPath expressions to identify the context for the modification being performed, and then have specific activities to perform the modification.

One possibility would be to change the current assign/copy construct, so that copy would be a 'replace' operator, while 'insert'/'delete'/'append' operators could be used for list manipulation.

For example,

<update> <!-- instead of 'assign' -->
    <copy target="cdl:getVariable('newNode')" expression=" '<listItem>16</listItem>' " />
    <append context="cdl:getVariable('doc')/listA" expression="cdl:getVariable('newNode')" />
    <delete expression="cdl:getVariable('doc')/listA/listItem[2]" />
</update>

The 'insert' could be the same syntax as the append, with the addition of a 'position' attribute to indicate the insertion point.

If it is considered necessary to separate the variable from the context node within the variable's document, then that could be easily achieved. For example,

<append variable="cdl:getVariable('doc')" context="/listA" expression="cdl:getVariable('newNode')" />

NOTE: To support concurrent update of any information (but especially the lists), within a choreography, we should stipulate that the 'update' activity is atomic and isolated.



Regards

Gary
Received on Wednesday, 9 March 2005 13:32:23 GMT

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