Re: [SCXML] Incorrect XPath expressions in IR tests 153 & 155

Ate,
   Questions about the XPath datamodel are always tricky, because the 
group's XPath experts haven't participated in several years.  ( I'm 
certainly not an XPath expert.)  However as I recall our past 
discussions of this issue, the consensus was with Zjnue's 
interpretation.  Specifically, the requirement for the creation of a 
<data> node holds for top-level <data> elements only. Furthermore, the 
intent was that <foreach> would have what you call 'transient pointers' 
to the array elements.  In particular, we intended that modifications to 
those array elements in the course of <foreach> would remain in effect 
after the <foreach> terminated.  Specifically, when we say "The SCXML 
processor /MUST/ act as if it has made a shallow copy of the collection 
produced by the evaluation of 'array'" the intent was that _only_ a  
shallow copy be made and that it be possible to access the array 
elements directly.

Tests 150 and 151 use the same list as tests 153 and 155.  (The .txml 
file has <conf:array123> and the xslt transform produces the <node> 
elements.)  So I would expect all 4 tests to work the same way, with the 
XPath variable bound to the <node> elements in turn.  150 and 151 do not 
test that the newly created variable is initially bound to a <data> 
element.  (Actually, all tests 150 and 151 do is check that using a 
<foreach> with a previously undeclared variable does not raise an 
error.  The tests are rather bogus because the <foreach> doesn't do 
anything, but I was trying to keep the test simple and avoid extra logic 
that might introduce subtle errors.)

  As for test 463, it is explicitly testing the structure of a variable 
created by the <data> element.  I don't think it is required that _any_ 
XPath variable that occurs in an SCXML document have that structure.  I 
admit that this leaves open the question of what structure such other 
variables should have. As the spec stands, it is 
implementation-specific.  However, we did intend for the behavior inside 
<foreach> to be what Zjnue describes.

I'm willing to be persuaded that we're wrong, though.  It may well be 
that our intended interpretation causes problems that we did not foresee.

- Jim
P.S. Everyone who has worked with the XPath data model has had problems 
with it.



On 2/9/2015 4:20 PM, Ate Douma wrote:
> Hi Zjnue,
>
>
> On 2015-02-09 19:28, Zjnue Brzavi wrote:
>> Hi Ate,
>>
>>        <datamodel>
>>          <data id="Var1" expr="0"/>
>>          <data id="Var2"/>
>>          <data id="Var3">
>>            <node xmlns="">1</node><node xmlns="">2</node><node 
>> xmlns="">3</node>
>>          </data>
>>          <data id="Var4" expr="1"/>
>>        </datamodel>
>>
>> [..]
>>
>>     The "Var2/text()" xpath expressions in the if condition check and 
>> the
>>     assignment value expression above are not valid/usable in this 
>> context.
>>
>>     The foreach element will assign each of the Var3 <node>x</node> 
>> children to
>>     the Var2 variable, and the Var2 variable (its data node) thus 
>> will contains
>>     no (direct) text node children, only a (single) "node" child.
>>
>>     To access the actual text value of that "node" child, the 
>> expression must
>>     be: "$Var2/*/text()" or if desired "$Var2/node[1]/text()".
>>
>>
>> This is something I've implemented also, and on this rare occasion I 
>> have to
>> disagree with the suggestion.
>>
>> Let's take this SO post as context:
>> http://stackoverflow.com/questions/11744465/xpath-difference-between-node-and-text 
>>
>>
>> Now back to the example you've stated.
>> Var3 evaluates to an XMLList, which has this simplified structure:
>>
>> element node (name="node")
>>      text node (value="1")
>> element node (name="node")
>>      text node (value="2")
>> element node (name="node")
>>      text node (value="3")
>>
>> Now, in the foreach structure, as we iterate through this list, we 
>> assign a
>> reference to the next element node to Var2.
>> In turn, $Var2/text() evaluates to 1, 2 and 3 as we expect.
>>
>> My guess is that your implementation creates a new XML instance when 
>> assigning
>> values to Var2, giving it values such as:
>>
>> root node
>>      element node (name="node")
>>          text node (value="1")
>>
>> root node
>>      element node (name="node")
>>          text node (value="2")
>>
>> and so on, thereby requiring the extra axis specifier as you suggested:
>> $Var2/*/text()   OR  $Var2/node[1]/text()
>>
>> However, as we are dealing with a complex type, I believe it is wrong 
>> to create
>> new instances and that we should use references to the existing 
>> nodes, making
>> the tests valid as they are currently specified.
>>
>> In my implementation, I have a normalized foreach routine for the 
>> different
>> datamodels:
>> https://github.com/zjnue/hscxml/blob/master/src/hsm/scxml/Interp.hx#L935-L972 
>>
>>
>> This works well for the tests mentioned, when I feed it with an array 
>> containing
>> references to the different nodes in the XMLList, formed here:
>> https://github.com/zjnue/hscxml/blob/master/src/hsm/scxml/Model.hx#L354-L366 
>>
>>
>> Do you agree?
>
> I agree such a solution was what I also implemented initially.
>
> However I changed my view on it and now implemented an actual item 
> copy assignment to the variable <data> node, and even create such a 
> <data> node first if it doesn't yet exist (as required and tested by 
> tests 150 and 151).
>
> The reason I changed my implementation is that the wording in the 
> specification and the tests made me conclude that the intent is that 
> XPath variables (in SCXML) always (must) refer to an actual <data> 
> node with an id equal to the variable name.
>
> In the solution you implemented the XPath variable *initially* refers 
> to such a <data> node, but then loses its binding/reference to a 
> datamodel <data> node and becomes a 'transient' reference to the 
> intermediate array items.
>
> Although I'd also rather and more optimally would like to use the 
> XPath variables as transient pointers, the spec wording and IR tests 
> don't seem to agree with that.
> Your implementation maybe passes each individual tests, but if rules 
> and semantics checked in one test should also apply in other tests, 
> then IMO your solution no longer is or would be valid.
>
> Note for example that such transient XPath variable no longer will 
> agree to an XPath test like "local-name($Var2)=='data' and 
> '$Var2@id='Var2'", something test 463 is testing. Of course not in the 
> context of a <foreach>, but I assume(d?) such XPath variable 
> conditions should always be true.
>
> One thing which seems to be required anyway is that if a <foreach> 
> item or index doesn't exist yet, at least a Node element must be 
> created to hold the variable data, as for example is checked by tests 
> 150 and 151.
>
> Note also that in test 151 the index variable, which just 'holds' a 
> number, still requires creating a Node variable because of the final 
> test condition ""$Var5/* or $Var5/text()".
> A 'normal' XPath variable perfectly well can point to just a number 
> value, however the SCXML spec (and/or tests) require even for such 
> variables an actual (data) node element.
>
> But, maybe I've been assuming and interpreting too much into this...
> Trying to get the xpath datamodel implementation working properly, and 
> as intended, has been (and still is) quite a challenge so say the least.
>
> Maybe Jim can chime in and help clarify what the actual or intended 
> requirements concerning XPath variables are.
>
> Thanks, Ate
>
>>
>> Best regards,
>> Zjnue
>>
>
>