> On 07 Sep 2016, at 19:44, Vladimir Vassilev <[email protected]> wrote:
>
> On 09/07/2016 02:18 PM, Martin Bjorklund wrote:
>> Hi,
>>
>> Your example is not circular, and it is legal. However, the 'when'
>> expression refers to the node in which the when expression is defined.
>> Note that this expression will always evaluates to 'false' (see the
>> third bullet in 7.21.5 in RFC 7950).
> This is true for YANG 1.1 due to that 3rd bullet in 7.21.5.
>
> However it should be noted that in YANG 1.0 nothing says the data node for
> which the "when" statement is defined will be replaced with dummy node
> without a value so that this expression always evaluates to 'false'. In YANG
> 1.0 the expression can evaluate to 'true' when a+b=100.
Using "when" expressions that refer to the context node or its descendants
could easily lead to deadlocks in YANG 1.0, so this part was underspecifed and
ill-defined. The new 1.1 procedure for evaluating "when" expressions is thus a
bug fix. Although it is somewhat tricky and sometimes (as in your example)
counter-intuitive at first sight, it leads to unambiguous and stable results.
>>
>> Take a step back and consider what the 'when' statement means - it is
>> used to indicate if the node can be present or not. As such, it
>> doesn't make any sense to refer to the node itself in the xpath
>> expression.
>
>> In your case, you probably want to use a 'must' expression. This is
>> evaluated once the node is present, in order to enforce some
>> constraint.
>>
> I agree. I think the conclusion can be generalized to: there is no point in
> using "when" expressions directly depending on the value of the data node for
> which the "when" statement is defined or its children when the parent is not
> 'augment', 'uses', 'case' or 'choice' statement in YANG 1.1 because the
> 'when' statement will always evaluate to 'false' and the data node will never
> exist.
Not necessarily. The following expression is always true:
leaf foo {
when ".";
...
}
>
> Would be helpful if model validation tools could issue at least warning in
> such cases.
This is rather difficult to detect in general. XPath is a complicated beast.
Lada
>
> Vladimir
>>
>> /martin
>>
>>
>> Vladimir Vassilev <[email protected]> wrote:
>>> Hi,
>>>
>>> Is there any practical value of 'when' statements with circular
>>> dependency to the value of the parent (in case it is a leaf) or any
>>> children of the parent?
>>>
>>> container circular-dependency-when {
>>> leaf a {
>>> when "(. + ../b) = 100";
>>> type uint16 {
>>> range "0 .. 100";
>>> }
>>> }
>>> leaf b {
>>> type uint16 {
>>> range "0 .. 100";
>>> }
>>> }
>>> }
>>>
>>>
>
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--
Ladislav Lhotka, CZ.NIC Labs
PGP Key ID: E74E8C0C
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