Re: [netmod] false() vs 'false' in 'when' statements

[Martin Bjorklund <[e-mail]>]

Andy Bierman <[e-mail]> wrote:
> On Wed, Aug 8, 2018 at 6:50 AM, Martin Bjorklund <[e-mail]> wrote:
> 
> > Hi,
> >
> > "Sterne, Jason (Nokia - CA/Ottawa)" <[e-mail]> wrote:
> > > Thanks William.
> > > Please see below.
> > > Regards,
> > > Jason
> > >
> > > > -----Original Message-----
> > > > From: Ivory, William <[e-mail]>
> > > > Sent: Wednesday, August 8, 2018 3:27 AM
> > > > To: Sterne, Jason (Nokia - CA/Ottawa) <[e-mail]>;
> > > > [e-mail]
> > > > Subject: Re: [netmod] false() vs 'false' in 'when' statements
> > > >
> > > > On 07/08/18 18:37, Sterne, Jason (Nokia - CA/Ottawa) wrote:
> > > > Hi all,
> > > >
> > > > When comparing the value of a boolean leaf in a 'when' statement (or a
> > > > 'must'
> > > > statement), is an equality comparison to 'true' or 'false' the same as
> > > > a check
> > > > against true() or false() ?
> > > >
> > > > For example:
> > > >
> > > > leaf a {
> > > >     type boolean;
> > > > }
> > > > leaf b {
> > > >     type uint32;
> > > >     when ".../a = 'true'";
> > > > }
> > > > leaf c {
> > > >     type uint32;
> > > >     when ".../a = 'false'";
> > > > }
> > > > (note that neither b nor c can be configured if leaf a isn't
> > > > explicitly configured
> > > > with a value since there is no default)
> > > >
> > > > If I replace the two 'when' statements above with the following
> > > > instead, is the
> > > > behavior the same ?
> > > >     ...
> > > >     when ".../a = true()";
> > > >     ...
> > > >     when ".../a = false()";
> > > >     ...
> > > >
> > > > Or does "../a = true()" behave as an existence check on leaf a and
> > > > would be
> > > > enabled whether a is configured as true or false ?
> > > >
> > > > Regards,
> > > > Jason
> > > >
> > > >
> > > >
> > > > See https://www.w3.org/TR/1999/REC-xpath-19991116/#booleans
> > > >
> > > > (a) "../a = 'true'"
> > > >
> > > > Compares 'string-value' (same as value of node if a leaf, rather more
> > > > interesting if a list / container!) of each node in nodeset generated
> > > > by '../a' and
> > > > if any node in the nodeset matches 'true', expression is true.
> > >
> > > [>>JTS: ] In my specific example, leaf a is a single Boolean leaf.  In
> > > that case, are these equivalent ?
> > >     when ".../a = 'true'";
> > >     when ".../a = true()";
> >
> > No...
> >
> > > and are these also equivalent ?
> > >     when ".../a = 'false'";
> > >     when ".../a = false()";
> >
> > .... and no.
> >
> > What you probably want is ".../a = 'true'" (or 'false').
> >
> > The expression:
> >
> >     ".../a = true()";
> >
> > will first take the string value of ".../a", which e.g. is "false".
> > This string value is then converted to a boolean, resulting in the
> > boolean value True (b/c the string is non-empty).
> >
> > This means that if "a" is set at all (to true or false), the
> > expression ".../a = true()" returns true.  The expression ".../a =
> > false()" will only return true if "a" is unset.
> >
> >
> 
> I suggest reading the XPath definition of boolean comparisons:
> https://www.w3.org/TR/1999/REC-xpath-19991116/#booleans
> 
> It is a bit complicated, but generally, a node-set is converted
> to a boolean (==false for empty, ==true for not empty) when
> compared to a boolean for equality/inequality.

You're right.   But the result is the same in the example above; i.e.,
"../a = true()" is true iff a exists.


>  false is a boolean

There are no boolean literals in XPath 1.0.  So false would refer to a
node with the name "false".

>  'false' is a string
>  false() is a function that return a boolean


/martin


> 
> 
> Andy
> 
> 
> > (sorry if I'm being dense here, but I'm not 100% positive if I'm
> > > interpreting your explanations correctly and getting the right
> > > conclusion.  I've never seen must or when statements that use =
> > > false() or = true()).
> > >
> > > [>>JTS: ] Are the strings 'true' and 'false' the "string-value" (that
> > > you mention above) for a Boolean leaf ?
> >
> > Yes, according to 9.5.1 in RFC 7950.
> >
> > > 'a' is a single Boolean leaf so the nodeset is that single leaf and
> > > the value is 'true' or 'false'.
> > >
> > > >
> > > > (b) "../a = true()"
> > > >
> > > > Similar to above but after getting the string-value, this is converted
> > > > to a
> > > > boolean before comparing with the boolean true.
> > >
> > > [>>JTS: ] I'm a bit confused here.  Are you saying that the
> > > string-value of leaf a is converted to a Boolean ?
> > > Do the string-values of a convert to something that is equal to true()
> > > or false() ?
> >
> > See above.
> >
> >
> > /martin
> >
> >
> > >
> > > >
> > > > (c) '../a'
> > > >
> > > > True if the nodeset generated by this expression is not empty.
> > > >
> > > > ---
> > > >
> > > > Caveats alluded to above are that if '../a' matches multiple nodes,
> > > > then only
> > > > one of these nodes needs to pass the test for the test to be true.
> > > > Separately, if
> > > > any of the nodes are not leaves, the 'string-value' is a concatenated
> > > > string of
> > > > the values of the node's child / grandchild leaves.
> > > >
> > > > Hope that helps.  You wouldn't believe the number of unit tests I have
> > > > to make
> > > > sure we get this logic right!  Using '!=' when comparing a nodeset
> > > > with >1 node
> > > > with other values is very likely not to give the answer you expect (-:
> > > >
> > > > Regards,
> > > >
> > > > William
> > > >
> > >
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