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

Hi,

"Sterne, Jason (Nokia - CA/Ottawa)" <jason.sterne@nokia.com> wrote:
> Thanks William.
> Please see below.
> Regards,
> Jason
> 
> > -----Original Message-----
> > From: Ivory, William <william.ivory@intl.att.com>
> > Sent: Wednesday, August 8, 2018 3:27 AM
> > To: Sterne, Jason (Nokia - CA/Ottawa) <jason.sterne@nokia.com>;
> > netmod@ietf.org
> > 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.

> (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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