Re: [xml-dev] Are there invariants in XML processing?

["G. Ken Holman" <gkholman@CraneSoftwrights.com>]

Thank you, Stefan! Interesting points made. I 
think it will take me a while to internalize the distinctions.

. . . . . Ken

At 2022-05-01 17:47 +0200, Stefan Hagen wrote:
> On Sun, May 1, 2022, at 15:44, G. Ken Holman wrote:
> > Forgive me, but I'm trying to understand the use
> > of the noun "invariant". This is the first I've
> > seen it used, and it is being bandied about.
> > Rick's post using it extensively is dated 2016,
> > so I feel very much out of the loop.
> >
> > Is an invariant any different than a constraint?
> >
> > Does the use of this noun come from another 
> > discipline where the use is common?
> >
> > When does it help the listener/reader to use the
> > noun "invariant" rather than "constraint" or some other word?
> >
> > Thanks for your patience with my questions ... I'm just curious.
> >
> > . . . . . Ken
> >
> > At 2022-05-01 14:08 +1000, Rick Jelliffe wrote:
> > >For a Schematron introduction to this, including
> > >document invariants (fixed and co-occurrence)
> > >and input/output invariants, and round-trip
> > >invariants, see Six Kinds of Validation using
> > >Schematron at
> > ><<https://www.schematron.com/document/279.html 
> >  >https://www.schematron.com/document/279.html>https://www.schematron.com/document/279.html
> > >
> > >For people interest in the practical theory of
> > >invariants, Bertam Meyers' work on Design by
> > >Contact still is a great place to start: he
> > >couches it in functional terms of pre-condition,
> > >invariant,  post-condition, which I think is
> > >more systematic than just "invariants".
> > >
> > >For the example, the Schematron IO validation
> > >(output to input) might be (roughly)
> > >
> > ><sch:pattern
> > >id="publicMilitaryIndicator-nullTransform" documents="'input.xml'">
> > >
> > >Â  Â  Â <sch:rule context="publicMilitaryIndicator">
> > >Â  Â  Â  Â  Â  Â  Â <sch:let
> > >name="matching-output" value="myFunc:findMatchingRow(., 'output.xml')" />
> > >Â  Â  Â  Â  Â Â
> > >Â  Â  Â  Â  Â  Â  Â  <sch:assert test="TYPE =
> > >'A' or TYPE = 'B' or TYPE = 'C'"
> > >role="pre-condition" >Every
> > >publicMilitaryIndicator input should have a
> > >TYPEÂ  with value A, b, or C (i.e., Civil, Joint, Military)</sch:assert>
> > >
> > >Â  Â  Â  Â  Â  Â  Â Â Â <sch:assert
> > >test="$matching-output/TYPE = 'A' orÂ
> > >$matching-output/TYPE = 'B' orÂ
> > >$matching-output/TYPE = 'C'"
> > >role="post-condition" >Every
> > >publicMilitaryIndicator output should have a
> > >TYPEÂ  with value A, b, or C (i.e., Civil, Joint, Military)</sch:assert>
> > >
> > >Â  Â  Â Â Â  Â  Â  Â <sch:assert
> > >test="$matching-output" role="invariant">Every
> > >input publicMilitaryIndicator should have a matching output</sch:assert>
> > >
> > >Â  Â  Â  Â Â Â Â  Â Â <sch:assert
> > >test="$matching-output/TYPE= ./TYPE"
> > >role="invariant" >Every input
> > >publicMilitaryIndicator TYPE value should be
> > >carried through to the output</sch:assert>
> > >
> > ></sch:rule></sch:pattern>
> > >
> > >Rick
> >
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> Invariant to me signals not varying under some assumptions.
> Constraints are external to, while invariants 
> are internal to the system we observe.
> Constraints may result in invariants.
> If cows cannot leave a fenced field, the number of cows is the invariant.
> Constraints can be many, like no one opening the 
> fence, no cow strong and willing enough to break 
> through, no big enough holes appearing, no bird 
> of prey large enough striking and lifting, …
> That is what I am used to.
>
> Best,
> Stefan

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