Re: Are there invariants in XML processing?

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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