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

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
>
>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="$matching-output/TYPE = 'A' or 
>$matching-output/TYPE = 'B' or 
>$matching-output/TYPE = 'C'" 
>role="post-condition" >Every 
>publicMilitaryIndicator output should have a 
>TYPEÂ  with value A, b, or C (i.e., Civil, Joint, Military)</sch:assert>
>
>Â  Â  Â Â Â  Â  Â  Â <sch:assert 
>test="$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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