Re: Is XML Schema a constraint language or a specification lan

From: "G. Ken Holman" <gkholman@CraneSoftwrights.com>
To: Roger L Costello <costello@mitre.org>,"xml-dev@l..." <xml-dev@l...>
Date: Wed, 21 Apr 2021 09:14:10 -0400

Play the video

My perspective is that any schema language is a constraint language.

When you say "Determining if an instance is valid is a matter of checking that the instance adheres to the specification." I would say "Determining if an instance is valid is a matter of checking that the instance does not violate any of the schema's constraints."

And one can say "An authoring tool directs the creation of content that does not violate any of the schema's constraints".

Different uses put different spins on the activity, but at the core is the activity is guided by sets of constraints.

That way you can have many sets of constraints using different technologies (W3C XML Schema, Schematron, etc.), and then selectively decide if an instance is valid against any or all sets of constraints.

RELAX-NG and DTDs define constraints as pattern grammars, W3C XML Schema defines constraints as type hierarchies, and Schematron defines constraints as content assertions.

Horses for courses.

I hope this helps.

. . . . . . Ken

At 2021-04-21 12:23 +0000, Roger L Costello wrote:

Hi Folks,

Is an XML Schema a constraint language? An XML Schema states constraints that must be satisfied by instances for them to be "valid". Determining if an instance is valid is a constraint satisfaction problem.

Or, is an XML Schema a specification language? An XML Schema is a specification that informs instance document authors how to construct instances. Determining if an instance is valid is a matter of checking that the instance adheres to the specification.

/Roger

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