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Re: Need a language whiz: An XML Schema "specifies" howdata is

  • From: Eliot Kimber <ekimber@contrext.com>
  • To: <xml-dev@lists.xml.org>
  • Date: Tue, 14 Nov 2017 09:42:43 -0600

Re:  Need a language whiz: An XML Schema "specifies" howdata is
At the risk of splitting hairs (ok, at the certain risk of splitting hairs), I don’t think a schema can be said to “describe how data is to be structured”.

Any declarative schema is simply a set of constraints to which instances governed by the schema do or don’t conform.

Any descriptive aspect is entirely emergent from the details of the constraints or contained in any embedded documentation and there is no guarantee that a given grammar will provide any useful descriptive aspect. That is, a grammar is like any other code artifact: it can be authored in a way that is descriptive and instructive or it can be obtuse and opaque, but both artifacts may produce the correct result.

In addition, any declarative schema is necessarily an incomplete definition of the structural rules because there are always rules that cannot be directly expressed in a given declarative grammar, such as editorial rules or co-existence constraints or business rules that depend on knowledge of the state of the world outside the scope of the object being validated.

So a schema, by itself, can only be an incomplete definition of constraints, no more.

One of the things I’ve observed in my 30+ years in the structured markup world is the overvaluing of grammars (DTDs, schemas, etc.) as the primary source of truth in document processing. They are of course useful parts of any system but any “description” of “how data is to be structured” can only be done completely in the domain of prose, possibly augmented with formal grammars.


Eliot Kimber

On 11/14/17, 6:43 AM, "Thomas Passin" <list1@tompassin.net> wrote:

    Or, to put it in language similar to Roger's 8 statements,
    0. A schema describes how data is to be structured, and may be used to 
    constrain that structure.
    On 11/13/2017 9:26 PM, Rick Jelliffe wrote:
    > An XML Schema describes certain document structures and data typing. The
    > parties specify when, how and how much of it should be used.
    > A schema has no agency or will or knowledge of outcomes or intent. It
    > reveals/reflects the relative power and attitudes of the various
    > stakeholders.
    > Regards
    > Rick
    > On 14 Nov 2017 00:14, "Costello, Roger L." <costello@mitre.org
    > <mailto:costello@mitre.org>> wrote:
    >     Hi Folks,____
    >     __ __
    >     Which of the following is most accurate and why?____
    >     __ __
    >      1. An XML Schema specifies how data is to be structured.____
    >      2. An XML Schema describes how data is to be structured.____
    >      3. An XML Schema states how data is to be structured.____
    >      4. An XML Schema constrains the structure of data.____
    >      5. An XML Schema structures data.____
    >      6. An XML Schema describes data.____
    >      7. An XML Schema constrains data.____
    >      8. Other (what?)____
    >     __ __
    >     /Roger____
    >     __ __
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