Re: [xml-dev] Notable declarative expressions?

These words, "relationships between values in the value space and things in the world outside", are key.

If one thinks primarily of document-oriented XML, as for example docbook or XHTML, one may regard XSD as a way of constraining the way *how* one speaks about the world, but certainly not constraining *what* one says - XSD is a grammar constraining speech.

If one thinks of data-oriented XML, XSD usually amounts to abstractions about the world outside. If my XSD says a return flight consists of a mandatory outbound flight and a mandatory inbound flight, each of which consists of one or more flight legs, each one of which has properties like a cabin class - this is a model of the world outside.

Hans-Juergen Rennau

Michael Kay <mike@saxonica.com> schrieb am 19:04 Dienstag, 20.Dezember 2016:

> On 20 Dec 2016, at 17:57, Thomas Passin <list1@tompassin.net> wrote:
>
> I think you could (or at least I could) think of a schema as a model.

I'm inclined to agree. It's at least as much a data model as the schema of a relational database is. It defines a value space, and it provides mechanisms for documenting the relationships between values in the value space and things in the world outside. What else does it have to be to make it a data model?

The fact that XSDs primary mechanism for constraining the value space is by means of a grammar is a distinguishing feature, but I don't see anything in the theory of data modelling that says "the constraints on the value space must be defined using mechanisms other than grammars".

Michael Kay
Saxonica

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