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Oh the irony of this question appearing on XML-Dev ! Isn't the provision of 'Other' the whole point of *Extensible* markup language? Ironic too that without that hack-ish xsd:any (or xsd:anyAttribute) in a schema, XML constrained by an XSD schema is not particularly extensible. So what does XML mean by 'extensible'? I guess what people usually want is semi-constrained markup, codelists, etc. Fully-constrained is what the coder wants but the business rarely copes with fully controlled vocabularies. And yet what real value is there actually in a partially-controlled vocabulary? Less value perhaps than either fully controlled or uncontrolled. Yet so often it is the preferred choice of the analyst. ---- Stephen D Green On 9 February 2014 14:27, Costello, Roger L. <costello@m...> wrote: > Hi Folks, > > I see many XML instances (and corresponding XML Schemas) with an "other" catchall element. > > I wish to convince you that an "other" catchall element is contrary to one's objectives and should be avoided. > > First, a quote from the book, "Principles of Program Design" (Jackson Design Methodology): > > The structure of a program must be > based on the structures of all the data > it processes. > > A corollary to that is: > > Data should model the real world. > > So there is this deep connection: > > real world --> data model --> program structure > > In the real world there is no "other" component: When was the last time you drove an "other car" or purchased an "other vegetable"? > > So it is wrong to create a data model for a nonexistent real world entity. Likewise it is wrong to write a program component for "other" data. > > Let's take an example. Suppose we use XML Schema to model the countries of the world: > > <xs:element name="Country"> > <xs:complexType> > <xs:choice> > <xs:element name="Afghanistan" type="countryType" /> > <xs:element name="Albania" type="countryType" /> > ... > <xs:element name="Zimbabwe" type="countryType" /> > <xs:element name="Other" type="countryType" /> > </xs:choice> > </xs:complexType> > </xs:element> > > Since the countries of the world are ever-changing, we include an "Other" element in anticipation of some future, unforeseen country. > > It is easy to imagine assigning a program component to Iceland: > > <Country> > <Iceland>...</Iceland> > </Document> > > But what is a program component to do when it encounters the "Other" country: > > <Country> > <Other>...</Other> > </Document> > > There is no "Other" country and so any operation that we might assign to the data would be nonsensical. > > Stated another way: > > "Other" data is not machine-processable. The reason > for using XML is that to enable machine-processing. > But by incorporating non-machine-processable > "other" data one's own objectives are defeated. > > "Okay, suppose that I avoid using an <Other> element and, say, a new country emerges; what should I do?" > > Answer: the real world has changed (there is a new country). Your data model must be updated to reflect the real world. So update your XML Schema and in your program add a component for the new country. > > Comments welcome. > > /Roger > > _______________________________________________________________________ > > XML-DEV is a publicly archived, unmoderated list hosted by OASIS > to support XML implementation and development. To minimize > spam in the archives, you must subscribe before posting. > > [Un]Subscribe/change address: http://www.oasis-open.org/mlmanage/ > Or unsubscribe: xml-dev-unsubscribe@l... > subscribe: xml-dev-subscribe@l... > List archive: http://lists.xml.org/archives/xml-dev/ > List Guidelines: http://www.oasis-open.org/maillists/guidelines.php
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