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   Re: Patterns - What is it?

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  • From: Rick JELLIFFE <ricko@geotempo.com>
  • To: ",XML Developers List" <xml-dev@xml.org>
  • Date: Fri, 14 Jul 2000 03:13:58 +0800

Dan Mabbutt wrote:
> The first group is represented by the page http://www.xmlpatterns.com/.  A
> brief review of this page and some of the references on it suggest that a
> "Pattern" is sort of a "best practice" or "archetype software algorithm."

An "design pattern" is ultimately a literary form, used to capture
experience by forcing certain answers to certain questions about a topic
to be provided. The form is loose.

Design patterns were developed for architecture, but they have had a
major impact on object-oriented programming.  When you read someone
writing "this is a visitor class" or "that is a factory method" they are
refering to a well-known design pattern.

The reason design patterns are important is that without establishing a
common vocabulary there really can be no XML "professionals".  Without
such a vocabulary, people will attempt to use a secondary vocabulary,
which will usually be OO or databases or HTML; those vocabularies may
include enough baggage that they may allow communication while
preventing understanding. 

For example, one of the most basic patterns is the one variously called
the "document skeleton" or "document shell":  that is the simple
structure that whenever information units grow to a certain level of
size or have a strong semantic or processing cohesiveness compared to
the outside material, we will see a common structure  
     - head (with metadata)
     - body (with data)
At the lowest level, this is actually supported in the element tag
syntax, but it scales all the way up so that documents will often be
structured at the highest level in a similar way.  I think Fabio has a
design pattern for this at www.xmlpatterns.com  

It is such a common pattern, but I doubt that if we asked people on
XML-DEV or any of the W3C working groups what that was called, that we
would get a recognised term.  Without such a controlled vocabulary, 
discussions are forced to start from the beginning with actual examples:
this is such a burdon that it prevents discussion of issues.  This will
only get worse as more people con on board who cannot write content
models: not only will it be difficult to write prose, but difficult to
write formal notations.

Rick Jelliffe


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