Re: XML, Topic Maps and Everything (was Re: [xml-dev] Relationships [was RE: James Clark: XML versus the Web]

In message <58F37483-654F-479B-9515-7644C6582AF3-ThOqFuTjmkBg9hUCZPvPmw@public.gmane.org>, Andrew 
S. Townley <[email protected]> writes
>
>Here are some of the initial thoughts I had on how to apply TMRM 
>fundamentals to the problems being discussed on the list about where to 
>go with XML.  I don't claim these are fully baked and there's likely 
>holes large enough to drive a fleet of trucks through, but I did want 
>to try and explain some of the things I see based on my experience over 
>the last few years designing, implementing and using an information 
>management system based on the TMRM.  Part of my thinking isn't "pure" 
>TMRM and has also been influenced by some work by the authors of the 
>specification in both implementing and explaining it to others[1], so 
>that puts a particular slant on things as well.  Any errors in 
>expressing the TMRM or Topic Maps are purely my own and should not be 
>considered flaws of the specs themselves. ;)

Andrew,

Since you are using the fact that the TMRM expresses a directed graph 
model as a starting point, one issue that interests me is the extent to 
which the built-in TMRM features, such as subject identity and 
superclass-subclass relationships, add value that is relevant for 
generic information modelling. Value that can not, for example, be 
delivered by the widely-used RDF model.  (This isn't just relevant to 
the "XML 2.0" discussion; it also goes to the heart of why Topic Maps 
[may] have a contribution to make to the Linked Data paradigm.)

In the context of "XML 2.0", I don't see how your approach is going to 
apply to XML's (and SGML's) original job description, which is to mark 
up texts.

Finally, in terms of offering something new for XML 2.0 (though maybe 
something no-one really wants ;-) ), there is the thought that if we do 
move towards a directed graph approach, it becomes possible to address 
the "multiple views of a document" (aka overlapping hierarchies) issue 
which was last supported by the SGML CONCUR feature.

Richard

>The TMRM in 10 Minutes or Less
>
>The Topic Maps Reference Model (TMRM) is an abstract model in which the 
>rest of the current Topic Maps specifications can be described.  It 
>came after the original Topic Maps specifications (originally developed 
>in HyTime), and it describes a self-recursive mechanism for 
>representing directed graphs using both late binding and lazy resolution.
>
>The fundamental construct of the TMRM is the Subject Proxy.  Subject 
>Proxies are representations of anything you wish to talk about (the 
>Subject), expressing a relationship similar to Plato's shadows on the 
>wall between the subject and the proxy.
>
>Subject Proxies represent vertices and have zero or more Proxy 
>Properties defining directed edges of the graph representing a 
>particular Subject Map.  Each Proxy Property has a label (key) and a 
>value which is one of nil, a reference to another Subject Proxy or a 
>literal.  I generally use the term "links" to describe the properties 
>referencing other proxies to differentiate them from literal property 
>values.  Each label is a symbol referencing another Subject Proxy, and 
>all you need to do to declare a Subject Proxy is reference it.
>
>Subject Proxies have two pre-defined relationships: 
>superclass-subclass and class-instance that must be interpreted 
>relative to the particular Subject Map.  The superclass-subclass 
>relationship is reflexive and transitive and circular relationships are 
>possible in any given map.  The class-instance relationship is 
>non-reflexive, and where a proxy is an instance of another, it is also 
>an instance of any superclass of that proxy. The TMRM does not specify 
>a representation for either of these relationships.
>
>As a Subject Map is a directed graph, the TMRM also defines a basic 
>path language for navigating and extracting information from a Subject 
>Map.  The fundamental operations on the graph are (using the notation 
>in Annex C of TMRM v7):
>
>- keys ( p \ ) defines a postfix operator to return all of the property 
>labels defined for p
>
>- remote keys ( p / ) defines a postfix operator to return all of the 
>property labels whose value is the proxy p
>
>- values ( p -> k) defines a postfix operator to return all of the 
>values of the particular property label k for the proxy p
>
>- proxies ( v <- k) defines a postfix operator to return all of the 
>proxies having the value v for the specified property label k.  By 
>definition, this result is also a Subject Map.
>
>Subject Maps may be merged such that all proxies found to be about the 
>same subject may be combined and referenced as a single proxy label. 
>The operation is defined in terms of constraints applied to two proxies 
>to generate a third proxy and therefore may only apply to some proxies 
>and properties in a given subject map.
>
>Merge operations and identity constraints are not defined by the 
>specification, however they are defined in terms of a Subject Map 
>legend that contains a finite set of constraints that represent a 
>particular way of interpreting the information in the map.  Legends may 
>be applied to more than one map, and more than one legend can be 
>applied to a single map to produce the desired views of the information.
>
>Topic Maps Identity Semantics
>
>The Topic Maps specification defines two very precise ways to indicate 
>the identity of a subject for any given proxy.  Either the subject may 
>be indicated by a resource which is primarily about the subject, or the 
>subject may be an addressable resource.  Each relationship uses 
>particular constructs to avoid the ambiguity present in other systems 
>in identifying the subject a proxy represents.
>
>The TMRM in Practice
>
>Since the TMRM is abstract, you need to define specific syntax 
>representations of it in order to do anything useful.  As you are 
>dealing with the most fundamental conceptual way to represent 
>information through proxies and properties, key-value pairs or EAV/OAV 
>models, you are only limited by your imagination in terms of what this 
>representation can be.  You can also use the same mechanism to 
>represent information about anything of interest to you, and this 
>fundamental model can be expanded to more efficient and expressive 
>operations grounded in the fundamentals of proxies, the path language 
>and the particular legend you choose (this is the "Everything" part of 
>the subject with apologies to Douglas Adams).
>
>You can quite easily define formalizations (legends) to represent 
>proxies using programming language constructs from structures to 
>classes and objects to hashes and maps.  What matters most is what 
>labels you define as reserved for any particular application and the 
>semantics by which properties using those labels are interpreted by 
>your particular application.
>
>For example, you could represent the TMRM specification itself very 
>simply in JSON as:
>
>tmrmspec = {
>  isa: "document",
>  reifies: "http://www.isotopicmaps.org/TMRM/TMRM-7.0/tmrm7.pdf",
>  title: "Topic Maps Reference Model, 13250-5",
>  name: "The Topic Maps Reference Model, version 7",
>  authors: {
>       isa: "part-whole",
>        parts: {
>               patrick: {
>                       name: "Patrick Durusau",
>                       isa: "person",
>                       subject-indicator: "http://tm.durusau.net", ... },
>               steve: { name: "Steve Newcomb", ... },
>               robert: { name: "Robert Barta", ... }
>       }, ...
>};
>
>I've chosen to explicitly make the authorship relation a proxy 
>instance, but your legend could say that it was simply an array of 
>anonymous proxy objects.  In this example the Subject Map is defined in 
>terms of the JSON object itself, but you could relax the JSON 
>referential constraints to easily treat your entire program as a 
>subject map.
>
>You can also go backwards, so that given any JSON or JavaScript object, 
>you can treat it as a set of proxies and property values.  Of course, 
>the above example could use string keys to reference any existing 
>ontology for representing known information like Dublin Core. The 
>syntax of JSON requires the labels be represented as strings if they're 
>QNames, but that doesn't change the underlying model.
>
>For XML, you can even treat individual elements as proxies too:
>
><Book isbn="XXX" title="Life, the Universe and Everything" 
>myns:animal="dolphin" />
>
>might conceptually represent the following proxy (as a Ruby Hash):
>
>example2 = {
>       :isa => :Book,
>       :isbn => "...",
>       :title => "...",
>       "myns:animal" => "dolphin"
>}
>
>Or you could get really crazy and define a standard XML Element to 
>Proxy mapping (based on things we already know):
>
><xyzzy:Book isbn="12345">
>       I think this is a really cool book!
></xyzzy:Book>
>
>As JSON again:
>
>example3 = {
>       isa: "xyzzy:Book",
>       isbn: "...",
>       text: "I think..."
>}
>
>Or you can define a legend that maps particular elements to proxy types 
>and properties using XPath or Schematron, or whatever.
>
>Similar to the call for "just show me the nodes", I think the TMRM 
>would provide a consistent model where you could manipulate both the 
>structure of the encoding (or the representation) as well as the 
>information being encoded, so that you can represent what you like in 
>the format of your choice, but manipulate the information and the 
>serialization using the same fundamental constructs.
>
>Admittedly, the examples above are contrived, and, as I said, I'm not 
>saying this is a finished proposal, so there's plenty left to argue 
>about! :)
>
>One of the key things I think XML 2.0 should do is be less worried 
>about the syntax of a representation and be more focused on allowing 
>you to co-mingle any structured domain representations of information 
>and manipulate it using a consistent model.  If you need a binary 
>format, fine, then we can easily define a legend for ASN.1 
>representations or your own proprietary one.  If you need to 
>serialize/deserialize into programming language constructs, that's no 
>problem either.  There's a formalized legend for languages X, Y, Z and 
>even Q.  Writing a new language?  No problem, here's how you define a 
>legend so it all plays nicely.
>
>I also think it's possible to achieve the above with a very limited set 
>of pre-defined proxy types and labels.  Let the applications worry 
>about how to interpret higher-level things the same as now, but at 
>least give people a consistent way to work with information, slice and 
>dice it, transform it and materialize it on the other end.
>
>Expressing most of what XML technologies already do in terms of a 
>standard, unifying model would allow those technologies to be applied 
>to any structured data representation.  To me, that would be a real win 
>for XML 2.0 and, more importantly, a real win for the application 
>developers in the trenches every day.  The vendors might not be so keen 
>about it, but I think they'll get over that.
>
>Having worked with a TMRM based information management system and done 
>a lot of structured format transformations in and out of it (CSV, XML, 
>database tables, custom formats, JSON, Ruby objects), I believe that 
>what I'm describing is certainly possible.  It just needs to be seen as 
>a relevant goal for the community.
>
>Hopefully, the above is a reasonable enough balance between detail and 
>concepts to allow you to understand where I'm coming from and why I 
>think it's important.
>
>I look forward to your comments, flames and feedback. :)
>
>Cheers,
>
>ast
>
>[1] http://www.acs.org.au/documents/public/crpit/CRPITV43Barta.pdf
>--
>Andrew S. Townley <[email protected]>
>http://atownley.org
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-- 
Richard Light