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A quick link primer:
1. Links as URIs identify a location of a resource (usually a file with
data of some type, can be a location within a file of some type, etc). These
are untyped links, meaning, they don't declare a relationship.
2. Typed links name a relationship. A bibliographic reference is a
typed link. An inverted index is a typed link.
3. Links may have directionality regardless of the mechanical
implementation
of the link. Some links point in one direction (an HTML hyperlink). Some
go in two directions (say, Next/Back).
4. Links may have multiple targets. The index at the back of the book
(word plus every location word is used) is an example. Is that multiple
links or one link with multiple targets? That's an implementation detail.
5. A link may only name a relationship or it may be the control for
navigating among instances. The major confusion in linking concepts
is this aspect.
XLinks go beyond identification or the resource and enable typed links.
Given that a link for the web is the URI, the rest of that information
is annotative, that is, a link processor of some kind might use it.
For example, you often want the links in the bibliography, the links
in the table of contents, and the links in the index at the back of
a book to be displayed differently, and certainly, to behave differently.
A click on a table of contents should take you to the location of that
resource. A click on the inverted index might display all, none or
one of the resources.
So linking as a concept seems simple, but it has overtones of identity,
display and control over navigation. Not everyone agrees or can agree
on how that fits into a single set of concepts or implementations. Also,
most anything one can do with a multiway link, one can do with multiple
simple links given some control to display it in. The classic example
is a popup menu.
So XLink is neat conceptually, but in practical use, there are easy ways
to do the same thing without using XLinks. Without a compelling use case
shared by a near universal set in the user community, it gets very little
play outside implementations of link data bases (and again, you should
look at the inverted index and compare that to systems that implement
topic maps).
len
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