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132 lines
6.3 KiB
132 lines
6.3 KiB
<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta name="generator" content=
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"HTML Tidy for Mac OS X (vers 31 October 2006 - Apple Inc. build 13), see www.w3.org" />
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<title>
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HyperText Design Issues: Topology
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</title>
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<nextid />
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</head>
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<body bgcolor="#FFC060" text="#302005">
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<a href="OldDocs.html"><img src=
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"../Icons/WWW/arch1990" /></a>TimBL
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<hr />
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<h1>
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Topology
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</h1>Here are a few questions about the underlying connectivity
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of a hypertext web.
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<h2>
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Are links two- or multi-ended?
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</h2>The term "link" normally indeicates with two ends.
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Variations of this are liks with multiple sources and/or
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multiple destinations, and constructs which relate more than
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two anchors. The latter map onto logic description systems,
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predicate calculus, etc. See the "Aquanet" system from Xerox
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PARC - paper at HT91). This is a natural step from hypertext
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whose the links are typed with semantic content. For example,
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the relation "Document A is a basis for document B given
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argument C". From now on however, let us restrict ourselves to
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links in the conventional sense, that is, with two ends.
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<h2>
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<a name="14">Should the links be monodirectional or
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bidirectional</a>?
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</h2>If they are bidirectional, a link always exists in the
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reverse direction. A disadvantage of this being enforced is
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that it might constrain the author of a hypertext - he might
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want to constrain the reader. However, an advantage is that
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often, when a link is made between two nodes, it is made in one
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direction in the mind of its author, but another reader may be
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more interested in the reverse link. Put another way,
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bidirectional linking allows the system to deduce the inverse
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relationship, that if A includes B, for example, that B is part
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of A. This effectively adds information for free. This is
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important when a critical parameter of the system is how long
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it takes someone to create a link.
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<p>
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KMS and hypercard have one-way links; Enquire has two-way
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links.
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</p>
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<p>
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There is a question of how one can make a two-way link to a
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protected database. The automatic addition of the reverse
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link is very useful for enhancing the information content of
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the database. See also:<a name="2" href=
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"Multiuser.html#3">Private overlaid web</a> , <a name="3"
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href=
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"../../History/19921103-hypertext/hypertext/Products/Microcosm/Microcosm.html#GenericLinking">
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Generic Links</a> .
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</p>
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<p>
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It may be useful to have bidirectional links from the point
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of view of managing data. For example: if a document is
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destroyed or moved, one is aware of what dangling links will
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be created, and can possibly fix them.
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</p>
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<p>
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A compromise that links be one-way in the data model, but
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that a reverse link is created when any link is made, so long
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as this can be done without infringing protection. An
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alternative is for the reverse links to be gathered by a
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background process operating on a basically monodirectionally
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linked web. See <a name="13" href=
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"BuildingBackLinks.html">Building Back-links</a>.
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</p>
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<h2>
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<a name="12">Should anchors have more than one link?</a>
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</h2>There is a design issue in whether one anchor may lead to
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many links, and/or on link have many anchors. It seems
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reasonable for many anchors to lead to the same reference. If
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one source anchor leads to more than one destination anchor,
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then there will be ambiguity if the anchor is clicked on with a
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mouse. This could be resolved by providing a menu to the user,
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but I feel this would complicate it too much. I therefore
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suggest a many-to-one mapping. <a name="8" href=
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"../People.html#groff">JFG</a> disagrees and would like to see
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a small menu presented to the user if the link was
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ambiguous.<a name="9" href=
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"../../History/19921103-hypertext/hypertext/Products/Microcosm/Microcosm.html">Microcosm</a>
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does this.
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<h2>
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<a name="4">Should links be typed?</a>
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</h2>A typed link carries some semantic information, which
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allows the system to manage data more efficiently on behalf of
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the user. A default type ("untyped") normally exists in some
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form when types are implemented. See also a <a name="1" href=
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"LinkTypes.html">list of some types</a> . (Should a link be
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allowed to have many types? (- <a name="6" href=
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"../People.html#groff">JFG</a> ) I don't think so: that should
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be represented by more than one link.(- <a name="7" href=
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"../People.html#BernersLee">TBL</a> ))
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<p>
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Link typing helps with the generation of <a name="10" href=
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"Navigation.html#6">graphical overviews</a> , and with
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<a name="11" href="TracingLinks.html">automatic tracing</a> .
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</p>
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<h2>
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Should links contain ancillary information?
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</h2>Does the system allow dating, versioning, authorship,
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comment text on a link? If so, how is it displayed and
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accessed? This sort of information complicates the issue, in
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that readable information is no longer carried within node
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contents only. Pretty soon, following this path leads to a link
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becoming a node in itself, annotatable and all. This perverts
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the data model significantly, and I cannot see that that is a
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good idea. Information about the link can always be put in the
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source node, or in an intermediate node, for example an
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annotation. However, this makes tracing more difficult. It is
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certainly nice to be able to put a comment on a link. Perhaps
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one should make a link annotatable. I think not.
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<h2>
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Should a link contain Preview information?
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</h2>This is information stored at the source to allow the
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reader to check whether he wants to follow a link before he
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goes. I feel that the system may cache some data (such as the
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target node title), or the writer of the node may include some
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descriptive material in the highlighted spot, but it is not
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necessary to include preview information just because access
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may be slow. Caching should be done instead of corrupting the
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user interface. If you have a fast <a name="5" href=
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"Navigation.html#6">graphic overview</a> , this could remove
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the necessity for a preview function.
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</body>
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</html>
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