Date: December 19, 1996
Status: personal view. Editing status: Italic text is rough. Reques complete edit and possibly massaging, but content is basically there. Words such as "axiom" and "theorem" are used with gay abandon and the reverse of rigour here. This file was split off from Axioms.html on 2001/1/15 as matric URI parsing is not a feature of the web. This is just something which could have been.
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It is maybe obvious to note that there are many, many
hierarchical systems. An interesting analogy with a
hierarchical power is, in a programming language, a sequence
of parameters supplied to a command or a procedure. For
example, in some languages a procedure may take positional
parameters which may be optional so that any parameters from
a certain point on may be omitted. This syntax can be
compared with a hierarchical slash separated URL path. This
is an interesting analogy because looking at the alternative
representation for procedure parameters which consists of a
list of procedure name and value pairs. This leads us
naturally to a discussion of the use of the semi-colon in
URLs and the matrix syntax. Just as the slash
separated set of elements is useful for representing a tree,
so a set of names and equally significant parameter can
represent a space more like a (possible sparse) matrix. In
this case navigation to "close" locations is done by varying
one or more parameters which form the dimensions of the
matrix. This is the purpose of the
a=b; parts of
URL syntax which was added later in the URL's history. The
initial need was to put qualifiers onto URLs which were
The analogy with procedure call holds still when looking at
combined forms: The hierarchical part of the URL is paused
first, and then the semi-colon separated qualifiers are
paused as indicating positions in some matrix. As an example
let's imagine the URL of an automatically generated map in
which the parameters for latitude, longitude and scale are
given separately. Each may be named, and each if omitted may
take a default. So, for example,
might be the URL of an automatically generated map.
How should one pause relative URLs in this context? Clearly the most convenient is to be able to select a similar map of a different scale simply by specifying ;scale=64000 as a relative URL. The pausing of the syntax has not been well defined in the URL circles and to my knowledge, has not been consistently used. So, in January 1997, whereas this might be a good idea, it is not currently implemented. However, this is the natural way to pause relative URLs which contain named parameter values. Another unresolved question is whether the order of such parameters should be significant, as logically expressing the same set of parameters in a different order normally has the same semantics. When a relative URL starts with a semi-colon, one can imagine the significance of that to be that all parts of the URL other than the parameters remain unchanged; that the parameters which have not been specified in the relative URL remain unchanged and that the parameter given in the URL takes its given value in place of any GET value in the original URL. Clearly, when adding another parameter value, this does not allow one to specify where it takes its place in the set of parameter values, which then implies that the order of the parameters must be immaterial. There is also the question of how to logically, in a relative URL, remove a parameter, but this could clearly be done if the convention is that a missing parameter has the same significance as a present perimeter with the now string as its value. It is not at all evident that this is what is most wise, so perhaps a special syntax should be used for deleting perimeters.
Note: relative Matrix URLs are notgenerally implemented so this is just a theoretical discussion.
It is only sensible for relative URls which start with ";" to take a set of attribute values which are different from those of the base URL. This implies:
So, by these rules, relative to
or the equivalent
relative URLs would be parsed as follows.
|Relative URI||should be interpreted as|
This is worth insituting (1996) if it can be deployed without crisis, though it clearly cannot be relied on today in browsers.
$Id: MatrixURIs.html,v 1.2 2001/02/15 21:02:49 timbl Exp $
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