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Non-numeric feature tags

From: Koen Holtman <koen@win.tue.nl>
Date: Mon, 12 Aug 1996 19:59:50 +0200 (MET DST)
Message-Id: <199608121759.TAA20763@wsooti04.win.tue.nl>
To: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Cc: Koen Holtman <koen@win.tue.nl>

The need for non-numeric feature tags has recently been discussed on
the list.  This is my proposal for adding them to the content
negotiation draft.

The mechanism below allows a feature tag to have multiple values,
which need not be numeric.  By allowing for multiple values, you can
say something like

 Accept-features: paper=a4, paper=a5

if you can handle two different paper sizes.  

What follows are updated versions of Sections 6.2 and 6.3 of the
content negotiation draft (see http://gewis.win.tue.nl/~koen/conneg/).
Changebars were added by hand.

----snip----

6.2 Accept-Features header

   The Accept-Features request header can be used by a client to give
   information about the presence or absence of certain features.

|      Accept-Features = "Accept-Features" : 
|                  #( [ "!" ] ftag [ "=" value ]
|                   | ftag "=" "{" value "}"
|                   | ftag "<=" number
|                   | "*"
|                   )
|
|      value  = token

       number = 1*DIGIT

|  Values are case-insensitive.   An example is:
|
|      Accept-Features: blex, !blebber, colordepth<=5, !screenwidth,
|                 UA-media={stationary}, paper=a4, !paper=a0, *
|
|  The different syntactical forms have the following meaning:
|
|     ftag     ftag is present, but with no values
|     
|     !ftag    ftag is absent
|
|     ftag=V   ftag is present with the value V
|
|     !ftag=V  ftag is present, but not with the value V
|   
|     ftag={V} ftag is present with the value V, and not with any
|              other values
|
|     ftag<=N  ftag is present with the numeric values from 0 up to
|              and including N, and not with any other values
|
|     *        makes true all feature predicates (Section 6.3) which
|              were not assigned truth values by other elements of the
|              header

   Absence of the Accept-Features header in a request is equivalent to
   the inclusion of

       Accept-Features: *


6.3 Feature predicates

   Feature predicates are used in the features attribute of a variant
   description.

|     fpred = [ "!" ] ftag [ "=" value ]

   Examples feature predicates are

|     blebber, !blebber, paper=a4, colordepth=5, !blex=54

   The network negotiation algorithm can compute the truth value of a
   feature predicate by using the contents of the Accept-Features
   header of the current request.

|  The truth value of a predicate is assigned as follows, depending on
|  its form:
|
|     ftag     true if the feature is present, false if the feature is
|              absent according to the Accept-Features header
|
|     !ftag    true if the feature is absent, false if the feature is
|              present according to the Accept-Features header
|
|     ftag=V   true if the feature is present with the value V,
|              false if the feature is not present with the value V
|              according to the Accept-Features header
|
|     !ftag=V  true if the feature is not present with the value V,
|              false if the feature is present with the value V
|              according to the Accept-Features header
|
|  If the Accept-Features header does not contain sufficient
|  information to assign a value using the above rules, then the value
|  is true if there is a "*" in the Accept-Features header, false
|  otherwise.

   As an example, the header

|      Accept-Features: blex, !blebber, colordepth<=5, !screenwidth,
|                UA-media={stationary}, paper=a4, !paper=a0, *

   makes the following predicates true:

|      blex, colordepth=4, !colordepth=6, colordepth, !screenwidth,
|      UA-media=stationary, paper=a4, !paper=a0, paper=a5
|      frtnbf, !frtnbf, frtnbf>=4, frtnbf<4

   and makes the following predicates false:

|      !blex, blex=0, blebber, colordepth=6, !colordepth,
|      screenwidth, screenwidth=640, !screenwidth=640
|      !UA-media=screen, paper=a0


----snip----

Note that the mechanism above allows a smooth transition between use
of `paper' as a single-value tag to its use as a multi-value tag.
This is crucial because, as a general rule, the first feature tags for
an area of negotiation will be registered _before_ this area is well
understood.  If there were no transition to use as a multi-value tag,
then an initial incorrect decision to create something as a
single-value tag could block progress far to easily.

Also note that the `<=' has moved from the feature predicate syntax to
the Accept-Features header syntax.  With both '<=' and '=' being legal
in the feature predicate syntax, I think that a CGI programmer could
too easily make the error of writing

  {features colordepth=8}

instead of the correct

  {features colordepth<=8} .

With '<=' in the Accept-Feature header, it will be up to user agent
authors to make the mistake of generating

  Accept-features: colordepth=8, *

instead of the correct

  Accept-features: colordepth<=8, *

and user agent authors are generally in a better position to find the
error through testing.

A final note: I'm open for suggestions for improving the "ftag={V}"
notation.  It is a bit yucky, but at least has some mnemonic value
(being the notation for a one-element set).  Other things I considered
were

  ftag:=V  ftag@V  ftag==V  ftag:V  ftag=V!


Koen.
Received on Monday, 12 August 1996 11:05:11 EDT

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