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[css3-*] Review of functional syntax in CSS

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Thu, 19 Jan 2012 18:01:52 -0800
Message-ID: <CAAWBYDC7LZZunUH_OP1kS_2abMvFC51gCZfrq-CJbJ-tjxTB3g@mail.gmail.com>
To: www-style list <www-style@w3.org>
Fantasai and I spent the day reviewing the syntax of all the functions
currently defined in CSS, per ACTION-413.  We posted our results and
recommendations on the wiki at
<http://wiki.csswg.org/ideas/functional-notation>.  They are
reproduced below:


General Principles
==================

1. Functions group/namespace a set of CSS-like values, and so should
abide by general CSS value syntax principles
  1.1 Optionality is handled per #3, as far as possible
  1.2 Ordering should be flexible as much as possible/prudent
2. Lists of parallel items are comma-separated
3. Lowest operator is comma
4. Backwards compat should be preserved unless there's a very good
reason otherwise


Rounding Functions
==================

  Before
    ''roundup(<modulus>)'' (automatically applied to width/height
    ''only) also ''rounddown()'' and ''round()''
  After
    ''roundup(<css-value>, <modulus>)''
  Rationale
    Generalizing the round functions seems useful for calc() and other
    places.  The order "value then modulus" matches basically every
    programming language.  However, you need 1-token lookahead if you
    omit the comma, because the <css-value> could be any number of
    arbitrary tokens.  Also, the common representation of this in math
    uses commas.


Transforms
==========

  Before
    ''matrix(<number>, <number>, <number>, <number>, <number>,
    ''<number>)''
  After
    ''matrix(<number>{2}, <number>{2}, <number>{2})''
  Rationale
    Six comma-separated numbers make it difficult to discern the
    structure of the matrix: is it 1x6, 2x3, 3x2, or 6x1?  Other
    languages such as Matlab use different separators for numbers-
    in-a-row and rows-in-a-matrix (Matlab uses spaces and semicolons).
    We should match.  (Our matrices are column-major, but the point
    stands.)
  Extra Note
    The same reasoning applies, much more strongly, to the 4x4 3d
    matrix with 16 comma-separated values.



  Before
    ''translate(<x>, <y>)''
  After
    ''translate(<x> <y>)''
  Rationale
    The comma isn't needed for grouping or disambiguation. Other
    places in CSS that accept an x and y length space-separate, like
    'border-spacing' and 'background-position'.  (Principle 2)
  Extra Note
    The same applies to ''scale()''.


Animations
==========

  Before
    ''steps(<number> [, [start | end]]? )''
  After
    ''steps(<number> && [start | end]?)''
  Rationale
    The comma isn't needed for grouping or disambiguation.  The
    ordering constraint can also be relaxed without ambiguity.
    (Principles 1.1 and 2)


  Before
    ''cubic-bezier(<number>, <number>, <number>, <number>)''
  After
    ''cubic-bezier(<number>{2}, <number>{2})''
  Rationale
    Similar to ''matrix()'', the value here is two pairs of numbers,
    not four numbers, and so the grouping should reflect that.
    Positions are space-separated in CSS (though this is obviously a
    restricted form of "position").


Color
=====

  No change to Color as part of this effort.  (We believe that
  percentages should be usable for opacity, and angles for hue, but
  those will be pursued as part of Colors 4.)


Exclusions
==========

  Before
    ''rectangle(<length>, <length>, <length>, <length> [, [<length>,]
    ''<length>])''
  After
    ''rectangle(<length>{4} [<'border-corner-shape'> <length>{1,2}]?
    '')''
  Rationale
    SVG makes all commas optional.  It's most common to see viewBox
    specified with no commas at all.  As well, ''rect()'' in CSS
    already allows space separation.  The addition of border-corner-
    shape allows greater flexibility in the future and serves to make
    it clearer what the trailing 1 or 2 lengths mean.
  Extra Note
    We recommend shortening the name to ''rect()'', and unifying with
    the 'clip' value.


  Before
    ''circle(<length>, <length>, <length>)''
  After
    ''circle(<length>{3})''
  Rationale
    No need for commas for grouping or disambiguation.  Dropping
    commas is consistent with ''rectangle()''.


  Before
    ''ellipse(<length>, <length>, <length>, <length>)''
  After
    ''ellipse(<length>{4})''
  Rationale
    Same as ''circle()''


  Before
    ''polygon([<fill-rule>,]? [<length>, <length>]# )''
  After
    ''polygon([<fill-rule>,]? <length>{2}# )''
  Rationale
    Same as ''cubic-bezier()'', but moreso - a list of points should
    be indicated with different separators between the components and
    the points, or else a long list becomes *unreadable* without
    writing conventions or manual counting.  ''<fill-rule>'' doesn't
    need a comma for disambuation, but we left it in due to Principle
    3 and to match the gradient functions.


Fonts
=====

  No change suggested to any functions in Fonts - they all accept either
  a single argument, or a comma-separated list of parallel items.


GCPM
====

  We **would** recommend removing the commas from ''target-text()'', as
  they're not necessary for disambiguation or grouping, but we think it
  is more valuable to match the syntax of ''target-counter()'' (which is
  constrained by the syntax of ''counter()'') for consistency.


Grid
====

  No change suggested to any functions in Grid.  We would suggest
  removing the comma from ''minmax()'', as it's not necessary for
  disambiguation or grouping, but we feel this is a math-like function
  and, like ''round()'', may be more natural to see with commas.


Images
======

  No change suggested to any functions in Image Values. (''image()''
  takes a comma-separated list of parallel arguments, ''element()''
  takes a single argument, and the gradient functions already follow the
  principles above)


Template
========

  Same as Grid.


Lists
=====

  No change suggested to Lists.  We would suggest removing the commas
  from ''counter()'' and ''counters()'', but back-compat dictates they
  stay the same unless there's a good reason to change them.  Given
  that, we don't feel this change is significantly helpful enough to
  justify itself.  (''symbols()'' already follows the principles above.)


Position
========

  We suggest that ''rect()'' be defined such that space separation MUST
  be accepted.  We believe this matches all major UAs.  Also, maybe
  align with the suggestions for ''rectangle()'' from Exclusions.


Values & Units
==============

  Before
    ''attr(<name>[, <type> [, <default>]?]?)''
  After
    ''attr(<name> <type>? [, <default>]?)''
  Rationale
    The comma between the name and type was not necessary for
    disambiguation or grouping.  Removing it more closely represents
    the association between the name and the type, and lets the comma
    operate in its traditional role as a fallback operator (Principle
    2).  Finally, it lets the author omit type but specify default,
    which was not possible in the old grammar due to ambiguity.
Received on Friday, 20 January 2012 02:02:42 GMT

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