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[css3-values] proposal for handling division and units in calc() [ACTION-205]

From: L. David Baron <dbaron@dbaron.org>
Date: Sun, 25 Jul 2010 11:48:59 -0700
To: www-style@w3.org
Message-ID: <20100725184859.GA13032@pickering.dbaron.org>
In http://lists.w3.org/Archives/Public/www-style/2010Jan/0468.html I
was given an action item to write a proposal for how to handle
division and units inside calc().  The questions this proposal needs
to resolve are:

  (a) what values are accepted as the leaf values inside a calc()
      expression,

  (b) what units can be divided inside a calc expression, and

  (c) if calc() allows division by values, how is division by zero
      handled?

My tentative conclusion is that supporting division by values is not
among the top use cases for calc(), and since it poses considerable
difficulty for both specification and implementation, we should not
support division by values in the first level of the calc()
specification.

Therefore, I propose that:

 1. We drop the 'mod' operator since it only makes sense when
    both operands have the same units.

 2. The right operand to the '/' operator be required to be unitless
    (that is, a number or an expression composed of numbers).

 3. Division by zero be a parse error (as I think it was in earlier
    drafts of the specification, perhaps?).

 4. Leaf values inside a calc() expression must all be either (a)
    numbers or (b) legal values for the property, or things that
    would be legal values if it weren't for range restrictions (such
    as the restriction that 'width' accepts only positive values).

-David

-- 
L. David Baron                                 http://dbaron.org/
Mozilla Corporation                       http://www.mozilla.com/
Received on Sunday, 25 July 2010 18:49:29 GMT

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