Proposal to require ECMAScript constructor objects for SVG types
================================================================


0. Introduction

It is currently cumbersome to create SVG type objects in script, since
the objects must be created using factory methods on an SVGSVGElement
instance, and these factory methods do not take arguments to initialise
the newly created object.  For example, if you wanted to create a new
SVGMatrix object to pass to the matrixTransform() method of an SVGPoint
p, then you need to use eight statements:

  var m = document.documentElement.createSVGMatrix();
  m.a = a;
  m.b = b;
  m.c = c;
  m.d = d;
  m.e = e;
  m.f = f;
  p.matrixTransform(m);

With an appropriate constructor, this can be reduced to one statement:

  p.matrixTransform(new SVGMatrix(a, b, c, d, e, f));

This proposal suggests having constructors for each of the SVG data type
interfaces.  These constructors can be specified using Web IDL.

I haven't specified constructors for the various SVGPathSeg interfaces.
I'm not sure how useful they would be, since the factory methods on
SVGPathElement all take parameters.

Note that currently Web IDL disallows overloading between a DOMString
and a float.  I'm considering relaxing that requirement, though.


1. SVGNumber

  [Constructor,
   Constructor(in float value)]
  interface SVGNumber {
    /* ...existing SVGNumber members... */
  };

SVGNumber()
  Returns a newly created SVGNumber object whose value attribute is set
  to 0, just as if SVGSVGElement.createSVGNumber() were called.

SVGNumber(in float value)
  Returns a newly created SVGNumber object whose value attribute is set
  to value.


2. SVGLength

  [Constructor,
   Constructor(in float value),
   Constructor(in short unitType, in float value),
   Constructor(in DOMString value)]
  interface SVGLength {
    /* ...existing SVGLength members here... */
  };

SVGLength()
  Returns a newly created SVGLength object not associated with any
  element, whose value attribute is set to 0, and whose unitType
  attribute is set to SVGLength.SVG_LENGTHTYPE_NUMBER.

SVGLength(in float value)
  Returns a newly created SVGLength object not associated with any
  element, whose value attribute is set to value, and whose unitType
  attribute is set to SVGLength.SVG_LENGTHTYPE_NUMBER.

SVGLength(in short unitType, in float value)
  <!-- The order of parameters here matches
       SVGLength.newValueSpecifiedUnits().  But I think the reverse
       makes more sense. -->
  Returns a newly created SVGLength object not associated with any
  element, whose value attribute is set to value, and whose unitType
  attribute is set to unitType.

  The effect of invoking this constructor is the same as running the
  following steps:

    1. Create a new SVGLength object L by inovking the zero-argument
       SVGLength constructor.
    2. Call the newValueSpecifiedUnits() method on L, passing unitType
       and value as its two arguments.
    3. Return L.

SVGLength(in DOMString valueAsString)
  Returns a newly created SVGLength object not associated with any
  element, whose valueAsString attribute is set to valueAsString.

  The effect of invoking this constructor is the same as running the
  following steps:

    1. Create a new SVGLength object L by inovking the zero-argument
       SVGLength constructor.
    2. Assign valueAsString to the valueAsString attribute on L.
    3. Return L.

  Note that the second step above may result in an exception being
  thrown.


3. SVGAngle

  [Constructor,
   Constructor(in float value),
   Constructor(in short unitType, in float value),
   Constructor(in DOMString value)]
  interface SVGAngle {
    /* ...existing SVGAngle members here... */
  };

SVGAngle()
  Returns a newly created SVGAngle object whose value attribute is set
  to 0, and whose unitType attribute is set to
  SVGAngle.SVG_ANGLETYPE_UNSPECIFIED.

SVGAngle(in float value)
  Returns a newly created SVGAngle object whose value attribute is set
  to value, and whose unitType attribute is set to
  SVGAngle.SVG_ANGLETYPE_UNSPECIFIED.

SVGAngle(in short unitType, in float value)
  Returns a newly created SVGAngle object whose value attribute is
  set to value, and whose unitType attribute is set to unitType.

  The effect of invoking this constructor is the same as running the
  following steps:

    1. Create a new SVGAngle object A by inovking the zero-argument
       SVGAngle constructor.
    2. Call the newValueSpecifiedUnits() method on A, passing unitType
       and value as its two arguments.
    3. Return A.

SVGAngle(in DOMString valueAsString)
  Returns a newly created SVGAngle object whose valueAsString attribute
  is set to valueAsString.

  The effect of invoking this constructor is the same as running the
  following steps:

    1. Create a new SVGAngle object A by invoking the zero-argument
       SVGAngle constructor.
    2. Assign valueAsString to the valueAsString attribute on A.
    3. Return A.

  Note that the second step above may result in an exception being
  thrown.


4. SVGPoint

  [Constructor,
   Constructor(in float x, in float y),
   Constructor(in DOMString valueAsString)]
  interface SVGPoint {
    /* ...existing members of SVGPoint... */
  };

SVGPoint()
  Returns a newly created SVGPoint object whose x and y attributes are
  both set to 0.

SVGPoint(in float x, in float y)
  Returns a newly created SVGPoint object whose x and y attributes are
  set to x and y, respectively.

SVGPoint(in DOMString coordinates)
  Returns a newly created SVGPoint object whose x and y attributes are
  set to values determined by parsing coordinates as a
  <coordinate-pair>.  The first coordinate parsed from coordinates is
  used as the x attribute value, and the second coordinate parsed from
  coordinates is used as the y attribute value.

  If coordinates could not be parsed as a <coordinate-pair>, then a
  DOMException with code SYNTAX_ERR is thrown.

  <!--
    Note that this would allow the following code to work:

      var coords = [12, -4.3];
      var p = new SVGPoint(coords);

    since the array stringifies to "12,-4.3".
    -->


5. SVGMatrix

  [Constructor,
   Constructor(in float a, in float b, in float c,
               in float d, in float e, in float f),
   Constructor(in DOMString components)]
  interface SVGMatrix {
    /* ...existing members of SVGMatrix... */
  };

SVGMatrix()
  Returns a newly created SVGMatrix object whose a, b, c, d, e and f
  attributes are all set to 0.

SVGMatrix(in float a, in float b, in float c, in float d, in float e, in float f)
  Returns a newly created SVGMatrix object whose a, b, c, d, e and f
  attributes are set to the values of the respective argument passed to
  the constructor.

SVGMatrix(in DOMString components)
  Returns a newly created SVGMatrix object whose a, b, c, d, e and f
  attributes are set to values determined by parsing components as
  a <matrix-components>.  <!-- Rework the grammar in section 7.6 to
  define the matrix transform in terms of a matrix-components. -->
  The first number parsed from components is used as the a attribute
  value, the second as the b attribute value, and so on.

  If coordinates could not be parsed as a <matrix-components>, then a
  DOMException with code SYNTAX_ERR is thrown.


6. SVGRect

  [Constructor,
   Constructor(in float x, in float y, in float width, in float height),
   Constructor(in DOMString value)]
  interface SVGRect {
    /* ...existing members of SVGRect... */
  };

SVGRect()
  Returns a newly created SVGRect object whose x, y, width and height
  attributes are all set to 0.

SVGRect(in float x, in float y, in float width, in float height)
  Returns a newly created SVGRect object whose x, y, width and height
  attributes are set to x, y, width and height, respectively.

SVGRect(in DOMString value)
  Returns a newly created SVGPoint object whose x and y attributes are
  set to values determined by parsing value as a <rect-components>.
  <!-- Make the 'viewBox' attribute in the coords chapter explicitly
  take a rect-components value. -->  The first number parsed from value
  is is used as the x attribute value, the second number is used as the
  y attribute value, and so on.

  If value could not be parsed as a <rect-components>, then a
  DOMException with code SYNTAX_ERR is thrown.


7. SVGTransform

  [Constructor,
   Constructor(in SVGMatrix matrix),
   Constructor(in DOMString value)]
  interface SVGTransform {
    /* ...existing members of SVGTransform... */
  };

SVGTransform()
  Returns a newly created SVGTransform object whose type attribute is
  set to SVGTransform.SVG_TRANSFORM_MATRIX and whose matrix attribute
  is set to an SVGMatrix object that represents the identity matrix.

SVGTransform(in SVGMatrix matrix)
  Returns a newly created SVGTransform object whose type attribute is
  set to SVGTransform.SVG_TRANSFORM_MATRIX and whose matrix attribute
  is set to an SVGMatrix object whose attributes are all initialized
  to be the same as the corresponding attributes in matrix.

SVGTransform(in DOMString value)
  Returns a newly created SVGTransform object whose type and matrix
  attributes are set to values determined by parsing value as a
  <transform>.

  If value could not be parsed as a <transform>, then a DOMException
  with code SYNTAX_ERR is thrown.

  The effect of invoking this constructor is the same as running the
  following steps:

    1. If value does not match the <transform> production, throw a
       DOMException with code SYNTAX_ERR.
    2. Create a new SVGGElement object, G.
    3. Set the 'transform' attribute to value.
    4. Let T be the SVGTransform object obtained by calling
       G.transform.baseVal.getItem(0).
    4. Return a new SVGTransform object whose attributes are set to
       the corresponding attributes in T.