[Grace-core] Inheritance in Javascript version

[Kim Bruce]

Hi,

We seem to have a serious problem with inheritance in the Javadraw version of Grace.  It appears that to access an inherited feature in a class, you must prefix it be self.  Worse luck, when you are using a method inherited in a subclass, and then call it externally, then it doesn't find the correct method.

The following code works fine in the C version of the compiler, but breaks twice in Javascript (note that it uses the objectdraw library in directory Objectdraw2013 in the main repository.

// BoxBall
import "objectdraw" as od
import "random" as rand

// Application that plays boxball game
def BoxBall:od.GraphicApplication = object {
   inherits od.aGraphicApplication.size(500,600)

   def boxStartPosn:od.Location = od.aLocation.at(50,80)
   def theBox: od.Resizable2DObject = 
      od.aFilledRect.at(boxStartPosn)size(40,40)on(canvas)

   theBox.width:=70               // <<<====== Line 13

   // required to pop up window and start graphics
   self.startGraphics

}

==================================
The first error message in the javascript version is that it does not recognize "canvas".  This is fixed by replacing it by "self.canvas".  Canvas is a (readable) variable in the superclass, and hence should be accessible without using "self".

Even more troubling is that with this fix, the program breaks at runtime at the line:
     theBox.width := 70

The error message is:
"Error around line 674: RuntimeError: No such method 'theWidth:=' on Object. Did you mean the local theWidth:=? It is not annotated readable.
  From call to Object.width:= at line 13"

Line 674 of objectdraw is the marked line below:

// abstract class to be extended for 2 dimensional objects that can be 
// resized.
class aResizable2D.at(location':Location)size(width':Number,height':Number)
                   on(canvas':Canvas)  -> Resizable2DObject{
   inherits aDrawable2D.at(location')size(width',height')on(canvas')

   method width:=(w:Number){
      theWidth := w                          // <<<===== line 674
      setStateChanged
   }
..........

====
The class aFilledRect (which generates theBox) inherits from this class:

class aFilledRect.at(location':Location)size(width':Number,height':Number)
           on(canvas':Canvas) -> Resizable2DObject{
   inherits aResizable2D.at(location')size(width',height')on(canvas')

It does NOT override method width:=

Unfortunately this is catastrophic error as it means I cannot use Jameson's animator module to write the demo and lab programs for my course.  I would be grateful if someone could take a look at this.

Thank you!

Kim