From dougo@ccs.neu.edu Mon Sep  8 18:53:01 1997
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Date: Mon, 8 Sep 1997 18:52:42 -0400
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From: Doug Orleans <dougo@ccs.neu.edu>
To: Karl Lieberherr <lieber@ccs.neu.edu>
Cc: dem@ccs.neu.edu, lopes@parc.xerox.com
Subject: Re: explicit strategy rule
In-Reply-To: <199709041317.JAA06366@stockberg.ccs.neu.edu>
References: <199709041317.JAA06366@stockberg.ccs.neu.edu>
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Status: R

Karl Lieberherr writes:
 > Crista still thinks that the Explicit Strategy Rule should be the default.
 > Do you have examples which show that this would be too restrictive?
 > Show some strategy/class graph combinations. For example, for
 > the strategy graph
 > 
 > {A->B
 >     B->C}
 > 
 > why is it useful to have a class graph which has an A between B and C.

How about this:

    NestedShape : Circle | Square *common* NestedShape.

Now, say you want find all circles that are inside squares:

    NestedShape {
	traversal allCirclesInsideSquares(SquaredCircleVisitor) {
	    via Square to Circle;
	}
    }

This will go to all Circles that are inside Squares. This will also go
to outermost Circles, but once we have the capability to qualify a
wrapper with a particular strategy edge, we can tell the difference:

    SquaredCircleVisitor {
	before Circle : NestedShape->Square
	    (@ println("outermost circle"); @)
	before Circle : Square->Circle
	    (@ println("squared circle"); @)
    }

If this were an Explicit Strategy, i.e.

    { NestedShape -> Square bypassing { NestedShape, Square, Circle },
      Square -> Circle	    bypassing { NestedShape, Square, Circle } }

then we could only visit shapes whose outermost shape was a square.

I'm sorry this example is so contrived, but to be honest I don't have
much intuition when it comes to class graphs containing cycles.

--Doug