[Jastadd] Feature request: avoiding list rewrites when unnecessary

From: Jesper Mattsson <jesper.mattsson_at_modelon.com>
Date: Mon, 22 Oct 2012 09:41:49 +0000

Hi all

Given a list X*, both getXList() and addX() will trigger a rewrite of each child in the list. As far as I can tell, this is because a rewite can be declared for a specific list that returns a list of elements, that will then be inlined into the original list. I can see that the general behavior of List must keep these rewrites of all its children, even though they are only needed in special cases. However, since those cases can be detected at compile time, it should be possible to generate a use of getChildNoTransform() to get the list unless the rewrite of the list is needed.

Or am I missing some case where rewriting the entire list is needed?

One example where the list rewrites can create problems is:
A ::= B* /*... etc */;

And A is created by the code:
A a = new A();
for (BCreator b : getBCreators())
// ... fill in the rest of A

This approach seems very reasonable to the user, and is useful when there are subclasses that share some of that code, but not all. This will, however, create two problems:

1. Since the A node isn't final yet, the B list won't be final either. Thus each call to addB() will trigger a rewrite of the list, and in turn an attempt to rewrite each child already in the list. This will have an O(n2) complexity, which for us have proven significant for large models.

2. If B has a rewrite that uses inherited attributes that are not defined on A, then this code will crash with a NullPointerException.

Of course a simple solution is to use addBNoTransform() instead, but I feel that that this encourages users to routinely use the NoTransform versions of these methods - a habit that strikes me as potentially harmful.


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Phone direct: +46 73 324 5909
Email: jesper.mattsson_at_modelon.com<mailto:jesper.mattsson_at_modelon.com>

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Received on Mon Oct 22 2012 - 11:42:09 CEST

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