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Semantics of xmap on a null value [message #1754585]

Mon, 20 February 2017 14:59

Rolf Theunissen

Messages: 257
Registered: April 2012

Senior Member

Hi,

I wonder what the semantics should be of calling a xmap on a null value? For instance as in the example below. Should this raise an exception or should this return an invalid value? The current QVTo implementation returns 'invalid'. The specification only talks about checking the 'when' precondition. Though, I would not expect 'invalid' values to be returned by xmap.

Kind Regards,
Rolf Theunissen

modeltype ecore "strict" uses ecore('http://www.eclipse.org/emf/2002/Ecore');

transformation NewTransformation();

main() {
    var a : EAttribute;
    var b := a.xmap toEClass();
    log ("b", b);
}

mapping EAttribute::toEClass() : EClass
{}

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Re: Semantics of xmap on a null value [message #1754590 is a reply to message #1754585]

Mon, 20 February 2017 15:24

Ed Willink

Messages: 7467
Registered: July 2009

Senior Member

Hi

Doing things with null is generally BAD. Bad things in OCL lead to an invalid value. Bad things in QVTo lead to an exception.

The new QVT 1.3 8.1.20 wording doesn't help that much: Every use of a source value by an ImperativeExpression has an implicit assertion that the source value is not invalid,
therefore using invalid is an assertion failure.

But I'm inclined to take the view that the "a.xmap" is a QVTo extension that emulates an OCL navigation and so returns invalid. Therefore in your example any use of the invalid "b", should throw an exception. But of course "log" is perhaps an "access" rather than "use" and so we don't want addition of a log to a program to add an exception. http://issues.omg.org/browse/QVT14-49 raised.

Regards

Ed Willink

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