[DFDL-WG] assert and discriminator - no more before/after

[Steve Hanson]

Hi Mike

I believe the rule is that an assert/discriminator expression is evaluated 
"as soon as it can be". In IBM DFDL, we try to evaluate an expression when 
it is encountered, and if it can't be evaluated at that time because it 
references element(s) that do not appear in the infoset, the parser 
continues but the expression manager registers an interest in the 
element(s). The expression manager gets notified when the element(s) 
appear in the infoset and the expression is re-evaluated at that time. If 
the parser gets to the end of the scope for the assert/discriminator and 
it is still not evaluated, it is an error. (Tim please correct any 
mis-information).

Regards

Steve Hanson
Architect, Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh at uk.ibm.com
tel:+44-1962-815848



From:   Mike Beckerle <mbeckerle.dfdl at gmail.com>
To:     dfdl-wg at ogf.org
Date:   13/09/2012 16:32
Subject:        [DFDL-WG] assert and discriminator - no more before/after
Sent by:        dfdl-wg-bounces at ogf.org




I am looking in the spec for guidance about the evaluation order of assert 
statements.

We used to have before/after control properties, but eliminated them.

If I annotate a simpleType'd element with an assert that says { . eq 'x' 
}, that of necessity references the current value, so must execute after 
the value has been computed.

If on the other hand I annotate a complexType element with a discriminator 
that says { ../flag eq 'C1' } then this of necessity must execute before I 
go after the contents because the whole point is to evalutate the 
discriminator first.

Did we ever articulate exactly what the rules are here about order of 
evaluation?

Thanks for reminders

..mikeb

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Mike Beckerle | OGF DFDL WG Co-Chair 
Tel:  781-330-0412
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