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Last Call Comment: Mandate or prohibit short circuit evaluation of Boolean conditions

From: Tony Fletcher <tony_fletcher@btopenworld.com>
Date: Sat, 8 Jan 2005 01:18:25 -0000
To: "'Tony Fletcher'" <tony_fletcher@btopenworld.com>, <public-ws-chor-comments@w3.org>
Cc: <public-ws-chor@w3.org>
Message-ID: <001a01c4f51f$f3c71b80$6401a8c0@corp.choreology.com>
The CDL specification ( Web Services ChoreographyDescription Language
Version 1.0 W3C Last Call Draft 17 December 2004
<http://www.w3.org/TR/2004/WD-ws-cdl-10-20041217/>
http://www.w3.org/TR/2004/WD-ws-cdl-10-20041217/  ) should specifically
state whether the short circuit evaluation of Boolean conditions is mandated
or prohibited.  
 
Short circuit evaluation of Boolean conditions does not alter the value of
the condition but in CDL it can alter the time at which that condition
evaluation becomes available, which in turn can alter the evolution of a
choreography.  My concern is that if different endpoint projections make the
opposite assumption then erroneous behaviour could result.
 
Consider a Boolean condition of the form  Variable>100 AND
hasDurationPassed(2hours)
 
not exactly the correct syntax I am sure but hopefully you see the point.
With this condition as the value of a guard and block =true and assuming the
variable value is available immediately and is such that Variable>100 is
FALSE then if short circuit evaluation is mandated then the condition
evaluates to FALSE immediately and the choreography moves on.  However, if
short circuit evaluation is prohibited then the condition evaluates to FALSE
only after two hours has elapsed - very different behaviour in time, and as
other interactions may also be time dependent we have to remember that CDL
can yield descriptions with some dependency on time.
 
Therefore: (!!)
 
1)  Rationale: Short circuit evaluation of Boolean conditions should be
explicitly mandated or prohibited so that different tools, endpoint
projections, etc do not make contrary choices of how to evaluate.

 

Add to section 2.4.3 Expressions (or elsewhere if a better place can be
found):

 

Boolean conditional expressions MUST use short circuit evaluation techniques
whatever the setting of the block attribute.  That is once the value of a
conditional expression can be evaluated and the result will no longer change
if further variables become available or time based functions become true,
the conditional expression returns its value.

 

Note:  If it is thought this incorrect and we should not permit short
circuit evaluation for some good reason then we should specifically prohibit
it and add text similar to that above but stating MUST NOT instead.

 
 
Best Regards     Tony
A M Fletcher
 
Cohesions  (TM)
 
Business transaction management software for application coordination
www.choreology.com <http://www.choreology.com/> 
 
Choreology Ltd., 68 Lombard Street, London EC3V 9LJ     UK
Tel: +44 (0) 1473 729537   Mobile: +44 (0) 7801 948219
tony.fletcher@choreology.com     (Home: amfletcher@iee.org)
 
Received on Saturday, 8 January 2005 01:18:57 GMT

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