[xquery-talk] How much laziness is permitted?

John Snelson john.snelson at oracle.com
Thu Mar 12 10:11:38 PST 2009

Florent Georges wrote:
> John Snelson wrote:
>> try { 1, 2, 3, error() } catch * { "doh!" }
>> Typically, lazy evaluation will start returning items from
>> expressions one by one as they are requested. So lets say I
>> request an item from the expression above, and it gives me
>> 1. I do this a couple more times and it gives me 2 and 3.
>> Then the next time I request an item, the error() function
>> raises an error - and the implementation says "Oh -
>> wait a minute! Give me back those answers, I didn't mean
>> them - I meant to return 'doh!'.".
>   Thanks.  But I still fail to see the exact problem.  If you use the whole sequence, or its 4th item, then lazy evaluation is not concerned if I am right.  And if you just use its 1st item (for instance) then an implementation is allowed to NOT report the error if it does not want to (if I am right.)

If you only need three items from the expression above, then XQuery 1.1 
says it's valid to return (1, 2, 3). However it's unusual for an XQuery 
implementation to ask an expression for 3 items - it more likely asks 
for the "next" item three times.

In this latter case, given that in general the implementation won't know 
if an error will be raised or not, the implementation is forced into 
eager evaluation of the try expression.


John Snelson, Oracle Corporation            http://snelson.org.uk/john
Berkeley DB XML:            http://oracle.com/database/berkeley-db/xml
XQilla:                                  http://xqilla.sourceforge.net

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