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   Re: [xml-dev] [XQuery] FLWOR (for-let-where-order-return) equivalentto F

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Roger L. Costello wrote:

>Hi Folks,
>I am wondering about the value of the "let" and "where" clause in the XQuery
>FLWOR expression.  I believe that the same capability is possible simply by
>using a for-order-return expression.  If that is true, then I wonder about
>the value of FLWOR (recent discussions seem to imply that  FLWOR is one of
>the "key advantages" of XQuery).
>Let me give a couple of examples.
>Example 1.  Let's start with a simple example.  Consider this XML document:
><?xml version="1.0"?> 
>    <Number>0</Number> 
>    <Number>8</Number> 
>    <Number>23</Number> 
>    <Number>17</Number> 
>    <Number>5</Number> 
>    <Number>19</Number> 
>    <Number>44</Number> 
>    <Number>13</Number> 
>    <Number>78</Number> 
>    <Number>21</Number> 
>    <Number>2</Number> 
>    <Number>1</Number> 
>    <Number>15</Number> 
>    <Number>67</Number> 
>    <Number>99</Number> 
>    <Number>14</Number> 
>    <Number>8</Number> 
>    <Number>33</Number> 
>    <Number>50</Number> 
>Problem: output all the numbers less than 20, in ascending sorted order.
>Here's how to do it using for-where-order-return:
>for $i in //Number 
>    where number($i) <= 20 
>        order by number($i) ascending 
>            return $i/text()

>Is there something that can be done with for-let-where-order-return which
>simply cannot be done with for-order-return?  If not, then what value is
>for-let-where-order-return?  Is it simply "syntactic sugar"?  /Roger 
for $i in //Number 
    let $y = number($i) * number($i) in
    where number($y) <= 20 
        order by number($i) ascending 
            return $i/text()

It's possible but clumsy to achieve the same without let (imagine $y was 
something more complicated). In most programming languages, "let" and 
"where" are just there to make things more readable. If $y is used in 
several places, an advantage can be that you evaluate it only once.

There are a number of equivalences regarding for-comprehensions and 
XPath expressions - see Mary Fernandez, Jerome Simeon and Phil Wadler's 
XQuery algebra paper "A semi-monad for semistructured data". In 
principle, if one knows which alternative evaluates faster, one can use 
these equivalences for optimization.


Burak Emir



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