[xquery-talk] some ... satisfies with positional variable

[Michael Kay]

> 
> Here's a question about an XQuery language construct I would 
> appreciate.

You seem to be overlooking that XQuery has XPath as a subset. These queries
can all be expressed by simple and efficient XPath 1.0 expressions.

> 
> in for-clauses it is possible to include a positional 
> variable and return
> the context node $fs:dot - or any other node - based on the 
> position of
> its child elements.
> 
> for $x at $pos in child::tag0 return
>     if ($pos > 3) then return $fs:dot

> 
> If the context node has more than 4 children, the above 
> expression will
> return it more than once. This could be avoided by wrapping 
> the result in
> a distinct-doc-order function, but it wouldn't prevent the 
> compiler from
> executing extra work, does it?

You seem to be searching for the expression

(.)[tag0[4]]

(The parentheses are needed in XPath 1.0 but not in XPath 2.0)

Any decent XPath processor is going to execute this very efficiently.

> 
> Without positional variables extra work can be evaded by 
> using the some
> ... satisfies ... clause:
> 
> if (some $x in child::* satisfies (fn:node-name($x) == "tag2")))
> then $fs:dot
> else ()

Actually, you want "=" or "eq" rather than "=="; and node-name() returns a
QName not a string. Rather than getting a node-name as a run-time string or
QName, it's better to do ($x[self::tag2]).

> 
> This works fine, but I cannot include a positional variable in the
> conditional. What I need is a "some ... at ... in ... satisfies ... "
> construct similar to the following:
> 
> epxr1 =
> 
> if (some $x at $pos in child::* satisfies ($pos > 3))
> then $fs:dot
> else ()

Again, you're forgetting positional predicates: you can write this as

if (child::*[position() gt 3])

or more simply

if (*[4]) then . else ()

or just

.[*[4]]

> 
> But this is not provided by XQuery. XPath expressions won't help since
> they're normalized to for, if-clauses, etc.

That's an odd way of looking at the world: XPath expressions like the ones
above have been around since 1999, long before for expressions and if
clauses were invented.

Michael Kay