[xquery-talk] why must one have something inside {} ?

G. Ken Holman gkholman at CraneSoftwrights.com
Sat Dec 3 05:29:01 PST 2011

At 2011-12-03 21:20 +0800, jidanni at jidanni.org wrote:
>Gentlemen, what a spiteful language xquery is, not allowing an empty
>statement. I want to "comment out" a line,
>  <Document>
>  {doc("p.xml")//STYLES/*} <--- Let's comment out this line
>  <Document>
>  {(: doc("p.xml")//STYLES/* :)}
>BaseX makes [XPST0003] Expecting valid expression after "{".
>  <Document>
>  <!-- {doc("p.xml")//STYLES/*} -->
>Works but ends up in output.
>It's all because {} for some reason needs to be filled with some junk.
>OK, adding "" works:
>  <Document>
>  {""(: doc("p.xml")//STYLES/* :)}

In XPath syntax, the empty string is not a valid expression:


Your use of curly braces has to be populated with an XPath 
expression.  An XPath expression is evaluated after comments have 
been removed.

   "Comments are lexical constructs only, and do not affect expression

Thus you need *something* to prevent the contents of the curly braces 
to be empty after removing comments.

I tell my students to use () rather than "".  This is an explicit 
lesson in the class because of the intuition that a comment is an 
expression, when it isn't.

I hope this explanation helps.

. . . . . . . . .  Ken

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