why must one have something inside {} ?

[G. Ken Holman]

At 2011-12-03 21:20 +0800, http://x-query.com/mailman/listinfo/talk 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:

   http://www.w3.org/TR/2007/REC-xpath20-20070123/#prod-xpath-XPath

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

   http://www.w3.org/TR/2007/REC-xpath20-20070123/#comments
   "Comments are lexical constructs only, and do not affect expression
    processing."

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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