[Xsltforms-support] XPath and whitespace

[C. M. Sperberg-M. <cm...@bl...>]

Over the last few years, as I've used XSLTForms, I have occasionally
stumbled over the way whitespace appears to be handled in XPath
expressions.  The XPath specs are not always as clear about it as they
might be (in the sense that if you just look at the EBNF rules, you'll
never discover the rule), but the general rule is that whitespace is
allowed between any two tokens of an XPath.

The last paragraph of section 1 in XPath 1.0 [1] says

  Expressions are parsed by first dividing the character string to be
  parsed into tokens and then parsing the resulting sequence of
  tokens. Whitespace can be freely used between tokens. The
  tokenization process is described in [3.7 Lexical Structure].

[1] http://www.w3.org/TR/xpath/#section-Introduction

In XPath 2.0 [2], the clearest statement is in the definition of
'ignorable whitespace':

  [Definition: Ignorable whitespace consists of any whitespace
  characters that may occur between terminals, unless these characters
  occur in the context of a production marked with a ws:explicit
  annotation, in which case they can occur only where explicitly
  specified (see A.2.4.2 Explicit Whitespace Handling).] Ignorable
  whitespace characters are not significant to the semantics of an
  expression. Whitespace is allowed before the first terminal and
  after the last terminal of a module. Whitespace is allowed between
  any two terminals. Comments may also act as "whitespace" to prevent
  two adjacent terminals from being recognized as one.

[2] http://www.w3.org/TR/xpath20/#IgnorableWhitespace

As I understand these rules, they have the consequence that

  .[not(instance('trial')/self::trial/*[name(.)=name(current())])]
			     ]
is grammatically the same as

  .[not(
      instance('trial')
      /self::trial
      /*
      [name(.) = name(current())]
    )
   ]

Since I found the first one a bit convoluted, as I was constructing
it, I really wanted to use the second form, which I find slightly
easier to check for correctness.  

But the XPath parser used in XSLTForms (at least through 1.0RC2)
seems to take the view that whitespace can be ignored only inside
predicates. So to make the expression just given work in a form, I end
up rewriting it without whitespace, or (if I insist on breaking the
expression into multiple lines) in some form like

   .[ not(instance('trial')/self::trial/*[
			     name(.) =  name(current())])
			     ]
    ]

Is there any chance of an improvement in this situation?  In
principle, it's easy to work around; in practice, I forget the
limitation again and again, when I'm working with long convoluted
binding expressions, and end up with mysterious failures until I
remember "oh, yes, I'm not allowed to have whitespace where I want".
So fixing this would definitely make XSLTForms easier to use, for at
least this one user.

best,

Michael Sperberg-McQueen

-- 
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* C. M. Sperberg-McQueen, Black Mesa Technologies LLC
* http://www.blackmesatech.com 
* http://cmsmcq.com/mib                 
* http://balisage.net
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