- From: Michael Fuller <msf@io.mds.rmit.edu.au>
- Date: Sat, 5 Feb 2000 14:10:06 +1100
- To: www-xpath-comments@w3.org
- Cc: xsl-editors@w3.org
Two important characteristics of the XPath evaluation model can only be found in the XSLT Recommendation. 1. XSLT 14.2, para 2 states that: "If such an extension function occurs in an expression and the extension function is actually called, the XSLT processor must signal an error. An XSLT processor must not signal an error merely because an expression contains an extension function for which no implementation is available." Note the wording: "If [...] the extension function is actually called." This implies that XPath engine of an XSLT processor must allow expressions such as: true() or an-unknown-function() para[false()][an-unknown-function()] to be _evaluated_ because the function "an-unknown-function()" can never be called. If the intention of the XSLT and XPath editors is to forbid this, the wording should be changed to: "If such an extension function occurs in an expression and the _expression_ is actually _evaluated_, the XSLT processor must signal an error." making the above would be illegal. If the intention of the XSLT and XPath editors is to allow the currently-documented behaviour, then I suggest that it should be documented in the XPath Recommendation, rather than in the XSLT Recommendation, that function resolution and, by extension, function argument type-checking, must be lazily evaluated; it is inappropriate to leave this to be discovered in the dependent Recommendation. 2. The definition of the extension function "system-property()" [XSLT 12.4] as: "object system-property(string)" implies that the XPath data model is based on a dynamic type system. Unfortunately, there is no hint in XPath that developers must cater for dynamic typing; indeed, system-property() is at present that only aspect of either XPath and XSLT that would require it. Given that the requirement to support dynamic typing imposes quite distinct design and implementation constraints on the development of an XPath engine, again it seems inappropriate to leave this fact to be discovered in the Recommendation that may well not be implemented until after completion of an XPath engine. Michael Fuller ____________________________________________ http://www.mds.rmit.edu.au/~msf/ Multimedia Databases Group, RMIT, Australia.
Received on Friday, 4 February 2000 22:10:21 UTC