Re: [xsl] Avoiding boneheaded mistakes in XSLT?

Subject: Re: [xsl] Avoiding boneheaded mistakes in XSLT?
From: Michael Kay <mike@xxxxxxxxxxxx>
Date: Wed, 29 Dec 2010 23:36:35 +0000
Even without schema awareness/context, surely the processor can
tell when an xpath expression will result in zero match?

No, it's very rarely possible.

For example, consider

<xsl:template match="books">
<xsl:value-of select="book[1]/authour"/>

Even if you have a schema telling you that the children of books can include book but the children of book can't include authour, this doesn't tell you that this path will necessarily return nothing - because there is no way you can know that every books element that the stylesheet processes will be valid against this schema.

In fact it's quite common for a stylesheet to create intermediate results that (deliberately) aren't valid against the schema for the input or output document.

It would however be possible to make a guess that this is wrong, and issue a warning. Incorrect warnings are always very irritating, so this is not something to be done lightly, but it's something I've wanted to attempt for a long time.

Michael Kay

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