Shallow copy in XSLT with maps and arrays

[michaelhkay]

Currently the xsl:copy instruction, if applied to a map or array, does a deep copy, and ignores the content of the contained sequence constructor.

I propose that if the contained sequence constructor is non-empty then instead of ignoring it, we should use it to create the content of the new map or array. Specifically, for maps xsl:copy will behave essentially like xsl:map, and for arrays it will behave essentially like xsl:array.

This is an incompatibility with 3.1, but since a contained sequence constructor is currently totally useless in this situation, it doesn't seem likely to cause any trouble.

I also propose that rather than using the new built-in on-no-match="shallow-copy-all, we should extend the semantics of shallow-copy to cover maps and arrays (as currently defined for shallow-copy-all). Again, there is an incompatibility, but the current rules are so unhelpful that it's unlikely people are relying on them.

I also propose that when apply-templates is applied to a map or array, it should be automatically pinned if it is not pinned already. The means that match patterns can be used with a lot more context to match the deep contents of the map or array and override the processing of the built-in templates.

And I propose that when apply-templates is applied to a map or array and there is no select attribute, it should "do the right thing" by applying templates to the map or array contents, rather than using the useless default of child::node().

[michaelhkay]

As regards the built in template rule for shallow-copy with maps and arrays:

• The first principle is that if you don't write any template rules at all, you always end up with a (deep) copy of what you started with.

• It's a general principle that within a mode, all the template rules should return similar values so that they can be used interchangably.

• The most likely way for users to override this processing is with template rules of the form match="?key" as proposed in XSLT templates: Matching values in a map by key #1795.

The built-in template needs to do something like this:

* If processing an item with an associated key (in its label)
** Generate a map entry having the same key
** If the value is a map, apply-templates with select = ?* to construct the entry
** if the value is an array, do <xsl:array><xsl:apply-templates select="?*/><xsl:array>
** otherwise, copy the value to the new entry
* If processing an item with no associated key
** If the value is a map,  do <xsl:map><xsl:apply-templates select="?*/><xsl:map>
** if the value is an array, do <xsl:array><xsl:apply-templates select="?*/><xsl:array>

That's a significant departure from the current spec.

[michaelhkay]

See also #1238 which complements this line of attack.