xsl:xpath instruction

[michaelhkay]

In the case study #1786 (comment) I encountered a use case where an instruction xsl:xpath would be useful.

<xsl:xpath>
   { "class":
       { "name": f:degenerify(name/@identifier),
          "abstract": ? abstract,
           "extends": array{? extendedTypes =!> map:merge(apply-templates()) },
           "implements": array{? implementedTypes =!> map:merge(apply-templates()) }
             ...
       }
  }
 </xsl:xpath>

The instruction is very simple: <xsl:xpath>EXPR</xsl:xpath> is equivalent to <xsl:sequence select="EXPR"/>. It's particularly useful because XPath constructors for maps and arrays are so much more concise than the XSLT equivalents. Compared with using xsl:sequence, it means that:

• XML attribute value normalization doesn't kick in, so your formatting is better protected (meaning also that the system has some chance of computing line numbers correctly for diagnostics)
• You haven't tied up either single or double-quotes as an attribute delimiter; both can be freely used within the expression.
• You aren't creating the false impression that you're returning a (multi-item) sequence

Note that the content is NOT a sequence constructor; no child elements are allowed; and the content is not interpreted as a text value template. Unlike xsl:evaluate, the XPath expression is statically fixed.

(This example also introduces apply-templates as a function, but that will be a separate proposal).