Conditional output¶

A recurring annoyance in 1.0 is the empty wrapper: you build a <discounts> element, the source happens to have no discounts, and you emit <discounts/> anyway — or you wrap every constructor in an xsl:if that duplicates the selection logic. XSLT 3.0 adds three instructions that make output depend on whether other output was produced, removing that duplication.

xsl:where-populated — drop empty elements¶

xsl:where-populated evaluates the content inside it and discards any element it constructed that ended up with no attributes and no children. Wrap a maybe-empty element in it and the element simply disappears when there is nothing to put inside:

<xsl:where-populated>
  <expensive>
    <xsl:for-each select="cd[xs:decimal(price) > 10]">
      <cd>{title}</cd>
    </xsl:for-each>
  </expensive>
</xsl:where-populated>

If at least one CD is over 10, you get <expensive><cd>…</cd></expensive>. If none are, the <expensive> wrapper is omitted entirely — no <expensive/>, and no separate xsl:if test="cd[...]" repeating the predicate.

xsl:on-empty / xsl:on-non-empty — react to the result¶

These two are evaluated relative to the other items in the same sequence constructor. They let one constructor say "if the rest of this produced output, add a header; if it produced nothing, say so instead":

<table>
  <xsl:on-non-empty>
    <tr><th>Title</th><th>Price</th></tr>          <!-- header: only if rows exist -->
  </xsl:on-non-empty>

  <xsl:for-each select="cd[xs:decimal(price) > 10]">
    <tr><td>{title}</td><td>{price}</td></tr>
  </xsl:for-each>

  <xsl:on-empty>
    <tr><td>No expensive CDs.</td></tr>            <!-- fallback: only if no rows -->
  </xsl:on-empty>
</table>

The rule:

• xsl:on-non-empty contributes its content only if the other items in the constructor produced a non-empty result. Its own output does not count toward that test — so a table header never fools the check into thinking there are rows.
• xsl:on-empty contributes its content only if the other items produced nothing.

So the table above gets a header row and the data rows when there are matches, and just the "No expensive CDs." row when there are none — the classic report-with-fallback, without a guarding xsl:choose.

Where to go next¶

• Conditionals — xsl:if and xsl:choose, when the test is on input, not output.
• Modern identity and text — text value templates, used in the examples above.
• Grouping — often the source of the maybe-empty groups these instructions tidy up.