fn:filter

fn:filter(
    $seq as item()*, 
    $f as function(item()) as xs:boolean
) as item()*

This function is deterministic, context-independent, focus-independent, and higher-order.

The effect of the function is equivalent to the following implementation in XQuery:

declare function fn:filter(
        $seq as item()*,
        $f as function(item()) as xs:boolean)
        as item()* {
  if (fn:empty($seq))
  then ()
  else ( fn:head($seq)[$f(.) eq fn:true()], 
         fn:filter(fn:tail($seq), $f)
       )
};

or its equivalent in XSLT:

<xsl:function name="fn:filter" as="item()*">
  <xsl:param name="seq" as="item()*"/>
  <xsl:param name="f" as="function(item()) as xs:boolean"/>
  <xsl:if test="fn:exists($seq)">
    <xsl:sequence select="fn:head($seq)[$f(.) eq fn:true()], fn:filter(fn:tail($seq), $f)"/>
  </xsl:if>
</xsl:function>
         

As a consequence of the function signature and the function calling rules, a type error occurs if the supplied function $f returns anything other than a single xs:boolean item; there is no conversion to an effective boolean value.

The function call fn:filter($SEQ, $F) has a very similar effect to the expression $SEQ[$F(.)]. There are some differences, however. In the case of fn:filter, the function $F is required to return a boolean; there is no special treatment for numeric predicate values, and no conversion to an effective boolean value. Also, with a filter expression $SEQ[$F(.)], the focus within the predicate is different from that outside; this means that the use of a context-sensitive function such as fn:lang#1 will give different results in the two cases.

The expression fn:filter(1 to 10, function($a) {$a mod 2 = 0}) returns (2, 4, 6, 8, 10).

The expression fn:filter((), fn:lang("en", ?)) returns ().