Returns those items from the sequence $seq for which the supplied function $f returns true.
fn:filter( $seq as item()*,$f as function(item()) as xs:booleanitem()*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), $f)"/>
</xsl:if>
</xsl:function>The expression fn:filter(1 to 10, function($a) {$a mod 2 = 0}) returns (2, 4, 6, 8, 10).
The expression fn:filter((), fn:lang("en", ?)) returns ().
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.