Higher-order Functions
Passing an inline function as parameter
xquery version "3.0";
declare namespace ex="http://exist-db.org/xquery/ex";
declare function ex:apply($func, $list) {
for $item in $list return $func($item)
};
(: Create an inline function and assign it to $f2 :)
let $f2 := function($a) { upper-case($a) }
return
ex:apply($f2, ("Hello", "world!"))
Using a named function reference
xquery version "3.0";
declare namespace ex="http://exist-db.org/xquery/ex";
declare function ex:apply($func as function(item()) as item()*, $list) {
for $item in $list return $func($item)
};
(: Use function reference literal to find function at compile time :)
let $fApply := ex:apply#2
return
$fApply(function($a) { upper-case($a) }, ("Hello", "world!"))
Using a dynamic function lookup
xquery version "3.0";
declare namespace ex="http://exist-db.org/xquery/ex2";
declare function ex:fold-left(
$f as function(item()*, item()) as item()*,
$zero as item()*,
$seq as item()*) as item()* {
if (fn:empty($seq)) then $zero
else ex:fold-left($f, $f($zero, $seq[1]), subsequence($seq, 2))
};
(: Function reference is resolved dynamically at runtime :)
let $foldLeft := function-lookup(xs:QName("ex:fold-left"), 3)
return
$foldLeft(function($a, $b) { $a * $b}, 1, 1 to 5)
Basic higher-order functions
The default function library provides a number of basic functions which take function items as arguments:
- for-each
- filter
- fold-left
- fold-right
- for-each-pair
fn:for-each
fn:filter
Closures
Inline function using variable from local context
xquery version "3.0";
declare function local:apply($names as xs:string*, $f as function(xs:string) as xs:string) {
for $name in $names
return
$f($name)
};
let $string := "Hello "
let $f := function($name as xs:string) {
(: $string will be copied into the functions context :)
$string || $name
}
return
local:apply(("Hans", "Rudi"), $f)
Partial function application
When passing functions as arguments, it is often useful to be able to set certain fixed parameters when creating the function reference. XQuery 3.0 allows this using ? as an argument placeholder.