Changes

This [[Module Library|XQuery Module]] adds some useful higher-order functions, additional to the [[Higher-Order Functions]] provided by the official specification.

=Conventions=

| '''Summary'''

|This function is similar to [[Higher-Order Functions#fn:fold-left|fn:fold-left]], but it returns a list of successive reduced values from the left. It is equivalent to:

declare function hof:scan-left($input, $acc, $action) {

if(empty($input)) then $acc else (

)

};

|- valign="top"

| '''Examples'''

|

* Returns triangular numbers:

hof:scan-left(1 to 10, 0, function($a, $b) { $a + $b })

|}

| '''Summary'''

|Returns the {{Code|$k}} items in {{Code|$input}} that are greatest when sorted by the result of {{Code|$key}} applied to the item. The function is a much more efficient implementation of the following scheme:

(for $item in $input

order by $key($item) descending

return $item

)[position() <= $k]

|- valign="top"

| '''Examples'''

* {{Code|hof:const(42, 1337)}} returns {{Code|42}}.

* With higher-order functions:

let $zip-sum := function($f, $seq1, $seq2) {

sum(for-each-pair($seq1, $seq2, $f))

$sum-left((1, 1, 1, 1, 1), 1 to 5)

)

* Another use-case: When inserting a key into a map, {{Code|$f}} decides how to combine the new value with a possibly existing old one. {{Code|hof:const}} here means ignoring the old value, so that's normal insertion.

let $insert-with := function($f, $map, $k, $v) {

let $old := $map($k)

$ins($map, 'foo', 42)('foo')

)

returns {{Code|3 42}}

|}