Difference between revisions of "Higher-Order Functions Module"
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* <code>hof:const(42, 1337)</code> returns <code>42</code>. | * <code>hof:const(42, 1337)</code> returns <code>42</code>. | ||
+ | * With higher-order functions: | ||
+ | <pre class="brush:xquery"> | ||
+ | let $zip-sum := function($f, $seq1, $seq2) { | ||
+ | sum(map-pairs($f, $seq1, $seq2)) | ||
+ | } | ||
+ | let $sum-all := $zip-sum(function($a, $b) { $a + $b }, ?, ?), | ||
+ | $sum-left := $zip-sum(hof:const#2, ?, ?) | ||
+ | return ( | ||
+ | $sum-all((1, 1, 1, 1, 1), 1 to 5), | ||
+ | $sum-left((1, 1, 1, 1, 1), 1 to 5) | ||
+ | ) | ||
+ | </pre> | ||
+ | returns <code>20 5</code> | ||
|} | |} | ||
Revision as of 02:29, 1 March 2012
This module adds some useful higher-order XQuery functions that were left out of the official spec. All functions are introduced with the hof:
prefix, which is linked to the statically declared http://basex.org/modules/hof
namespace.
Contents
Functions
hof:id
Signatures | hof:id($expr as item()*) as item()*
|
Summary | Returns its argument unchanged. This function isn't useful on its own, but can be used as argument to other higher-order functions. |
Examples |
let $sort-by := function($f, $seq) { for $x in $seq order by $f($x) return $x } let $sort := $sort-by(hof:id#1, ?), $reverse-sort := $sort-by(function($x) { -$x }, ?) return ( $sort((1, 5, 3, 2, 4)), '|', $reverse-sort((1, 5, 3, 2, 4)) ) returns: |
hof:const
Signatures | hof:const($expr as item()*, $ignored as item()*) as item()*
|
Summary | Returns its first argument unchanged and irgores the second. This function isn't useful on its own, but can be used as argument to other higher-order functions. |
Examples |
let $zip-sum := function($f, $seq1, $seq2) { sum(map-pairs($f, $seq1, $seq2)) } let $sum-all := $zip-sum(function($a, $b) { $a + $b }, ?, ?), $sum-left := $zip-sum(hof:const#2, ?, ?) return ( $sum-all((1, 1, 1, 1, 1), 1 to 5), $sum-left((1, 1, 1, 1, 1), 1 to 5) ) returns |
hof:fold-left1
Signatures | hof:fold-left1($f as function(item()*, item()) as item()*, $seq as item()+) as item()*
|
Summary | Works the same as fn:fold-left($f, $seed, $seq), but doesn't need a seed, because the sequence must be non-empty. |
Errors | XPTY0004 if $seq is empty
|
Examples |
|
hof:until
Signatures | hof:until($pred as function(item()*) as xs:boolean, $f as function(item()*) as item()*, $start as item()*) as item()*
|
Summary | Applies the function $f to the initial value $start until the predicate $pred applied to the result returns true() .
|
Examples |
let $sqrt := function($x as xs:double) as xs:double { hof:until( function($res) { abs($res * $res - $x) < 0.00001 }, function($guess) { ($guess + $x div $guess) div 2 }, $x ) } return $sqrt(25) returns |
hof:top-k-by
Signatures | hof:top-k-by($sort-key as function(item()) as item(), $k as xs:integer, $seq as item()*) as item()*
|
Summary | Returns the $k items in $seq that are greatest when sorted by the result of $f applied to the item. The function is a much more efficient implementation of the following scheme:
( for $x in $seq order by $sort-key($x) descending return $x )[position() <= $k] |
Errors | XPTY0004 if $sort-key doesn't return exactly one item
|
Examples |
|
hof:top-k-with
Signatures | hof:top-k-with($lt as function(item(), item()) as xs:boolean, $k as xs:integer, $seq as item()*) as item()*
|
Summary | Returns the $k items in $seq that are greatest when sorted in the order of the less-than predicate $lt . The function is a general version of hof:top-k-by($sort-key, $k, $seq) .
|
Examples |
|
Recent Changes
- removed
hof:iterate
- added
hof:top-k-by
andhof:top-k-with