Higher-Order Functions Module
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This XQuery Module adds some useful higher-order functions, additional to the Higher-Order Functions provided by the official specification.
Contents
Conventions
All functions in this module are assigned to the http://basex.org/modules/hof
namespace, which is statically bound to the hof
prefix.
Functions
hof:id
Signatures | hof:id($expr as item()*) as item()*
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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 := sort(?, hof:id#1) let $reverse-sort := sort(?, function($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()*
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Summary | Returns its first argument unchanged and ignores the second. This function isn't useful on its own, but can be used as argument to other higher-order functions, e.g. when a function combining two values is expected and one only wants to retain the left one. |
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) )
let $insert-with := function($f, $map, $k, $v) { let $old := $map($k), $new := if($old) then $f($v, $old) else $v return map:new(($map, map { $k: $new })) } let $map := map { 'foo': 1 } let $add := $insert-with(function($a, $b) {$a + $b}, ?, ?, ?), $insert := $insert-with(hof:const#2, ?, ?, ?) return ( $add($map, 'foo', 2)('foo'), $insert($map, 'foo', 42)('foo') ) returns |
hof:fold-left1
Signatures | hof:fold-left1($seq as item()+, $f as function(item()*, item()) as item()*) as item()*
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Summary | Works the same as fn:fold-left, but doesn't need a seed, because the sequence must be non-empty. |
Examples |
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hof:until
Signatures | hof:until($pred as function(item()*) as xs:boolean, $f as function(item()*) as item()*, $start as item()*) as item()*
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Summary | Applies the function $f to the initial value $start until the predicate $pred applied to the result returns true() .
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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 $sqrt(25) returns |
hof:scan-left
Signatures | hof:scan-left($seq as item()*, $start as item()*, $f as function(item()*, item()) as item()*) as item()*
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Summary | This function is similar to fn:fold-left, but it returns a list of successive reduced values from the left. It is equivalent to:
declare function hof:scan-left($seq, $acc, $f) { if(empty($seq)) then $acc else ( $acc, hof:scan-left(tail($seq), $f($acc, head($seq)), $f) ) }; |
Examples |
hof:scan-left(1 to 10, 0, function($a, $b) { $a + $b }) |
hof:take-while
Signatures | hof:take-while($seq as item()*, $pred as function(item()) as xs:boolean) as item()*
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Summary | The function returns items of $seq as long as the predicate $pred is satisfied. It is equivalent to:
declare function hof:take-while($seq, $pred) { if(empty($seq) or not($pred(head($seq)))) then () else ( head($seq), hof:take-while(tail($seq), $pred) ) }; |
Examples |
hof:take-while( for $i in 1 to 100 return random:integer(50), function($x) { $x >= 10 } ) |
hof:top-k-by
Signatures | hof:top-k-by($seq as item()*, $sort-key as function(item()) as item(), $k as xs:integer) as item()*
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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() <= $k] |
Examples |
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hof:top-k-with
Signatures | hof:top-k-with($seq as item()*, $lt as function(item(), item()) as xs:boolean, $k as xs:integer) as item()*
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Summary | Returns the $k items in $seq that are greatest when sorted in the order of the less-than predicate $lt . The function is a general version of hof:top-k-by($seq, $sort-key, $k) .
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Examples |
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Changelog
- Version 8.1
- Added: hof:scan-left, hof:take-while
- Version 7.2
- Added: hof:top-k-by, hof:top-k-with
- Removed: hof:iterate
- Version 7.0
- module added