Higher-Order Functions Module
This XQuery Module adds some useful higher-order functions, additional to the Higher-Order Functions provided by the official specification.
Conventions
All functions in this module are assigned to the http://basex.org/modules/hof
namespace, which is statically bound to the hof
prefix.
Loops
hof:fold-left1
Signatures | hof:fold-left1($seq as item()+, $f as function(item()*, item()) as item()*) as item()*
|
Summary | Works the same as fn:fold-left, but does not need a seed, because the sequence must be non-empty. |
Examples |
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hof:until
Signatures | hof:until($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 predicate function $pred to $start . If the result is false , $f is invoked with the start value – or, subsequently, with the result of this function – until the predicate function returns true() .
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Examples |
<syntaxhighlight lang="xquery"> hof:until( function($output) { $output ge 1000 }, function($input ) { 2 * $input }, 1 ) </syntaxhighlight>
<syntaxhighlight lang="xquery"> let $sqrt := function($input as xs:double) as xs:double { hof:until( function($result) { abs($result * $result - $input) < 0.00001 }, function($guess) { ($guess + $input div $guess) div 2 }, $input ) } return $sqrt(25) </syntaxhighlight>
<syntaxhighlight lang="xquery"> hof:until( function($_) { true() }, function($_) { error() }, 'OK' ) </syntaxhighlight> |
hof:scan-left
Signatures | hof:scan-left($seq as item()*, $start as item()*, $f as function(item()*, item()) as item()*) as item()*
|
Summary | This function is similar to fn:fold-left, but it returns a list of successive reduced values from the left. It is equivalent to:
<syntaxhighlight lang="xquery"> declare function hof:scan-left($seq, $acc, $f) { if(empty($seq)) then $acc else ( $acc, hof:scan-left(tail($seq), $f($acc, head($seq)), $f) ) }; </syntaxhighlight> |
Examples |
<syntaxhighlight lang="xquery"> hof:scan-left(1 to 10, 0, function($a, $b) { $a + $b }) </syntaxhighlight> |
hof:take-while
Signatures | hof:take-while($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 $pred is satisfied. It is equivalent to:
<syntaxhighlight lang="xquery"> declare function hof:take-while($seq, $pred) { if(empty($seq) or not($pred(head($seq)))) then () else ( head($seq), hof:take-while(tail($seq), $pred) ) }; </syntaxhighlight> |
Examples |
<syntaxhighlight lang="xquery"> hof:take-while( (1 to 100) ! random:integer(50), function($x) { $x >= 10 } ) </syntaxhighlight> |
hof:take-while
Signatures | hof:drop-while($seq as item()*, $pred as function(item()) as xs:boolean) as item()*
|
Summary | The function skips all items of $seq until the predicate $pred is not satisfied anymore. It is equivalent to:
<syntaxhighlight lang="xquery"> declare function hof:drop-while($seq, $pred) { if($pred(head($seq))) then ( hof:drop-while(tail($seq), $pred) ) else ( $seq ) }; </syntaxhighlight> |
Examples | Returns the name of the first file that does not exist on disk:
<syntaxhighlight lang="xquery"> hof:drop-while( (1 to 1000) ! (. || '.log'), file:exists#1 )[1] </syntaxhighlight> |
Sorting
hof:top-k-by
Signatures | hof:top-k-by($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:
<syntaxhighlight lang="xquery"> (for $x in $seq order by $sort-key($x) descending return $x )[position() <= $k] </syntaxhighlight> |
Examples |
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hof:top-k-with
Signatures | hof:top-k-with($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, $k) .
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Examples |
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IDs
hof:id
Signatures | hof:id($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 |
<syntaxhighlight lang="xquery"> let $sort := sort(?, (), hof:id#1) let $reverse-sort := sort(?, (), function($x) { -$x }) return ( $sort((1, 5, 3, 2, 4)), '|', $reverse-sort((1, 5, 3, 2, 4)) )
</syntaxhighlight>
returns: |
hof:const
Signatures | hof:const($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 |
<syntaxhighlight lang="xquery"> let $zip-sum := function($f, $seq1, $seq2) { sum(for-each-pair($seq1, $seq2, $f)) } let $sum-all := $zip-sum(function($a, $b) { $a + $b }, ?, ?) let $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) ) </syntaxhighlight>
<syntaxhighlight lang="xquery"> let $insert-with := function($f, $map, $k, $v) { let $old := $map($k) let $new := if($old) then $f($v, $old) else $v return map:merge(($map, map:entry($k, $new))) } let $map := map { 'foo': 1 } let $add := $insert-with(function($a, $b) { $a + $b }, ?, ?, ?) let $ins := $insert-with(hof:const#2, ?, ?, ?) return ( $add($map, 'foo', 2)('foo'), $ins($map, 'foo', 42)('foo') )
</syntaxhighlight>
returns |
Changelog
- Version 9.5
- Added: hof:drop-while
- 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