Difference between revisions of "Higher-Order Functions Module"
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|- | |- | ||
| valign='top' | '''Summary''' | | valign='top' | '''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. | + | |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, e.g. when a function combining two values is expected and one only wants to retain the left one. |
|- | |- | ||
| valign='top' | '''Examples''' | | valign='top' | '''Examples''' | ||
Line 57: | Line 57: | ||
) | ) | ||
</pre> | </pre> | ||
− | returns <code> | + | * Another use-case: When inserting a key into a map, <code>$f</code> descides how to combine the new value with a possibly existing old one. <code>hof:const</code> here means ignoring the old value, so that's normal insertion. |
+ | <pre class="brush:xquery"> | ||
+ | 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') | ||
+ | ) | ||
+ | </pre> | ||
+ | returns <code>3 42</code> | ||
|} | |} | ||
Revision as of 02:47, 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, 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($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