Difference between revisions of "Higher-Order Functions Module"
Jump to navigation
Jump to search
LeoWoerteler (talk | contribs) |
LeoWoerteler (talk | contribs) |
||
| Line 40: | Line 40: | ||
|- | |- | ||
| 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-byandhof:top-k-with
